Programming With Sockets

Programming With Sockets
2
Programming With Sockets
This chapter presents the socket interface and illustrates them with sample
programs. The programs demonstrate the Internet domain sockets.
What Are Sockets
page 12
Socket Tutorial
page 14
Standard Routines
page 31
Client-Server Programs
page 34
Advanced Topics
page 41
Moving Socket Applications to Solaris 2.x
page 54
Sockets are Multithread Safe
The interface described in this chapter is multithread safe. Applications that
contain socket function calls can be used freely in a multithreaded application.
SunOS Binary Compatibility
There are two major changes from SunOS 4.x that hold true for Solaris 2.x
releases. The binary compatibility package allows SunOS 4.x–based
dynamically linked socket applications to run in Solaris 2.x.
1. You must explicitly specify the socket library (-lsocket) on the
compilation line.
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2. You must recompile all SunOS 4.x socket-based applications with the socket
library to run under Solaris 2.x. The differences in the two socket
implementations are outlined in “Moving Socket Applications to Solaris 2.x”
on page 54.
What Are Sockets
Sockets are the 4.2 Berkeley software distribution (BSD) UNIX interface to
network protocols. It has been an integral part of SunOS releases since 1981.
They are commonly referred to as Berkeley sockets or BSD sockets. Since the
days of early UNIX, applications have used the file system model of
input/output to access devices and files. The file system model is sometimes
called open-close-read-write after the basic system calls used in this model.
However, the interaction between user processes and network protocols are
more complex than the interaction between user processes and I/O devices.
A socket is an endpoint of communication to which a name can be bound. A
socket has a type and one associated process. Sockets were designed to
implement the client-server model for interprocess communication where:
•
The interface to network protocols needs to accommodate multiple
communication protocols, such as TCP/IP, XNS, and UNIX domain.
•
The interface to network protocols need to accommodate server code that
waits for connections and client code that initiates connections.
•
They also need to operate differently, depending on whether communication
is connection-oriented or connectionless.
•
Application programs may wish to specify the destination address of the
datagrams it delivers instead of binding the address with the open() call.
To address these issues and others, sockets are designed to accommodate
network protocols, while still behaving like UNIX files or devices whenever it
makes sense to. Applications create sockets when they need to. Sockets work
with the open(), close(), read(), and write() system calls, and the
operating system can differentiate between the file descriptors for files, and file
descriptors for sockets.
UNIX domain sockets are named with UNIX paths. For example, a socket may
be named /tmp/foo. UNIX domain sockets communicate only between
processes on a single host. Sockets in the UNIX domain are not considered part
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of the network protocols because they can only be used to communicate with
processes within the same UNIX system. They are rarely used today and are
only briefly covered in this manual.
Socket Libraries
The socket interface routines are in a library that must be linked with the
application. The libraries libsocket.so and libsocket.a are contained in
/usr/lib with the rest of the system service libraries. The difference is that
libsocket.so is used for dynamic linking, whereas libsocket.a is used
for static linking. Static linking is strongly discouraged.
Socket Types
Socket types define the communication properties visible to a user. The
Internet domain sockets provide access to the TCP/IP transport protocols. The
Internet domain is identified by the value AF_INET. Sockets exchange data
only with sockets in the same domain.
Three types of sockets are supported:
1. Stream sockets allow processes to communicate using TCP. A stream socket
provides bidirectional, reliable, sequenced, and unduplicated flow of data
with no record boundaries. Once the connection has been established, data
can be read from and written to these sockets as a byte stream. The socket
type is SOCK_STREAM.
2. Datagram sockets allow processes to use UDP to communicate. A datagram
socket supports bidirectional flow of messages. A process on a datagram
socket may receive messages in a different order from the sending sequence
and may receive duplicate messages. Record boundaries in the data are
preserved. The socket type is SOCK_DGRAM.
3. Raw sockets provide access to ICMP. These sockets are normally datagram
oriented, although their exact characteristics are dependent on the interface
provided by the protocol. Raw sockets are not for most applications. They
are provided to support developing new communication protocols or for
access to more esoteric facilities of an existing protocol. Only superuser
processes may use raw sockets. The socket type is SOCK_RAW. See “Selecting
Specific Protocols” on page 46 for further information.
Programming With Sockets
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Socket Tutorial
This section covers the basic methodologies of using sockets.
Socket Creation
The socket() call creates a socket,
s = socket(domain, type, protocol);
in the specified domain and of the specified type. If the protocol is unspecified
(a value of 0), the system selects a protocol that supports the requested socket
type. The socket handle (a file descriptor) is returned.
The domain is specified by one of the constants defined in <sys/socket.h>.
For the UNIX domain the constant is AF_UNIX. For the Internet domain it is
AF_INET. Constants named AF_<suite> specify the address format to use in
interpreting names.
Socket types are defined in <sys/socket.h>. SOCK_STREAM, SOCK_DGRAM,
or SOCK_RAW is supported by AF_INET and AF_UNIX. The following creates a
stream socket in the Internet domain:
s = socket(AF_INET, SOCK_STREAM, 0);
This call results in a stream socket with the TCP protocol providing the
underlying communication. A datagram socket for intramachine use is created
by:
s = socket(AF_UNIX, SOCK_DGRAM, 0);
Use the default protocol (the protocol argument is 0) in most situations. You can
specify a protocol other than the default, as described in “Advanced Topics” on
page 41.
Binding Local Names
A socket is created with no name. A remote process has no way to refer to a
socket until an address is bound to it. Communicating processes are connected
through addresses. In the Internet domain, a connection is composed of local
and remote addresses, and local and remote ports. In the UNIX domain, a
connection is composed of (usually) one or two path names. In most domains,
connections must be unique.
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In the Internet domain, there may never be duplicate ordered sets, such as:
<protocol, local address, local port, foreign address, foreign
port>. UNIX domain sockets need not always be bound to a name, but when
bound there may never be duplicate ordered sets such as: <local pathname,
foreign pathname>. The path names may not refer to existing files.
The bind() call allows a process to specify the local address of the socket. This
forms the set <local address, local port> (or <local pathname>)
while connect() and accept() complete a socket’s association. The bind()
system call is used as follows:
bind (s, name, namelen);
s is the socket handle. The bound name is a byte string that is interpreted by
the supporting protocol(s). Internet domain names contain an Internet address
and port number. UNIX domain names contain a path name and a family.
Code Example 2-1 binds the name /tmp/foo to a UNIX domain socket.
Code Example 2-1
Bind Name to Socket
#include <sys/un.h>
...
struct sockaddr_un addr;
...
strcpy(addr.sun_path, "/tmp/foo");
addr.sun_family = AF_UNIX;
bind (s, (struct sockaddr *) &addr,
strlen(addr.sun_path) + sizeof (addr.sun_family));
Note that in determining the size of an AF_UNIX socket address, null bytes are
not counted, which is why strlen() use is fine.
The file name referred to in addr.sun_path is created as a socket in the
system file name space. The caller must have write permission in the directory
where addr.sun_path is created. The file should be deleted by the caller
when it is no longer needed. AF_UNIX sockets can be deleted with unlink().
Binding an Internet address is more complicated. The call is similar:
#include <sys/types.h>
#include <netinet/in.h>
...
struct sockaddr_in sin;
...
bind (s, (struct sockaddr *) &sin, sizeof sin);
Programming With Sockets
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The content of the address sin is described in “Address Binding” on page 47,
where Internet address bindings are discussed.
Connection Establishment
Connection establishment is usually asymmetric, with one process acting as the
client and the other as the server. The server binds a socket to a well-known
address associated with the service and blocks on its socket for a connect
request. An unrelated process can then connect to the server. The client
requests services from the server by initiating a connection to the server’s
socket. On the client side, the connect() call initiates a connection. In the
UNIX domain, this might appear as:
struct sockaddr_un server;
server.sun.family = AF_UNIX;
...
connect(s, (struct sockaddr *)&server,
strlen(server.sun_path) + sizeof (server.sun_family));
while in the Internet domain it might be:
struct sockaddr_in server;
...
connect(s, (struct sockaddr *)&server, sizeof server);
If the client’s socket is unbound at the time of the connect call, the system
automatically selects and binds a name to the socket. See “Signals and Process
Group ID” on page 45. This is the usual way that local addresses are bound to
a socket on the client side.
