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For most programmers, the mechanisms already described are enough to build distributed applications. This section describes additional features.

Nonblocking Sockets

Some applications require sockets that do not block. For example, a server would return an error code, not executing a request that cannot complete immediately. This error could cause the process to be suspended, awaiting completion. After creating and connecting a socket, issuing a fcntl (2) call, as shown in the following example, makes the socket non-blocking.

#include <fcntl.h>#include <sys/file.h>...int fileflags;int s;...s = socket(AF_INET6, 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 performing I/O on a nonblocking socket, check for the error EWOULDBLOCK in errno.h , which occurs when an operation would normally block. accept (3SOCKET), connect (3SOCKET), send (3SOCKET), recv (3SOCKET), read (2), and write (2) can all return EWOULDBLOCK . If an operation such as a send (3SOCKET) cannot be done in its entirety but partial writes work, as when using a stream socket, all available data is processed. The return value is the amount of data actually sent.

Asynchronous Socket I/O

Asynchronous communication between processes is required in applications that simultaneously handle multiple requests. Asynchronous sockets must be of the SOCK_STREAM type. To make a socket asynchronous, you issue a fcntl (2) call, as shown in the following example.

#include <fcntl.h>#include <sys/file.h>...int fileflags;int s;...s = socket(AF_INET6, SOCK_STREAM, 0);...if (fileflags = fcntl(s, F_GETFL ) == -1) perror("fcntl F_GETFL"); exit(1);}if (fcntl(s, F_SETFL, fileflags | FNDELAY | FASYNC) == -1) perror("fcntl F_SETFL, FNDELAY | FASYNC"); exit(1);}...

After sockets are initialized, connected, and made nonblocking and asynchronous, communication is similar to reading and writing a file asynchronously. Initiate a data transfer by using send (3SOCKET), write (2), recv (3SOCKET), or read (2). A signal-driven I/O routine completes a data transfer, as described in the next section.

Interrupt-Driven Socket I/O

The SIGIO signal notifies a process when a socket, or any file descriptor, has finished a data transfer. The steps in using SIGIO are as follows:

1. Set up a SIGIO signal handler with the signal (3C) or sigvec (3UCB) calls.

2. Use fcntl (2) to set the process ID or process group ID to route the signal to its own process ID or process group ID. The default process group of a socket is group 0 .

3. Convert the socket to asynchronous, as shown in .

The following sample code enables receipt of information on pending requests as the requests occur for a socket by a given process. With the addition of a handler for SIGURG , this code can also be used to prepare for receipt of SIGURG signals.

#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);}

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