QNX Developer Support The Philosophy of QNX What is QNX

QNX Developer Support The Philosophy of QNX What is QNX
QNX Developer Support
QNX Developer Support
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The Philosophy of QNX
This chapter covers the following topics:
l What is QNX?
l QNX's microkernel architecture
l Interprocess communication (IPC)
l QNX as a network
What is QNX?
The main responsibility of an operating system is to manage a computer's resources. All activities in
the system - scheduling application programs, writing files to disk, sending data across a network, and
so on - should function together as seamlessly and transparently as possible.
Some environments call for more rigorous resource management and scheduling than others. Realtime
applications, for instance, depend on the operating system to handle multiple events within fixed time
constraints. The more responsive the OS, the more "room" a realtime application has to maneuver
when meeting its deadlines.
The QNX Operating System is ideal for realtime applications. It provides multitasking, priority-driven
preemptive scheduling, and fast context switching - all essential ingredients of a realtime system.
QNX is also remarkably flexible. Developers can easily customize the operating system to meet the
needs of their application. From a "bare-bones" configuration of a kernel with a few small modules to a
full-blown network-wide system equipped to serve hundreds of users, QNX lets you set up your system
to use only those resources you require to tackle the job at hand.
QNX achieves its unique degree of efficiency, modularity, and simplicity through two fundamental
principles:
l microkernel architecture
l message-based interprocess communication
QNX's microkernel architecture
QNX consists of a small kernel in charge of a group of cooperating processes. As the following
illustration shows, the structure looks more like a team than a hierarchy, as several players of equal
rank interact with each other and with their "quarterback" kernel.
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The QNX Microkernel coordinating the system managers.
A true kernel
The kernel is the heart of any operating system. In some systems the "kernel" comprises so many
functions, that for all intents and purposes it is the entire operating system!
But the QNX Microkernel is truly a kernel. First of all, like the kernel of a realtime executive, the QNX
Microkernel is very small. Secondly, it's dedicated to only two essential functions:
l message passing - the Microkernel handles the routing of all messages among all processes
throughout the entire system
l scheduling - the scheduler is a part of the Microkernel and is invoked whenever a process
changes state as the result of a message or interrupt
Unlike processes, the Microkernel itself is never scheduled for execution. It is entered only as the
direct result of kernel calls, either from a process or from a hardware interrupt.
System processes
All QNX services, except those provided by the Microkernel, are handled via standard QNX processes.
A typical QNX configuration has the following system processes:
l Process Manager (Proc)
l Filesystem Manager (Fsys)
l Device Manager (Dev)
l Network Manager (Net)
System processes vs. user-written processes
System processes are practically no different from any user-written program - they have no private or
hidden interfaces that are unavailable to user processes.
It is this architecture that gives QNX unparalleled extensibility. Since most OS services are provided by
standard QNX processes, it's a very simple matter to augment the OS itself: you just write new
programs to provide new services!
In fact, the boundary between the operating system and the application can become very blurred. The
only real difference between system services and applications is that OS services manage resources for
clients.
Let's suppose you've written a database server. How should such a process be classified?
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Just as a filesystem accepts requests (messages in QNX) to open files and read or write data, so too
would a database server. While the requests to the database server may be more sophisticated, both
servers are very much the same in that they provide a set of primitives (implemented by messages)
which in turn provide access to a resource. Both are independent processes that can be written by an
end-user and started on an as-needed basis.
A database server might be considered a system process at one installation, and an application at
another. It really doesn't matter! The important point is that QNX allows such processes to be
implemented cleanly, with no need at all for modifications to the standard components of the operating
system.
Device drivers
Device drivers are processes that shield the operating system from dealing with all the details required
for supporting specific hardware.
Since drivers start up as standard processes, adding a new driver to QNX doesn't affect any other part
of the operating system. The only change you need to make to your QNX environment is to actually
start the new driver.
Once they've completed their initialization, drivers can do either of the following:
l choose to disappear as standard processes, simply becoming extensions to the system process
they're associated with
l retain their individual identity as standard processes
Interprocess communication (IPC)
When several processes run concurrently, as in typical realtime multitasking environments, the
operating system must provide mechanisms to allow processes to communicate with each other.
IPC is the key to designing an application as a set of cooperating processes in which each process
handles one well-defined part of the whole.
QNX provides a simple but powerful set of IPC capabilities that greatly simplify the job of developing
applications made up of cooperating processes.
QNX as a message-passing operating system
QNX was the first commercial operating system of its kind to make use of message passing as the
fundamental means of IPC. QNX owes much of its power, simplicity, and elegance to the complete
integration of the message-passing method throughout the entire system.
In QNX, a message is a packet of bytes passed from one process to another. QNX attaches no special
meaning to the content of a message - the data in a message has meaning for the sender of the
message and for its receiver, but for no one else.
Message passing not only allows processes to pass data to each other, but also provides a means of
synchronizing the execution of several processes. As they send, receive, and reply to messages,
processes undergo various "changes of state" that affect when, and for how long, they may run.
Knowing their states and priorities, the Microkernel can schedule all processes as efficiently as possible
to make the most of available CPU resources. This single, consistent method - message-passing - is
thus constantly operative throughout the entire system.
Realtime and other mission-critical applications generally require a dependable form of IPC, because
the processes that make up such applications are so strongly interrelated. The discipline imposed by
QNX's message-passing design helps bring order and greater reliability to applications.
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QNX as a network
In its simplest form, local area networking provides a mechanism for sharing files and peripheral
devices among several interconnected computers. QNX goes far beyond this simple concept and
integrates the entire network into a single, homogeneous set of resources.
Any process on any machine in the network can directly make use of any resource on any other
machine. From the application's perspective, there is no difference between a local or remote resource
- no special facilities need to be built into applications to make use of remote resources. In fact, a
program would need special code to be able to tell whether a resource such as a file or device was
present on the local computer or was on some other node on the network!
Users may access files anywhere on the network, take advantage of any peripheral device, and run
applications on any machine on the network (provided they have the appropriate authority). Processes
can communicate in the same manner anywhere throughout the entire network. Again, QNX's allpervasive message-passing IPC accounts for such fluid, transparent networking.
Single-computer model
QNX is designed from the ground up as a network-wide operating system. In some ways, a QNX
network feels more like a mainframe computer than a set of micros. Users are simply aware of a large
set of resources available for use by any application. But unlike a mainframe, QNX provides a highly
responsive environment, since the appropriate amount of computing power can be made available at
each node to meet the needs of each user.
In a process control environment, for example, PLCs and other realtime I/O devices may require more
resources than other, less critical, applications, such as a word processor. The QNX network is
responsive enough to support both types of applications at the same time - QNX lets you focus
computing power on the plant floor where and when it's needed, without sacrificing concurrent
connectivity to the desktop.
Flexible networking
QNX networks can be put together using various hardware and industry-standard protocols. Since
these are completely transparent to application programs and users, new network architectures can be
introduced at any time without disturbing the operating system.
The list of specific network hardware that QNX supports may grow over time. For details,
consult the documentation for the network hardware you will be using.
Each node in a QNX network is assigned a unique number that becomes its identifier. This number is
the only visible means to determine whether QNX is running as a network or as a single-processor
operating system.
This degree of transparency is yet another example of the distinctive power of QNX's message-passing
architecture. In many systems, important functions such as networking, IPC, or even message passing
are built on top of the OS, rather than integrated directly into its core. The result is often an awkward,
inefficient "double standard" interface, whereby communication between processes is one thing, while
penetrating the private interface of a mysterious monolithic kernel is another matter altogether!
QNX, on the other hand, is grounded on the principle that effective communication is the key to
effective operation. Message passing thus forms the cornerstone of QNX's architecture and enhances
the efficiency of all transactions among all processes throughout the entire system, whether across a
PC backplane or across a mile of coax.
Let's now take a closer look at the structure and functions of QNX.
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All content ©2004, QNX Software Systems
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The Microkernel
This chapter covers the following topics:
l Introduction
l Interprocess communication
l IPC via messages
l IPC via proxies
l IPC via signals
l IPC across the network
l IPC via semaphores
l Process scheduling
l A word about realtime performance
Introduction
The QNX Microkernel is responsible for the following:
l IPC - the Microkernel supervises the routing of messages; it also manages two other forms of
IPC: proxies and signals
l low-level network communication - the Microkernel delivers all messages destined for processes
on other nodes
l process scheduling - the Microkernel's scheduler decides which process will execute next
l first-level interrupt handling - all hardware interrupts and faults are first routed through the
Microkernel, then passed on to the appropriate driver or system manager
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Inside the QNX Microkernel.
Interprocess communication
The QNX Microkernel supports three essential types of IPC: messages, proxies, and signals.
l Messages - the fundamental form of IPC in QNX. They provide synchronous communication
between cooperating processes where the process sending the message requires proof of
receipt and potentially a reply to the message.
l Proxies - a special form of message. They're especially suited for event notification where the
sending process doesn't need to interact with the recipient.
l Signals - a traditional form of IPC. They're used to support asynchronous interprocess
communication.
IPC via messages
In QNX, a message is a packet of bytes that's synchronously transmitted from one process to another.
QNX attaches no meaning to the content of a message. The data in a message has meaning for the
sender and for the recipient, but for no one else.
Message-passing primitives
To communicate directly with one another, cooperating processes use these C language functions:
C Function:
Purpose:
Send()
to send messages
Receive()
to receive messages
Reply()
to reply to processes that have sent messages
These functions may be used locally or across the network.
Note also that unless processes want to communicate directly with each other, they don't need to use
Send(), Receive(), and Reply(). The QNX C Library is built on top of messaging - processes use
messaging indirectly when they use standard services, such as pipes.
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Process A sends a message to Process B, which subsequently receives, processes, then replies to the
message.
The above illustration outlines a simple sequence of events in which two processes, Process A and
Process B, use Send(), Receive(), and Reply() to communicate with each other:
1.
2.
Process A sends a message to Process B by issuing a Send() request to the Microkernel. At this
point, Process A becomes SEND-blocked until Process B issues a Receive() to receive the
message.
Process B issues a Receive() and receives Process A's waiting message. Process A changes to a
REPLY-blocked state. Since a message was waiting, Process B doesn't block.
(Note that if Process B had issued the Receive() before a message was sent, it would become
RECEIVE-blocked until a message arrived. In this case, the sender would immediately go into
the REPLY-blocked state when it sent its message.)
3.
Process B completes the processing associated with the message it received from Process A and
issues a Reply(). The reply message is copied to Process A, which is made ready to run. A Reply
() doesn't block, so Process B is also ready to run. Who runs depends on the relative priorities
of Process A and Process B.
Process synchronization
Message passing not only allows processes to pass data to each other, but also provides a means of
synchronizing the execution of several cooperating processes.
Let's look at the above illustration again. Once Process A issues a Send() request, it's unable to resume
execution until it has received the reply to the message it sent. This ensures that the processing
performed by Process B for Process A is complete before Process A can resume executing. Moreover,
once Process B has issued its Receive() request, it can't continue processing until it receives another
message.
For details on how QNX schedules processes, see "Process scheduling" in this chapter.
Blocked states
When a process isn't allowed to continue executing - because it must wait for some part of the
message protocol to end - the process is said to be blocked.
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The following table summarizes the blocked states of processes:
If a process has issued a:
The process
is:
Send() request, and the message it has sent hasn't yet been received by the
recipient process
SEND-blocked
Send() request, and the message has been received by the recipient process,
but that process hasn't yet replied
REPLY-blocked
Receive() request, but hasn't yet received a message
RECEIVE-blocked
A process undergoing state changes in a typical send-receive-reply transaction.
For information on all possible process states, see Chapter 3, "The Process Manager."
Using Send(), Receive(), and Reply()
Let's now take a closer look at the Send(), Receive(), and Reply() function calls. We'll stay with our
example of Process A and Process B.
Send()
Let's assume Process A issues a request to send a message to Process B. It issues the request by
means of a Send() function call:
Send( pid, smsg, rmsg, smsg_len, rmsg_len );
The Send() call contains these arguments:
pid
the process ID of the process that is to receive the message (i.e. Process B); a pid is the
identifier by which the process is known to the operating system and to other processes
smsg
the message buffer (i.e. the message to be sent)
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rmsg
the reply buffer (will contain the reply from Process B)
smsg_len
the length of the message being sent
rmsg_len
the maximum length of the reply that Process A will accept
Note that no more than smsg_len bytes will be sent, and no more than rmsg_len bytes will be
accepted in the reply - this ensures that buffers aren't accidentally overwritten.
Receive()
Process B can receive the Send() issued from Process A by issuing a Receive() call:
pid = Receive( 0, msg, msg_len );
The Receive() call contains these arguments:
pid
the process ID of the process that sent the message (i.e. Process A) is returned
0
(zero) specifies that Process B is willing to accept a message from any process
msg
the buffer where the message will be received
msg_len
the maximum amount of data that will be accepted in the receive buffer
If the smsg_len in the Send() call and the msg_len in the Receive() call differ in size, the smaller of
the two determines the amount of data that will be transferred.
Reply()
Having successfully received the message from Process A, Process B should reply to Process A by
issuing a Reply() function call:
Reply( pid, reply, reply_len );
The Reply() call contains these arguments:
pid
the process ID of the process to which the reply is directed (i.e. Process A)
reply the reply buffer
reply_len
the length of the data to be transmitted in the reply
If the reply_len in the Reply() call and the rmsg_len in the Send() call differ in size, the smaller of the
two determines how much data will be transferred.
Reply-driven messaging
The messaging example we just looked at illustrates the most common use of messaging - that in
which a server process is normally RECEIVE-blocked for a request from a client in order to perform
some task. This is called send-driven messaging: the client process initiates the action by sending a
message, and the action is finished by the server replying to the message.
Although not as common as send-driven messaging, another form of messaging is also possible - and
often desirable - to use: reply-driven messaging, in which the action is initiated with a Reply() instead.
Under this method, a "worker" process sends a message to the server indicating that it's available for
work. The server doesn't reply immediately, but rather "remembers" that the worker has sent an
arming message. At some future time, the server may decide to initiate some action by replying to the
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available worker process. The worker process will do the work, then finish the action by sending a
message containing the results to the server.
Additional points to remember
Here are some more things to keep in mind about message passing:
l The message data is maintained in the sending process until the receiver is ready to process
l
l
l
l
the message. There is no copying of the message into the Microkernel. This is safe since the
sending process is SEND-blocked and is unable to inadvertently modify the message data.
The message reply data is copied from the replying process to the REPLY-blocked process as an
atomic operation when the Reply() request is issued. The Reply() doesn't block the replying
process - the REPLY-blocked process becomes unblocked after the data is copied into its space.
The sending process doesn't need to know anything about the state of the receiving process
before sending a message. If the receiving process isn't prepared to receive a message when
the sending process issues it, the sending process simply becomes SEND-blocked.
If necessary, a process can send a zero-length message, a zero-length reply, or both.
From the developer's point of view, issuing a Send() to a server process to get a service is
virtually identical to calling a library subroutine to get the same service. In either case, you set
up some data structures, then make the Send() or the library call. All of the service code
between two well-defined points - Receive() and Reply() for a server process, function entry
and return statement for a library call - then executes while your code waits. When the service
call returns, your code "knows" where results are stored and can proceed to check for error
conditions, process results, or whatever.
Despite this apparent simplicity, the code does much more than a simple library call. The Send
() may transparently go across the network to another machine where the service code actually
executes. It can also exploit parallel processing without the overhead of creating a new process.
The server process can issue a Reply(), allowing the caller to resume execution as soon as it is
safe to do so, and meanwhile continue its own execution.
l There may be messages outstanding from many processes for a single receiving process.
Normally, the receiving process receives the messages in the order they were sent by other
processes; however, the receiving process can specify that messages be received in an order
based on the priority of the sending processes.
Server has received (but not replied to) messages from Client A and Client B. Server has not yet
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received messages from Client C, Client D, and Client E.
Advanced facilities
QNX also provides these advanced message-passing facilities:
l conditional message reception
l reading or writing part of a message
l multipart messages
Conditional message reception
Generally, when a process wants to receive messages, it uses Receive() to wait for a message to
arrive. This is the normal way of receiving messages and is appropriate in most circumstances.
In some cases, however, a process may need to determine whether messages are pending, yet may
not want to become RECEIVE-blocked in the absence of a pending message. For example, a process
needs to poll a free-running device at high speed - the device isn't capable of generating interrupts but the process still has to respond to messages from other processes. In this case, the process could
use the Creceive() function to read a message, if one became available, yet return immediately if no
further messages were pending.
You should avoid Creceive(), if possible, since it allows a process to consume the processor
continuously at its priority level.
Reading or writing part of a message
Sometimes it's desirable to read or write only part of a message at a time so that you can use the
buffer space already allocated for the message instead of allocating a separate work buffer.
