CS 640 Fall 1994 BSD Socket Reference Page 1

CS 640 Fall 1994 BSD Socket Reference Page 1
CS 640 Fall 1994
BSD Socket Reference
Page 1
Berkeley UNIX† System Calls and Interprocess Communication
by Lawrence Besaw
January, 1987
Revised, September 1987, January 1991 by Marvin Solomon
The purpose of this paper is to discuss interprocess communication in the context of Berkeley UNIX. Special
emphasis will be given to those system calls concerned with the creation, management, and use of sockets. There
will also be a discussion of signals and selected other system calls that will be useful to those working on the network project assigned for this course. More information on all the system calls mentioned below can be found in
the UNIX Programmer’s Manual. Periodic mention will be made of other manual page entries that might be consulted. System header files are designated by enclosing angle brackets; they reside in /usr/include and its subdirectories.
1. Socket Creation
The most general mechanism for interprocess communication offered by Berkeley UNIX is the socket. A
socket is an endpoint for communication. Two processes can communicate by creating sockets and sending messages between them. There are a variety of different types of sockets, differing in the way the address space of the
sockets is defined and the kind of communication that is allowed between sockets. A socket type is uniquely determined by a <domain, type, protocol> triple. In order for a remote socket to be reached, it must be possible to assign
a name to it. The form that this name assumes is determined by the communication domain or address family to
which the socket belongs. There is also an abstract type or style of communication associated with each socket.
This dictates the semantics of communication for that socket. Finally, there is a specific protocol that is used with
the socket. A socket can be created with the socket system call by specifying the desired address family, socket
type, and protocol.
socket_descriptor = socket(domain, type, protocol)
int socket_descriptor, domain, type, protocol;
This call returns a small positive integer called a socket descriptor that can be used as a parameter to reference the
socket in subsequent system calls. Socket descriptors are similar to file descriptors returned by the open system call.
Each open or socket call will return the smallest unused integer. Thus a given number denotes either an open file, a
socket, or neither (but never both). Socket and file descriptors may be used interchangeably in many system calls.
For example, the close system call is used to destroy sockets.
1.1. Domains
The communication domain or address family to which a socket belongs specifies a certain address format.
All later operations on a socket will interpret the supplied address according to this specified format. The various
address formats are defined as manifest constants in the file <sys/socket.h>.1 Examples are AF_UNIX (UNIX path
names), AF_INET (DARPA Internet addresses), and AF_OSI (as specified by the international standards for Open
Systems Interconnection). AF_UNIX and AF_INET are the most important address families. The general form of
an address is represented by the sockaddr structure defined in <sys/socket.h>.
struct sockaddr {
short sa_family;
char sa_data[14];
†UNIX is a trademark of AT&T Bell Laboratories.
/* address family */
/* up to 14 bytes of direct address */
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When a socket is created, it does not initially have an address associated with it. However, since it is impossible for
a remote host or process to find a socket unless it has an address, it is important to bind an address to the socket. A
socket does not have a name until an address is explicitly assigned to it with the bind system call.
status = bind(sock, address, addrlen)
int status; /* status returns 0 for success, −1 otherwise */
int sock;
/* descriptor returned by socket() */
struct sockaddr *address;
int addrlen; /* size of address (in bytes) */
This call fails if the address is already in use, the address is not in the correct format for the address family specified,
or the socket already has an address bound to it.
1.1.1. UNIX domain
In the UNIX domain, a socket is addressed by a UNIX path name that may be up to 108 characters long. The
binding of a path name to a socket results in the allocation of an inode and an entry of the path name into the file
system. This necessitates removing the path name from the file system (using the unlink system call) when the
socket is closed. The created file is only used to provide a name for the socket and does not play a role in the actual
transfer of data. When using sockets in the UNIX domain, it is advisable to only use path names for directories
(such as /tmp) directly mounted on the local disk. The UNIX domain only allows interprocess communication for
processes working on the same machine. The structure sockaddr_un used to define the UNIX address format can be
found in <sys/un.h>.
struct sockaddr_un {
/* AF_UNIX */
sun_path[108-4]; /* path name */
NB: When specifying the length of UNIX domain addresses for system calls, use
sockaddr_un). Using the size of sockaddr will cause the call to fail.