In the examples that follow, only AF_INET sockets are described.
To receive a client’s connection, a server must perform two steps after binding
its socket. The first is to indicate how many connection requests can be queued.
The second step is to accept a connection:
struct sockaddr_in from;
...
listen(s, 5);
/* Allow queue of 5 connections */
fromlen = sizeof(from);
newsock = accept(s, (struct sockaddr *) &from, &fromlen);
s is the socket bound to the address to which the connection request is sent.
The second parameter of listen() specifies the maximum number of
outstanding connections that may be queued. from is a structure that is filled
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with the address of the client. A NULL pointer may be passed. fromlen is the
length of the structure. (In the UNIX domain, from is declared a struct
sockaddr_un.)
accept() normally blocks. accept() returns a new socket descriptor that is
connected to the requesting client. The value of fromlen is changed to the actual
size of the address.
There is no way for a server to indicate that it will accept connections only
from specific addresses. The server can check the from-address returned by
accept() and close a connection with an unacceptable client. A server can
accept connections on more than one socket, or avoid blocking on the accept
call. These techniques are presented in “Advanced Topics” on page 41.
Connection Errors
An error is returned if the connection is unsuccessful (however, an address
bound by the system remains). Otherwise, the socket is associated with the
server and data transfer may begin.
Table 2-2 lists some of the more common errors returned when a connection
attempt fails.
Table 2-2
Socket Connection Errors
Socket Errors
Error Description
ENOBUFS
Lack of memory available to support the call.
EPROTONOSUPPORT
Request for an unknown protocol.
EPROTOTYPE
Request for an unsupported type of socket.
ETIMEDOUT
No connection established in specified time. This happens
when the destination host is down or when problems in
the network result in lost transmissions.
ECONNREFUSED
The host refused service. This happens when a server
process is not present at the requested address.
ENETDOWN or
EHOSTDOWN
These errors are caused by status information delivered by
the underlying communication interface.
Programming With Sockets
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Table 2-2
Socket Connection Errors (Continued)
Socket Errors
Error Description
ENETUNREACH or
EHOSTUNREACH
These operational errors can occur either because there is
no route to the network or host, or because of status
information returned by intermediate gateways or
switching nodes. The status returned is not always
sufficient to distinguish between a network that is down
and a host that is down.
Data Transfer
This section describes the functions to send and receive data. You can send or
receive a message with the normal read() and write() system calls:
write(s, buf, sizeof buf);
read(s, buf, sizeof buf);
Or the calls send() and recv() can be used:
send(s, buf, sizeof buf, flags);
recv(s, buf, sizeof buf, flags);
send() and recv() are very similar to read() and write(), but the flags
argument is important. The flags, defined in <sys/socket.h>, can be
specified as a nonzero value if one or more of the following is required:
MSG_OOB
MSG_PEEK
MSG_DONTROUTE
send and receive out-of-band data
look at data without reading
send data without routing packets
Out-of-band data is specific to stream sockets. When MSG_PEEK is specified
with a recv() call, any data present is returned to the user but treated as still
unread. The next read() or recv() call on the socket returns the same data.
The option to send data without routing applied to the outgoing packets is
currently used only by the routing table management process and is unlikely
to be interesting to most users.
Closing Sockets
A SOCK_STREAM socket can be discarded by a close() system call. If data is
queued to a socket that promises reliable delivery after a close(), the
protocol continues to try to transfer the data. If the data is still undelivered
after an arbitrary period, it is discarded.
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shutdown() closes SOCK_STREAM sockets gracefully. Both processes can
acknowledge that they are no longer sending. This call has the form:
shutdown(s, how);
where how is 0 disallows further receives, 1 disallows further sends, and 2
disallows both.
Connecting Stream Sockets
Figure 2-1 and the next two code examples illustrate initiating and accepting
an Internet domain stream connection.
To initiate a connection, the client program in Code Example 2-2 creates a
stream socket and calls connect(), specifying the address of the socket to
connect to. If the target socket exists and the request is accepted, the
connection is complete and the program can send data. Data are delivered in
sequence with no message boundaries. The connection is destroyed when
either socket is closed. For more information about data representation
routines, such as ntohl(), ntohs(), htons(), and htonl(), in this
program, see the byteorder(3N)man page.
The program in Code Example 2-3 is a server. It creates a socket and binds a
name to it, then displays the port number. The program calls listen() to
mark the socket ready to accept connection requests and initialize a queue for
the requests. The rest of the program is an infinite loop. Each pass of the loop
accepts a new connection and removes it from the queue, creating a new
socket. The server reads and displays the messages from the socket and closes
it. The use of INADDR_ANY is explained in “Address Binding” on page 47.
Programming With Sockets
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Server
socket()
bind()
Client
socket()
listen()
connect()
accept()
Connection
establishment
Data
transfer
read()/
write()
shutdown()
and/or
close()
Figure 2-1
shutdown()
and/or
close()
Connection-Oriented Communication Using Stream Sockets
Code Example 2-2
#include
#include
#include
#include
#include
read()/
write()
Internet Domain Stream Connection (Client)
<sys/types.h>
<sys/socket.h>
<netinet/in.h>
<netdb.h>
<stdio.h>
#define DATA "Half a league, half a league . . ."
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/*
*
*
*
*
*
This program creates a socket and initiates a connection with the
socket given in the command line. Some data are sent over the
connection and then the socket is closed, ending the connection.
The form of the command line is: streamwrite hostname portnumber
Usage: pgm host port
*/
main(argc, argv)
int argc;
char *argv[];
{
int sock;
struct sockaddr_in server;
struct hostent *hp, *gethostbyname();
char buf[1024];
/* Create socket. */
sock = socket( AF_INET, SOCK_STREAM, 0 );
if (sock == -1) {
perror("opening stream socket");
exit(1);
}
/* Connect socket using name specified by command line. */
server.sin_family = AF_INET;
hp = gethostbyname(argv[1] );
/*
* gethostbyname returns a structure including the network address
* of the specified host.
*/
if (hp == (struct hostent *) 0) {
fprintf(stderr, "%s: unknown host\n", argv[1]);
exit(2);
}
memcpy((char *) &server.sin_addr, (char *) hp->h_addr,
hp->h_length);
server.sin_port = htons(atoi( argv[2]));
if (connect(sock, (struct sockaddr *) &server, sizeof server)
== -1) {
perror("connecting stream socket");
exit(1);
}
if (write( sock, DATA, sizeof DATA ) == -1)
perror("writing on stream socket");
Programming With Sockets
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close(sock);
exit(0);
}
Code Example 2-3
#include
#include
#include
#include
#include
Accepting an Internet Stream Connection (Server)
<sys/types.h>
<sys/socket.h>
<netinet/in.h>
<netdb.h>
<stdio.h>
#define TRUE 1
/*
* This program creates a socket and then begins an infinite loop.
* Each time through the loop it accepts a connection and prints
* data from it. When the connection breaks, or the client closes
* the connection, the program accepts a new connection.
*/
main()
{
int sock, length;
struct sockaddr_in server;
int msgsock;
char buf[1024];
int rval;
/* Create socket. */
sock = socket(AF_INET, SOCK_STREAM, 0);
if (sock == -1) {
perror("opening stream socket");
exit(1);
}
/* Bind socket using wildcards.*/
server.sin_family = AF_INET;
server.sin_addr.s_addr = INADDR_ANY;
server.sin_port = 0;
if (bind(sock, (struct sockaddr *) &server, sizeof server)
== -1)
perror("binding stream socket");
exit(1);
}
/* Find out assigned port number and print it out. */
length = sizeof server;
if (getsockname(sock,(struct sockaddr *) &server,&length)
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== -1) {
perror("getting socket name");
exit(1);
}
printf("Socket port #%d\n", ntohs(server.sin_port));
/* Start accepting connections. */
listen(sock, 5);
do {
msgsock = accept(sock,(struct sockaddr *) 0,(int *) 0);
if (msgsock == -1
perror("accept");
else do {
memset(buf, 0, sizeof buf);
if ((rval = read(msgsock,buf, 1024)) == -1)
perror("reading stream message");
if (rval == 0)
printf("Ending connection\n");
else
printf("-->%s\n", buf);
} while (rval != 0);
close(msgsock);
} while(TRUE);
/*
* Since this program has an infinite loop, the socket "sock" is
* never explicitly closed. However, all sockets will be closed
* automatically when a process is killed or terminates normally.