For example, an I/O manager may accept messages of data to be written that consist of a fixed-size
header followed by a variable amount of data. The header contains the byte count of the data (0 to
64K bytes). The I/O manager may elect to receive only the header and then use the Readmsg()
function to read the variable-length data directly into an appropriate output buffer. If the sent data
exceeds the size of the I/O manager's buffer, the manager may issue several Readmsg() requests over
time to transfer the data as space becomes available. Likewise, the Writemsg() function can be used to
collect data over time and copy it back to the sender's reply buffer as it becomes available, thus
reducing the I/O manager's internal buffer requirements.
Multipart messages
Up to now, messages have been discussed as single packets of bytes. However, messages often
consist of two or more discrete components. For example, a message may have a fixed-length header
followed by a variable amount of data. To ensure that its components will be efficiently sent or
received without being copied into a temporary work buffer, a multipart message can be constructed
from two or more separate message buffers. This facility helps QNX I/O managers, such as Dev and
Fsys, achieve their high performance.
The following functions are available to handle multipart messages:
l Creceivemx()
l Readmsgmx()
l Receivemx()
l Replymx()
l Sendmx()
l Writemsgmx()
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Multipart messages can be specified with an mx control structure. The Microkernel assembles these
into a single data stream.
Reserved message codes
Although you aren't required to do so, QNX begins all of its messages with a 16-bit word called a
message code. Note that QNX system processes use message codes in the following ranges:
Reserved range:
Description:
0x0000 to 0x00FF
Process Manager messages
0x0100 to 0x01FF
I/O messages (common to all I/O servers)
0x0200 to 0x02FF
Filesystem Manager messages
0x0300 to 0x03FF
Device Manager messages
0x0400 to 0x04FF
Network Manager messages
0x0500 to 0x0FFF
Reserved for future QNX system processes
IPC via proxies
A proxy is a form of non-blocking message especially suited for event notification where the sending
process doesn't need to interact with the recipient. The only function of a proxy is to send a fixed
message to a specific process that owns the proxy. Like messages, proxies work across the network.
By using a proxy, a process or an interrupt handler can send a message to another process without
blocking or having to wait for a reply.
Here are some examples of when proxies are used:
l A process wants to notify another process that an event has occurred, but can't afford to send a
message (which would cause it to become blocked until the recipient issues a Receive() and a
Reply()).
l A process wants to send data to another process, but needs neither a reply nor any other
acknowledgment that the recipient has received the message.
l An interrupt handler wants to tell a process that some data is available for processing.
Proxies are created with the qnx_proxy_attach() function. Any other process or any interrupt handler
that knows the identification of the proxy can then cause the proxy to deliver its predefined message
by using the Trigger() function. The Microkernel handles the Trigger() request.
A proxy can be triggered more than once - it sends a message for each time it's triggered. A proxy
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process can queue up to 65,535 messages for delivery.
A client process triggers a proxy three times, which causes the server to receive three "canned"
messages from the proxy.
IPC via signals
Signals are a traditional method of asynchronous communication that have been available for many
years in a variety of operating systems.
QNX supports a rich set of POSIX-compliant signals, some historical UNIX signals, as well as some
QNX-specific signals.
Generating signals
A signal is considered to have been delivered to a process when the process-defined action for that
signal is taken. A process can set a signal on itself.
If you want to:
Use the:
Generate a signal from the shell
kill or slay utilities
Generate a signal from within a process
kill() or raise() C functions
Receiving signals
A process can receive a signal in one of three ways, depending on how it has defined its signalhandling environment:
l If the process has taken no special action to handle signals, the default action for the signal is
taken - usually, this default action is to terminate the process.
l The process can ignore the signal. If a process ignores a signal, there's no effect on the process
when the signal is delivered (note that the SIGCONT, SIGKILL, and SIGSTOP signals can't be
ignored under normal circumstances).
l The process can provide a signal handler for the signal - a signal handler is a function in the
process that is invoked when the signal is delivered. When a process contains a signal handler
for a signal, it is said to be able to "catch" the signal. Any process that catches a signal is, in
effect, receiving a form of software interrupt. No data is transferred with the signal.
Between the time that a signal is generated and the time that it's delivered, the signal is said to be
pending. Several distinct signals can be pending for a process at a given time. Signals are delivered to
a process when the process is made ready to run by the Microkernel's scheduler. A process should
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make no assumptions about the order in which pending signals are delivered.
Summary of signals
Signal:
Description:
SIGABRT
Abnormal termination signal such as issued by the abort() function.
SIGALRM
Timeout signal such as issued by the alarm() function.
SIGBUS
Indicates a memory parity error (QNX-specific interpretation). Note that if a second fault
occurs while your process is in a signal handler for this fault, the process will be
terminated.
SIGCHLD
Child process terminated. The default action is to ignore the signal.
SIGCONT
Continue if HELD. The default action is to ignore the signal if the process isn't HELD.
SIGDEV
Generated when a significant and requested event occurs in the Device Manager
SIGFPE
Erroneous arithmetic operation (integer or floating point), such as division by zero or an
operation resulting in overflow. Note that if a second fault occurs while your process is in
a signal handler for this fault, the process will be terminated.
SIGHUP
Death of session leader, or hangup detected on controlling terminal.
SIGILL
Detection of an invalid hardware instruction. Note that if a second fault occurs while your
process is in a signal handler for this fault, the process will be terminated.
SIGINT
Interactive attention signal (Break)
SIGKILL
Termination signal - should be used only for emergency situations. This signal cannot be
caught or ignored. Note that a server with superuser privileges may protect itself from
this signal via the qnx_pflags() function.
SIGPIPE
Attempt to write on a pipe with no readers.
SIGPWR
Soft boot requested via Ctrl -Alt -Shift -Del or shutdown utility.
SIGQUIT
Interactive termination signal.
SIGSEGV
Detection of an invalid memory reference. Note that if a second fault occurs while your
process is in a signal handler for this fault, the process will be terminated.
SIGSTOP
HOLD process signal. The default action is to hold the process. Note that a server with
superuser privileges may protect itself from this signal via the qnx_pflags() function.
SIGTERM
Termination signal
SIGTSTP
Not supported by QNX.
SIGTTIN
Not supported by QNX.
SIGTTOU
Not supported by QNX.
SIGUSR1
Reserved as application-defined signal 1
SIGUSR2
Reserved as application-defined signal 2
SIGWINCH Window size changed
Defining signal processing
To define the type of handling you want for each signal, you use the ANSI C signal() function or the
POSIX sigaction() function.
The sigaction() function gives you greater control over the signal-handling environment.
You may change the type of handling for a signal at any time. If you set the signal handling for a
function to ignore the signal, any pending signals of that type will be immediately discarded.
Catching signals
Some special considerations apply to processes that catch signals with a signal-handling function.
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The signal-handling function is similar to a software interrupt. It is executed asynchronously to the rest
of the process. Therefore, it's possible for a signal handler to be entered while any function in the
program is running (including library functions).
If your process doesn't return from the signal handler, it can use either siglongjmp() or longjmp(), but
siglongjmp() is preferred. With longjmp(), the signal remains blocked.
Blocking signals
Sometimes you may want to temporarily prevent a signal from being delivered, without changing the
method of how the signal is handled when it is delivered. QNX provides a set of functions that let you
block delivery of signals. A signal that is blocked remains pending; once unblocked, it is delivered to
your program.
While your process is executing a signal handler for a particular signal, QNX automatically blocks that
signal. This means that you don't have to worry about setting up nested invocations of your handler.
Each invocation of your signal handler is an atomic operation with respect to the delivery of further
signals of that type. If your process returns normally from the handler, the signal is automatically
unblocked.
Some UNIX systems have a flawed implementation of signal handlers in that they reset the
signal to the default action rather than block the signal. As a result, some UNIX applications call
the signal() function within the signal handler to re-arm the handler. This has two windows of
failure. First, if another signal arrives while your program is in the handler but before signal() is
called, your program may be killed. Second, if a signal arrives just after the call to signal() in
the handler, you might enter your handler recursively. QNX supports signal blocking and
therefore avoids these problems. You don't need to call signal() within your handler. If you
leave your handler via a long jump, you should use the siglongjmp() function.
Signals and messages
There's an important interaction between signals and messages. If your process is SEND-blocked or
RECEIVE-blocked when a signal is generated - and you have a signal handler - the following actions
occur:
1.
2.
3.
The process is unblocked.
Signal-handling processing takes place
The Send() or Receive() returns with an error
If your process was SEND-blocked at the time, this doesn't represent a problem, because the recipient
wouldn't have received a message. But if your process was REPLY-blocked, you won't know whether
the sent message had been handled or not, and therefore won't know whether to retry the Send().
It's possible for a process acting as a server (i.e. it is receiving messages) to ask that it be notified
when a client process is signaled while in the REPLY-blocked state. In this case, the client process is
made SIGNAL-blocked with a pending signal and the server process receives a special message
describing the type of signal. The server process can then decide to do either of the following:
l Complete the original request normally - the sender is assured that the message was handled
properly.
OR
l Release any resources tied up and return an error indicating that the process was unblocked by
a signal - the sender receives a clear-error indication.
When the server replies to a process that was SIGNAL-blocked, the signal will take effect immediately
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after the sender's Send() returns.
IPC across the network
Virtual circuits
A QNX application can talk to a process on another computer on the network just as if it were talking
to another process on the same machine. As a matter of fact, from the application's perspective,
there's no difference between a local and remote resource.
This remarkable degree of transparency is made possible by virtual circuits (VCs), which are paths the
Network Manager provides to transmit messages, proxies, and signals across the network.
VCs contribute to efficient overall use of resources in a QNX network for several reasons:
1.
2.
3.
4.
When a VC is created, it's given the ability to handle messages up to a specified size; this
means you can preallocate resources to handle the message. Nevertheless, if you need to send
a message larger than the maximum specified size, the VC is automatically resized to
accommodate the larger message.
If two processes residing on different nodes are communicating with each other via more than
one VC, the VCs are shared - only one real virtual circuit exists between the processes. This
situation occurs commonly when a process accesses several files on a remote filesystem.
If a process attaches to an existing shared VC and it requests a buffer size larger than that
currently in use, the buffer size is automatically increased.
When a process terminates, its associated VCs are automatically released.
Virtual processes
A sending process is responsible for setting up the VC between itself and the process it wants to
communicate with. To do so, the sending process usually issues a qnx_vc_attach() function call. In
addition to creating a VC, this call also creates a virtual process ID, or VID, at each end of the circuit.
To the process at either end of the virtual circuit, the VID on its end appears to have the process ID of
the remote process it wants to communicate with. Processes communicate with each other via these
VIDs.
For example, in the following illustration, a virtual circuit connects PID 1 to PID 2. On node 20 - where
PID 1 resides - a VID represents PID 2. On node 40 - where PID 2 resides - a VID represents PID 1.
Both PID 1 and PID 2 can refer to the VID on their node as if it were any other local process (sending
messages, receiving messages, raising signals, waiting, etc.). So, for example, PID 1 can send a
message to the VID on its end, and this VID will relay the message across the network to the VID
representing PID 1 on the other end. This VID will then route the message to PID 2.
Network communications is handled with virtual circuits. When PID 1 sends to VID 2, the send request
is relayed across the virtual circuit causing VID 1 to send to PID 2.
Each VID maintains a connection that contains the following information:
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l local pid
l remote pid
l remote nid (node ID)
l remote vid
You probably won't come into direct contact with VCs very often. For example, when an application
wants to access an I/O resource across the network, a VC is created by the open() library function on
the application's behalf. The application has no direct part in the creation or use of the VC. Again,
when an application establishes the location of a server with qnx_name_locate(), a VC is automatically
created on behalf of the application. To the application, the VC simply appears to be a PID.
For more information on qnx_name_locate(), see the discussion of process symbolic names in Chapter
3.
Virtual proxies
A virtual proxy allows a proxy to be triggered from a remote node, much like a virtual circuit allows a
process to exchange messages with a remote node.
Unlike a virtual circuit, which binds two processes together, a virtual proxy allows any process on the
remote node to trigger it.
Virtual proxies are created by qnx_proxy_rem_attach(), which takes a node (nid_t) and proxy (pid_t)
as arguments. A virtual proxy is created on the remote node, which refers to the proxy on the caller's
node.
A virtual proxy is created on the remote node, which refers to the proxy on the caller's node.
Note that the virtual circuit is created automatically on the caller's node by qnx_proxy_rem_attach().
Terminating virtual circuits
A process might become unable to communicate over an established VC for various reasons:
l The computer it was running on was powered down.
l The network cable to the computer was disconnected.
l The remote process it was communicating with was terminated.
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Any of these conditions can prevent messages from being transmitted over a VC. It's necessary to
detect these situations so that applications can take remedial action or terminate themselves
gracefully. If this isn't done, valuable resources can be unnecessarily tied up.
The Process Manager on each node checks the integrity of the VCs on its node. It does this as follows:
1.
2.
3.
4.
Each time a successful transmission takes place on a VC, a time stamp associated with the VC
is updated to indicate the time of last activity.
At installation-defined intervals, the Process Manager looks at each VC. If there's been no
activity on a circuit, the Process Manager sends a network integrity packet to the Process
Manager on the node at the other end of the circuit.
If no response comes back, or if a problem is indicated, the VC is flagged as having a problem.
An installation-defined number of attempts are then made to re-establish contact.
If the attempts fail, the VC is dismantled; any process blocked on the VC is made READY. (The
process sees a failure return code from the communication primitive on the VC.)
To control parameters related to this integrity check, you use the netpoll utility.
IPC via semaphores
Semaphores are another common form of synchronization that allows processes to "post" (sem_post
()) and "wait" (sem_wait()) on a semaphore to control when processes wake or sleep. The post
operation increments the semaphore; the wait operation decrements it.
If you wait on a semaphore that's positive, you won't block. Waiting on a non-positive semaphore will
block until some other process executes a post. It's valid to post one or more times before a wait - this
will allow one or more processes to execute the wait without blocking.
A significant difference between semaphores and other synchronization primitives is that semaphores
are "async safe" and can be manipulated by signal handlers. If the desired effect is to have a signal
handler wake a process, semaphores are the right choice.
Process scheduling
When scheduling decisions are made
The Microkernel's scheduler makes scheduling decisions when:
l a process becomes unblocked
l the timeslice for a running process expires
l a running process is preempted
Process priorities
In QNX, every process is assigned a priority. The scheduler selects the next process to run by looking
at the priority assigned to every process that is READY (a READY process is one capable of using the
CPU). The process with the highest priority is selected to run.
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The ready queue for six processes (A-F) which are READY. All other processes (G-Z) are BLOCKED.
Process A is currently running. Processes A, B, and C are at the highest priority, so will share the
processor based on the running process's scheduling algorithm.
The priorities assigned to processes range from 0 (the lowest) to 31 (the highest). The default priority
for a new process is inherited from its parent; this is normally set to 10 for applications started by the
Shell.
If you want to:
Use this function:
Determine the priority of a process
getprio()
Set the priority of a process
setprio()
Scheduling methods
To meet the needs of various applications, QNX provides three scheduling methods:
l FIFO scheduling
l round-robin scheduling
l adaptive scheduling
Each process on the system may run using any one of these methods. They are effective on a perprocess basis, not on a global basis for all processes on a node.
Remember that these scheduling methods apply only when two or more processes that share the same
priority are READY (i.e. the processes are directly competing with each other). If a higher-priority
process becomes READY, it immediately preempts all lower-priority processes.
In the following diagram, three processes of equal priority are READY. If Process A blocks, Process B
will run.
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Process A blocks, Process B runs.
Although a process inherits its scheduling method from its parent process, you can change the
method.
If you want to:
Use this function:
Determine the scheduling method for a process
getscheduler()
Set the scheduling method for a process
setscheduler()
FIFO scheduling
In FIFO scheduling, a process selected to run continues executing until it:
l voluntarily relinquishes control (e.g. it makes any kernel call)
l is preempted by a higher-priority process
FIFO scheduling. Process A runs until it blocks.
Two processes that run at the same priority can use FIFO scheduling to ensure mutual exclusion to a
shared resource. Neither process will be preempted by the other while it is executing. For example, if
they shared a memory segment, each of the two processes could update the segment without
resorting to some form of semaphoring.
Round-robin scheduling
In round-robin scheduling, a process selected to run continues executing until it:
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l voluntarily relinquishes control
l is preempted by a higher-priority process
l consumes its timeslice
Round-robin scheduling. Process A ran until it consumed its timeslice; the next READY process
(Process B) now runs.
A timeslice is the unit of time assigned to every process. Once it consumes its timeslice, a process is
preempted and the next READY process at the same priority level is given control. A timeslice is 50
milliseconds.
Apart from time slicing, round-robin scheduling is identical to FIFO scheduling.
Adaptive scheduling
In adaptive scheduling, a process behaves as follows:
l If the process consumes its timeslice (i.e. it doesn't block), its priority is reduced by 1. This is
known as priority decay. Note that a "decayed" process won't continue decaying, even if it
consumes yet another timeslice without blocking - it will drop only one level below its original
priority.
l If the process blocks, it immediately reverts to its original priority.
