Programming

/*
 * one-way-pipe.c  - example of using a pipe to communicate data between a
 *                   process and its child process. The parent reads input
 *                   from the user, and sends it to the child via a pipe.
 *                   The child prints the received data to the screen.
 */

#include <stdio.h>    /* standard I/O routines.                */
#include <unistd.h>   /* defines pipe(), amongst other things. */

/* this routine handles the work of the child process. */
void do_child(int data_pipe[]) {
    int c;	/* data received from the parent. */
    int rc;	/* return status of read().       */

    /* first, close the un-needed write-part of the pipe. */
    close(data_pipe[1]);

    /* now enter a loop of reading data from the pipe, and printing it */
    while ((rc = read(data_pipe[0], &c, 1)) > 0) {
	putchar(c);
    }

    /* probably pipe was broken, or got EOF via the pipe. */
    exit(0);
}

/* this routine handles the work of the parent process. */
void do_parent(int data_pipe[])
{
    int c;	/* data received from the user. */
    int rc;	/* return status of getchar().  */

    /* first, close the un-needed read-part of the pipe. */
    close(data_pipe[0]);

    /* now enter a loop of read user input, and writing it to the pipe. */
    while ((c = getchar()) > 0) {
	/* write the character to the pipe. */
        rc = write(data_pipe[1], &c, 1);
	if (rc == -1) { /* write failed - notify the user and exit */
	    perror("Parent: write");
	    close(data_pipe[1]);
	    exit(1);
        }
    }

    /* probably got EOF from the user. */
    close(data_pipe[1]); /* close the pipe, to let the child know we're done. */
    exit(0);
}

/* and the main function. */
int main(int argc, char* argv[])
{
    int data_pipe[2]; /* an array to store the file descriptors of the pipe. */
    int pid;       /* pid of child process, or 0, as returned via fork.    */
    int rc;        /* stores return values of various routines.            */

    /* first, create a pipe. */
    rc = pipe(data_pipe);
    if (rc == -1) {
	perror("pipe");
	exit(1);
    }

    /* now fork off a child process, and set their handling routines. */
    pid = fork();

    switch (pid) {
	case -1:	/* fork failed. */
	    perror("fork");
	    exit(1);
	case 0:		/* inside child process.  */
	    do_child(data_pipe);
	    /* NOT REACHED */
	default:	/* inside parent process. */
	    do_parent(data_pipe);
	    /* NOT REACHED */
    }

    return 0;	/* NOT REACHED */
}