This is pipes/change_case.c, an example to accompany the book, The Linux Programming Interface.

This file is not printed in the book; it is the solution to Exercise 44-1 (page 919).

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

In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.


Download pipes/change_case.c

  Cover of The Linux Programming Interface

Function list (Bold in this list means a function is not static)

/* change_case.c

   Demonstrate the use of two pipes for bidirectional communication
   between a parent and child process. The parent reads text from
   standard input and sends it to the child via one of the pipes.
   The child reads text from this pipe, converts it to uppercase, and
   sends it back to the parent using the other pipe. The parent reads
   the text returned by the child and echoes it on standard output.
#include <ctype.h>
#include "tlpi_hdr.h"

#define BUF_SIZE 100    /* Should be <= PIPE_BUF bytes */
main(int argc, char *argv[])
    char buf[BUF_SIZE];
    ssize_t cnt;

    int outbound[2];            /* Pipe to send data from parent to child */
    int inbound[2];             /* Pipe to send data from child to parent */
    if (pipe(outbound) == -1)
    if (pipe(inbound) == -1)

    switch (fork()) {
    case -1:

    case 0: /* Child */

        /* Close unused pipe descriptors */

        if (close(outbound[1]) == -1)
        if (close(inbound[0]) == -1)

        /* Read data from outbound pipe, convert to uppercase,
           and send back to parent on inbound pipe */

        while ((cnt = read(outbound[0], buf, BUF_SIZE)) > 0) {
            for (int j = 0; j < cnt; j++)
                buf[j] = toupper((unsigned char) buf[j]);
            if (write(inbound[1], buf, cnt) != cnt)
                fatal("failed/partial write(): inbound pipe");

        if (cnt == -1)


        /* Close unused pipe descriptors */

        if (close(outbound[0]) == -1)
        if (close(inbound[1]) == -1)

        /* Read data from stdin, send to the child via the
           outbound pipe, read the results back from the child
           on the inbound pipe, and print them on stdout */

        while ((cnt = read(STDIN_FILENO, buf, BUF_SIZE)) > 0) {
            if (write(outbound[1], buf, cnt) != cnt)
                fatal("failed/partial write(): outbound pipe");

            cnt = read(inbound[0], buf, BUF_SIZE);
            if (cnt == -1)
            if (cnt > 0)
                if (write(STDOUT_FILENO, buf, cnt) != cnt)
                    fatal("failed/partial write(): STDOUT_FILENO");

        if (cnt == -1)

        /* Exiting will close write end of outbound pipe, so that
           child see EOF */



Download pipes/change_case.c

Note that, in most cases, the programs rendered in these web pages are not free standing: you'll typically also need a few other source files (mostly in the lib/ subdirectory) as well. Generally, it's easier to just download the entire source tarball and build the programs with make(1). By hovering your mouse over the various hyperlinked include files and function calls above, you can see which other source files this file depends on.

Valid XHTML 1.1