Adaptive scheduling. Process A consumed its timeslice; its priority was then dropped by 1. The next
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READY process (Process B) runs.
You can use adaptive scheduling in environments where potentially compute-intensive background
processes are sharing the computer with interactive users. You should find that adaptive scheduling
gives the compute-intensive processes sufficient access to the CPU, yet retains fast interactive
response for other processes.
Adaptive scheduling is the default scheduling method for programs created by the Shell.
Client-driven priority
In QNX, most transactions between processes follow a client/server model. Servers provide some form
of service and clients send messages to these servers to request service. In general, servers are more
trusted and vital than clients.
Clients usually outnumber servers. As a result, a server will likely run at a priority that exceeds the
priorities of all its clients. The scheduling method may be any of the three previously described, but
round-robin is probably the most common.
If a low-priority client sends a message to the server, then its request will by default be handled at the
higher priority of the server. This has indirectly boosted the priority of the client, because the client's
request is what causes the server to run.
As long as the server runs for a short period of time to satisfy the request, this usually isn't a concern.
If the server runs for a more extended period, then a low-priority client may adversely affect other
processes at priorities higher than the client but lower than the server.
To solve this dilemma, a server may elect to have its priority driven by the priority of the clients that
send it messages. When the server receives a message, its priority will be set to that of the client.
Note that only its priority is changed - its scheduling method stays the same. If another message
arrives while the server is running, the server's priority will be boosted if the new client's priority is
greater than the server's. In effect, the new client "turbocharges" the server to its priority, allowing it
to finish the current request so it can handle the new client's request. If this weren't done, the new
client would have its priority lowered as it blocked on a lower-priority server.
If you select client-driven priorities for your server, you should also request that messages be
delivered in priority order (as opposed to time order).
To enable client-driven priority, you use the qnx_pflags() function as follows:
qnx_pflags(~0, _PPF_PRIORITY_FLOAT
| _PPF_PRIORITY_REC, 0, 0);
A word about realtime performance
No matter how much we wish it were so, computers are not infinitely fast. In a realtime system, it's
absolutely crucial that CPU cycles aren't unnecessarily spent. It's also crucial that you minimize the
time it takes from the occurrence of an external event to the actual execution of code within the
program responsible for reacting to that event. This time is referred to as latency.
Several forms of latency are encountered in a QNX system.
Interrupt latency
Interrupt latency is the time from the reception of a hardware interrupt until the first instruction of a
software interrupt handler is executed. QNX leaves interrupts fully enabled almost all the time, so that
interrupt latency is typically insignificant. But certain critical sections of code do require that interrupts
be temporarily disabled. The maximum such disable time usually defines the worst-case interrupt
latency - in QNX this is very small.
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The following diagrams illustrate the case where a hardware interrupt is processed by an established
interrupt handler. The interrupt handler either will simply return, or it will return and cause a proxy to
be triggered.
Interrupt handler simply terminates.
The interrupt latency (Til) in the above diagram represents the minimum latency - that which occurs
when interrupts were fully enabled at the time the interrupt occurred. Worst-case interrupt latency will
be this time plus the longest time in which QNX, or the running QNX process, disables CPU interrupts.
Til on various CPUs
The following table shows typical interrupt-latency times (Til) for a range of processors:
Interrupt latency (Til):
Processor:
3.3 microsec
166 MHz Pentium
4.4 microsec
100 MHz Pentium
5.6 microsec
100 MHz 486DX4
22.5 microsec
33 MHz 386EX
Scheduling latency
In some cases, the low-level hardware interrupt handler must schedule a higher-level process to run.
In this scenario, the interrupt handler will return and indicate that a proxy is to be triggered. This
introduces a second form of latency - scheduling latency - which must be accounted for.
Scheduling latency is the time between the termination of an interrupt handler and the execution of
the first instruction of a driver process. This usually means the time it takes to save the context of the
currently executing process and restore the context of the required driver process. Although larger
than interrupt latency, this time is also kept small in a QNX system.
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Interrupt handler terminates, triggering a proxy.
It's important to note that most interrupts terminate without triggering a proxy. In a large number of
cases, the interrupt handler can take care of all hardware-related issues. Triggering a proxy to kick a
higher-level driver process occurs only when a significant event occurs. For example, the interrupt
handler for a serial device driver would feed one byte of data to the hardware upon each received
transmit interrupt, and would trigger the higher-level process (Dev) only when the output buffer is
finally empty.
Tsl on various CPUs
This table shows typical scheduling-latency times (Tsl) for a range of processors:
Scheduling latency (Tsl):
Processor:
4.7 microsec
166 MHz Pentium
6.7 microsec
100 MHz Pentium
11.1 microsec
100 MHz 486DX4
74.2 microsec
33 MHz 386EX
Stacked interrupts
Since microcomputer architectures allow hardware interrupts to be given priorities, higher-priority
interrupts can preempt a lower-priority interrupt.
This mechanism is fully supported in QNX. The previous scenarios describe the simplest - and most
common - situation where only one interrupt occurs. Substantially similar timing is true for the
highest-priority interrupt. Worst-case timing considerations for lower-priority interrupts must take into
account the time for all higher-priority interrupts to be processed, since in QNX, a higher-priority
interrupt will preempt a lower-priority interrupt.
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Process A is running. Interrupt IRQx causes interrupt handler Intx to run, which is preempted by IRQy
and its handler Inty. Inty triggers a proxy causing Process B to run; Intx triggers a proxy causing
Process C to run.
All content ©2004, QNX Software Systems
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The Process Manager
This chapter covers the following topics:
l Introduction
l The life cycle of a process
l Process states
l Process symbolic names
l Timers
l Interrupt handlers
Introduction
Process Manager responsibilities
The Process Manager works closely with the Microkernel to provide essential operating system
services. Although it shares the same address space as the Microkernel (and is the only process to do
so), the Process Manager runs as a true process. As such, it is scheduled to run by the Microkernel like
all other processes and it uses the Microkernel's message-passing primitives to communicate with
other processes in the system.
The Process Manager is responsible for creating new processes in the system and managing the most
fundamental resources associated with a process. These services are all provided via messages. For
example, if a running process wants to create a new process, it does so by sending a message
containing the details of the new process to be created. Note that since messages are network-wide,
you can easily create a process on another node by sending the process-creation message to the
Process Manager on that node.
Process creation primitives
QNX supports three process-creation primitives:
l fork()
l exec()
l spawn()
Both fork() and exec() are defined by POSIX, while the implementation of spawn() is unique to QNX.
fork()
The fork() primitive creates a new process that is an exact image of the calling process. The new
process shares the same code as the calling process and inherits a copy of all of the calling process's
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data.
exec()
The exec() primitive replaces the calling process image with a new process image. There's no return
from a successful exec(), because the new process image overlays the calling process image. It's
common practice in POSIX systems to create a new process- without removing the calling process-by
first calling fork(), and then having the child of the fork() call exec().
spawn()
The spawn() primitive creates a new process as a child of the calling process. It can avoid the need to
fork() and exec(), resulting in a faster and more efficient means for creating new processes. Unlike
fork() and exec(), which by their very nature operate on the same node as the calling process, the
spawn() primitive can create processes on any node in the network.
Process inheritance
When a process is created by one of the three primitives just described, it inherits much of its
environment from its parent. This is summarized in the following table:
Item inherited
fork()
exec()
spawn()
process ID
no
yes
no
open files
yes
optional*
optional
file locks
no
yes
no
pending signals
no
yes
no
signal mask
yes
optional
optional
ignored signals
yes
optional
optional
signal handlers
yes
no
no
environment variables
yes
optional
optional
session ID
yes
yes
optional
process group
yes
yes
optional
real UID, GID
yes
yes
yes
effective UID, GID
yes
optional
optional
current working directory
yes
optional
optional
file creation mask
yes
yes
yes
priority
yes
optional
optional
scheduler policy
yes
optional
optional
virtual circuits
no
no
no
symbolic names
no
no
no
realtime timers
no
no
no
*optional: the caller may select either yes or no, as needed.
The life cycle of a process
A process goes through four phases:
1.
2.
3.
Creation
Loading
Execution
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4.
Termination
Creation
Creating a process consists of allocating a process ID for the new process and setting up the
information that defines the environment of the new process. Most of this information is inherited from
the parent of the new process (see the previous section on "Process inheritance").
Loading
The loading of process images is done by a loader thread. The loader code resides in the Process
Manager, but the thread runs under the process ID of the new process. This lets the Process Manager
handle other requests while loading programs.
Execution
Once the program code has been loaded, the process is ready for execution; it begins to compete with
other processes for CPU resources. Note that all processes run concurrently with their parents. In
addition, the death of a parent process does not automatically cause the death of its child processes.
Termination
A process is terminated in either of two ways:
l a signal whose defined action is to cause process termination is delivered to the process
l the process invokes exit(), either explicitly or by default action when returning from main()
Termination involves two stages:
1.
2.
A termination thread in the Process Manager is run. This "trusted" code is in the Process
Manager but the thread runs with the process ID of the terminating process. This thread closes
all open file descriptors and releases the following:
¡ any virtual circuits held by the process
¡ all memory allocated to the process
¡ any symbolic names
¡ any major device numbers (I/O managers only)
¡ any interrupt handlers
¡ any proxies
¡ any timers
After the termination thread is run, notification of process termination is sent to the parent
process (this phase runs inside the Process Manager).
If the parent process hasn't issued a wait() or waitpid() call, the child process becomes a
"zombie" process and won't terminate until the parent process issues a wait() or terminates. (If
you don't want a process to wait on the death of children, you should either set the
_SPAWN_NOZOMBIE flag with qnx_spawn() or set the action for SIGCHLD to be SIG_IGN via
signal(). This way, children won't become zombies when they die.)
A parent process can wait() on the death of a child spawned on a remote node. If the parent of
a zombie process terminates, the zombie is released.
If a process is terminated by the delivery of a termination signal and the dumper utility is running, a
memory-image dump is generated. You can examine this dump with the symbolic debugger.
Process states
A process is always in one of the following states:
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l READY - the process is capable of using the CPU (i.e. it isn't waiting for any event to occur).
l BLOCKED - the process is in one of the following blocked states:
¡ SEND-blocked
¡ RECEIVE-blocked
¡ REPLY-blocked
¡ SIGNAL-blocked
¡ SEMAPHORE-blocked
l HELD - the process has received a SIGSTOP signal. Until it's removed from the HELD state, a
process isn't eligible to use the CPU; the only way to remove it from the HELD state is to either
deliver a SIGCONT signal or terminate the process via a signal.
l WAIT-blocked - the process has issued a wait() or waitpid() call to wait for status from one or
more of its child processes.
l DEAD - the process has terminated but is unable to send its exit status to its parent because
the parent hasn't issued a wait() or waitpid(). A DEAD process has a state, but the memory it
once occupied has been released. A DEAD process is also known as a zombie process.
For more information on blocked states, see Chapter 2, The Microkernel.
Possible process states in a QNX system.
The transactions depicted in the previous diagram are as follows:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
Process sends message.
Target process receives message.
Target process replies.
Process waits for message.
Process receives message.
Signal unblocks process.
Signal attempts to unblock process; target has requested message signal catching.
Target process receives signal message.
Process waits on death of child.
Child dies or signal unblocks process.
SIGSTOP set on process.
SIGCONT set on process.
Process dies.
Parent waits on death, terminates itself or has already terminated.
Process calls semwait() on a non-positive semaphore.
Another process calls sempost() or an unmasked signal is delivered.
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Determining process states
To determine the state of an individual process from within the Shell, use the ps and sin utilities (from
within applications, use the qnx_psinfo() function).
To determine the state of the operating system as a whole from within the Shell, use the sin utility
(from within applications, use the qnx_osinfo() function).
The ps utility is defined by POSIX; its use in command scripts may be portable. The sin utility, on the
other hand, is unique to QNX; it gives you helpful QNX-specific information that you can't get from the
ps utility.
Process symbolic names
QNX encourages the development of applications that are split up into cooperating processes. An
application that exists as a team of cooperating processes exhibits greater concurrency and can be
network-distributed for greater performance.
Splitting up applications into cooperating processes requires special considerations, however. If
cooperating processes are to communicate reliably with each other, they must be able to determine
each other's process ID. For example, let's say you have a database server process that provides
services to an arbitrary number of clients. The clients start and stop at any time, but the server always
remains available. How do client processes discover the process ID of the database server so they can
send messages to it?
In QNX, the solution is to let processes give themselves a symbolic name. In the context of a single
node, processes can register this name with the Process Manager on the node where they're running.
Other processes can then ask the Process Manager for the process ID associated with that name.
The situation becomes more complex in a network environment where a server may need to service
clients from several nodes across a network. QNX accordingly provides the ability to support both
global names and local names. Global names are known across the network, whereas local names are
known only on the node where they're registered. Global names start with a slash (/). For example:
qnx/fsys
local name
company/xyz
local name
/company/xyz
global name
We recommend that you prefix all your names with your company name to reduce name
conflicts among vendors.
In order for global names to be used, a process known as a process name locator (i.e. the nameloc
utility) must be running on at least one node of a network. This process maintains a record of all global
names that have been registered.
Up to ten process name locators may be active on a network at a particular time. Each maintains an
identical copy of all active global names. This redundancy ensures that a network can continue to
function properly even if one or more nodes supporting process name location fail simultaneously.
To attach a name, a server process uses the qnx_name_attach() function. To locate a process by
name, a client process uses the qnx_name_locate() function.
Timers
Time management
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In QNX, time management is based on a system timer maintained by the operating system. The timer
contains the current Coordinated Universal Time (UTC) relative to 0 hours, 0 minutes, 0 seconds,
January 1, 1970. To establish local time, time management functions use the TZ environment variable
(which is described in the Installation & Configuration guide).
Simple timing facilities
Shell scripts and processes can pause for a specific number of seconds, using a simple timing facility.
For Shell scripts, this facility is provided by the sleep utility; for processes, it's provided by the sleep()
function. You can also use the delay() function, which takes a time interval specified in milliseconds.
Advanced timing facilities
A process can also create timers, arm them with a time interval, and remove timers. These advanced
timing facilities are based on the POSIX Std 1003.1b specification.
Creating timers
A process can create one or more timers. These timers can be any mix of supported timer types,
subject to a configurable limit on the total number of timers supported by the operating system (see
Proc in the Utilities Reference). To create timers, you use the timer_create() function.
Arming timers
You can arm timers with the following time intervals:
l absolute-the time relative to 0 hours, 0 minutes, 0 seconds, January 1, 1970.
l relative-the time relative to the current clock value.
You can also have a timer go off repeatedly at a specified interval. For example, let's say you have a
timer armed to go off at 9 am tomorrow morning. You can specify that it should also go off every five
minutes thereafter.
You can also set a new time interval on an existing timer. The effect of the new time interval depends
on the interval type:
l for an absolute timer, the new interval replaces the current time interval
l for a relative timer, the new interval is added to any remaining time interval
To arm timers with:
an absolute or relative time interval
Use this function:
timer_settime()
Removing timers
To remove timers, you use the timer_delete() function.
Setting timer resolution
You can set the resolution of timers by using the ticksize utility or the qnx_ticksize() function. You
can adjust the resolution from 500 microseconds to 50 milliseconds.
Reading timers
To inspect the interval outstanding on a timer, or to check if the timer has been removed, you use the
timer_gettime() function.
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Interrupt handlers
Interrupt handlers service the computer's hardware interrupt system - they react to hardware
interrupts and manage the low-level transfer of data between the computer and external devices.
Interrupt handlers are physically packaged as part of a standard QNX process (e.g. a driver), but they
always run asynchronously to the process they're associated with.
An interrupt handler:
l is entered with a far call, not directly from the interrupt itself (this can be written in C, rather
than in assembler)
l runs in the context of the process it is embedded in, so it has access to all the global variables
of the process
l runs with interrupts enabled, but is preempted only if a higher-priority interrupt occurs
l shouldn't talk directly to the 8259 interrupt hardware (the operating system takes care of this)
l should be as short as possible.
Several processes may attach to the same interrupt (if supported by the hardware). When a physical
interrupt occurs, each interrupt handler in turn will be given control. No assumptions should be made
about the order in which interrupt handlers sharing an interrupt are invoked.
If you want to:
Use this function:
Establish a hardware interrupt
qnx_hint_attach()
Remove a hardware interrupt
qnx_hint_detach()
Timer interrupt handlers
You can attach an interrupt handler directly to the system timer so that the handler will be invoked on
each timer interrupt. To set the period, you can use the ticksize utility.
You can also attach to a scaled timer interrupt that will activate every 50 milliseconds, regardless of
the tick size. These timers offer a lower-level alternative to the POSIX 1003.4 timers.