1.1.2. Internet domain
In the DARPA Internet domain, addresses consist of two parts—a host address (consisting of a network
number and a host number) and a port number (also known as a transport suffix). This host address allows
processes on different machines to communicate. The port number in turn is like a mail box that allows multiple
addresses on the same host. The structure sockaddr_in describing an address in the Internet domain is defined in the
file <netinet/in.h>.
struct sockaddr_in {
in_addr sin_addr;
port number */
see struct below */
not used */
struct in_addr {
union {
struct { u_char s_b1,s_b2,s_b3,s_b4; } S_un_b;
struct { u_short s_w1,s_w2; } S_un_w;
u_long S_addr;
} S_un;
S_un.S_addr /* can be used for most tcp & ip code */
You must include <sys/types.h> before <sys/socket.h>.
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It is often useful to bind a specific service to a “well-known” port, enabling remote processes to locate the
required server easily. Examples of well-known ports are 79 for the “finger” service and 513 for remote login. The
kernel reserves the first 1024 port numbers for its own use. There is a network services database in /etc/services that
can be queried by using the system calls getservbyname and getservbyport (see getservent(3N) and services(5)).
Each of these routines returns a pointer to the structure servent defined in <netdb.h>. If the port field in the address
parameter to bind is specified as zero, the system will assign an unused port number. The assigned port number can
be discovered by calling getsockname. In the Internet domain, UDP and TCP can use the same port numbers.
There is no confusion in naming because ports bound to sockets with different protocols cannot communicate. Ports
numbered less than 1024 are “reserved”—only processes running as superuser may bind to them.
Internet host addresses are specified in four bytes (32 bits). They are typically represented by a standard ‘.’
notation, a.b.c.d. The bytes of the address are represented by decimal integers, separated by periods, and ordered
from high order to low order. This ordering is called network order and is the order in which addresses are
transmitted over the network. For example, the Internet address for the host garfield.cs.wisc.edu is
which corresponds to the unsigned integer 80690103 in hex, or 2154365187 in decimal. Some hosts (such as the
VAX), however, have a host order for these integer values that reverses the order of the bytes. When a word is
transmitted from such a host (or when a C program treats a word as a sequence of bytes), the low-order byte is
transmitted first (has the lowest address). Thus, it is necessary to reverse the bytes of an address, stored in an
integer, before transmitting it. The system routines htonl and ntohl are provided to convert long (32-bit) integers
from host to network order and vice versa. Similarly, htos and ntohs swap the bytes of short (16-bit) integers, such
as port numbers. System calls that return or demand Internet addresses and port numbers (such as gethostbyname or
bind, which is described above), deal entirely in network order, so you normally don’t have to worry about all this.
However, if you want to print out one of these values or read it in, you will have to convert from/to host order.
An Internet host address can be decomposed into two parts—a network number and a local address. There
are three formats for Internet addresses (see inet(3N)), each of which partitions the 32 bits differently. “Class A”
addresses have a one-byte network number (the high order byte), and a 3-byte host number. “Class B” addresses
have two bytes each of network and host number, and “class C” networks have three bytes of network number and
one byte of host number. (Thus a class C network can have at most 256 hosts). The high-order bits of an address
determine its class: Addresses starting with a zero are class A addresses, addresses starting with 10 are class B, and
addresses starting with 110 are class C. Thus there can be at most 128 class A networks and at most 214 = 16,384
class B networks.
The system calls gethostbyaddr and gethostbyname can be used to look up the host address (see
gethostent(3N) and host(5)). Each of these calls returns a pointer to the structure hostent defined in <netdb.h>.
Host addresses are returned in network order, the proper format for a call to bind. The system routine memcpy can
be used to copy the host address into the sockaddr_in structure. If a zero host address is given (the wildcard value
INADDR_ANY can be used) in the call to bind, the local host address will be automatically supplied. Another reason for using INADDR_ANY is that a host may have multiple Internet addresses; addresses specified this way will
match any incoming messages with a valid Internet address.
1.2. Styles of Communication
The type field of the socket call specifies the abstract “style of communication” to be used with that socket.
