This is threads/thread_incr.c (Listing 30-1, page 632), 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 threads/thread_incr.c

  Cover of The Linux Programming Interface

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

/* thread_incr.c

   This program employs two POSIX threads that increment the same global
   variable, without using any synchronization method. As a consequence,
   updates are sometimes lost.

   See also thread_incr_mutex.c.
#include <pthread.h>
#include "tlpi_hdr.h"

static volatile int glob = 0;   /* "volatile" prevents compiler optimizations
                                   of arithmetic operations on 'glob' */
static void *                   /* Loop 'arg' times incrementing 'glob' */
threadFunc(void *arg)
    int loops = *((int *) arg);
    int loc, j;

    for (j = 0; j < loops; j++) {
        loc = glob;
        glob = loc;

    return NULL;
main(int argc, char *argv[])
    pthread_t t1, t2;
    int loops, s;

    loops = (argc > 1) ? getInt(argv[1], GN_GT_0, "num-loops") : 10000000;

    s = pthread_create(&t1, NULL, threadFunc, &loops);
    if (s != 0)
        errExitEN(s, "pthread_create");
    s = pthread_create(&t2, NULL, threadFunc, &loops);
    if (s != 0)
        errExitEN(s, "pthread_create");

    s = pthread_join(t1, NULL);
    if (s != 0)
        errExitEN(s, "pthread_join");
    s = pthread_join(t2, NULL);
    if (s != 0)
        errExitEN(s, "pthread_join");

    printf("glob = %d\n", glob);


Download threads/thread_incr.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