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I/O Namespace
This chapter covers the following topics:
l Introduction
l Resolving pathnames
l File descriptor namespace
Introduction
I/O namespace
The I/O resources of QNX are not built into the Microkernel. Instead, the I/O services are provided by
processes that may be started dynamically while the system is running. Since the QNX filesystem is
optional, the pathname space isn't built into the filesystem as it is in most monolithic systems.
Prefixes and regions of authority
With QNX, the pathname space is divided into regions of authority. Any process that wishes to provide
file-oriented I/O services will register a prefix with the Process Manager defining the portion of the
namespace it wishes to administer (i.e. its region of authority). These prefixes make up a prefix tree
that is maintained in memory on each computer running QNX.
Resolving pathnames
I/O manager prefixes
When a process opens a file, the file's pathname is applied against the prefix tree in order to direct the
open() to the appropriate I/O resource manager. For example, the character Device Manager (Dev)
usually registers the prefix /dev. If a process calls open() with /dev/xxx, a prefix match of /dev will
occur and the open() will be directed to Dev (its owner).
The prefix tree may contain partially overlapping regions of authority. In this case, the longest match
wins. For example, suppose we have three prefixes registered:
/
disk-based filesystem (Fsys)
/dev
character device system (Dev)
/dev/hd0
raw disk volume (Fsys)
The Filesystem Manager has registered two prefixes, one for a mounted QNX filesystem (i.e. /) and
one for a block special file that represents an entire physical hard drive (i.e. /dev/hd0). The character
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Device Manager has registered a single prefix. The following table illustrates the longest-match rule for
pathname resolution.
This pathname:
matches:
and resolves to:
/dev/con1
/dev
Dev
/dev/hd0
/dev/hd0
Fsys
/usr/dtdodge/test
/
Fsys
The prefix tree is managed as a list of prefixes separated by colons as follows:
prefix=pid,unit:prefix=pid,unit:prefix=pid,unit
where pid is the process ID of the I/O resource manager, and the unit number is a single character
used by the manager to select one of possibly several prefixes it owns. In the above example, if Fsys
were process 3 and Dev were process 5, then the system prefix tree might look like this:
/dev/hd0=3,a:/dev=5,a:/=3,e
If you want to:
Use the:
Display the prefix tree
prefix utility
Obtain the prefix tree from within a C program
qnx_prefix_query() function
Network root
QNX supports the concept of a super or network root that lets you apply a pathname against a specific
node's prefix tree, which is specified by a pathname that starts with two slashes followed by a node
number. This also lets you easily access files and devices that aren't in your node's normal pathname
space. For example, in a typical QNX network the following paths would map as follows:
/dev/ser1
local serial port
//10/dev/ser1
serial port on node 10
//0/dev/ser1
local serial port
//20/usr/dtdodge/test
file on node 20
Note that //0 always refers to the local node.
Default network root
When a program is executed remotely, you typically want any given pathnames to be resolved within
the context of your own node's pathname space. For example, this command:
//5 ls /
which executes ls on node 5, should return the same thing as:
ls /
which executes ls on your node. In both cases, / should be resolved through your node's prefix tree,
not through the one on node 5. Otherwise, imagine the chaos that would result if node 5's root (/)
were its local hard disk and your node's root were a hard disk local to your machine-executing
remotely would get files from a completely different filesystem!
To resolve pathnames properly, each process has associated with it a default network root that defines
which node's prefix tree will be used to resolve any pathnames starting with a single slash. When a
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pathname starting with a single / is resolved, the default network root is prepended to it. For example,
if a process had a default network root of //9, then:
/usr/home/luc
would be resolved as:
//9/usr/home/luc
which can be interpreted as "resolve via node 9's prefix tree the pathname /usr/home/luc".
The default network root is inherited by new processes when they're created, and is initialized to your
local node when your system starts up. For example, let's say you're running on node 9, sitting in a
shell with its default network root set to node 9 (a very typical case). If you were to run the command:
ls /
the command would inherit the default network root of //9 and you would get back the equivalent of:
ls //9/
Likewise, if you were to enter this command:
//5 ls /
you would be starting the ls command on node 5, but it would still inherit a default network root
of //9, so you would again get back the equivalent of ls //9/. In both cases the pathname is resolved
according to the same pathname space.
If you want to:
Use the:
Get your current default network root
qnx_prefix_getroot() function
Set your default network root
qnx_prefix_setroot() function
Run a program with a new default network root
on utility
Passing pathnames between processes
If you have several processes running, they may not all have the same default network root-even if
they're running on the same node. For example, one process may have inherited a default network
root from its parent process elsewhere on the network or it may have had its default network root
explicitly overridden by its parent process.
When passing a pathname between processes whose network roots may differ (e.g. when submitting a
file to a spooler for printing), you should prepend the default network root to the pathname before
passing the pathname to the recipient process. If you're sure the sending process and the recipient
process have the same default network root (or if the pathname already has a leading //node/), then
you may omit this step in the sending process.
Alias prefixes
We have discussed prefixes that map to an I/O resource manager. A second form of prefix, known as
an alias prefix, is a simple string substitution for a matched prefix. An alias prefix is of the form:
prefix=replacement-string
For example, assume you're running on a machine that doesn't have a local filesystem (so there's no
process to administer "/"). However, there's a filesystem on another node (say 10) that you wish to
access as "/". You accomplish this using the following alias prefix:
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/=//10/
This will cause the leading slash (/) to be mapped into the //10/ prefix. For
example, /usr/dtdodge/test will be replaced with the following:
//10/usr/dtdodge/test
This new pathname will again be applied against the prefix tree, but this time the prefix tree on node
10 will be used because of the leading //10 pathname. This will resolve to the Filesystem Manager on
node 10, where the open() request will be directed. With just a few characters, this alias has allowed
us to access a remote filesystem as though it were local.
It's not necessary to run a local filesystem process to perform the redirection. A diskless workstation's
prefix tree might look something like this:
/dev=5,a:/=//10/
With this prefix tree, pathnames under /dev will be routed to the local character device manager,
while requests for other pathnames will be routed to the remote filesystem.
Creating special device names
You can also use aliases to create special device names. For example, if a print spooler were running
on node 20, you could alias a local printer pathname to it as follows:
/dev/printer=//20/dev/spool
Any request to open /dev/printer will be directed across the network to the real spooler. Likewise, if
a local floppy drive didn't exist, an alias to a remote floppy on node 20 could be made as follows:
/dev/fd0=//20/dev/fd0
In both cases above, the alias redirection could be bypassed and the remote resource could be directly
named as:
//20/dev/spool OR //20/dev/fd0
Relative pathnames
Pathnames need not start with a single or a double slash. In such cases, the path is considered relative
to the current working directory. QNX maintains the current working directory as a character string.
Relative pathnames are always converted to full network pathnames by prepending the current
working directory string to the relative pathname.
Note that different behaviors result when your current working directory starts with a single slash
versus starting with a network root.
Current working directory
If the current working directory has a leading double slash (network root), it is said to be specific and
locked to the pathname space of the specified node. If instead it has a single leading slash, the default
network root is prepended.
For example, this command:
cd //18/
is an example of the first (specific) form, and would lock future relative pathname evaluation to be on
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node 18, no matter what your default network root happens to be. Subsequently entering cd dev
would put you in //18/dev.
On the other hand, this command:
cd /
would be of the second form, where the default network root would affect the relative pathname
resolution. For example, if your default network root were //9, then entering cd dev would put you
in //9/dev. Because the current working directory doesn't start with a node override, the default
network root is prepended to create a fully specified network pathname.
This really isn't as complicated as it may seem. Typically, network roots (//node/) are not specified,
and everything you do will simply work within your namespace (defined by your default network root).
Most users will log in, accept the normal default network root (i.e. the namespace of their own node),
and work within that environment.
A note about cd
In some traditional UNIX systems, the cd (change directory) command modifies the pathname given to
it if that pathname contains symbolic links. As a result, the pathname of the new current working
directory-which you can display with pwd - may differ from the one given to cd.
In QNX, however, cd doesn't modify the pathname - aside from collapsing .. references. For example:
cd /usr/home/luc/test/../doc
would result in a current working directory of /usr/home/luc/doc, even if some of the elements in the
pathname were symbolic links.
For more information about symbolic links and .. references, see Chapter 5, The Filesystem Manager.
To display a fully resolved network pathname, you can use the fullpath utility.
File descriptor namespace
Once an I/O resource has been opened, a different namespace comes into play. The open() returns an
integer referred to as a file descriptor (FD) which is used to direct all further I/O requests to that
manager. (Note that the Sendfd() kernel call is used within library routines to direct the request.)
The file descriptor namespace, unlike the pathname space, is completely local to each process. The
manager uses the combination of a PID and FD to identify the control structure associated with the
previous open() call. This structure is referred to as an open control block (OCB) and is contained
within the I/O manager.
The following diagram shows an I/O manager taking some PID, FD pairs and mapping them to OCBs.
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The PID and FD map to an OCB of an I/O Manager.
Open control blocks
The OCB contains active information about the open resource. For example, the filesystem keeps the
current seek point within the file here. Each open() creates a new OCB. Therefore, if a process opens
the same file twice, any calls to lseek() using one FD will not affect the seek point of the other FD.
The same is true for different processes opening the same file.
The following diagram shows two processes, in which one opens the same file twice, and the other
opens it once. There are no shared FDs.
Process A opens the file /tmp/file twice. Process B opens the same file once.
Several file descriptors in one or more processes can refer to the same OCB. This is accomplished by
two means:
l A process may use the dup(), dup2(), or fcntl() functions to create a duplicate file descriptor
that refers to the same OCB.
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l When a new process is created via fork(), spawn(), or exec(), all open file descriptors are by
default inherited by the new process; these inherited descriptors refer to the same OCBs as the
corresponding file descriptors in the parent process.
When several FDs refer to the same OCB, then any change in the state of the OCB is immediately seen
by all processes that have file descriptors linked to the same OCB.
For example, if one process uses the lseek() function to change the position of the seek point, then
reading or writing takes place from the new position no matter which linked file descriptor is used.
The following diagram shows two processes in which one opens a file twice, then does a dup() to get a
third. The process then creates a child that inherits all open files.
dup() function">
A process opens a file twice, then is duplicated via dup() to get a third FD. Its child process will inherit
these three file descriptors.
You can prevent a file descriptor from being inherited when you spawn() or exec() by calling the fcntl()
function and setting the FD_CLOEXEC flag.
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The Filesystem Manager
This chapter covers the following topics:
l Introduction
l What is a file?
l Regular files and directories
l Links and inodes
l Symbolic links
l Pipes and FIFOs
l Filesystem Manager performance
l Filesystem robustness
l Raw volumes
l Key components of a QNX partition
l The DOS Filesystem Manager
l CD-ROM Filesystem
l Flash filesystem
l NFS filesystem
l SMB filesystem
Introduction
The Filesystem Manager (Fsys) provides a standardized means of storing and accessing data on disk
subsystems. Fsys is responsible for handling all requests to open, close, read, and write files.
What is a file?
In QNX, a file is an object that can be written to, read from, or both. QNX implements at least six
types of files; five of these are managed by Fsys:
l Regular files-consist of randomly accessible sequences of bytes and have no other predefined
structure.
l Directories-contain the information needed to locate regular files; they also contain status and
attribute information for each regular file.
l Symbolic links-contain a pathname to a file or directory that is to be accessed in place of the
symbolic link file. These files are often used to provide multiple paths to a single file.
l Pipes and FIFOs-serve as I/O channels between cooperating processes.
l Block special files-refer to devices, such as disk drives, tapes, and disk drive partitions. These
files are normally accessed in a manner that hides the hardware characteristics of the device
from applications.
All of these filetypes are described in detail in this chapter. The sixth filetype, the character special file,
is managed by the Device Manager. Other filetypes may be managed by other managers.
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Date and time stamps
Fsys maintains four different times for each file:
l date of last access (read)
l date of last write
l date of last modification
l date of creation (unique to QNX)
File access
Access to regular files and directories is controlled by mode bits stored in the file's inode (see the
section on "Links and inodes"). These bits permit read, write, and execute capability based on effective
user and group IDs. There are three access qualifiers:
l user only
l group only
l others
A process can also run with the user ID or group ID of a file rather than those of its parent process.
The mechanism that allows this is referred to as setuid (set user ID on execution) and setgid (set
group ID on execution).
Regular files and directories
Regular files
QNX views a regular file as a randomly accessible sequence of bytes that has no other predefined
internal structure. Application programs are responsible for understanding the structure and content of
any specific regular file.
Regular files constitute the majority of files found in filesystems. Filesystems are supported by the
Filesystem Manager and are implemented on top of the block special files that define disk partitions
(see the section on "Raw volumes").
Directories
A directory is a file that contains directory entries. Each directory entry associates a filename with a
file. A filename is the symbolic name that lets you identify and access a file. A file may be identified by
more than one filename (see the sections on "Links and inodes" and "Symbolic links").
The following diagram shows how the directory structure is navigated to locate the
file /usr/bill/file2.
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The path through the QNX directory structure to the file usr/bill/file2.
Directory operations
Although a directory behaves much like a standard file, the Filesystem Manager imposes some
restrictions on the operations you can perform on a directory. Specifically, you can't open a directory
for writing, nor can you link to a directory with the link() function.
Reading directory entries
To read directory entries, you use a set of POSIX-defined functions that provide access to directory
entries in an OS-independent fashion. These functions include:
l opendir()
l readdir()
l rewinddir()
l closedir()
Since QNX directories are simply files that contain "known" information, you can also read directory
entries directly using the open() and read() functions. This technique isn't portable, however-the
format of directory entries varies from operating system to operating system.
Extents
In QNX, regular files and directory files are stored as a sequence of extents. An extent is a contiguous
set of blocks on disk.
Where extents are stored
Files that have only a single extent store the extent information in the directory entry. But if more than
one extent is needed to hold the file, the extent location information is stored in one or more linked
extent blocks. Each extent block can hold location information for up to 60 extents.
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A file consisting of multiple consecutive regions on a disk - called extents in QNX.
Extending files
When the Filesystem Manager needs to extend a file whose last extent is full, it first tries to extend the
last extent, even if only by one block. But if the last extent can't be extended, a new extent is
allocated to extend the file.
To allocate new extents, the Filesystem Manager uses a "first fit" policy. A special table in the
Filesystem Manager contains an entry for each block represented in the /.bitmap file (this file is
described in the section on "Key components of a QNX partition"). Each of these entries defines the
largest contiguous free extent in the area defined by its corresponding block. The Filesystem Manager
chooses the first entry in this table large enough to satisfy the request for a new extent.
Links and inodes
In QNX, file data can be referenced by more than one name. Each filename is called a link. (There are
actually two kinds of links: hard links, which we refer to simply as "links," and symbolic links. Symbolic
links are described in the next section.)
In order to support links for each file, the filename is separated from the other information that
describes a file. The non-filename information is kept in a storage table called an inode (for
"information node").
If a file has only one link (i.e. one filename), the inode information (i.e. the non-filename information)
is stored in the directory entry for the file. If the file has more than one link, the inode is stored as a
record in a special file named /.inodes, as are the file's directory entry points to the inode record.
Note that you can create a link to a file only if the file and the link are in the same filesystem.
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The same file is referenced by two links named "more" and "less."
There are two other situations in which a file can have an entry in the /.inodes file:
l If a file's filename is longer than 16 characters, the inode information is stored in the /.inodes
file, making room for a 48-character filename in the directory entry.
l If a file has had more than one link and all links but one have been removed, the file continues
to have a separate /.inodes file entry. This is done because the overhead of searching for the
directory entry that points to the inode entry would be prohibitive (there are no back links from
inode entries to directory entries).
If you want to:
Use the:
Create links from within the Shell
ln utility
Create links from within programs
link() function
Removing links
When a file is created, it is given a link count of one. As links to the file are added, this link count is
incremented; as links are removed, the link count is decremented. The filespace isn't removed from
disk until its link count goes to zero and all programs using the file have closed it. This allows an open
file to remain in use, even though it has been completely unlinked.
If you want to:
Use the:
Remove links from within the Shell
rm utility
Remove links from within programs
remove() or unlink() functions
Directory links
Although you can't create hard links to directories, each directory has two hard-coded links already
built in:
l . ("dot")
l .. ("dot dot")
The filename "dot" specifies the current directory; "dot dot" specifies the directory above the current
one.
Note that the "dot dot" entry of "/" is simply "/"-you can't go further up the path.
Symbolic links
A symbolic link is a special file that has a pathname as its data. When the symbolic link is named in an
I/O request-by open(), for example-the link portion of the pathname is replaced by the link's "data"
and the path is re-evaluated. Symbolic links are a flexible means of pathname indirection and are often
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used to provide multiple paths to a single file. Unlike hard links, symbolic links can cross filesystems
and can also create links to directories.