These types are defined as manifest constants in <sys/socket.h>. The following types are currently defined:
SOCK_STREAM (stream), SOCK_DGRAM (datagram), SOCK_RAW (raw protocol interface), SOCK_RDM
(reliable datagrams), and SOCK_SEQPACKET (sequenced packet stream). Each of these abstractions is supported
by different protocols. We will be primarily concerned with the SOCK_STREAM and SOCK_DGRAM abstractions.
1.2.1. Datagram Sockets
The SOCK_DGRAM type provides a datagram model of communication. A datagram service is connectionless and unreliable. Independent (and usually small) messages called datagrams are accepted by the transport protocol and sent to the address specified. These messages may be lost, duplicated, or received out of order. An important characteristic of datagrams is that message boundaries are maintained. This means that individual datagrams
(i.e., messages sent with separate calls) will be kept separate when they are received at the other end. A revcfrom
call on a datagram socket will only return the next datagram available. The SOCK_DGRAM type can be used in
the UNIX or Internet domain. When used in the Internet domain, it is supported by the Internet transport protocol
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UDP. The call
sock = socket(AF_INET, SOCK_DGRAM, 0)
returns a UDP datagram socket.
1.2.2. Stream Sockets
The STREAM_SOCK type provides a stream model of communication. This service is reliable and
connection-oriented. Data is transmitted on a full-duplex, flow-controlled byte stream. The transport protocol supporting this socket type must ensure that data is delivered in order without loss, error, or duplication. Otherwise, it
must abort the connection and report an error to the user. Message boundaries are not preserved on a stream socket.
Before data can be sent or received on a stream socket, the socket must be made the active or passive end of a connection using the system calls listen, accept, and connect discussed in Section 2. Again, the SOCK_STREAM
abstraction can be used in both the UNIX and Internet domains. In the Internet domain, it is supported by the Internet transport protocol TCP. The call
sock = socket(AF_INET, SOCK_STREAM, 0)
returns a TCP stream socket.
1.3. Protocols
A protocol is a set of communication conventions for regulating the exchange of information between two
parties. Transport protocols that support the styles of communication described above are implemented as code in
the UNIX kernel. These concrete protocols actually realize the semantics defined by the socket type. User
Datagram Protocol (UDP), Transmission Control Protocol (TCP), and Internet Protocol (IP) all belong to the Internet family of protocols. Each member of this protocol family supports a different abstract type (see tcp(4P),
udp(4P), and ip(4P)). UDP supports datagram sockets, TCP supports stream sockets, and the SOCK_RAW type of
socket provides a raw interface to IP. TP, the ISO connection-based transport protocol, conceptually supports the
SOCK_SEQPACKET abstraction, although it is not currently implemented in Berkeley Unix,
There is currently only one protocol supporting a given socket abstraction in a particular domain. Nothing,
however, prevents future protocols from being implemented that support the same abstract type in the same domain
as an existing protocol. The protocol to be used is specified in the protocol field of the socket call. There are three
ways to specify the protocol. First, if a zero is supplied, the system provides the default protocol for that address
family and socket type; this is the most common method. Second, manifest constants describing protocols, such as
IPPROTO_UDP, can be found in <netinet/in.h>. Third, the network protocol database /etc/protocols can be consulted by calling getprotobyname and getprotobynumber (see getprotoent(3N) and protocols(5)).
2. Connection Establishment
Stream sockets must establish a connection before data can be transferred. A stream socket is either “active”
or “passive”. A socket is initially active and only becomes passive after a listen call. Only active sockets can be
mentioned in a connect call and only passive sockets can be mentioned in accept. These connection establishment
calls are intended for use with stream sockets, but datagram sockets can call connect to permanently fix the destination for future send calls.
2.1. Active End
An active socket establishes a connection to a named passive socket by using the connect system call.
status = connect(sock, name, namelen)
int sock; /* socket descriptor */
struct sockaddr *name;
int namelen;
The name parameter is the address of the remote socket interpreted according to the communication domain in
which the socket exists. If the domain is AF_UNIX, this should be a sockaddr_un structure; if the domain is
AF_INET, it should be a sockaddr_in structure.
