This is pipes/simple_pipe.c (Listing 44-2, page 896), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/* simple_pipe.c

   Simple demonstration of the use of a pipe to communicate
   between a parent and a child process.

   Usage: simple_pipe "string"

   The program creates a pipe, and then calls fork() to create a child process.
   After the fork(), the parent writes the string given on the command line
   to the pipe, and the child uses a loop to read data from the pipe and
   print it on standard output.
#include <sys/wait.h>
#include "tlpi_hdr.h"

#define BUF_SIZE 10
main(int argc, char *argv[])
    int pfd[2];                             /* Pipe file descriptors */
    char buf[BUF_SIZE];
    ssize_t numRead;

    if (argc != 2 || strcmp(argv[1], "--help") == 0)
        usageErr("%s string\n", argv[0]);

    if (pipe(pfd) == -1)                    /* Create the pipe */

    switch (fork()) {
    case -1:

    case 0:             /* Child  - reads from pipe */
        if (close(pfd[1]) == -1)            /* Write end is unused */
            errExit("close - child");

        for (;;) {              /* Read data from pipe, echo on stdout */
            numRead = read(pfd[0], buf, BUF_SIZE);
            if (numRead == -1)
            if (numRead == 0)
                break;                      /* End-of-file */
            if (write(STDOUT_FILENO, buf, numRead) != numRead)
                fatal("child - partial/failed write");

        write(STDOUT_FILENO, "\n", 1);
        if (close(pfd[0]) == -1)

    default:            /* Parent - writes to pipe */
        if (close(pfd[0]) == -1)            /* Read end is unused */
            errExit("close - parent");

        if (write(pfd[1], argv[1], strlen(argv[1])) != strlen(argv[1]))
            fatal("parent - partial/failed write");

        if (close(pfd[1]) == -1)            /* Child will see EOF */
        wait(NULL);                         /* Wait for child to finish */


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