In the following example, the directories //1/usr/fred and //2/usr/barney are linked even though
they reside on different filesystems-they're even on different nodes (see the following diagram). This
couldn't be done using hard links:
//1/usr/fred --> //2/usr/barney
Note how the symbolic link and the target directory need not share the same name. In most cases,
you use a symbolic link for linking one directory to another directory. However, you can also use
symbolic links for files, as in this example:
//1/usr/eric/src/test.c --> //1/usr/src/game.c
If you want to:
Use this utility:
Create symbolic links
ln (with -s option)
Remove symbolic links*
rm
Query whether a file is a symbolic link
ls
* Remember that removing a symbolic link acts on the link, not the target.
Several functions operate directly on the symbolic link. For these functions, the replacement of the
symbolic element of the pathname with its target is not performed. These functions include unlink()
(which removes the symbolic link), lstat(), and readlink().
Since symbolic links can point to directories, incorrect configurations can result in problems such as
circular directory links. To recover from circular references, the system imposes a limit on the number
of hops; this limit is defined as SYMLOOP_MAX in the <limits.h> include file.
Pipes and FIFOs
Pipes
A pipe is an unnamed file that serves as an I/O channel between two or more cooperating processesone process writes into the pipe, the other reads from the pipe. The Filesystem Manager takes care of
buffering the data. The buffer size is defined as PIPE_BUF in the <limits.h> file. A pipe is removed
once both of its ends have closed.
Pipes are normally used when two processes want to run in parallel, with data moving from one
process to the other in a single direction. (If bidirectional communication is required, messages should
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be used instead.)
A typical application for a pipe is connecting the output of one program to the input of another
program. This connection is often made by the Shell. For example:
ls | more
directs the standard output from the ls utility through a pipe to the standard input of the more utility.
If you want to:
Use the:
Create pipes from within the Shell
pipe symbol ("|")
Create pipes from within programs
pipe() or popen() functions
On diskless workstations, you can run the Pipe Manager (Pipe) in place of the Filesystem
Manager when only pipes are required. The Pipe Manager is optimized for pipe I/O and may
achieve better pipe throughput than the Filesystem Manager.
FIFOs
FIFOs are essentially the same as pipes, except that FIFOs are named permanent files that are stored
in filesystem directories.
If you want to:
Use the:
Create FIFOs from within the Shell
mkfifo utility
Create FIFOs from within programs
mkfifo() function
Remove FIFOs from within the Shell
rm utility
Remove FIFOs from within programs
remove() or unlink() function
Filesystem Manager performance
The Filesystem Manager has several features that contribute to high-performance disk access:
l elevator seeking
l buffer cache
l multi-threading
l client-driven priority
l temporary files
l ramdisks
Elevator seeking
Elevator seeking minimizes the overall seek time required to read or write data from or to disk.
Outstanding I/O requests are ordered such that they can all be performed with one sweep of the disk
head assembly, from the lowest to the highest disk address.
Elevator seeking also has integrated enhancements to ensure that multi-sector I/O is performed
whenever possible.
Buffer cache
The buffer cache is an intelligent buffer between the Filesystem Manager and the disk driver. The
buffer cache attempts to store filesystem blocks in order to minimize the number of times the
Filesystem Manager has to access the disk. By default, the size of the cache is determined by total
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system memory, but you can specify a different size via an option to Fsys.
Read operations are synchronous. Write operations, on the other hand, are usually asynchronous.
When the data enters the cache, the Filesystem Manager replies to the client process to indicate that
the data is written. The data is then written to the disk as soon as possible, typically less than five
seconds later.
Applications can modify write behavior on a file-by-file basis. For example, a database application can
cause all writes for a given file to be performed synchronously. This would ensure a high level of file
integrity in the face of potential hardware or power problems that might otherwise leave a database in
an inconsistent state.
Multi-threading
The Filesystem Manager is a multi-threaded process. That is, it can manage several I/O requests
simultaneously. This allows the Filesystem Manager to fully exploit potential parallelism since it can do
both of the following:
l access several devices in parallel
l satisfy I/O requests from the buffer cache while other I/O requests that access physical disks
are in progress
Client-driven priority
The Filesystem Manager may have its priority driven by the priority of the processes that send it
messages. When the Filesystem Manager receives a message, its priority is set to that of the process
that sent the message. For more information, see "Process scheduling" in Chapter 2.
Temporary files
QNX has a performance option for opening temporary files that are written and then reread in a short
period of time. For such files, the Filesystem Manager attempts to keep the data blocks in the cache
and will write the blocks to disk only if absolutely necessary.
Ramdisks
The Filesystem Manager has an integrated ramdisk capability that allows up to 8M of memory to be
used as a simulated disk. Since the Filesystem Manager uses highly efficient multipart messaging, data
moves from the ramdisk directly to the application buffers.
The Filesystem Manager is able to bypass the buffer cache because the ramdisk is built in, not
implemented as a driver. (For information on multipart messaging, see the section on "Advanced
facilities" in Chapter 2.)
Because they eliminate the delays of physical hardware and don't rely on the filesystem cache,
ramdisks provide greater determinism in read/write operations than hard disks.
Filesystem robustness
The QNX filesystem achieves high throughput without sacrificing reliability. This has been accomplished
in several ways.
While most data is held in the buffer cache and written after only a short delay, critical filesystem data
is written immediately. Updates to directories, inodes, extent blocks, and the bitmap are forced to disk
to ensure that the filesystem structure on disk is never corrupt (i.e. the data on disk should never be
internally inconsistent).
Sometimes all of the above structures must be updated. For example, if you move a file to a directory
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and the last extent of that directory is full, the directory must grow. In such cases, the order of
operations has been carefully chosen such that if a catastrophic failure occurs with the operation only
partially completed (e.g. a power failure), the filesystem, upon rebooting, would still be "sane." At
worst, some blocks may have been allocated, but not used. You can recover these for later use by
running the chkfsys utility.
Filesystem recovery
Even in the best systems, true catastrophes such as these may happen:
l Bad blocks may develop on a disk because of power surges or brownouts.
l A naive or malicious user with access to superuser privileges might reinitialize the filesystem
(via the dinit utility).
l An errant program (especially one run in a non-QNX environment) may ignore the disk
partitioning information and overwrite a portion of the QNX partition.
So that you can recover as many of your files as possible if such events ever occur, unique
"signatures" have been written on the disk to aid in the automatic identification and recovery of the
critical filesystem pieces. The inodes file (/.inodes), as well as each directory and extent block, all
contain unique patterns of data that the chkfsys utility can use to reassemble a truly damaged
filesystem.
For details on filesystem recovery, see the documentation for the chkfsys utility.
Raw volumes
The Filesystem Manager manages block special files. These files define disks and disk partitions.
Disks and disk subsystems
QNX considers each physical disk on a computer to be a block special file. As a block special file, a disk
is viewed by a QNX filesystem as a sequential set of blocks, each 512 bytes in size, regardless of the
size and capacities of the disk. Blocks are numbered, beginning with the first block on the disk (block
1).
Because each disk is a block special file, it can be opened as an entity for raw-level access using
standard POSIX C functions such as open(), close(), read(), and write(). At the level of a block special
file that defines an entire disk, QNX makes absolutely no assumptions about any data structure that
may reside on the disk.
A computer running QNX may have one or more disk subsystems. Each disk subsystem consists of a
controller and one or more disks. You start a device driver process for each disk subsystem that is to
be managed by the Filesystem Manager.
OS partitions
QNX complies with a de facto industry standard that allows a number of operating systems to share
the same physical disk. According to this standard, a partition table can define up to four primary
partitions on the disk. The table is stored on the first disk block.
Each partition must be given a "type" recognized by the operating system prepared to handle that
partition. The following list shows some of the partition types that are currently used:
Type
Filesystem
1
DOS (12-bit FAT)
4
DOS (16-bit FAT; partitions <32M)
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5
DOS Extended Partition
6
DOS 4.0 (16-bit FAT; partitions >=32M)
7
OS/2 HPFS
7
Previous QNX version 2 (pre-1988)
8
QNX 1.x and 2.x ("qny")
9
QNX 1.x and 2.x ("qnz")
11
DOS 32-bit FAT; partitions up to 2047G
12
Same as Type 11, but uses Logical Block Address Int 13h extensions
14
Same as Type 6, but uses Logical Block Address Int 13h extensions
15
Same as Type 5, but uses Logical Block Address Int 13h extensions
77
QNX POSIX partition
78
QNX POSIX partition (secondary)
79
QNX POSIX partition (secondary)
99
UNIX
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If you want more than one QNX 4.x partition on a single physical disk, you would use type 77 for your
first QNX partition, type 78 for your second QNX partition, and type 79 for your third QNX partition.
You can use other types for your second and third partitions, but 78 and 79 are preferred. To mark any
of these partitions as bootable, you use the fdisk utility.
At boot time, the QNX boot loader (optionally installed by fdisk) lets you override the default boot
partition selection in the partition table.
You can use the fdisk utility to create, modify, or delete partitions.
Because QNX treats each partition on a disk as a block special file, you can access either of the
following:
l the complete disk-disregarding partitions-as a block special file
l a single partition as a block special file; this block special file will be a subset of the block
special file that defines the entire disk
Two physical disk drives. The first drive contains DOS, QNX, and UNIX partitions. The second has DOS
and QNX partitions.
Defining block special files
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The names of all block special files are placed in the prefix tree for the computer where the block
special files reside (the prefix tree is described in Chapter 3, I/O Namespace). When a device driver for
a disk subsystem is started, the Filesystem Manager automatically registers prefixes that define a
block special file for each physical drive attached to the disk subsystem. The mount utility is then used
to register a prefix for a block special file for each partition on the disk subsystem.
For example, let's say you have a standard IDE interface with two attached drives. On one drive, you
want to mount a DOS partition, a QNX partition, and a UNIX partition. On the other drive, you want to
mount a DOS partition and a QNX partition.
The Filesystem Manager would define the block special files /dev/hd0 and /dev/hd1 for the two drives
on the controller where the driver was started.
You would then use the mount utility to define a block special file for each partition. For example:
mount -p /dev/hd0 -p /dev/hd1
would yield the following block special files:
OS partition:
Block special file:
DOS partition on drive hd0
/dev/hd0t4
QNX partition on drive hd0
/dev/hd0t77
UNIX partition on drive hd0
/dev/hd0t99
DOS partition on drive hd1
/dev/hd1t4
QNX partition on drive hd1
/dev/hd1t77
Note that the tn convention is used to refer to disk partitions used by certain operating systems. For
example a DOS partition is t4, a UNIX partition is t99, etc.
Mounting a filesystem
You typically mount a QNX filesystem on a block special file. To mount a filesystem, you again use the
mount utility-it lets you specify the prefix that identifies the filesystem. For example:
mount /dev/hd0t77 /
mounts a filesystem with a prefix of / on the partition defined by the block special file named hd0t77.
If a disk has been partitioned, you must mount a partition block special file (e.g. /dev/hd0t77
that defines a QNX 4.x partition, not the base block special file that defines the entire raw disk
(e.g. /dev/hd0. If you attempt to mount the base block special file for the entire disk, you'll get
a "corrupt filesystem" message when you try to access the filesystem.
Unmounting a filesystem
To unmount a filesystem you use the umount utility. For example, the following command will unmount
the filesystem on your primary QNX partition:
umount /dev/hd0t77
Once a filesystem is unmounted, files on that partition are no longer accessible.
Key components of a QNX partition
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Several key components found at the beginning of every QNX partition tie the filesystem together:
l loader block
l root block
l bitmap
l root directory
These structures are created when you initialize the filesystem with the dinit utility.
Structure of a QNX filesystem within a disk partition.
Loader block
This is the first physical block of a disk partition. This block contains the code that is loaded and then
executed by the BIOS of the computer to load an operating system from the partition. If a disk hasn't
been partitioned (e.g. as in a floppy diskette), this block is the first physical block on the disk.
Root block
The root block is structured as a standard directory. It contains inode information for these four special
files:
l the root directory of the filesystem (/)
l /.inodes
l /.boot
l /.altboot
The files /.boot and /.altboot contain images of the operating system that can be loaded by the
QNX bootstrap loader.
Normally, the QNX loader loads the OS image stored in the /.boot file. But if the /.altboot file isn't
empty, you'll be given the option to load the image stored in the /.altboot file.
Bitmap
To allocate space on a disk, QNX uses a bitmap stored in the /.bitmap file. This file contains a map of
all the blocks on the disk, indicating which blocks are used. Each block is represented by a bit. If the
value of a bit is 1, its corresponding block on the disk is being used.
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Root directory
The root directory of a partition behaves as a normal directory file with two exceptions:
l Both "dot" and "dot dot" are links to the same inode information, namely the root directory
inode in the root block.
l The root directory always has entries for the /.bitmap, /.inodes, /.boot, and /.altboot files.
These entries are provided so programs that report information on filesystem usage will see the
entries as normal files.
The DOS Filesystem Manager
In QNX, the I/O namespace is managed through prefixes that direct file requests to the appropriate
manager process. A process that takes advantage of this is the DOS Filesystem Manager (Dosfsys).
Dosfsys administers the /dos namespace prefix and presents DOS drives within the QNX namespace
as "guest" filesystems.
Dosfsys provides transparent access to DOS disks, so you can treat DOS filesystems as though they
were QNX filesystems. This transparency allows processes to operate on DOS files without any special
knowledge or work on their part. Standard I/O library functions, such as open(), close(), read(), and
write(), operate identically for a file on a DOS partition as for a file on a QNX partition. For example, to
copy a file from your QNX partition to your DOS partition, you would simply enter:
cp /usr/luc/file.dat /dos/c/file.dat
Note that /dos/c is the pathname of the DOS drive C. The cp command contains no special code to
detect whether the file it's copying is located on a DOS disk. Other commands also work with equal
transparency (e.g. cd, ls, mkdir).
If there's no DOS equivalent to a QNX feature, such as mkfifo() or link(), an appropriate error code
(i.e. errno) is returned by Dosfsys.
Dosfsys works both with floppies and with hard disk partitions. All of the low-level disk access that
Dosfsys requires is done using standard functions provided by the Filesystem Manager. Thus, with no
low-level code, Dosfsys is able to integrate a seamless interface between QNX applications and a DOS
filesystem.
CD-ROM filesystem
The CD-ROM Filesystem, Iso9660fsys, provides transparent access to CD-ROM media, so you can
treat CD-ROM filesystems as though they were POSIX filesystems. This transparency allows processes
to operate on CD-ROM files without any special knowledge or work on their part.
Iso9660fsys implements the ISO 9660 standard, including Rock Ridge extensions. DOS and Windows
Compact Discs follow this standard. In addition to normal files, Iso9660fsys also supports audio.
Flash
The Flash filesystem manager, Efsys.*, was explicitly designed to work with Flash memory, whether
built-in or removable. Files written to removable Flash media (PC-Card cards) are portable to other
systems that also support this standard.
Efsys.* combines the functions of a filesystem manager and a device driver to manage a Flash
filesystem on memory-based media. Because Efsys.* includes the device driver for controlling the
hardware, there are separate versions of Efsys.* for different embedded systems hardware. For
example:
l Efsys.explr2 for the Intel EXPLR2 evaluation board (with on-board RFA)
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l Efsys.elansc400 for the AMD Élan SC400 evaluation board
l Efsys.pcmcia for mixed use of Flash filesystems and PCMCIA devices
Restrictions
The functionality of the filesystem is restricted by the memory devices used. For example, if the
medium contains ROM devices only, the filesystem is read-only.
Memory-based Flash devices have restrictions on file writing. You can only append to the file. In
addition, file access times aren't updated. Currently, these restrictions apply even if devices such as
SRAM are used.
Space reclamation
Efsys.* stores directories and files using linked lists of data structures, rather than the fixed-size disk
blocks used in traditional rotating-medium filesystems. When a directory or file is deleted, the data
structures representing it are marked as deleted, but aren't removed. This avoids continuously erasing
and rewriting the medium.
Eventually, the medium will run out of free space and the filesystem manager will perform space
reclamation (or "garbage collection"). During this process, Efsys.* recovers the space occupied by
deleted files and directories. To perform space reclamation, the filesystem manager requires at least
one spare block on the medium. The mkffs utility automatically reserves one spare block when you
make a filesystem.
Compression and decompression
Efsys.* supports decompression, which increases the amount of data that can be stored on a
medium. Decompression operates transparently to applications.
For files to be decompressed transparently by the filesystem manager, you must compress them
explicitly before using mkffs. You do this with the bpe utility. If you copy a compressed file to a live
Flash filesystem, it will remain compressed when it's read from the filesystem.
File permissions
If you disable the POSIX extensions, file ownership is permanently set to root and all permissions are
set to rwx. The chgrp, chmod, and chown commands won't work.
Mounting
You can set mountpoints only when initializing the partitions or when starting the filesystem manager.
Raw device access
When you start Efsys.*, it creates a special device file in the /dev directory for each medium. In a
system with two memory-based media, Efsys.* would create the special files /dev/skt1
and /dev/skt2. The special device ignores the partitioning, allowing raw device access to the medium.