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2.2. Passive End
A socket becomes the passive end of a connection by first doing a listen and then an accept.
status = listen(sock, queuelen) int sock, queuelen;
Listen initializes a queue for waiting connection requests. The parameter queuelen specifies the maximum number
of queued connections that will be allowed. The maximum queue length is currently limited by the system to 5.
The manifest constant SOMAXCONN from <sys/socket.h> defines this maximum. A connection can then be established using the system call accept.
new_socket = accept(old_sock, name, namelen) int new_socket, old_sock; /* socket descriptors */ struct sockaddr
*name; /* name of peer socket on new connection */ int *namelen;
/* length of name in bytes */
Accept takes the first pending connection request off the queue and returns a socket descriptor for a new, connected
socket. This socket has the same properties as the old socket. The address of the socket at the active end is returned
in name. Namelen is a value/result parameter that should be initialized to the size of the address structure being
passed; upon return it will be set to the actual length of the address returned. The old socket remains unaffected and
can be used to accept more connections. If there are no pending connection requests, accept blocks. If necessary, a
select can be done first to see if there are any connection requests (see Section 4.2). A socket with pending connections will show up as being ready for reading.
3. Data Transfer
The system call pairs (read, write), (recv, send), (recvfrom, sendto), (recvmsg, sendmsg), and (readv, writev)
can all be used to transfer data on sockets. The most appropriate call depends on the exact functionality required.
Send and recv are typically used with connected stream sockets. They can also be used with datagram sockets if the
sender has previously done a connect or the receiver does not care who the sender is. Sendto and recvfrom are used
with datagram sockets. Sendto allows one to specify the destination of the datagram, while revcfrom returns the
name of the remote socket sending the message. Read and write can be used with any connected socket. These two
calls may be chosen for efficiency considerations. The remaining data transfer calls can be used for more specialized purposes. Writev and readv make it possible to scatter and gather data to/from separate buffers. Sendmsg and
recvmsg allow scatter/gather capability as well as the ability to exchange access rights. The calls read, write, readv,
and writev take either a socket descriptor or a file descriptor as their first argument; all the rest of the calls require a
socket descriptor.
count = send(sock, buf, buflen, flags)
int count, sock, buflen, flags;
char *buf;
count = recv(sock, buf, buflen, flags)
int count, sock, buflen, flags;
char *buf;
count = sendto(sock, buf, buflen, flags, to, tolen)
int count, sock, buflen, flags, tolen;
char *buf;
struct sockaddr *to;
count = recvfrom(sock, buf, buflen, flags, from, fromlen)
int count, sock, buflen, flags, *fromlen;
char *buf;
struct sockaddr *from;
For the send calls, count returns the number of bytes accepted by the transport layer, or −1 if some error is
detected locally. A positive return count is no indication of the success of the data transfer. NB: If made nonblocking, send may accept some but not all of the bytes in the data buffer (see Section 4.1). The return value should
be checked so that the remaining bytes can be sent if necessary. For receive calls, count returns the number of bytes
actually received, or −1 if some error is detected.
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The first parameter for each call is a valid socket descriptor. The parameter buf is a pointer to the caller’s data
buffer. In the send calls, buflen is the number of bytes being sent; in the receive calls, it indicates the size of the data
area and the maximum number of bytes the caller is willing to receive. The parameter to in the sendto call specifies
the destination address (conforming to a particular address family) and tolen specifies its length. The parameter
from in the recvfrom call specifies the source address of the message. Fromlen is a value/result parameter that initially gives the size of the structure pointed to by from and then is modified on return to indicate the actual length of
the address.
The flags parameter, which is usually given zero as an argument, allows several special operations on stream
sockets. It is possible to send out-of-band data or “peek” at the incoming message without actually reading it. The
flags MSG_OOB and MSG_PEEK are defined in <sys/sockets.h>. Out-of-band data is high priority data (such as
an interrupt character) that a user might want to process quickly before all the intervening data on the stream. If
out-of-band data were present, a SIGURG signal could be delivered to the user. The actual semantics of out-ofband data is determined by the relevant protocol. ISO protocols treat it as expedited data, while Internet protocols
treat it as urgent data.
