socketpair()

Create a pair of connected sockets

Synopsis:

#include <sys/types.h>
#include <sys/socket.h>

int socketpair( int domain, 
                int type, 
                int protocol, 
                int * fd[2] );

Arguments:

domain: The communications domain where the sockets are to be created.
type: The type of sockets to create.
protocol: The protocol to use with the sockets. A protocol of 0 causes socketpair() to use a default protocol that's appropriate for the requested socket type.
fd[2]: The 2-digit integer array where the file descriptors of the created socket pair are to be held.

Library:

libsocket

Use the -l socket option to qcc to link against this library.

Description:

The socketpair() call creates an unnamed pair of connected sockets in the specified domain, of the specified type, using the optionally specified protocol argument. The file descriptors are returned in the vector fd and are identical. Data that's written to one of the file descriptors can be read from the other.

Valid types are described in socket().

If the protocol argument is nonzero, it must be a protocol that's understood by the address family. No such protocols are defined at this time.

Returns:

0: Success.
-1: An error occurred (errno is set).

Errors:

EAFNOSUPPORT: The specified address family isn't supported on this machine.
EFAULT: The address sv doesn't specify a valid part of the process address space.
EMFILE: All, or all but one, of the file descriptors available to the process are currently open.
EOPNOTSUPP: The specified protocol doesn't support creation of socket pairs.
EPROTONOSUPPORT: The specified protocol isn't supported on this machine.

Examples:

#include <stdio.h>
#include <unistd.h>
#include <sys/socket.h>

#define CHAR_BUFSIZE 50

int main(int argc, char **argv) {
    int fd[2], len;
    char message[CHAR_BUFSIZE];
    
    if(socketpair(AF_LOCAL, SOCK_STREAM, 0, fd) == -1) {
        return 1;
    }
    
    /* If you write to fd[0], you read from fd[1], and vice versa. */
    
    /* Print a message into one end of the socket. */
    snprintf(message, CHAR_BUFSIZE, "A message written to fd[0]");
    write(fd[0], message, strlen(message) + 1);
    
    /* Print a message into the other end of the socket. */
    snprintf(message, CHAR_BUFSIZE, "A message written to fd[1]");
    write(fd[1], message, strlen(message) + 1);
    
    /* Read from the first socket the data written to the second. */
    len = read(fd[0], message, CHAR_BUFSIZE-1);
    message[len] = '\0';
    printf("Read from fd[0]: %s \n", message); 
    
    /* Read from the second socket the data written to the first. */
    len = read(fd[1], message, CHAR_BUFSIZE-1);
    message[len] = '\0';
    printf("Read from fd[1]: %s \n", message); 
    
    close(fd[0]);
    close(fd[1]);
    
    return 0;
}

This program produces the following output:

Read from fd[0]: A message written to fd[1]
Read from fd[1]: A message written to fd[0]

Classification:

POSIX 1003.1

Safety:
Cancellation point	No
Interrupt handler	No
Signal handler	No
Thread	Yes