This is pmsg/pmsg_send.c (Listing 52-4, page 1074), 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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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.


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

/* pmsg_send.c

   Usage as shown in usageError().

   Send a message (specified as a command line argument) to a
   POSIX message queue.

   See also pmsg_receive.c.

   Linux supports POSIX message queues since kernel 2.6.6.
#include <mqueue.h>
#include <fcntl.h>              /* For definition of O_NONBLOCK */
#include "tlpi_hdr.h"
static void
usageError(const char *progName)
    fprintf(stderr, "Usage: %s [-n] mq-name msg [prio]\n", progName);
    fprintf(stderr, "    -n           Use O_NONBLOCK flag\n");
main(int argc, char *argv[])
    int flags, opt;
    mqd_t mqd;
    unsigned int prio;

    flags = O_WRONLY;
    while ((opt = getopt(argc, argv, "n")) != -1) {
        switch (opt) {
        case 'n':   flags |= O_NONBLOCK;        break;
        default:    usageError(argv[0]);

    if (optind + 1 >= argc)

    mqd = mq_open(argv[optind], flags);
    if (mqd == (mqd_t) -1)

    prio = (argc > optind + 2) ? atoi(argv[optind + 2]) : 0;

    if (mq_send(mqd, argv[optind + 1], strlen(argv[optind + 1]), prio) == -1)


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