*/
exit(0);
}
Datagram Sockets
A datagram socket provides a symmetric data exchange interface. There is no
requirement for connection establishment. Each message carries the destination
address. Figure 2-2 shows the flow of communication between server and
client.
Datagram sockets are created as described in “Socket Creation” on page 14. If a
particular local address is needed, the bind() operation must precede the first
data transmission. Otherwise, the system sets the local address and/or port
when data is first sent. To send data, the sendto() call is used:
sendto(s, buf, buflen, flags, (struct sockaddr *) &to, tolen);
Programming With Sockets
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The s, buf, buflen, and flags parameters are the same as in connection-oriented
sockets. The to and tolen values indicate the address of the intended recipient
of the message. A locally detected error condition (such as an unreachable
network) causes a return of –1 and errno to be set to the error number.
To receive messages on a datagram socket, the recvfrom() call is used:
recvfrom(s, buf, buflen, flags, (struct sockaddr *) &from,
&fromlen);
Before the call, fromlen is set to the size of the from buffer. On return it is set to
the size of the address from which the datagram was received.
Datagram sockets can also use the connect() call to associate a socket with a
specific destination address. It can then use the send() call. Any data sent on
the socket without explicitly specifying a destination address is addressed to
the connected peer, and only data received from that peer is delivered. Only
one connected address is permitted for one socket at a time. A second
connect() call changes the destination address. Connect requests on
datagram sockets return immediately. The system simply records the peer’s
address. accept(), and listen() are not used with datagram sockets.
While a datagram socket is connected, errors from previous send() calls may
be returned asynchronously. These errors can be reported on subsequent
operations on the socket, or an option of getsockopt, SO_ERROR, can be used
to interrogate the error status.
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Server
socket()
Client
socket()
bind()
recvfrom()
sendto()
normally block until a
request is received
data
process
the request
recvfrom()
sendto()
data
normally block
waiting
for reply
Figure 2-2
Connectionless Communication Using Datagram Sockets
Code Example 2-4 shows how to read an Internet call, and Code Example 2-5
shows how to send an Internet call.
Code Example 2-4
Reading Internet Domain Datagrams
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
Programming With Sockets
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2
#include <stdio.h>
/*
* The include file <netinet/in.h> defines sockaddr_in as:
* struct sockaddr_in {
*
short
sin_family;
*
u_short
sin_port;
*
struct
in_addr sin_addr;
*
char
sin_zero[8];
* };
* This program creates a datagram socket, binds a name to it, then
* reads from the socket.
*/
main()
{
int sock, length;
struct sockaddr_in name;
char buf[1024];
/* Create socket from which to read. */
sock = socket(AF_INET, SOCK_DGRAM, 0);
if (sock == -1) {
perror("opening datagram socket");
exit(1);
}
/* Create name with wildcards. */
name.sin_family = AF_INET;
name.sin_addr.s_addr = INADDR_ANY;
name.sin_port = 0;
if (bind(sock,(struct sockaddr *)&name, sizeof name) == -1) {
perror("binding datagram socket");
exit(1);
}
/* Find assigned port value and print it out. */
length = sizeof(name);
if (getsockname(sock,(struct sockaddr *) &name, &length)
== -1) {
perror("getting socket name");
exit(1);
}
printf("Socket port #%d\n", ntohs( name.sin_port));
/* Read from the socket. */
if ( read(sock, buf, 1024) == -1 )
perror("receiving datagram packet");
printf("-->%s\n", buf);
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close(sock);
exit(0);
}
Code Example 2-5
#include
#include
#include
#include
#include
Sending an Internet Domain Datagram
<sys/types.h>
<sys/socket.h>
<netinet/in.h>
<netdb.h>
<stdio.h>
#define DATA "The sea is calm, the tide is full . . ."
/*
* Here I send a datagram to a receiver whose name I get from the
* command line arguments. The form of the command line is:
* dgramsend hostname portnumber
*/
main(argc, argv)
int argc;
char *argv[];
{
int sock;
struct sockaddr_in name;
struct hostent *hp, *gethostbyname();
/* Create socket on which to send. */
sock = socket(AF_INET,SOCK_DGRAM, 0);
if (sock == -1) {
perror("opening datagram socket");
exit(1);
}
/*
* Construct name, with no wildcards, of the socket to ‘‘send’’
* to. gethostbyname returns a structure including the network
* address of the specified host. The port number is taken from
* the command line.
*/
hp = gethostbyname(argv[1]);
if (hp == (struct hostent *) 0) {
fprintf(stderr, "%s: unknown host\n", argv[1]);
exit(2);
}
memcpy((char *) &name.sin_addr, (char *) hp->h_addr,
hp->h_length);
name.sin_family = AF_INET;
Programming With Sockets
27
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name.sin_port = htons( atoi( argv[2] ));
/* Send message. */
if (sendto(sock,DATA, sizeof DATA ,0,
(struct sockaddr *) &name,sizeof name) == -1)
perror("sending datagram message");
close(sock);
exit(0);
}
Input/Output Multiplexing
Requests can be multiplexed among multiple sockets or files. The select()
call is used to do this:
#include <sys/time.h>
#include <sys/types.h>
#include <sys/select.h>
...
fd_set readmask, writemask, exceptmask;
struct timeval timeout;
...
select(nfds, &readmask, &writemask, &exceptmask, &timeout);
The first argument of select() is the number of file descriptors in the lists
pointed to by the next three arguments.
The second, third, and fourth arguments of select() are pointers to three sets
of file descriptors: a set of descriptors to read on, a set to write on, and a set on
which exception conditions are accepted. Out-of-band data is the only
exceptional condition. Any of these pointers can be a properly cast null. Each
set is a structure containing an array of long integer bit masks. The size of the
array is set by FD_SETSIZE (defined in select.h). The array is long enough
to hold one bit for each FD_SETSIZE file descriptor.
The macros FD_SET(fd, &mask) and FD_CLR(fd, &mask) add and delete,
respectively, the file descriptor fd in the set mask. The set should be zeroed
before use, and the macro FD_ZERO(&mask) clears the set mask.
A time-out value may be specified. If the timeout pointer is NULL, select()
blocks until a descriptor is selectable, or until a signal is received. If the fields
in timeout are set to 0, select() polls and returns immediately.
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select() normally returns the number of file descriptors selected. select()
returns a 0 if the time-out has expired. select() returns -1 for an error or
interrupt with the error number in errno and the file descriptor masks
unchanged.
For a successful return, the three sets indicate which file descriptors are ready
to be read from, written to, or have exceptional conditions pending.
Test the status of a file descriptor in a select mask with the FD_ISSET(fd,
&mask) macro. It returns a nonzero value if fd is in the set mask, and 0 if it is
not. Use select() followed by a FD_ISSET(fd, &mask) macro on the read set
to check for queued connect requests on a socket.
Code Example 2-6 shows how to select on a “listening” socket for readability
to determine when a new connection can be picked up with a call to
accept(). The program accepts connection requests, reads data, and
disconnects on a single socket.
Code Example 2-6
#include
#include
#include
#include
#include
#include
Check for Pending Connections With select()
<sys/types.h>
<sys/socket.h>
<sys/time.h>
<netinet/in.h>
<netdb.h>
<stdio.h>
#define TRUE 1
/*
* This program uses select to check that someone is
* trying to connect before calling accept.