An image filesystem partition can be accessed only as a raw device. For each image filesystem
partition, Efsys.* creates a special file in the format /dev/sktXimgY, where X is the medium socket
number and Y is the number of the image filesystem partition in that medium.
NFS filesystem
Originally developed by Sun Microsystems, the Network File System (NFS) is a TCP/IP application that
has since been implemented on most DOS and Unix systems. You should find the QNX implementation
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invaluable for transparently accessing filesystems on other operating systems that support NFS.
QNX inherently supports networked filesystems. You need to use NFS only if you're accessing
non-QNX NFS filesystems, or if you want to let NFS clients access your QNX namespace.
NFS lets you graft remote filesystems - or portions of them - onto your local namespace. Directories
on the remote systems appear as part of your local filesystem and all the utilities you use for listing
and managing files (e.g. ls, cp, mv) operate on the remote files exactly as they do on your local files.
In QNX 4, NFS requires the Socket manager, which implements the networking protocols used
in TCP/IP for QNX. Note that a "lighter" version of the socket manager, known as Socklet, is
also available if you don't need NFS.
SMB filesystem
SMBfsys implements the SMB (Server Message Block) file-sharing protocol, which is used by a number
of different servers such as Windows NT, Windows 95, Windows for Workgroups, LAN Manager, and
Samba. SMBfsys allows a QNX client to transparently access remote drives residing on such servers.
SMBfsys implements this protocol using NetBIOS on TCP/IP only, not NetBEUI. Accordingly, you need
TCP/IP installed on both the QNX and remote server side. Once SMBfsys is running and a remote
server has been mounted, the server's filesystem appears in the local directory structure.
All content ©2004, QNX Software Systems
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The Device Manager
This chapter covers the following topics:
l Introduction
l Device services
l Edited input mode
l Raw input mode
l Device drivers
l The QNX console
l Serial devices
l Parallel devices
l Device subsystem performance
Introduction
The QNX Device Manager (Dev) is the interface between processes and terminal devices. These
terminal devices are located in the I/O namespace with names starting with /dev. For example, a
console device on QNX would have a name such as:
/dev/con1
Device services
QNX programs access terminal devices using the standard read(), write(), open(), and close()
functions. A terminal device is presented to a QNX process as a bidirectional stream of bytes that can
be read or written by the process.
The Device Manager regulates the flow of data between an application and the device. Some
processing of this data is performed by Dev according to parameters in a terminal control structure
(called termios), which exists for each device. Users can query and/or change these parameters by
using the stty utility; programs can use the tcgetattr() and tcsetattr() functions.
The termios parameters control low-level functionality such as:
l line-control discipline (including baud rate, parity, stop bits, and data bits)
l echoing of characters
l input line editing
l recognizing, and acting on, breaks and hangups
l software and hardware flow control
l translation of output characters
The Device Manager also provides a set of auxiliary services available to processes for managing
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terminal devices. The following table summarizes some of these services.
A process can:
via this C function:
Perform timed read operations
dev_read() or read() and tcsetattr
()
Asynchronously notify a process of data available on one or
more input devices
dev_arm()
Wait for output to be completely transmitted
tcdrain()
Send breaks across a communications channel
tcsendbreak()
Disconnect a communications channel
tcdropline()
Insert input data
dev_insert_chars()
Perform non-blocking reads and writes
open() and fcntl() (O_NONBLOCK
mode)
Edited input mode
The most significant mode of device processing is controlled by the ICANON bit in the termios control
structure. If this control bit is set, the Device Manager performs line-editing functions on received
characters. Only when a line is "entered" - typically when a carriage return (CR) is received - will the
processed data be made available to application processes. This mode of operation is referred to as
edited, canonical, or sometimes cooked mode.
Most non-full-screen applications run in edited mode. The Shell is a typical example.
The following table shows several special control characters that may be set in the termios control
structure to define how Dev performs this editing.
Dev
When it
receives:
will:
Move the cursor one character to the left
LEFT
Move the cursor one character to the right
RIGHT
Move the cursor to the beginning of the line
HOME
Move the cursor to the end of the line
END
Rub out the character to the left of the cursor
ERASE
Delete the character at the current cursor position
DEL
Rub out the entire input line
KILL
Erase the current line and recall a previous line
UP
Erase the current line and recall the next line
DOWN
Toggle between insert mode and typeover mode (every new line starts in
insert mode)
INS
Line-editing characters vary from terminal to terminal. The QNX console always starts out with a full
set of editing keys defined.
If a terminal is connected to QNX via a serial channel, you need to define the editing characters that
apply to that particular terminal. To do this, you can use the stty utility. For example, if you have a
VT100 terminal connected to a serial port (called /dev/ser1), you would use the following statement
to extract the appropriate editing keys from the terminfo database and apply them to /dev/ser1:
stty term=vt100 </dev/ser1
If, instead, you had a modem connected to that serial port, which in turn was connected to another
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QNX system running the qtalk utility, you'd want to set the line-editing keys as follows:
stty term=qnx </dev/ser1
Raw input mode
When ICANON isn't set, the device is said to be in raw mode. In this mode, no input editing is
performed, and any received data is made immediately available to QNX processes.
Full-screen programs and serial communications programs are examples of QNX applications that put a
device in raw mode.
When reading from a raw device, the application is capable of specifying under what conditions an
input request is to be satisfied. The criteria used for accepting raw input data are based on two
members of the termios control structure: MIN and TIME. The application can specify a further qualifier
of accepting input data when it issues its read request via dev_read(). This qualifier, TIMEOUT, is
useful when writing protocols or realtime applications. Note that TIMEOUT is always 0 for read().
When a QNX process issues a read request for n bytes of data, these three parameters define when
that read request is to be satisfied:
MIN TIME TIMEOUT
Description:
0
0
0
Return immediately with as many bytes as are currently available
(up to n bytes).
M
0
0
Return with up to n bytes only, when at least M bytes are available.
0
T
0
Return with up to n bytes when at least one byte is available, or
T * .1 second has expired.
M
T
0
Return with up to n bytes when either M bytes are available or at
least one byte has been received and the interbyte time between
any subsequently received characters exceeds T * .1 second.
0
0
t
RESERVED.
M
0
t
Return with up to n bytes when t * .1 second has expired, or M
bytes are available.
0
T
t
RESERVED.
t
Return with up to n bytes when M bytes are available, or t * .1
second has expired and no characters are received, or at least one
byte has been received and the interbyte time between any
subsequently received characters exceeds T * .1 second.
M
T
Device drivers
The following illustration shows a typical QNX device subsystem.
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The Device Manager process (Dev) manages the flow of data to and from the QNX application
processes. The hardware interface is managed by individual driver processes. Data flows between Dev
and its drivers through a set of shared memory queues for each terminal device.
Since shared memory queues are used, it's necessary that Dev and all its drivers reside on the
same physical CPU. The advantage, of course, is increased performance.
Three queues are used for each device. Each queue is implemented using a first-in, first-out
mechanism. A control structure is also associated with each queue.
Received data is placed into the raw input queue by the driver and is consumed by Dev only when
application processes request data. Interrupt handlers within drivers typically call a trusted library
routine within Dev to add data to this queue - this ensures a consistent input discipline and greatly
minimizes the responsibility of the driver.
Dev places output data into the output queue; the data is consumed by the driver as characters are
physically transmitted to the device. Dev calls a trusted routine within the driver process each time
new data is added so it can "kick" the driver into operation (in the event that it was idle). Since output
queues are used, Dev implements full write-behind for all terminal devices. Only when the output
buffers are full will Dev cause a process to block while writing.
The canonical queue is managed entirely by Dev and is used while processing input data in edited
mode. The size of this queue determines the maximum edited input line that can be processed for a
particular device.
The sizes of all these queues are configurable by the system administrator; the only restriction is that
the sum total of all three queues can't exceed 64K. Default values are usually more than adequate to
handle most hardware configurations, but you can "tune" these either to reduce overall system
memory requirements or to accommodate unusual hardware situations.
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Device control
Device drivers simply add received data to the raw input queue or consume and transmit data from the
output queue. Dev decides when (and if) output transmission is to be suspended, how (and if) received
data is echoed, etc.
To ensure good interactive response to input events, Dev must run at a reasonably high priority. Dev
typically has very little actual work to do when it does run, so it seldom impedes overall system
performance.
The drivers themselves are just like any other QNX process - they can run at different priorities
according to the nature of the hardware they're serving.
Low-level device control is implemented with a far call into an ioctl entry point within each driver. A
common set of ioctl commands are supported by most drivers used directly by Dev. Device-specific
ioctl commands can also be sent through Dev to the drivers by QNX processes (via the qnx_ioctl()
function).
The QNX console
System consoles are managed by the Dev.con driver process. The display adapter and the screen, plus
the system keyboard, are collectively referred to as the console.
QNX permits multiple sessions to be run concurrently on consoles by means of virtual consoles. The
Dev.con console driver process typically manages more than one set of I/O queues to Dev, which are
made available to user processes as a set of terminal devices with names like /dev/con1, /dev/con2,
etc. From the application's point of view, there "really are" multiple consoles available to be used.
Of course, there's only one physical screen and keyboard, so only one of these virtual consoles is
actually displayed at any one time. The keyboard is "attached" to whichever virtual console is currently
visible.
Console-specific functions
In addition to implementing the standard QNX Terminal (as defined in the QNX Installation &
Configuration guide), the console driver also provides a set of console-specific functions that let
application processes communicate via messages directly to a console driver process. Communication
is established with the console_open() function. Once communication is established, a QNX process
has access to the following capabilities:
A process can:
via this C
function:
Read directly from the console screen
console_read()
Write directly to the console screen
console_write()
Be asynchronously notified of significant events (e.g. display data or size has
console_arm()
changed, cursor has moved, visible console has changed, etc.)
Control the console size
console_size()
Switch the visible console
console_active()
The QNX console driver runs as a normal QNX process. Input characters from the keyboard are
mapped by the keyboard interrupt handler and placed directly into the input queue. Output data is
consumed and displayed by Dev.con while it is executing as a process.
Serial devices
Serial communication channels are managed by the Dev.ser driver process. This driver can manage
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more than one physical channel; it provides terminal devices with names such
as /dev/ser1, /dev/ser2, etc.
When you start Dev.ser, you can specify command-line arguments that determine which - and how
many - serial ports are installed. To see what serial ports may be available on a QNX system, use the
ls utility:
ls /dev/ser*
Dev.ser is an example of a purely interrupt-driven I/O server. After initializing the hardware, the
process itself goes to sleep. Received interrupts place input data directly into the input queue. The first
output character on an idle channel is transmitted to the hardware when Dev issues the first kick call
into the driver. Subsequent characters are transmitted by the appropriate interrupt being received.
Parallel devices
Parallel printer ports are managed by the Dev.par driver process. When you start Dev.par, you specify
a command-line argument that determines which parallel port is installed. To see if a parallel port is
available on a QNX system, use the ls utility:
ls /dev/par*
Dev.par is an output-only driver, so it has no input or canonical input queues. You can configure the
size of the output buffer with a command-line argument when you start Dev.par. The output buffer
can be made very large if you wish to provide a form of software print buffer.
Dev.par is an example of a completely non-interrupt I/O server. The parallel printer process normally
remains RECEIVE-blocked, waiting for data to appear in its output queue and a kick from Dev. When
data is available to print, Dev.par runs in a busy-wait loop (at relatively low adaptive priority), while
waiting for the printer hardware to accept characters. This low-priority busy-wait loop ensures that
overall system performance isn't affected, yet on the average produces the maximum possible
throughput to the parallel device.
Device subsystem performance
The flow of events within the device subsystem is engineered to minimize overhead and maximize
throughput when a device is in raw mode. To accomplish this, the following rules are used:
l Interrupt handlers place received data directly into a memory queue. Only when a read
operation is pending, and that read operation can now be satisfied, will the interrupt handler
schedule Dev to run. In all other cases, the interrupt simply returns. Moreover, if Dev is already
running, no scheduling takes place.
l When a read operation is satisfied, Dev replies to the application process directly from the raw
input buffer into the application's receive buffer. The net result is that the data is copied only
once.
These rules - coupled with the extremely small interrupt and scheduling latencies inherent within QNX
- result in a very lean input model.
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The Network Manager
This chapter covers the following topics:
l Introduction
l Network Manager responsibilities
l Microkernel/Network Manager interface
l Network drivers
l Node and network identifiers
l Choosing a network
l TCP/IP networking
Introduction
The Network Manager (Net) gives QNX users a seamless extension of the operating system's powerful
messaging capabilities. Communicating directly with the Microkernel, the Network Manager enhances
QNX's message-passing IPC by efficiently propagating messages to remote machines. In addition, the
Network Manager offers three advanced features:
l increased throughput via load balancing
l fault tolerance via redundant connectivity
l bridging between QNX networks
Network Manager responsibilities
The Network Manager is responsible for propagating the QNX messaging primitives across a local area
network. The standard messaging primitives used in local messaging are used unmodified in remote
messaging. In other words, there's no special "network" Send(), Receive(), or Reply().
An independent module
The Network Manager does not have to be built into the operating system image. It may be started
and stopped at any time to provide or remove network messaging capabilities.
When the Network Manager starts, it registers with the Process Manager and Microkernel. This
activates existing code within the two that interfaces to the Network Manager. This means that
network messaging and remote process creation are not just a layer added onto the operating system.
Network messaging is integrated into the very heart of the messaging and process-management
primitives themselves.
This deep integration at the lowest level gives QNX its network transparency and qualifies it as a fully
distributed operating system. Since applications access all services via messaging, and since the
Network Manager allows messages to flow transparently on the network, QNX nodes perform together
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as a single, logical computer.
Since the Network Manager and Microkernel are distinct, the Microkernel can achieve independence
from the network hardware, and non-networked machines can benefit from reduced code
requirements.
Microkernel/Network Manager interface
The Microkernel and Process Manager interface to the Network Manager via a special nonblocking
memory queue. This queue is essentially a list of transmissions to be performed by the Network
Manager. The entries contain all the information for a particular operation (e.g. Send(), Reply(), VC
creation, remote signal propagation, etc.).
Another resource the operating system uses to provide transparent messaging is the virtual circuit
buffer. Allocated when a process creates a VC, virtual circuit buffers hold the data during the
completion of a messaging transaction to another node.
The following diagrams outline the flow of data and control for sending and receiving remote
messages.
Sending a message to a remote node
Send() or Reply() to a remote node">
A process issues a Send() or Reply() to a remote node.
In the case of a Send() or Reply() to a remote node, the following events occur:
1.
2.
The process calls Send() or Reply() and the Microkernel puts the data from the process's data
space into the virtual circuit buffer associated with the VC specified by the Send() or Reply().
The Microkernel enqueues to the Network Manager a time-ordered queue entry identifying the
sender, the remote receiver, and pointers to the data in the virtual circuit buffer. If the queue
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3.
4.
5.
was previously empty, the Network Manager's proxy is triggered to let it know new work has
arrived.
The Network Manager dequeues the queue entry.
The Network Manager passes the entry to an appropriate network driver.
The network driver begins transmission on the network media and is responsible for delivery.
In the case of a signal propagation or VC creation, the Process Manager rather than the Microkernel
would enqueue a control packet. Nevertheless, the Network Manager would transmit the packet to its
destination.
Receiving the message on the remote node
Send() or Reply()">
A process receives a remote Send() or Reply().
Assuming the remote node will have sent a message as previously described, the following events
occur on the node receiving the message:
1.
2.
3.
4.
The Network driver puts the message data from the network media into the appropriate virtual
circuit buffer.
The Network driver informs the Network Manager that the reception is complete.
The Network Manager uses a private kernel call to inform the Microkernel that the reception is
complete.
The Microkernel puts the data from the virtual circuit buffer into the process's buffer (assuming
it was RECEIVE- or REPLY-blocked on this virtual circuit).
Any control packets the Network Manager receives are forwarded immediately to the Process Manager
via the standard Send() primitive. These control packets are used for signal propagation and VC
creation.
Network drivers
Like the Filesystem Manager and the Device Manager, the Network Manager contains no hardwarespecific code. This functionality is provided by network card drivers. The Network Manager can support
multiple network drivers at one time. Each driver typically supports a single network card. You may
have drivers/cards of the same type or different types-for example, two Ethernet drivers/cards or
perhaps an Ethernet driver/card and an Arcnet driver/card.
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Shared memory queues provide the interface between the Network Manager and drivers. This interface
has been designed to obtain the maximum performance possible. The driver determines the
appropriate protocol for the network media.
The driver is responsible for packetization, sequencing, and retransmission if reliable guaranteed data
transmission to a remote physical node is requested. This is the default for all QNX messaging
primitives. This design allows QNX to easily support new network hardware and protocols by writing or
modifying only a network driver.
Node and network identifiers
Each node in a local area network is identified by two numbers:
l its physical node ID (or IDs if the node has more than one network card)
l a combination of its logical node ID and logical network ID.
