This is procexec/fork_file_sharing.c (Listing 24-2, page 518), 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.

This page shows the "distribution" or "book" version of the file (why are there two versions?), or the differences between the two versions. You can switch between the views using the tabs below.

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 procexec/fork_file_sharing.c

  Cover of The Linux Programming Interface

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

/* fork_file_sharing.c

   Show that the file descriptors of a forked child refer to the
   same open file objects as the parent.
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/wait.h>
#include "tlpi_hdr.h"
main(int argc, char *argv[])
    int fd, flags;
    char template[] = "/tmp/testXXXXXX";

    setbuf(stdout, NULL);                   /* Disable buffering of stdout */

    /* Open a temporary file, set its file offset to some arbitrary value,
       and change the setting of one of the open file status flags. */

    fd = mkstemp(template);
    if (fd == -1)

    printf("File offset before fork(): %lld\n",
            (long long) lseek(fd, 0, SEEK_CUR));

    flags = fcntl(fd, F_GETFL);
    if (flags == -1)
        errExit("fcntl - F_GETFL");
    printf("O_APPEND flag before fork() is: %s\n",
            (flags & O_APPEND) ? "on" : "off");

    switch (fork()) {
    case -1:

    case 0:     /* Child: change file offset and status flags */
        if (lseek(fd, 1000, SEEK_SET) == -1)

        flags = fcntl(fd, F_GETFL);         /* Fetch current flags */
        if (flags == -1)
            errExit("fcntl - F_GETFL");
        flags |= O_APPEND;                  /* Turn O_APPEND on */
        if (fcntl(fd, F_SETFL, flags) == -1)
            errExit("fcntl - F_SETFL");

    default:    /* Parent: can see file changes made by child */
        if (wait(NULL) == -1)
            errExit("wait");                /* Wait for child exit */
        printf("Child has exited\n");

        printf("File offset in parent: %lld\n",
                (long long) lseek(fd, 0, SEEK_CUR));

        flags = fcntl(fd, F_GETFL);
        if (flags == -1)
            errExit("fcntl - F_GETFL");
        printf("O_APPEND flag in parent is: %s\n",
                (flags & O_APPEND) ? "on" : "off");


Download procexec/fork_file_sharing.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