*/
main()
{
int sock, length;
struct sockaddr_in server;
int msgsock;
char buf[1024];
int rval;
fd_set ready;
struct timeval to;
/* Open a socket and bind it as in previous examples. */
Programming With Sockets
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2
/* Start accepting connections. */
listen(sock, 5);
do {
FD_ZERO(&ready);
FD_SET(sock, &ready);
to.tv_sec = 5;
to.tv_usec = 0;
if (select(1, &ready, (fd_set *)0, (fd_set *)0, &to) == -1) {
perror("select");
continue;
}
if (FD_ISSET(sock, &ready)) {
msgsock = accept(sock, (struct sockaddr *)0,
(int *)0);
if (msgsock == -1)
perror("accept");
else do {
memset(buf, 0, sizeof buf);
if ((rval = read(msgsock, buf, 1024)) == -1)
perror("reading stream message");
else if (rval == 0)
printf("Ending connection\n");
else
printf("-->%s\n", buf);
} while (rval > 0);
close(msgsock);
} else
printf("Do something else\n");
} while (TRUE);
exit(0);
}
In previous versions of the select() routine, its arguments were pointers to
integers instead of pointers to fd_sets. This style of call still works if the
number of file descriptors is smaller than the number of bits in an integer.
select() provides a synchronous multiplexing scheme. The SIGIO and
SIGURG signals described in “Advanced Topics” on page 41 provide
asynchronous notification of output completion, input availability, and
exceptional conditions.
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Standard Routines
You may need to locate and construct network addresses. This section
describes the new routines that manipulate network addresses. Unless
otherwise stated, functions presented in this section apply only to the Internet
domain.
Locating a service on a remote host requires many levels of mapping before
client and server communicate. A service has a name for human use. The
service and host names must be translated to network addresses. Finally, the
address is used to locate and route to the host. The specifics of the mappings
may vary between network architectures. Preferably, a network will not require
that hosts be named, thus protecting the identity of their physical locations.
It is more flexible to discover the location of the host when it is addressed.
Standard routines map host names to network addresses, network names to
network numbers, protocol names to protocol numbers, and service names to
port numbers, and the appropriate protocol to use in communicating with the
server process. The file <netdb.h> must be included when using any of these
routines.
Host Names
An Internet host name to address mapping is represented by the hostent
structure:
struct hostent {
char
*h_name;
/*
char
**h_aliases;
/*
int
h_addrtype;
/*
int
h_length;
/*
char
**h_addr_list;
/*
};
/*1st addr, net byte order*/
#define h_addr h_addr_list[0]
official name of host */
alias list */
hostaddrtype(e.g.,AF_INET) */
length of address */
list of addrs, null terminated */
gethostbyname() maps an Internet host name to a hostent structure.
gethostbyaddr() maps an Internet host address to a hostent structure.
inet_ntoa() maps an Internet host address to a displayable string.
Programming With Sockets
31
2
The routines return a hostent structure containing the name of the host, its
aliases, the address type (address family), and a NULL-terminated list of
variable length addresses. The list of addresses is required because a host can
have many addresses. The h_addr definition is for backward compatibility,
and is the first address in the list of addresses in the hostent structure.
Network Names
There are routines to map network names to numbers, and back. These
routines return a netent structure:
/*
* Assumes that a network number fits in 32
*/
struct netent {
char
*n_name;
/* official name of
char
**n_aliases; /* alias list */
int
n_addrtype; /* net address type
int
n_net;
/* net number, host
};
bits.
net */
*/
byte order */
getnetbyname(), getnetbyaddr(), and getnetent() are the network
counterparts to the host routines described above.
Protocol Names
The protoent structure defines the protocol-name mapping used with
getprotobyname(), getprotobynumber(), and getprotoent():
struct protoent {
char
*p_name;
/* official protocol name */
char
**p_aliases; /* alias list */
int
p_proto;
/* protocol number */
};
In the UNIX domain, no protocol database exists.
Service Names
An Internet domain service resides at a specific, well-known port and uses a
particular protocol. A service name to port number mapping is described by
the servent structure:
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struct servent {
char
*s_name;
char
**s_aliases;
int
s_port;
char
*s_proto;
};
/*
/*
/*
/*
official service name */
alias list */
port number, network byte order */
protocol to use */
getservbyname() maps service names and, optionally, a qualifying protocol
to a servent structure. The call
sp = getservbyname("telnet", (char *) 0);
returns the service specification of a telnet server using any protocol. The call
sp = getservbyname("telnet", "tcp");
returns the telnet server that uses the TCP protocol. getservbyport() and
getservent() are also provided. getservbyport() has an interface similar
to that of getservbyname(); an optional protocol name may be specified to
qualify lookups.
Other Routines
In addition to address-related database routines, there are several other
routines that simplify manipulating names and addresses. Table 2-3
summarizes the routines for manipulating variable-length byte strings and
byte-swapping network addresses and values.
Table 2-3
Run-Time Library Routines
Call
Synopsis
memcmp(s1, s2, n)
Compares byte-strings; 0 if same, not 0 otherwise
memcpy(s1, s2, n)
Copies n bytes from s2 to s1
memset(base, value, n)
Sets n bytes to value starting at base
htonl(val)
32-bit quantity from host into network byte order
htons(val)
16-bit quantity from host into network byte order
ntohl(val)
32-bit quantity from network into host byte order
ntohs(val)
16-bit quantity from network into host byte order
Programming With Sockets
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2
The byte-swapping routines are provided because the operating system
expects addresses to be supplied in network order. On some architectures the
host byte ordering is different from network byte order, so, programs must
sometimes byte-swap values. Routines that return network addresses do so in
network order. There are byte-swapping problems only when interpreting
network addresses. For example, the following code formats a TCP or UDP
port:
printf("port number %d\n", ntohs(sp->s_port));
On certain machines, where these routines are not needed, they are defined as
null macros.
Client-Server Programs
The most common form of distributed application is the client/server model.
In this scheme, client processes request services from a server process.
An alternate scheme is a service server that can eliminate dormant server
processes. An example is inetd, the Internet service daemon. inetd listens at
a variety of ports, determined at start up by reading a configuration file. When
a connection is requested on an inetd serviced port, inetd spawns the
appropriate server to serve the client. Clients are unaware that an intermediary
has played any part in the connection. inetd is described in more detail in
“inetd Daemon” on page 53.
Servers
Most servers are accessed at well-known Internet port numbers or UNIX
domain names. Code Example 2-7 illustrates the main loop of a remote-login
server.
Code Example 2-7
Remote Login Server
main(argc, argv)
int argc;
char *argv[];
{
int f;
struct sockaddr_in from;
struct sockaddr_in sin;
struct servent *sp;
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sp = getservbyname("login", "tcp");
if (sp == (struct servent *) NULL) {
fprintf(stderr, "rlogind: tcp/login: unknown service");
exit(1);
}
...
#ifndef DEBUG
/* Disassociate server from controlling terminal. */
...
#endif
sin.sin_port = sp->s_port;/* Restricted port */
sin.sin_addr.s_addr = INADDR_ANY;
...
f = socket(AF_INET, SOCK_STREAM, 0);
...
if (bind( f, (struct sockaddr *) &sin, sizeof sin ) == -1) {
...
}
...
listen(f, 5);
while (TRUE) {
int g, len = sizeof from;
g = accept(f, (struct sockaddr *) &from, &len);
if (g == -1) {
if (errno != EINTR)
syslog(LOG_ERR, "rlogind: accept: %m");
continue;
}
if (fork() == 0) {
close(f);
doit(g, &from);
}
close(g);
}
exit(0);
}
First, the server gets its service definition, as Code Example 2-8 shows.
Code Example 2-8
Remote Login Server: Step 1
sp = getservbyname("login", "tcp");
if (sp == (struct servent *) NULL) {
fprintf(stderr, "rlogind: tcp/login: unknown service\n");
exit(1);
}
Programming With Sockets
35
2
The result from getservbyname() is used later to define the Internet port at
which the program listens for service requests. Some standard port numbers
are in /usr/include/netinet/in.h.
In the non-DEBUG mode of operation, the server dissociates from the
controlling terminal of its invoker, shown in Code Example 2-9.
Code Example 2-9
Dissociating from the Controlling Terminal
(void) close(0);
(void) close(1);
(void) close(2);
(void) open("/", O_RDONLY);
(void) dup2(0, 1);
(void) dup2(0, 2);
setsid();
This prevents the server from receiving signals to the process group of the
controlling terminal. Once a server has dissociated itself, it cannot send reports
of errors to a terminal and must log errors with syslog().
A server next creates a socket and listens for service requests. bind() insures
that the server listens at the expected location. (The remote login server listens
at a restricted port number, so it runs as super-user.)
Code Example 2-10 illustrates the main body of the loop.