If any of these (as well as other) system calls is interrupted by a signal, such as SIGALRM or SIGIO, the call
will return −1 and the variable errno will be set to EINTR.2 The system call will be automatically restarted. It may
be advisable to reset errno to zero.
4. Synchronization
By default, all the reading and writing calls are blocking: Read calls do not return until at least one byte of
data is available for reading and write calls block until there is enough buffer space to accept some or all of the data
being sent. Some applications need to service several network connections simultaneously, performing operations
on connections as they become enabled. There are three techniques available to support such applications: nonblocking sockets, asynchronous notifications, and the select system call. The select system call is by far the most
commonly used; non-blocking sockets are less common, and asynchronous notifications are rarely used.
4.1. Socket Options
The system calls getsockopt and setsockopt can be used to set and inspect special options associated with
sockets. These might be general options for all sockets or implementation-specific options. Sample options taken
from <sys/sockets.h> are SO_DEBUG and SO_REUSEADDR (allow the reuse of local addresses).
Fcntl and ioctl are system calls that make it possible to control files, sockets, and devices in a variety of ways.
status = fcntl(sock, command, argument)
int status, sock, command, argument;
status = ioctl(sock, request, buffer)
int status, sock, request;
char *buffer;
The command and argument parameters for fcntl can be supplied with manifest constants found in <fcntl.h>. The
manifest constants for ioctl’s request parameter are located in <sys/ioctl.h>. This parameter also specifies how the
buffer argument of the call is to be used.
Either one of these system calls can be used to enable asynchronous notifications on a socket. Whenever data
arrives on such a socket a SIGIO signal will be delivered to the process. The process should have already declared
a signal handler for this signal (see Section 6.1). This signal handler can then read the data from the socket. Program execution continues at the point of interruption. The calling sequence
fcntl(sock, F_SETOWN, getpid());
fcntl(sock, F_SETFL, FASYNC);
enables asynchronous i/o on the given socket. The first call is necessary to specify the process to be signalled. The
second call sets the descriptor status flags to enable SIGIO.
Error numbers are defined in the file <errno.h>.
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The same calling sequence can be used to make a socket non-blocking; the only change is that the flag FNDELAY is used in the second call instead of FASYNC. In this case, if a read or write operation on the socket would
normally block, −1 is returned and the external system variable errno is set to EWOULDBLOCK. This error
number can be checked and appropriate action taken.
4.2. Multiplexing File Descriptors
The system call select makes possible synchronous multiplexing of socket and file descriptors. It can be used
to determine when there is data to read or when it is possible to send more data.
nfound = select(numdes, readmask, writemask, exceptmask, timeout)
int nfound, numdes;
fd_set *readmask, *writemask, *exceptmask;
struct timeval *timeout;
The masks in this call are value/result parameters through which file and socket descriptors are indicated. The possibility of a particular operation—reading, writing, or the presence of an exceptional condition—is investigated by
setting the bit for that socket in the corresponding mask. A zero pointer can used if a given condition is of no
interest. For example, if writemask is zero, sockets will not be checked for writing.
The type fd_set is defined in <sys/types.h> to be a struct containing a single field, which is an array of
integers. The array is interpreted as a bitmask, with one bit for each possible file or socket descriptor. (Representing the mask as struct rather than a simple array allows fd_set values to be assigned to each other, without necessitating a call to memcpy). Four macros are defined in <sys/types.h> for setting, clearing, and testing individual bits
in the mask:
FD_SET(n, p)
FD_CLR(n, p)
result = FD_ISSET(n, p)
int n;
fd_set *p;
set bit n */
clear bit n */
test bit n */
clear all bits */
Multiple bits can be set in a given mask, but each must correspond to a valid descriptor.