Physical node ID
The physical node ID is determined by the hardware. Network cards communicate with each other by
specifying the physical node ID of the remote node they wish to talk to. In the case of Ethernet and
Token Ring, this represents a large number that is difficult for people-and utilities-to deal with. For
example, each Ethernet and Token Ring card is shipped with a unique 48-bit physical node ID,
conforming to the IEEE 802 standard. Arcnet cards, on the other hand, have only an 8-bit ID.
Physical node IDs entail a significant drawback: when interconnecting some networks, addresses may
conflict (especially in the case of Arcnet) or be of a radically different format.
Logical node ID
To overcome the above problems with physical node IDs, each QNX node is given a logical node ID. All
QNX processes deal with logical node IDs. The physical node IDs are hidden from processes running on
QNX.
Logical node IDs simplify network and application licensing. They also make it easy for some utilities
that may wish to poll the network using a simple loop, where the logical node ID goes from 1 to the
number of nodes.
The mapping between logical and physical node IDs is done by the Network Manager. The driver is
given a physical ID by the Network Manager when asked to transmit data to another node.
The logical node ID is typically assigned sequential numbers starting at 1. For example, a node with an
Ethernet card may be given a logical node ID of 2, which is mapped to the physical node ID of
00:00:c0:46:93:30.
Note that the logical node ID must be unique for all nodes across all interconnected QNX networks in
order for network bridging to work.
Logical network ID
The network ID identifies a particular logical network. A logical network is any hardware that allows a
network driver to directly communicate with a network driver on another node. This may be as simple
as a serial port or as complex as an Ethernet network with hardware bridges.
In the following diagram, node 7 has two network cards that allow the node to access nodes on logical
networks 1 and 2. Nodes 8 and 9 both have three cards, connecting them to networks 1, 2, and 3.
Note that every logical node ID is unique across all three logical networks.
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Logical network and logical node IDs are assigned by the system administrator. For more
information, see Network Installation in the Installation & Configuration guide.
Multiple physical networks happily coexist via logical networks.
Choosing a network
In the case where nodes are connected by more than one logical network, the Network Manager has a
choice of which network to use when transmitting to a remote node. For example, in the above
diagram, node 7 could transmit to node 8 using the driver attached to logical network 1 or logical
network 2.
Load balancing
Network throughput is determined by a combination of the speed of the computer and the speed of the
network. If your computer can provide data faster than the network can accept it, then the network
will limit your throughput.
For example, two Pentium computers connected by a 10BASE-T Ethernet network will be limited to a
maximum of 1.1 million bytes per second, which is the data rate provided by the Ethernet hardware.
If, however, you were to place two Ethernet cards in each computer and connect them with separate
cables, the Network Manager could transmit data over both networks simultaneously. Under a heavy
load, this would provide up to twice as much throughput as a single network.
The Network Manager will automatically load-balance by choosing an appropriate network driver. In
our example above, if a transmission is in progress from node 7 to node 8 on logical network 1, and
another transmission to node 8 is initiated on node 7, then logical network 2 will automatically be
chosen to transmit the data.
Fault tolerance
When nodes are connected by two or more networks, there's more than a single path to use for
communication. If a card in one network fails in a way that prevents any communication on that
network, the Network Manager will automatically re-route all data through another network. This
happens automatically without any involvement on the part of the application software, and results in
transparent network fault tolerance. If cables for the different networks are run using separate routes,
you'll also be protected against an accidental cable cut.
You can also construct tandem systems in which two machines are connected by a fast network for
normal operation and by a cheaper, slower network (e.g. a serial link) that remains on standby. If the
first network fails, communication will continue, although throughput would naturally be reduced.
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Bridging between QNX LANs
The Network Manager allows any node to act as a bridge between two separate IEEE 802-based QNX
networks.
Since QNX uses the same packet format and protocol on all IEEE 802-based networks, you can
create bridges between Ethernet, Token Ring, and FDDI networks.
Arcnet networks cannot be bridged.
Consider the following diagram, where node 17 and node 18 are on one network, and node 18 and
node 19 are on another network:
Network bridging between two IEEE 802-based QNX LANs.
Nodes 17 and 18 are on the same network, so they can talk directly to each other. The same is true
for nodes 18 and 19. But how can node 17 talk to node 19?
Since both the LANS involved are IEEE 802-based, node 18 automatically bridges packets, allowing
node 17 and node 19 to create a virtual circuit. Although they're not connected to the same LAN,
nodes 17 and 19 can nevertheless talk to each other as if they were.
TCP/IP networking
QNX's inherent network support implements a LAN that relies on its own proprietary protocol and is
optimized to provide a fast, seamless interface between QNX computers. But to communicate with
non-QNX systems, QNX uses the industry-standard set of networking protocols collectively known as
TCP/IP.
As the Internet has grown to become more and more visible in our daily lives, the protocol it's based
on - IP (Internet Protocol) - has become increasingly important. Even if you're not connecting to "The
Internet" per se, the IP protocol and tools that go with it are ubiquitous, making IP the de facto choice
for many private networks.
IP is used for everything from simple tasks (e.g. remote login) to more complicated tasks (e.g.
delivering realtime stock quotes). More and more businesses are turning to the World Wide Web,
which commonly rides on IP, for communication with their customers, advertising, and other business
connectivity.
TCP/IP Manager
The QNX TCP/IP Manager is derived from the Berkeley BSD 4.3 stack, which is the most common
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TCP/IP stack on the Internet and has been used as the basis for many systems.
Socket API
The BSD Socket API was the obvious choice for QNX 4. The Socket API is the standard API for TCP/IP
programming in the Unix world. In the Windows world, the Winsock API is based on and shares a lot
with the BSD Socket API. This makes conversion between the two fairly easy.
All the routines that application programmers would expect are available, including:
accept()
bind()
bindresvport()
connect()
dn_comp()
dn_expand()
endprotoent()
endservent()
gethostbyaddr()
gethostbyname()
getpeername()
getprotobyname()
getprotobynumber()
getprotoent()
getservbyname()
getservent()
getsockname()
getsockopt()
herror()
hstrerror()
htonl()
htons()
h_errlist()
h_errno()
h_nerr()
inet_addr()
inet_aton()
inet_lnaof()
inet_makeaddr()
inet_netof()
inet_network()
inet_ntoa()
ioctl()
listen()
ntohl()
ntohs()
recv()
recvfrom()
res_init()
res_mkquery()
res_query()
res_querydomain()
res_search()
res_send()
select()
send()
sendto()
setprotoent()
setservent()
setsockopt()
shutdown()
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socket()
The common daemons and utilities from the Internet will easily port or just compile in this
environment. This makes leveraging what already exists for your applications a snap.
Network interoperability
The QNX TCP/IP Manager was designed and implemented with interoperability utmost in mind. The
code takes into account both the RFCs and the real world. The TCP/IP Manager addresses all the
functionality proposed in RFC 1122. The ARP, IP, ICMP, UDP, and TCP protocols are supported as well.
NFS
The Network FileSystem (NFS) is a TCP/IP application that has been implemented on most DOS and
Unix systems. NFS lets you graft remote filesystems - or portions of them - onto your local
namespace. Files on remote systems appear as part of your local QNX filesystem.
In QNX 4, NFS requires the Socket manager. Note that a "lighter" version of the socket
manager, known as Socklet, is also available if you don't need NFS.
SMB
QNX also supports the Server Message Block (SMB) file-sharing protocol, which is used by a number of
different servers such as Windows NT, Windows 95, Windows for Workgroups, LAN Manager, and
Samba. The SMBfsys filesystem allows a QNX client to transparently access remote drives residing on
such servers.
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The Photon microGUI Windowing System
This chapter covers the following topics:
l A graphical microkernel
l The Photon event space
l Graphics drivers
l Scalable fonts
l Unicode multilingual support
l Animation support
l Printing support
l The Photon Window Manager
l Widget library
l Summary
A graphical microkernel
Many embedded systems require a UI for interaction with the application. For complex applications, or for maximum
ease of use, a graphical windowing system is a natural choice. However, the windowing systems on desktop PCs
simply require too much in the way of system resources to be practical in an embedded system where memory and
cost are limited.
Drawing upon the successful approach of the QNX microkernel architecture to achieve a POSIX OS environment for
embedded systems, we have followed a similar course in creating the Photon microGUI windowing system.
To successfully implement a microkernel OS, we first had to tune the microkernel so that the kernel calls for IPC were
as lean and efficient as possible (since the performance of the whole OS rests on this message-based IPC). Having
implemented this low-overhead IPC, we were able to structure a GUI as a graphical "microkernel" process with a
team of cooperating processes around it, communicating via that IPC.
While at first glance this might seem similar to building a graphical system around the classic client/server paradigm
already used by the X Window System, the Photon architecture differentiates itself by restricting the functionality
implemented within the graphical microkernel (or server) itself and distributing the bulk of the GUI functionality
among the cooperating processes.
The Photon microkernel runs as a tiny process, implementing only a few fundamental primitives that external,
optional processes use to construct the higher-level functionality of a windowing system. Ironically, for the Photon
microkernel itself, "windows" do not exist. Nor can the Photon microkernel "draw" anything, or manage a pen, mouse,
or keyboard.
To manage a GUI environment, Photon creates a 3-dimensional "event space" and confines itself only to handling
regions and processing the clipping and steering of various events as they flow through the regions in this event
space.
This abstraction is roughly parallel to the concept of a microkernel OS being incapable of filesystem or device I/O, but
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relying instead on external processes to provide these high-level services. Just as this allows a microkernel OS to
scale up or down in size and functionality, so also a microkernel GUI.
The core microkernel "abstraction" implemented by the Photon microkernel is that of an abstract, graphical event
space that other processes can populate with regions. Using QNX IPC to communicate with the Photon microkernel,
these other processes manipulate their regions to provide higher-level graphical services or to act as user
applications. By removing service-providing processes, Photon can be scaled down for limited-resource systems; by
adding service-providing processes, Photon can be scaled up to full desktop functionality.
The Photon event space
The "event space" managed by the Photon microkernel can be visualized as an empty void with a "root region" at the
back. The end-user can be imagined to be "looking into" this event space. Applications place "regions" into the 3dimensional space between the root region and the end-user; they use those regions to generate and accept various
types of "events" within this space.
Processes that provide device driver services place regions at the front of the event space. In addition to managing
the event space and root region, the Photon microkernel maintains an on-screen pointer, projected as draw events
towards the user.
Photon uses a series of regions, ranging from the Root region at the back of the Photon event space to the Graphics
region at the front. Draw events start at an application's region and move forward to the Graphics region. Input
events start at the Pointer/Keyboard region and travel towards the Root region.
We can think of these events that travel through this space as "photons" (from which this windowing system gets its
name). Events themselves consist of a list of rectangles with some attached data. As these events flow through the
event space, their rectangle lists intersect the "regions" placed there by various processes (applications).
Events traveling away from the root region of the event space are said to be traveling outwards (or towards the
user), while events from the user are said to be traveling inwards towards the root region at the back of the event
space.
The interaction between events and regions is the basis for the input and output facilities in Photon. Pen, mouse, and
keyboard events travel away from the user towards the root plane, with the location of the on-screen pointer
associated with them. Draw events originate from regions and travel towards the device plane and the user.
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Regions
Regions are managed in a hierarchy associated as a family of rectangles that define their location in the 3dimensional event space. A region also has attributes that define how it interacts with various classes of events as
they intersect the region. The interactions a region can have with events are defined by two bitmasks:
l sensitivity bitmask
l opaque bitmask.
The sensitivity bitmask uses specific event types to define which intersections the process owning the region wishes
to be informed of. A bit in the sensitivity bitmask defines whether or not the region is sensitive to each event type.
When an event intersects a region for which the bit is set, a copy of that event is enqueued to the application process
that owns the region, notifying the application of events traveling through the region. This notification doesn't modify
the event in any way.
The opaque bitmask is used to define which events the region is opaque to. For each event type, a bit in the opaque
mask defines whether or not the region is opaque or transparent to that event. The optical property of "opaqueness"
is implemented by modifying the event itself as it passes through the intersection.
These two bitmasks can be combined to accomplish a variety of effects in the event space. The four possible
combinations are:
Bitmask
combination:
Description:
Not sensitive,
transparent
The event passes through the region, unmodified, without the region owner being notified.
The process owning the region simply isn't interested in the event.
Not sensitive, opaque
The event is clipped by the region as it passes through; the region owner isn't notified. For
example, most applications would use this attribute combination for draw event clipping, so
that an application's window wouldn't be overwritten by draw events coming from
underlying windows.
A copy of the event will be sent to the region owner; the event will continue, unmodified,
Sensitive, transparent through the event space. A process wishing to log the flow of events through the event
space could use this combination.
Sensitive, opaque
A copy of the event will be sent to the region owner; the event will also be clipped by the
region as it passes through. By setting this bitmask combination, an application can act as
an event filter or translator. For every event received, the application can process and
regenerate it, arbitrarily transformed in some manner, possibly traveling in a new direction,
and perhaps sourced from a new coordinate in the event space. For example, a region could
absorb pen events, perform handwriting recognition on those events, and then generate the
equivalent keystroke events.
Events
Like regions, events also come in various classes and have various attributes. An event is defined by:
l an originating region
l a type
l a direction
l an attached list of rectangles
l some event-specific data (optional).
Unlike most windowing systems, Photon classifies both input (pen, mouse, keyboard, etc.) and output (drawing
requests) as events. Events can be generated either from the regions that processes have placed in the event space
or by the Photon microkernel itself. Event types are defined for:
l keystrokes
l pen and mouse button actions
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l pen and mouse motion
l region boundary crossings
l expose and covered events
l draw events
l drag events.
Application processes can either poll for these events, block and wait for them to occur, or be asynchronously notified
of a pending event.
The rectangle list attached to the event can describe one or more rectangular regions, or it can be a "point-source" a single rectangle where the upper-left corner is the same as the lower-right corner.
When an event intersects a region that is opaque to it, that region's rectangle is "clipped out" of the event's list of
rectangles such that the list describes only the portion of the event that would ultimately be visible.
The best way to illustrate how this clipping is performed is to examine the changes in the rectangle list of a draw
event as it passes through various intersecting regions. When the draw event is first generated, the rectangle list
consists of only a single, simple rectangle describing the region that the event originated from.
If the event goes through a region that clips the upper-left corner out of the draw event, the rectangle list is modified
to contain only the two rectangles that would define the area remaining to be drawn. These resulting rectangles are
called "tiles."
Likewise, every time the draw event intersects a region opaque to draw events, the rectangle list will be modified to
represent what will remain visible of the draw event after the opaque region has been "clipped out." Ultimately, when
the draw event arrives at a graphics driver ready to be drawn, the rectangle list will precisely define only the portion
of the draw event that is to be visible.
Regions opaque to draw events are clipped out, resulting in an area consisting of rectangular tiles.
If the event is entirely clipped by the intersection of a region, the draw event will cease to exist. This mechanism of
"opaque" windows modifying the rectangle list of a draw event is how draw events from an underlying region (and its
attached process) are properly clipped for display as they travel towards the user.
Graphics drivers
Graphics drivers are implemented as processes that place a region in front of the event space. Rather than inject pen,
mouse, or keyboard events, a graphics driver's region is sensitive to draw events coming out of the event space. As
draw events intersect the region, those events are received by the graphics driver process. In effect, the region can
be imagined to be coated in "phosphor," which is illuminated by the impact of "photons."
Since the Photon drawing API accumulates draw requests into batches emitted as single draw events, each draw
event received by the driver contains a list of individual graphical primitives to be rendered. By the time the draw
event intersects the graphics driver region, its rectangle list will also contain a "clip list" describing exactly which
portions of the draw list are to be rendered to the display. The driver's job is to transform this clipped draw list into a
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visual representation on whatever graphics hardware the driver is controlling.
One advantage of delivering a "clip list" within the event passed to the driver is that each draw request then
represents a significant "batch" of work. As graphics hardware advances, more and more of this "batch" of work can
be pushed directly into the graphics hardware. Many display controller chips already handle a single clip rectangle;
some handle multiple clip rectangles.
Although using the QNX IPC services to pass draw requests from the application to the graphics driver may appear to
be an unacceptable overhead, our performance measurements demonstrate that this implementation performs as well
as having the application make direct calls into a graphics driver. One reason for such performance is that multiple
draw calls are batched with the event mechanism, allowing the already minimal overhead of the QNX lightweight IPC
to be amortized over many draw calls.
Multiple graphics drivers
Since graphics drivers simply put a region into the Photon event space, and since that region describes a rectangle to
be intersected by draw events, it naturally follows that multiple graphics drivers can be started for multiple graphics
controller cards, all with their draw-event-sensitive regions present in the same event space.