Code Example 2-10 Remote Login Server: Main Body
while(TRUE) {
int g, len = sizeof(from);
if (g = accept(f, (struct sockaddr *) &from, &len) == -1) {
if (errno != EINTR)
syslog(LOG_ERR, "rlogind: accept: %m");
continue;
}
if (fork() == 0) {
/* Child */
close(f);
doit(g, &from);
}
close(g);
/* Parent */
}
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accept() blocks until a client requests service. accept() returns a failure
indication if it is interrupted by a signal such as SIGCHLD. The return value
from accept() is checked and an error is logged with syslog() if an error
has occurred.
The server then forks a child process and invokes the main body of the remote
login protocol processing. The socket used by the parent to queue connection
requests is closed in the child. The socket created by accept() is closed in the
parent. The address of the client is passed to doit() for authenticating clients.
Clients
This section describes the steps taken by the client remote login process. As in
the server, the first step is to locate the service definition for a remote login:
sp = getservbyname("login", "tcp");
if (sp == (struct servent *) NULL) {
fprintf(stderr,"rlogin: tcp/login: unknown service");
exit(1);
}
Next, the destination host is looked up with a gethostbyname() call:
hp = gethostbyname(argv[1]);
if (hp == (struct hostent *) NULL) {
fprintf(stderr, "rlogin: %s: unknown host", argv[1]);
exit(2);
}
Then, connect to the server at the requested host and start the remote login
protocol. The address buffer is cleared and filled with the Internet address of
the foreign host and the port number at which the login server listens:
memset((char *) &server, 0, sizeof server);
memcpy((char*) &server.sin_addr,hp->h_addr,hp->h_length);
server.sin_family = hp->h_addrtype;
server.sin_port = sp->s_port;
A socket is created, and a connection initiated. connect() implicitly does a
bind(), since s is unbound.
s = socket(hp->h_addrtype, SOCK_STREAM, 0);
if (s < 0) {
perror("rlogin: socket");
exit(3);
}
Programming With Sockets
37
2
...
if (connect(s, (struct sockaddr *) &server, sizeof server) < 0) {
perror("rlogin: connect");
exit(4);
}
Connectionless Servers
Some services use datagram sockets. The rwho service provides status
information on hosts connected to a local area network. (Avoid running
in.rwho; it causes heavy network traffic.) This service requires the ability to
broadcast information to all hosts connected to a particular network. It is an
example of datagram socket use.
A user on a host running the rwho server may get the current status of another
host with ruptime. Typical output is illustrated in Code Example 2-11.
Code Example 2-11 Output of ruptime Program
itchy up 9:45, 5 users, load 1.15, 1.39, 1.31
scratchy up 2+12:04, 8 users, load 4.67, 5.13, 4.59
click up 10:10, 0 users, load 0.27, 0.15, 0.14
clack up 2+06:28, 9 users, load 1.04, 1.20, 1.65
ezekiel up 25+09:48, 0 users, load 1.49, 1.43, 1.41
dandy 5+00:05, 0 users, load 1.51, 1.54, 1.56
peninsula down 0:24
wood down 17:04
carpediem down 16:09
chances up 2+15:57, 3 users, load 1.52, 1.81, 1.86
Status information is periodically broadcast by the rwho server processes on
each host. The server process also receives the status information and updates
a database. This database is interpreted for the status of each host. Servers
operate autonomously, coupled only by the local network and its broadcast
capabilities.
Use of broadcast is fairly inefficient, since a lot of net traffic is generated.
Unless the service is used widely and frequently, the expense of periodic
broadcasts outweighs the simplicity.
A simplified version of the rwho server is shown in Code Example 2-12. It does
two tasks: receives status information broadcast by other hosts on the network
and supplies the status of its host. The first task is done in the main loop of the
program: Packets received at the rwho port are checked to be sure they were
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2
sent by another rwho server process, and are stamped with the arrival time.
They then update a file with the status of the host. When a host has not been
heard from for an extended time, the database routines assume the host is
down and logs it. This application is prone to error, as a server may be down
while a host is up.
Code Example 2-12 rwho Server
main()
{
...
sp = getservbyname("who", "udp");
net = getnetbyname("localnet");
sin.sin_addr = inet_makeaddr(net->n_net, INADDR_ANY);
sin.sin_port = sp->s_port;
...
s = socket(AF_INET, SOCK_DGRAM, 0);
...
on = 1;
if (setsockopt(s, SOL_SOCKET, SO_BROADCAST, &on, sizeof on)
== -1) {
syslog(LOG_ERR, "setsockopt SO_BROADCAST: %m");
exit(1);
}
bind(s, (struct sockaddr *) &sin, sizeof sin);
...
signal(SIGALRM, onalrm);
onalrm();
while(1) {
struct whod wd;
int cc, whod, len = sizeof from;
cc = recvfrom(s, (char *) &wd, sizeof(struct whod), 0,
(struct sockaddr *) &from, &len);
if (cc <= 0) {
if (cc == -1 && errno != EINTR)
syslog(LOG_ERR, "rwhod: recv: %m");
continue;
}
if (from.sin_port != sp->s_port) {
syslog(LOG_ERR, "rwhod: %d: bad from port",
ntohs(from.sin_port));
continue;
}
...
if (!verify( wd.wd_hostname)) {
Programming With Sockets
39
2
syslog(LOG_ERR, "rwhod: bad host name from %x",
ntohl(from.sin_addr.s_addr));
continue;
}
(void)
whod =
...
(void)
(void)
(void)
sprintf(path, "%s/whod.%s", RWHODIR, wd.wd_hostname);
open(path, O_WRONLY|O_CREAT|O_TRUNC, 0666);
time(&wd.wd_recvtime);
write(whod, (char *) &wd, cc);
close(whod);
}
exit(0);
}
The second server task is to supply the status of its host. This requires
periodically acquiring system status information, packaging it in a message,
and broadcasting it on the local network for other rwho servers to hear. This
task is run by a timer and triggered with a signal. Locating the system status
information is involved but uninteresting.
Status information is broadcast on the local network. For networks that do not
support broadcast, another scheme must be used.
It is important that software operating in a distributed environment not have
any site-dependent information compiled into it. This would require a separate
copy of the server at each host and make maintenance a severe problem. The
system isolates host-specific data from applications by providing system calls
that return the required data. (For example, uname() returns the host’s official
name.) The ioctl() call lets you find the networks to which a host is directly
connected. A local network broadcasting mechanism has been implemented at
the socket level. Combining these two features lets a process broadcast on any
directly connected local network that supports broadcasting in a siteindependent manner. This solves the problem of deciding how to propagate
status with rwho, or more generally in broadcasting. Such status is broadcast
to connected networks at the socket level, where the connected networks have
been obtained through the appropriate ioctl() calls. “Broadcasting and
Determining Network Configuration” on page 49 details the specifics of
broadcasting.
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2
Advanced Topics
For most programmers, the mechanisms already described are enough to build
distributed applications. Others will need some of the features in this section.
Out-of-Band Data
The stream socket abstraction includes out-of-band data. Out-of-band data is a
logically independent transmission channel between a pair of connected
stream sockets. Out-of-band data is delivered independently of normal data.
The out-of-band data facilities must support the reliable delivery of at least one
out-of-band message at a time. This message can contain at least one byte of
data, and at least one message may be pending delivery at any time.
For communications protocols that support only in-band signaling (that is,
urgent data is delivered in sequence with normal data), the message is
extracted from the normal data stream and stored separately. This lets users
choose between receiving the urgent data in order and receiving it out of
sequence, without having to buffer the intervening data.
You can peek (with MSG_PEEK) at out-of-band data. If the socket has a process
group, a SIGURG signal is generated when the protocol is notified of its
existence. A process can set the process group or process id to be informed by
SIGURG with the appropriate fcntl() call, as described in “Interrupt Driven
Socket I/O” on page 44 for SIGIO. If multiple sockets have out-of-band data
waiting delivery, a select() call for exceptional conditions can be used to
determine the sockets with such data pending.
A logical mark is placed in the data stream at the point at which the out-ofband data was sent. The remote login and remote shell applications use this
facility to propagate signals between client and server processes. When a signal
is received, all data up to the mark in the data stream is discarded.