The parameter numdes indicates that bits 0 through numdes − 1 should be examined. The manifest constant
FD_SETSIZE, defined in <sys/types.h>, indicates the maximum number of descriptors that can be represented by an
fd_set. Thus setting numdes = FD_SETSIZE will ensure that no descriptor is overlooked. The timeout parameter is
a pointer to a timeval structure defined in <sys/time.h>. It is used to specify the maximum time interval that the call
will wait before returning. If timeout is zero (a null pointer), then select blocks indefinitely. If timeout points to a
zero timeval structure, then the call returns immediately, even if no descriptor is ready. Select returns when a condition has been discovered for one or more of the sockets or the specified time interval has elapsed. The return value
nfound indicates the number of conditions that were satisfied. The masks are modified to indicate those sockets for
which the respective conditions hold.
The most common reason for including file descriptors in the masks passed to select is to respond to interactive terminal activity. For example,
FD_SET(fileno(stdin), readmask)
may be used to check whether anything has been typed on the terminal.
5. Connection Dissolution
A connection can be dissolved by simply using the system call close to close one of the sockets involved.
status = close(sock)
int status, sock;
The precise semantics of the connection close are determined by the responsible protocol. It may be a “graceful
close” or an abort that loses data in transit.
The shutdown system call can be used to selectively close a full-duplex socket.
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status = shutdown(sock, how)
int status, sock, how;
The how parameter indicates either that data will no longer be sent on the socket (0), that data will no longer be
received on the socket to (1), or that the socket should be completely closed (2).
In the case of sockets created in the UNIX domain, the system call unlink should be used to remove the path
name to which the socket was bound from the file system.
status = unlink(pathname)
char *pathname;
These path names are not automatically removed when the socket is closed.
6. Signals
Berkeley UNIX provides a set of signals that may be delivered to a process for various reasons, such as an
interrupt key being typed or a bus error occurring. These signals—examples of which are SIGIO and SIGALRM—
are defined in the file <signal.h>. Typically, the default action is that the delivery of the signal causes a process to
terminate. This default can be changed so that a signal is caught or ignored. If the signal is caught, a signal handler
is declared as the location where control is transferred at the time of interrupt. The arrival of a signal is thus similar
to a hardware interrupt. When a signal is delivered to a process, the program state is saved, the delivered signal is
blocked from further occurrence, and program control is transferred to the designated handler. If the handler returns
normally, the signal is once again enabled and program execution resumes from the point where it was interrupted.
If a signal that is currently blocked arrives, it is queued for later delivery.
6.1. Signal Handlers
A signal handler can be declared with either the signal or sigvec system calls. Signal is a simplified version of
the more general sigvec call.
oldhandler = signal(sig, handler)
int sig;
int (*handler)(), (*oldhandler)();
The sig parameter is a manifest constant found in <signal.h> describing the signal. Handler is the name of the routine that will be called when the signal is delivered. SIG_DFL (default action) and SIG_IGN (ignore) can also be
specified as arguments to this parameter. The value returned is the previous handler (if any). The handler routine
itself has the form
sighandler(sig, code, scp)
int sig, code;
struct sigcontext *scp;
In the signal handler for SIGIO, it is advisable to remove all the data possible from the socket before exiting the routine. This is eliminates the possibility of accidentally losing data because of a missed signal. If more than one event
for a given signal occurs while that signal is blocked, only one signal is saved to deliver to the process.
6.2. Blocking Signals
There is a global mask that specifies which signals are currently blocked. The system call sigblock can be
used to block signals, while the system calls sigsetmask and sigpause can be used to unblock signals by restoring the
original mask. In the file <signal.h>, there is a macro sigmask that makes it convenient to set the signal mask for the
call to sigblock. (It is defined as
#define sigmask(m) (1 << ((m)-1))
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oldmask = sigblock(mask)
int oldmask, mask;
oldmask = sigsetblock(mask)
int oldmask, mask;
result = sigpause(mask)
int result, mask; /* call always returns EINTR */
For sigblock, mask specifies those signals to be blocked. Sigsetmask and sigpause set mask as the current signal
mask. The call sigsetmask returns immediately, while sigpause waits for a signal to arrive. For critical sections
where it is necessary to block a signal such as SIGIO, the following sequence can be used.
newmask = sigmask(SIGIO);
oldmask = sigblock(newmask);
7. Timers
There are two system calls that can be used to deliver a SIGALRM signal to a calling process, alarm and setitimer. Alarm, which causes a single SIGALRM to be sent to a process when the specified time expires, provides no
resolution finer than a second. Setitimer, on the other hand, provides periodic clock interrupts (via SIGALRM) at
regular intervals and has a time resolution as small as 10 milliseconds. This is ideal for updating program timers.
status = setitimer(which, value, oldvalue)
int status, which;
struct itimerval *value, *oldvalue;
It is only necessary to call setitimer once and it will continue to send SIGALRM signals at regular intervals.
