intro_socket1.c

#include <stdio.h>

#define DATA "This is from introductory 4.4BSD IPC"

/*
 * This program creates a pipe, then forks. The child communicates
 * to the parent over the pipe. Notice that a pipe is a one-way
 * communications device. I can write to the output socket (
 * sockets[1], the second socket of the array returned by pipe())
 * and read from the input socket (sockets[0]), but not vice versa.
 */

main()
{
    int sockets[2], child;

    /* Create a pipe */
    if (pipe(sockets) < 0) {
        perror("opening stream socket pair");
        exit(10);
    }

    if ((child = fork()) == -1)
        perror("fork");
    else if (child) {
        char buf[1024];

        /* This is still the parent. It reads the child's message. */
        close(sockets[1]);
        if (read(sockets[0], buf, 1024) < 0)
            perror("reading message");
        printf("-->%s\n", buf);
        close(sockets[0]);

    } else {
        /* This is the child. It writes a message to its parent. */
        close(sockets[0]);
        if (write(sockets[1], DATA, sizeof(DATA)) < 0 )
            perror("writing message");
        close(sockets[1]);
    }
}