To send an out-of-band message, the MSG_OOB flag is applied to send() or
sendto(). To receive out-of-band data, specify MSG_OOB to recvfrom() or
recv() (unless out-of-band data is taken in line, in which case the MSG_OOB
flag is not needed). The SIOCATMARK ioctl tells whether the read pointer
currently points at the mark in the data stream:
ioctl(s, SIOCATMARK, &yes);
Programming With Sockets
41
2
If yes is 1 on return, the next read returns data after the mark. Otherwise,
assuming out-of-band data has arrived, the next read provides data sent by the
client before sending the out-of-band signal. The routine in the remote login
process that flushes output on receipt of an interrupt or quit signal is shown in
Code Example 2-13. This code reads the normal data up to the mark (to discard
it), then reads the out-of-band byte.
Code Example 2-13 Flushing Terminal I/O on Receipt of Out-of-Band Data
#include <sys/ioctl.h>
#include <sys/file.h>
...
oob()
{
int out = FWRITE;
char waste[BUFSIZ];
int mark;
/* flush local terminal output */
ioctl(1, TIOCFLUSH, (char *) &out);
while(1) {
if (ioctl(rem, SIOCATMARK, &mark) == -1) {
perror("ioctl");
break;
}
if (mark)
break;
(void) read(rem, waste, sizeof waste);
}
if (recv(rem, &mark, 1, MSG_OOB) == -1) {
perror("recv");
...
}
...
}
A process can also read or peek at the out-of-band data without first reading
up to the mark. This is more difficult when the underlying protocol delivers
the urgent data in-band with the normal data, and only sends notification of its
presence ahead of time (for example, TCP, the protocol used to provide socket
streams in the Internet domain). With such protocols, the out-of-band byte may
not yet have arrived when a recv() is done with the MSG_OOB flag. In that
case, the call returns the error of EWOULDBLOCK. Also, there may be enough in-
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2
band data in the input buffer that normal flow control prevents the peer from
sending the urgent data until the buffer is cleared. The process must then read
enough of the queued data before the urgent data can be delivered.
There is also a facility to retain the position of urgent in-line data in the socket
stream. This is available as a socket-level option, SO_OOBINLINE. See the
getsockopt(3N)manpage for usage. With this option, the position of urgent
data (the mark) is retained, but the urgent data immediately follows the mark
in the normal data stream returned without the MSG_OOB flag. Reception of
multiple urgent indications causes the mark to move, but no out-of-band data
are lost.
Nonblocking Sockets
Some applications require sockets that do not block. For example, requests that
cannot complete immediately and would cause the process to be suspended
(awaiting completion) are not executed. An error code would be returned.
Once a socket is created and any connection to another socket is made, it may
be made nonblocking by fcntl() as Code Example 2-14 shows.
Code Example 2-14 Set Nonblocking Socket
#include <fcntl.h>
#include <sys/file.h>
...
int fileflags;
int s;
...
s = socket(AF_INET, SOCK_STREAM, 0);
...
if (fileflags = fcntl(s, F_GETFL, 0) == -1)
perror("fcntl F_GETFL");
exit(1);
}
if (fcntl(s, F_SETFL, fileflags | FNDELAY) == -1)
perror("fcntl F_SETFL, FNDELAY");
exit(1);
}
...
When doing I/O on a nonblocking socket, check for the error EWOULDBLOCK
(in <errno.h>), which occurs when an operation would normally block.
accept(), connect(), send(), recv(), read(), and write() can all
Programming With Sockets
43
2
return EWOULDBLOCK. If an operation such as a send() cannot be done in its
entirety, but partial writes work (such as when using a stream socket), the data
that can be sent immediately are processed, and the return value is the amount
actually sent.
Asynchronous Sockets
Asynchronous communication between processes is required in real-time
applications. Asynchronous sockets must be SOCK_STREAM type. Make a
socket asynchronous as shown in Code Example 2-15.
Code Example 2-15 Making a Socket Asynchronous
#include <fcntl.h>
#include <sys/file.h>
...
int fileflags;
int s;
...
s = socket(AF_INET, SOCK_STREAM, 0);
...
if (fileflags = fcntl(s, F_GETFL ) == -1)
perror("fcntl F_GETFL");
exit(1);
}
if (fcntl(s, F_SETFL, fileflags | FNDELAY | FASYNC) < 0)
perror("fcntl F_SETFL, FNDELAY | FASYNC");
exit(1);
}
...
After sockets are initialized, connected, and made asynchronous,
communication is similar to reading and writing a file asynchronously. A
send(), write(), recv(), or read() initiates a data transfer. A data transfer
is completed by a signal-driven I/O routine, described in the next section.
Interrupt Driven Socket I/O
The SIGIO signal notifies a process when a socket (actually any file descriptor)
has finished a data transfer. There are three steps in using SIGIO:
•
44
Set up a SIGIO signal handler with the signal() or sigvec() calls.
Transport Interfaces Programming Guide—November 1995
2
•
Use fcntl() to set the process ID or process group ID to receive the signal
to its own process ID or process group ID (the default process group of a
socket is group 0).
•
Convert the socket to asynchronous as shown in “Asynchronous Sockets”
on page 44.
Sample code to allow a given process to receive information on pending
requests as they occur for a socket is shown in Code Example 2-16. With the
addition of a handler for SIGURG, this code can also be used to prepare for
receipt of SIGURG signals.
Code Example 2-16 Asynchronous Notification of I/O Requests
#include <fcntl.h>
#include <sys/file.h>
...
signal(SIGIO, io_handler);
/* Set the process receiving SIGIO/SIGURG signals to us. */
if (fcntl(s, F_SETOWN, getpid()) < 0) {
perror("fcntl F_SETOWN");
exit(1);
}
Signals and Process Group ID
For SIGURG and SIGIO, each socket has a process number and a process group
ID. These values are initialized to zero, but may be redefined at a later time
with the F_SETOWN fcntl(), as in the previous example. A positive third
argument to fcntl() sets the socket’s process ID. A negative third argument
to fcntl() sets the socket’s process group ID. The only allowed recipient of
SIGURG and SIGIO signals is the calling process.
A similar fcntl(), F_GETOWN, returns the process number of a socket.
Reception of SIGURG and SIGIO can also be enabled by using ioctl() to
assign the socket to the user’s process group:
/* oobdata is the out-of-band data handling routine */
sigset(SIGURG, oobdata);
int pid = -getpid();
if (ioctl(client, SIOCSPGRP, (char *) &pid) < 0) {
perror("ioctl: SIOCSPGRP");
}
Programming With Sockets
45
2
Another signal that is useful in server processes is SIGCHLD. This signal is
delivered to a process when any child process changes state. Normally, servers
use the signal to “reap” child processes that have exited without explicitly
awaiting their termination or periodically polling for exit status. For example,
the remote login server loop shown previously can be augmented as shown in
Code Example 2-17.
Code Example 2-17 SIGCHLD Signal
int reaper();
...
sigset(SIGCHLD, reaper);
listen(f, 5);
while (1) {
int g, len = sizeof from;
g = accept(f, (struct sockaddr *) &from, &len);
if (g < 0) {
if (errno != EINTR)
syslog(LOG_ERR, "rlogind: accept: %m");
continue;
}
...
}
#include <wait.h>
reaper()
{
int options;
int error;
siginfo_t info;
options = WNOHANG | WEXITED;
bzero((char *) &info, sizeof(info));
error = waitid(P_ALL, 0, &info, options);
}
If the parent server process fails to reap its children, zombie processes result.
Selecting Specific Protocols
If the third argument of the socket() call is 0, socket() selects a default
protocol to use with the returned socket of the type requested. The default
protocol is usually correct, and alternate choices are not usually available.
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When using “raw” sockets to communicate directly with lower-level protocols
or hardware interfaces, it may be important for the protocol argument to set up
de-multiplexing. For example, raw sockets in the Internet domain can be used
to implement a new protocol on IP, and the socket will receive packets only for
the protocol specified. To obtain a particular protocol, determine the protocol
number as defined in the protocol domain. For the Internet domain, use one of
the library routines discussed in “Standard Routines” on page 31, such as
getprotobyname():
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
...
pp = getprotobyname("newtcp");
s = socket(AF_INET, SOCK_STREAM, pp->p_proto) ;
This results in a socket s using a stream-based connection, but with protocol
type of newtcp instead of the default tcp.