There should be a handler declared to catch this signal, otherwise it will terminate the process. The which parameter specifies which interval timer to use. Two possibilities defined in <sys/time.h> are ITIMER_REAL and
ITIMER_VIRTUAL. ITIMER_REAL causes the timer to decrement in real time, while an ITIMER_VIRTUAL
timer only runs while the process is active. The other two parameters are pointers to a structure defined in
struct itimerval {
struct timeval it_interval;
struct timeval it_value;
/* timer interval */
/* current value */
struct timeval {
tv_sec; /* seconds */
tv_usec; /* microseconds */
Setitimer sets the specified interval timer to the time values specified by value and returns the previous value
in oldvalue. A SIGALRM signal (SIGVTALRM for ITIMER_VIRTUAL) will be sent to the process when the time
value specified by value->it_value becomes zero. The interval timer will then be loaded with the time value designated by value->it_interval, and so on repeatedly. Thus, in order to provide a periodic clock interrupt, it is only
necessary to call setitimer once with value->it_interval and value->it_value initialized to the same desired time
value. If it_interval is zero, the timer will only fire once. A zero value for it_value turns off the timer. There are
three macros defined in <sys/time.h> that are quite useful in manipulating timeval structures. They are timerisset,
timerclear, and timercmp.
8. Example Programs
The following two programs demonstrate the use of TCP STREAM sockets. The first program, client.c,
establishes a connection to a port and host specified as command-line arguments and then sits in a loop sending
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everything typed at the terminal to the connection and prints everything that comes back from the connection. It terminates when end-of-file is encountered on stdin (control-D typed on the terminal). The second program, server.c,
is a primitive echo server. It listens for TCP connections on a port specified as a command-line argument and reads
data from them. Data is printed and then sent back over the connection from which it came. Both programs use
select with a read mask: client.c uses it to choose between keyboard and network input, while server.c uses it to
select among exsting connections and requests for new connections. They both do unconditionally blocking output.
A client that sends a large amount of data to the server and then fails to read the echo can cause the server to hang.
In fact, because a client may be blocked sending to the server and thus unable to receive the echo, deadlock is possible. These problems could be solved (at the cost of additional complexity in the programs) by buffering data internally and using write masks in the select statements.
8.1. client.c
main(argc, argv)
int argc;
char *argv[];
struct hostent *hostp;
struct servent *servp;
struct sockaddr_in server;
int sock;
static struct timeval timeout = { 5, 0 }; /* five seconds */
fd_set rmask, xmask, mask;
char buf[BUFSIZ];
int nfound, bytesread;
if (argc != 3) {
(void) fprintf(stderr,"usage: %s service host\n",argv[0]);
if ((sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0) {
if (isdigit(argv[1][0])) {
static struct servent s;
servp = &s;
s.s_port = htons((u_short)atoi(argv[1]));
} else if ((servp = getservbyname(argv[1], "tcp")) == 0) {
fprintf(stderr,"%s: unknown service\n",argv[1]);
if ((hostp = gethostbyname(argv[2])) == 0) {
fprintf(stderr,"%s: unknown host\n",argv[2]);
memset((void *) &server, 0, sizeof server);
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server.sin_family = AF_INET;
memcpy((void *) &server.sin_addr, hostp->h_addr, hostp->h_length);
server.sin_port = servp->s_port;
if (connect(sock, (struct sockaddr *)&server, sizeof server) < 0) {
(void) close(sock);