These multiple regions could be placed adjacent to each other, describing an array of "drawable" tiles, or overlapped
in various ways. Since Photon inherits QNX's network transparency, Photon applications or drivers can run on any
network node, allowing additional graphics drivers to extend the graphical space of Photon to the physical displays of
many networked computers. By having the graphics driver regions overlap, the draw events can be replicated onto
multiple display screens.
Many interesting applications become possible with this capability. For example, a factory operator with a wirelessLAN handheld computer could walk up to a workstation and drag a window from a plant control screen onto the
handheld, and then walk out onto the plant floor and interact with the control system.
In other environments, an embedded system without a UI could project a display onto any network-connected
computer. This connectivity also enables useful collaborative modes of work for people using computers - a group of
people could simultaneously see the same application screen and cooperatively operate the application.
From the application's perspective, this looks like a single unified graphical space. From the user's perspective, this
looks like a seamlessly connected set of computers, where windows can be dragged from physical screen to physical
screen across network links.
Color model
Colors processed by the graphics drivers are defined by a 24-bit RGB quantity (8 bits for each of red, green, and
blue), providing a total range of 16,777,216 colors. Depending on the actual display hardware, the driver will either
invoke the 24-bit color directly from the underlying hardware or use various dithering techniques to create the
requested color from less-capable hardware.
Since the graphics drivers use a hardware-independent color representation, applications can be displayed without
modifications on hardware that has varied color models. This allows applications to be "dragged" from screen to
screen, without concern for what the underlying display hardware's color model might be.
Scalable fonts
In addition to full support of bitmap fonts, Photon also provides scalable fonts. These fonts can be scaled to virtually
any point size as well as anti-aliased (16 shades) for crisp, clean output on screen at practically any resolution.
Photon's scalable fonts are achieved by means of a high-speed font server that retrieves highly compressed font data
from Portable Font Resource files (*.pfr), and then renders the font character shapes at any point size and
resolution. Note that PFRs provide better than 2-to-1 compression over PostScript fonts.
Font sets
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Core Latin set
The Photon Core Latin set (latin1.pfr), which comprises the Unicode Basic Latin (U+0000 - U+007F) and the Latin1 Supplement (U+0080 - U+00FF) character sets, includes the following scalable fonts:
l Dutch
l Dutch Bold
l Dutch Italic
l Dutch Bold Italic
l Swiss
l Swiss Bold
l Swiss Italic
l Swiss Bold Italic
l Courier
l Courier Bold
l Courier Italic
l Courier Bold Italic
Extended Latin set
The Photon Extended Latin set (latinx.pfr) comprises the Unicode Latin Extended-A (U+0100 - U+017F) and Latin
Extended-B (U+0180 - U+0217) sets. The Extended Latin set includes these fonts:
l Dutch
l Dutch Bold
l Dutch Italic (algorithmically generated)
l Dutch Bold Italic (algorithmically generated)
l Swiss
l Swiss Bold
l Swiss Italic (algorithmically generated)
l Swiss Bold Italic (algorithmically generated)
Languages supported
Armed with the Photon Core Latin set, the developer can support a host of languages, including:
Danish
Dutch
English
Finnish
Flemish
French
German
Hawaiian
Icelandic
Indonesian
Irish
Italian
Norwegian
Portuguese
Spanish
Swahili
Swedish
By adding the Photon Extended Latin set (latinx.pfr), the developer can support several additional languages,
including:
Afrikaans
Basque
Catalan
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Croatian
Czech
Esperanto
Estonian
Greenlandic
Hungarian
Latvian
Lithuanian
Maltese
Polish
Romanian
Slovak
Turkish
Welsh
International language supplements
Several language supplement packages are available for Photon:
l Japanese Language Supplement - offers developers full support for Japanese characters.
Besides a complete set of PFRs (Hon Mincho Kanji set), the package also includes a front-end processor that
allows users to provide input using either an English or a Japanese keyboard (via the popular VJE input
method used in Japan).
l Chinese Language Supplement - offers developers full support for Chinese characters as well as a
corresponding input method.
l Korean Language Supplement - offers developers full support for Korean characters as well as a corresponding
input method.
l Cyrillic Language Supplement - offers developers full support for Russian or any language that relies on the
Cyrillic alphabet. This package also includes the appropriate keyboard drivers to support English or Cyrillic
keyboards.
Unicode multilingual support
Photon is designed to handle international characters. Following the Unicode Standard (ISO/IEC 10646), Photon
provides developers with the ability to create applications that can easily support the world's major languages and
scripts.
Unicode is modeled on the ASCII character set, but uses a 16-bit encoding to support full multilingual text. There's no
need for escape sequences or control codes when specifying any character in any language. Note that Unicode
encoding conveniently treats all characters - whether alphabetic, ideographs, or symbols - in exactly the same way.
UTF-8 encoding
Formerly known as UTF-2, the UTF-8 (for "8-bit form") transformation format is designed to address the use of
Unicode character data in 8-bit UNIX environments.
Here are some of the main features of UTF-8:
l Unicode characters from U+0000 to U+007E (ASCII set) map to UTF-8 bytes 00 to 7E (ASCII values).
l ASCII values don't otherwise occur in a UTF-8 transformation, giving complete compatibility with historical
filesystems that parse for ASCII bytes.
l UTF-8 simplifies conversions to and from Unicode text.
l The first byte indicates the number of bytes to follow in a multibyte sequence, allowing for efficient forward
parsing.
l Finding the start of a character from an arbitrary location in a byte stream is efficient, because you need to
search at most four bytes backwards to find an easily recognizable initial byte. For example:
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isInitialByte = ((byte & 0xC0) != 0x80);
l UTF-8 is reasonably compact in terms of the number of bytes used for encoding.
The OS library includes a convenient set of conversion functions:
Function
Description
mblen()
Length of a multibyte string in characters
mbtowc()
Convert multibyte character to wide character
mbstowcs()
Convert multibyte string to wide character string
wctomb()
Convert wide character to its multibyte representation
wcstombs()
Convert wide character string to multibyte string
In addition to the conversion functions listed above, developers can also rely on Photon's own PxTranslate library
functions, which will translate various character set encodings to and from UTF-8.
Animation support
Photon provides flicker-free animations via a special "double-buffer" container widget (PtDBContainer) that creates a
dedicated off-screen memory context for drawing images.
The PtDBContainer widget uses a block of shared memory large enough to hold an image the size of its canvas, and
will also dynamically load the render shared library.
Printing support
Photon provides built-in printing support for a variety of outputs, including:
l bitmap files
l PostScript
l Hewlett-Packard PCL
l Epson ESC/P2
l Canon
l Lexmark
Photon also comes with a print-selection widget/convenience dialog to make printing simpler within developers' own
applications.
The Photon Window Manager
Adding a window manager to Photon creates a fully functional desktop-class GUI. The window manager is entirely
optional and can be omitted for most classes of embedded systems. If present, the window manager allows the user
to manipulate application windows by resizing, moving, and iconifying them.
The window manager is built on the concept of filtering events with additional regions placed behind application
regions, upon which a title bar, resize handles, and other "gadgets" are drawn and interacted with. Since the
replaceable window manager implements the actual "look and feel" of the environment, various UI flavors can be
optionally installed.
Widget library
Photon provides a library of components known as widgets - objects that can manage much of their on-screen
behavior automatically, without explicit programming effort. As a result, a complete application can be quickly
assembled by combining widgets in various ways and then attaching C code to the appropriate callbacks the widgets
provide. The Photon Application Builder (PhAB), which is included as part of the Photon development system, provides
an extensive widget palette in its visual development environment.
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Photon provides a wide range of widgets:
l basic widgets (e.g. a button)
l container widgets (e.g. a window widget)
l advanced widgets (e.g. an HTML display widget).
Basic widgets
Label widget (PtLabel)
The label widget is mainly used to display textual information. The PtLabel widget is the superclass for all text-based
widgets, providing many customizable attributes (e.g. font typeface, pop-up balloons, colors, borders, alignment,
margins, etc.), all of which are inherited by all its subclasses.
Push button widget (PtButton)
Push buttons are a necessary component in every windowing system. They have a raised look that changes to
depressed when pushed, giving a visual cue to let the user know the button has been selected. In addition to this
visual behavior, push buttons automatically invoke an application-defined callback when they're selected.
Text input widgets (PtText, PtMultiText)
Photon provides two text-input widgets:
l a simple single-line input widget (PtText) commonly used in forms
l a powerful wordprocessor-like multi-line widget (PtMultiText) providing full editing capabilities, word
wrapping, automatic scrolling, and multi-font line segments.
Toggle button widgets (PtToggleButton, PtOnOffButton)
Toggle buttons are objects that display two states - either on or off. Photon provides two different types of toggle
buttons, each with a different visual appearance. Toggle buttons are used to display or request state information
related to a command or action about to be performed.
Graphical widgets (PtArc, PtPixel, PtRectangle, PtLine, PtPolygon, PtEllipse,
PtBezier, PtGrid)
Photon has no shortage of graphical widgets. There's a widget to accomplish everything from simple lines and
rectangles to complex multi-segmented bézier curves. Graphical widgets provide attributes for color, fills, patterns,
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line thickness, joins, and much more.
Scrollbar widget (PtScrollbar)
A scrollbar widget is used to scroll the display of a viewable area. The scrollbar is combined with other widgets (e.g.
PtList, PtScrollArea) to allow scrolling.
Separator widget (PtSeparator)
The separator widget is used to separate two or more different areas, giving a better visual appearance. The
separator can be customized for many different styles and looks.
Slider widget (PtSlider)
Sliders are different from scrollbars. A scrollbar defines a range, whereas a slider defines a single value. The slider
widget provides a rich list of customizable attributes.
Timer widget (PtTimer)
The timer widget makes using timers a snap. This widget has no visual appearance - it simply defines a callback
whenever a timer event is triggered. The application sets the timer value and optional repeat value. When the timer
goes off, the application is notified.
Graphic image widgets (PtBitmap, PtLabel, PtButton)
Photon supports every major graphic file standard, so you can import graphics and display them inside widgets. Many
Photon widgets directly support displaying graphics - the most common are PtButton for making push-button
toolbars and PtLabel for displaying images.
Progress bar widget (RtProgress)
If an application needs to do something that takes a fair amount of time (e.g. loading a file), it can use the progress
bar widget to let the user know what's happening and, more importantly, how much longer the process is going to
take. The progress bar can be horizontal or vertical and has many attributes for customization.
Message widget (PtMessage)
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Pop-up messages and notifications are quite common in a windowing environment. Photon provides a very handy
message dialog widget that displays a message and up to 3 user-response buttons. There's also a very useful modal
dialog function call (PtAskQuestion()) based on the message widget.
Numeric widgets (PtNumericInteger, PtNumericFloat)
The PtNumericInteger class lets the user specify integer values between given minimum and maximum values. The
PtNumericFloat class lets the user enter floating-point values.
A PtUpDown widget is also shown to allow the user to increase or decrease the value by a set amount.
Container widgets
Window and icon widgets (PtWindow, PtIcon)
Windows are the main application containers. The main UI components (menu bars, toolbars, etc.) appear with the
window widget. The widget automatically handles all the necessary interactions with the Photon Window Manager
(PWM) - all you need to specify is what should and shouldn't be rendered or managed.
Icon widgets are closely associated with windows and are displayed in the Photon Desktop Manager launch folders
and PWM taskbar.
Bulletin board widgets (PtPane)
Bulletin board widgets are simple container widgets that are used to hold other widgets. Although these are parent
widgets, they don't manage their child widgets in any way. They are quite useful for designing form layouts
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commonly found in dialog windows.
Group widget (PtGroup)
The group widget is a very powerful widget that manages the geometry of all its child widgets. It can align the
widgets horizontally, vertically, or in a matrix. The group widget can be anchored to the side of any other container
(like a window) so that it automatically resizes when the window resizes. The group widget also provides attributes
that let you specify whether the children should be stretched to fit the group if it's resized larger due to anchoring.
Scrolling area widget (PtScrollArea)
The scrolling area widget is a very powerful widget that provides a viewport into a potentially larger container. You
can place any number of widgets inside a scrolling area and it will automatically display a scrollbar if the widgets are
contained within the viewable area. Scroll area widgets could be used to implement a text file viewer, wordprocessor,
customized list display, and so on.
To scroll child widgets quickly, the scrolling area widget uses a hardware blitter (provided the underlying graphics
driver supports it).
Background widget (PtBkgd)
The background widget provides a way to create fancy background displays, from simple color gradations to tiled
textures. Just about any background requirement is handled by this widget.
Advanced widgets
Menu related widgets (PtMenu, PtMenuBar, PtMenuButton)
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Photon provides for every menu related requirement. There's a widget to simplify the creation of a standard menu
bar. The menu widget handles the pop-up display, press-drag-release, point and click, keyboard traversal, and
selection of menu items. The menu button widget is used for creating individual menu items.
List widget (PtList)
The list widget is a very powerful widget that manages a list of items. It provides many different selection modes,
including single selection, multiple selection and range selection. The list widget also supports multi-columned lists
through the use of a divider widget (PtDivider).
Pulldown list widget (PtComboBox)
The pulldown list widget combines the PtText widget (for text input) with a pulldown button for displaying a list
widget. When the user selects from the list, the text widget is automatically updated with the current selection. The
pulldown list widget is very useful for displaying a list of items using a small space. Dialogs and containers use a lot
less screen real-estate, which is important in embedded environments.
Tree widget (PtTree)
The tree widget is similar to the list widget - in fact they both have the same ancestors. The main difference is that
the tree widget displays the items in a hierarchical manner. Items, called branches, can be expanded or collapsed;
any number of tree branches can be created. Each branch can define its own unique image to display. Trees are
useful because they display information in a very logical manner.
Photon applications that use the tree widget include: the File Manager (directory display), PhAB (widget layout), vsin
(process list), and many others.
Terminal widgets (PtTty, PtTerminal)
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Imagine having a text console inside your application. That's exactly what this widget does. It creates and manages
an entire text-mode terminal inside a widget. Just drop it into your application and you've created your very own
pterm (our terminal application).
The terminal widget doesn't stop there - it also provides complete cut-and-paste functionality and quick-launch help
by highlighting any text within the widget.
Divider widget (PtDivider)
This powerful widget manages its children in a unique and useful way. When you place two or more widgets inside a
divider widget, it automatically puts little separators in between the child widgets. Using these separators, the user
can drag back and forth, causing the child widgets on either side of the separator to be resized. This is very useful for
creating resizable column headings for lists. In fact, if you drop a divider widget into a list widget, it will automatically
turn your simple list into a resizable multi-column list.
Dividers aren't limited to just labels or buttons. Any widgets can be placed inside to create side-by-side resizable
trees, scroll areas, and so on.
Trend graph widget (RtTrend)
Realtime systems often require trend graphs. Photon comes with a trend bar widget that supports the display of
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multiple trend lines simultaneously. If your graphics hardware supports masked blits, it can even smooth-scroll the
trend across grid lines.
Realtime meter widget (RtMeter)
The RtMeter widget is drawn as a half circle with divisional ticks at 1/3, 1/2, and 2/3 of the arc. The needle can be
moved with the mouse, the keyboard, or programmatically. A single mouse click moves the meter to the current
mouse position; a mouse "drag" causes the needle to follow the mouse through meter values.
Font selection dialog (PtFontSel)
To accommodate the wide selection of fonts available for Photon, a Font Selection widget is provided. This widget can
read the standard font-mapping files and display a list of all available fonts. It allows you to choose the typeface,
style (bold, italic, etc.) and also indicate whether the font should be anti-aliased.
File selection widget (PtFileSel)
The PtFileSel widget is a tree widget that displays files, directories, links to files or directories, or custom entries.
Besides selecting a particular file in response to an application prompt, users can also use this widget to navigate an
entire filesystem and choose their own file and directory.
Print selection dialog (PtPrintSel)
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The PtPrintSel widget lets a user select a printer or control its properties. The user may also select a range of pages
to print as well as the number of copies to submit.
HTML viewer widget (PtHtml)
Using the HTML widget makes it easy to create a customized help viewer. It will format a standard HTML file and even
autoload all the images. It handles resizing, scrolling, just about everything you would need to do. This widget is ideal
for creating helpviewers for touchscreen environments.
Widget building toolkit
If all the standard Photon widgets aren't enough, you can easily build your own! The Photon Development System
comes with complete documentation and sample source code to create your own custom widgets. You can sub-class
off of existing widgets to inherit their functionality or create a complete new widget class tree. Your widget
possibilities are endless.
Summary
Photon represents a new approach to GUI building - using a microkernel and a team of cooperating processes, rather
than the monolithic approach typified by other windowing systems. As a result, Photon exhibits a unique set of
capabilities:
l Low memory requirements enable Photon to deliver a high level of windowing functionality to environments
where only a graphics library might have been allowed within the memory constraints.
l Photon provides a very flexible, user-extensible architecture that allows developers to extend the GUI in
directions unique to their applications.
l With flexible cross-platform connectivity, Photon applications can be used from virtually any connected
desktop environment.
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