Address Binding
In the Internet Protocol family, bindings are composed of local and foreign IP
addresses, and of local and foreign port numbers. Port numbers are allocated
in separate spaces, one for each system and one for each transport protocol
(TCP or UDP). Through bind(), a process specifies the <local IP address, local
port number> half of an association, while connect() and accept() complete
a socket’s association by specifying the <foreign IP address, foreign port number>
part. Since the association is created in two steps, the association-uniqueness
requirement might be violated, unless care is taken. It is unrealistic to expect
user programs to always know proper values to use for the local address and
local port, since a host may reside on multiple networks and the set of
allocated port numbers is not directly accessible to a user.
The wildcard address simplifies local address binding in the Internet domain.
When an address is specified as INADDR_ANY (a constant defined in
<netinet/in.h>), the system interprets the address as any valid address.
Code Example 2-18 binds a specific port number to a socket, and leaves the
local address unspecified.
Programming With Sockets
47
2
Code Example 2-18 Bind Port Number to Socket
#include <sys/types.h>
#include <netinet/in.h>
...
struct sockaddr_in sin;
...
s = socket(AF_INET, SOCK_STREAM, 0);
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = htonl(INADDR_ANY);
sin.sin_port = htons(MYPORT);
bind(s, (struct sockaddr *) &sin, sizeof sin);
Each network interface on a host typically has a unique IP address. Sockets
with wildcard local addresses may receive messages directed to the specified
port number and sent to any of the possible addresses assigned to a host. For
example, if a host has two interfaces with addresses 128.32.0.4 and 10.0.0.78,
and a socket is bound as in Code Example 2-18, the process can accept
connection requests addressed to 128.32.0.4 or 10.0.0.78. To allow only hosts on
a specific network to connect to it, a server binds the address of the interface
on the appropriate network.
Similarly, a local port number can be left unspecified (specified as 0), in which
case the system selects a port number. For example, to bind a specific local
address to a socket, but to leave the local port number unspecified:
sin.sin_addr.s_addr = inet_addr("127.0.0.1");
sin.sin_family = AF_INET;
sin.sin_port = htons(0);
bind(s, (struct sockaddr *) &sin, sizeof sin);
The system uses two criteria to select the local port number:
•
The first is that Internet port numbers less than 1024(IPPORT_RESERVED)
are reserved for privileged users (that is, the superuser). Nonprivileged
users may use any Internet port number greater than 1024. The largest
Internet port number is 65535.
•
The second criterion is that the port number is not currently bound to some
other socket.
The port number and IP address of the client is found through either
accept() (the from result) or getpeername().
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Transport Interfaces Programming Guide—November 1995
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In certain cases, the algorithm used by the system to select port numbers is
unsuitable for an application. This is because associations are created in a twostep process. For example, the Internet file transfer protocol specifies that data
connections must always originate from the same local port. However,
duplicate associations are avoided by connecting to different foreign ports. In
this situation the system would disallow binding the same local address and
port number to a socket if a previous data connection’s socket still existed. To
override the default port selection algorithm, an option call must be performed
before address binding:
...
int on = 1;
...
setsockopt(s, SOL_SOCKET, SO_REUSEADDR, &on, sizeof on);
bind(s, (struct sockaddr *) &sin, sizeof sin);
With this call, local addresses can be bound that are already in use. This does
not violate the uniqueness requirement, because the system still verifies at
connect time that any other sockets with the same local address and port do
not have the same foreign address and port. If the association already exists,
the error EADDRINUSE is returned.
Broadcasting and Determining Network Configuration
Messages sent by datagram sockets can be broadcast to reach all of the hosts on
an attached network. The network must support broadcast; the system
provides no simulation of broadcast in software. Broadcast messages can place
a high load on a network since they force every host on the network to service
them. Broadcasting is usually used for either of two reasons: to find a resource
on a local network without having its address, or functions like routing require
that information be sent to all accessible neighbors.
To send a broadcast message, create an Internet datagram socket:
s = socket(AF_INET, SOCK_DGRAM, 0);
and bind a port number to the socket:
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = htonl(INADDR_ANY);
sin.sin_port = htons(MYPORT);
bind(s, (struct sockaddr *) &sin, sizeof sin);
Programming With Sockets
49
2
The datagram can be broadcast on only one network by sending to the
network’s broadcast address. A datagram can also be broadcast on all attached
networks by sending to the special address INADDR_BROADCAST, defined in
<netinet/in.h>.
The system provides a mechanism to determine a number of pieces of
information (including the IP address and broadcast address) about the
network interfaces on the system. The SIOCGIFCONF ioctl call returns the
interface configuration of a host in a single ifconf structure. This structure
contains an array of ifreq structures, one for each address domain supported
by each network interface to which the host is connected. Code Example 2-19
shows these structures defined in <net/if.h>.
Code Example 2-19 net/if.h Header File
struct ifreq {
#define IFNAMSIZ 16
char ifr_name[IFNAMSIZ]; /* if name, e.g., "en0" */
union {
struct sockaddr ifru_addr;
struct sockaddr ifru_dstaddr;
char ifru_oname[IFNAMSIZ]; /* other if name */
struct sockaddr ifru_broadaddr;
short ifru_flags;
int ifru_metric;
char ifru_data[1]; /* interface dependent data */
char ifru_enaddr[6];
} ifr_ifru;
#define ifr_addr ifr_ifru.ifru_addr
#define ifr_dstaddr ifr_ifru.ifru_dstaddr
#define ifr_oname ifr_ifru.ifru_oname
#define ifr_broadaddr ifr_ifru.ifru_broadaddr
#define ifr_flags ifr_ifru.ifru_flags
#define ifr_metric ifr_ifru.ifru_metric
#define ifr_data ifr_ifru.ifru_data
#define ifr_enaddr ifr_ifru.ifru_enaddr
};
The call that obtains the interface configuration is:
struct ifconf ifc;
char buf[BUFSIZ];
ifc.ifc_len = sizeof buf;
ifc.ifc_buf = buf;
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Transport Interfaces Programming Guide—November 1995
2
if (ioctl(s, SIOCGIFCONF, (char *) &ifc ) < 0) {
...
}
After this call, buf contains an array of ifreq structures, one for each network
to which the host is connected. These structures are ordered first by interface
name and then by supported address families. ifc.ifc_len is set to the
number of bytes used by the ifreq structures.
Each structure has a set of interface flags that tell whether the corresponding
network is up or down, point to point or broadcast, and so on. The
SIOCGIFFLAGS ioctl returns these flags for an interface specified by an
ifreq structure shown in Code Example 2-20.
Code Example 2-20 Obtaining Interface Flags
struct ifreq *ifr;
ifr = ifc.ifc_req;
for (n = ifc.ifc_len/sizeof (struct ifreq); --n >= 0; ifr++) {
/*
* Be careful not to use an interface devoted to an address
* domain other than those intended.
*/
if (ifr->ifr_addr.sa_family != AF_INET)
continue;
if (ioctl(s, SIOCGIFFLAGS, (char *) ifr) < 0) {
...
}
/* Skip boring cases */
if ((ifr->ifr_flags & IFF_UP) == 0 ||
(ifr->ifr_flags & IFF_LOOPBACK) ||
(ifr->ifr_flags & (IFF_BROADCAST | IFF_POINTOPOINT)) == 0)
continue;
}
Code Example 2-21 shows the broadcast of an interface can be obtained with
the SIOGGIFBRDADDR ioctl().
Code Example 2-21 Broadcast Address of an Interface
if (ioctl(s, SIOCGIFBRDADDR, (char *) ifr) < 0) {
...
}
memcpy((char *) &dst, (char *) &ifr->ifr_broadaddr,
sizeof ifr->ifr_broadaddr);
Programming With Sockets
51
2
The SIOGGIFBRDADDR ioctl can also be used to get the destination address
of a point-to-point interface.
After the interface broadcast address is obtained, transmit the broadcast
datagram with sendto():
sendto(s, buf, buflen, 0, (struct sockaddr *)&dst, sizeof dst);
Use one sendto() for each interface to which the host is connected that
supports the broadcast or point-to-point addressing.