FD_SET(sock, &mask);
FD_SET(fileno(stdin), &mask);
for (;;) {
rmask = mask;
nfound = select(FD_SETSIZE, &rmask, (fd_set *)0, (fd_set *)0, &timeout);
if (nfound < 0) {
if (errno == EINTR) {
printf("interrupted system call\n");
/* something is very wrong! */
if (nfound == 0) {
/* timer expired */
printf("Please type something!\n");
if (FD_ISSET(fileno(stdin), &rmask)) {
/* data from keyboard */
if (!fgets(buf, sizeof buf, stdin)) {
if (ferror(stdin)) {
if (write(sock, buf, strlen(buf)) < 0) {
if (FD_ISSET(sock,&rmask)) {
/* data from network */
bytesread = read(sock, buf, sizeof buf);
buf[bytesread] = ’\0’;
printf("%s: got %d bytes: %s\n", argv[0], bytesread, buf);
} /* main - client.c */
8.2. server.c
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/time.h>
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main(argc, argv)
int argc;
char *argv[];
struct servent *servp;
struct sockaddr_in server, remote;
int request_sock, new_sock;
int nfound, fd, maxfd, bytesread, addrlen;
fd_set rmask, mask;
static struct timeval timeout = { 0, 500000 }; /* one half second */
char buf[BUFSIZ];
if (argc != 2) {
(void) fprintf(stderr,"usage: %s service\n",argv[0]);
if ((request_sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)) < 0) {
if (isdigit(argv[1][0])) {
static struct servent s;
servp = &s;
s.s_port = htons((u_short)atoi(argv[1]));
} else if ((servp = getservbyname(argv[1], "tcp")) == 0) {
fprintf(stderr,"%s: unknown service\n");
memset((void *) &server, sizeof server);
server.sin_family = AF_INET;
server.sin_addr.s_addr = INADDR_ANY;
server.sin_port = servp->s_port;
if (bind(request_sock, (struct sockaddr *)&server, sizeof server) < 0) {
if (listen(request_sock, SOMAXCONN) < 0) {
FD_SET(request_sock, &mask);
maxfd = request_sock;
for (;;) {
rmask = mask;
nfound = select(maxfd+1, &rmask, (fd_set *)0, (fd_set *)0, &timeout);
if (nfound < 0) {
if (errno == EINTR) {
printf("interrupted system call\n");
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BSD Socket Reference
/* something is very wrong! */
if (nfound == 0) {
/* timeout */
printf("."); fflush(stdout);
if (FD_ISSET(request_sock, &rmask)) {
/* a new connection is available on the connetion socket */
addrlen = sizeof(remote);
new_sock = accept(request_sock,
(struct sockaddr *)&remote, &addrlen);
if (new_sock < 0) {
printf("connection from host %s, port %d, socket %d\n",
inet_ntoa(remote.sin_addr), ntohs(remote.sin_port),
FD_SET(new_sock, &mask);
if (new_sock > maxfd)
maxfd = new_sock;
FD_CLR(request_sock, &rmask);
for (fd=0; fd <= maxfd ; fd++) {
/* look for other sockets that have data available */
if (FD_ISSET(fd, &rmask)) {
/* process the data */
bytesread = read(fd, buf, sizeof buf - 1);
if (bytesread<0) {
/* fall through */
if (bytesread<=0) {
printf("server: end of file on %d\n",fd);
FD_CLR(fd, &mask);
if (close(fd)) perror("close");
buf[bytesread] = ’\0’;
printf("%s: %d bytes from %d: %s\n",
argv[0], bytesread, fd, buf);
/* echo it back */
if (write(fd, buf, bytesread)!=bytesread)
} /* main - server.c */
Page 13
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Page 14
9. References
[1] N. Hall, "The IPC Interface Under Berkeley Unit", CS838 Handout #3, UW−Madison Computer Science
Department, April 1986.
[2] B.W. Kernighan and R. Pike, The UNIX Programming Environment, Prentice-Hall, Englewood Cliffs, New Jersey, 1984.
[3] S.J Leffler, W.N. Joy, and M.K. McKusick, UNIX Programmer’s Manual, Computer Systems Research Group,
Department of Electrical Engineering and Computer Science, University of California, Berkeley, 1983.
[4] S. Sechrest, "Tutorial Examples of Interprocess Communication in Berkeley UNIX 4.2bsd", Computer Systems
Research Group, Department of Electrical Engineering and Computer Science, University of California, Berkeley,
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