Socket Options
You can set and get several options on sockets through setsockopt() and
getsockopt(); for example changing the send or receive buffer space. The
general forms of the calls are:
setsockopt(s, level, optname, optval, optlen);
and
getsockopt(s, level, optname, optval, optlen);
Table 2-4 shows the arguments of the calls.
Table 2-4
setsockopt() and getsockopt() Arguments
Arguments
Description
s
Socket on which the option is to be applied
level
Specifies the protocol level, i.e. socket level, indicated by the
symbolic constant SOL_SOCKET in <sys/socket.h>
optname
Symbolic constant defined in <sys/socket.h> that specifies the
option
optval
Points to the value of the option
optlen
Points to the length of the value of the option
For getsockopt(), optlen is a value-result argument, initially set to the size of
the storage area pointed to by optval and set on return to the length of storage
used.
It is sometimes useful to determine the type (for example, stream or datagram)
of an existing socket. Programs invoked by inetd may need to do this by
using the SO_TYPE socket option and the getsockopt() call:
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Transport Interfaces Programming Guide—November 1995
2
#include <sys/types.h>
#include <sys/socket.h>
int type, size;
size = sizeof (int);
if (getsockopt(s, SOL_SOCKET, SO_TYPE, (char *) &type, &size) < 0) {
...
}
After getsockopt(), type is set to the value of the socket type, as defined in
<sys/socket.h>. For a datagram socket, type would be SOCK_DGRAM.
inetd Daemon
One of the daemons provided with the system is inetd. It is invoked at startup time, and gets the services for which it listens from the /etc/inetd.conf
file. The daemon creates one socket for each service listed in
/etc/inetd.conf, binding the appropriate port number to each socket. See
the inetd(1M)man page for details.
inetd does a select() on each socket, waiting for a connection request to
the service corresponding to that socket. For SOCK_STREAM type sockets,
inetd does an accept() on the listening socket, fork()s, dup()s the new
socket to file descriptors 0 and 1 (stdin and stdout), closes other open file
descriptors, and exec()s the appropriate server.
The primary benefit of inetd is that services that are not in use are not taking
up machine resources. A secondary benefit is that inetd does most of the
work to establish a connection. The server started by inetd has the socket
connected to its client on file descriptors 0 and 1, and can immediately
read(), write(), send(), or recv(). Servers can use buffered I/O as
provided by the stdio conventions, as long as they use fflush() when
appropriate.
getpeername() returns the address of the peer (process) connected to a
socket; it is useful in servers started by inetd. For example, to log the Internet
address in decimal dot notation (such as 128.32.0.4, which is conventional for
representing an IP address of a client), an inetd server could use the
following:
Programming With Sockets
53
2
struct sockaddr_in name;
int namelen = sizeof name;
...
if (getpeername(0, (struct sockaddr *) &name, &namelen) < 0) {
syslog(LOG_ERR, "getpeername: %m");
exit(1);
} else
syslog(LOG_INFO, "Connection from %s",
inet_ntoa(name.sin_addr));
...
Moving Socket Applications to Solaris 2.x
Sockets and the socket implementation are mostly compatible with previous
releases of SunOS. But, an application programmer must be aware of some
differences that are listed in the tables provided in this section.
Table 2-5
Connection-Mode Primitives (SunOS 4.x/Solaris 2.x)
SunOS 4.x (BSD)
Solaris 2.x
connect()
When connect() is called on an
unbound socket, the protocol determines
whether the endpoint is bound before the
connection takes place.
Table 2-6
When connect() is called on an
unbound socket, that socket is always
bound to an address selected by the
protocol.
Data Transfer Primitives (SunOS 4.x/Solaris 2.x)
SunOS 4.x (BSD)
Solaris 2.x
write()
write() fails with errno set to
ENOTCONN if it is used on an
unconnected socket.
54
A call to write() appears to succeed,
but the data are discarded. The socket
error option SO_ERROR returns
ENOTCONN if this happens.
Transport Interfaces Programming Guide—November 1995
2
Table 2-6
Data Transfer Primitives (SunOS 4.x/Solaris 2.x)
SunOS 4.x (BSD)
Solaris 2.x
write() can be used on type
SOCK_DGRAM sockets (either AF_UNIX or
AF_INET domains) to send zero-length
data.
A call to write() returns -1, with errno
set to ERANGE. send(), sendto(), or
sendmsg() should be used to send zerolength data.
read()
read() fails with errno set to ENOTCONN
if read() is used on an unconnected
socket.
Table 2-7
read() returns zero bytes read if the
socket is in blocking mode. If the socket
is in non-blocking mode, it returns -1
with errno set to EAGAIN.
Information Primitives (SunOS 4.x/Solaris 2.x)
SunOS 4.x (BSD)
Solaris 2.x
getsockname()
getsockname() works even when a
previously existing connection has been
closed.
getsockname() returns -1 and errno is
set to EPIPE if a previously existing
connection has been closed.
ioctl() and fcntl()
The argument of the
SIOCSPGRP/FIOSETOWN/F_SETOWN
ioctl()s and the F_SETOWN fcntl()
are a positive process ID or negative
process group ID of the intended
recipient list of subsequent SIGURG and
SIGIO signals.
Table 2-8
This is not the case in Solaris 2.x. The
only acceptable argument of these system
calls is the caller's process ID or a
negative of the caller's process group ID.
So, the only recipient of SIGURG and
SIGIO is the calling process.
Local Management (SunOS 4.x/Solaris 2.x)
SunOS 4.x (BSD)
Solaris 2.x
bind()
bind() uses the credentials of the user
at the time of the bind() call to
determine if the requested address is
allocated or not.
socket() causes the user's credentials
to be found and used to validate
addresses used later in bind().
setsockopt()
Programming With Sockets
55
2
Table 2-8
Local Management (SunOS 4.x/Solaris 2.x)
SunOS 4.x (BSD)
setsockopt() can be used at any time
during the life of a socket.
Solaris 2.x
If a socket is unbound and
setsockopt() is used, the operation
succeeds in the AF_INET domain but
fails in the AF_UNIX domain.
shutdown()
If shutdown() is called with how set to
zero, further tries to receive data returns
zero bytes (EOF).
If a shutdown() call with how set to
zero is followed by a read(2) call and
the socket is in nonblocking mode,
read() returns -1 with errno set to
EAGAIN. If one of the socket receive
primitives is used, the correct result
(EOF) is returned.
If shutdown() is called with the value of
2 for how, further tries to receive data
return EOF. Tries to send data return -1
with errno set to EPIPE and a SIGPIPE is
issued.
The same result happens, but tries to
send data using write(2) cause errno to
be set to EIO. If a socket primitive is
used, the correct errno is returned.
Table 2-9
Signals (SunOS 4.x/Solaris 2.x)
SunOS 4.x (BSD)
Solaris 2.x
SIGIO
SIGIO is delivered every time new data
are appended to the socket input queue.
SIGIO is delivered only when data are
appended to a socket queue that was
previously empty.
SIGURG
A SIGURG is delivered every time new
data are anticipated or actually arrive.
A SUGURG is delivered only when data
are already pending.
S_ISSOCK()
The S_ISSOCK macro takes the mode of
a file as an argument. It returns 1 if the
file is a socket and 0 otherwise.
56
The S_ISSOCK macro does not exist.
Here, a socket is a file descriptor
associated with a STREAMS character
device that has the socket module
pushed onto it.
Transport Interfaces Programming Guide—November 1995
2
Table 2-10 Miscellaneous Socket Issues (SunOS 4.x/Solaris 2.x)
SunOS 4.x (BSD)
Solaris 2.x
Invalid buffers
If an invalid buffer is specified in a
function, the function returns -1 with
errno set to EFAULT.
If an invalid buffer is specified in a
function, the user's program probably
dumps core.
Sockets in Directories
If ls -l is executed in a directory
containing a UNIX domain socket, an s is
displayed on the left side of the mode
field.
If ls -l is executed in a directory that
contains a UNIX domain socket, a p is
displayed on the left side of the mode
field.
An ls -F causes an equal sign (=) to be
displayed after any file name of a UNIX
domain socket.
Nothing is displayed after the file name.
Programming With Sockets
57
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Transport Interfaces Programming Guide—November 1995
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