This is pmsg/mq_notify_sigwaitinfo.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 52-6 (page 1088).

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 pmsg/mq_notify_sigwaitinfo.c

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

/* mq_notify_sigwaitinfo.c

   Usage: mq_notify_sigwaitinfo mq-name

   Demonstrate message notification via signals (accepting the signals with
   sigwaitinfo()) on a POSIX message queue.
#define _POSIX_C_SOURCE 199309
#include <signal.h>
#include <mqueue.h>
#include <fcntl.h>              /* For definition of O_NONBLOCK */
#include "tlpi_hdr.h"

#define NOTIFY_SIG SIGRTMIN     /* Signal used for message notifications */
/* This program does not handle the case where a message already exists on
   the queue by the time the first attempt is made to register for message
   notification. In that case, the program would never receive a notification.
   Compare mq_notify_sig.c and mq_notify_via_signal.c for some hints on how
   this program might be modified to handle that case. */

main(int argc, char *argv[])
    if (argc != 2 || strcmp(argv[1], "--help") == 0)
        usageErr("%s mq-name\n", argv[0]);

    mqd_t mqd = mq_open(argv[1], O_RDONLY | O_NONBLOCK);
    if (mqd == (mqd_t) -1)

    /* Determine mq_msgsize for message queue, and allocate an input buffer
       of that size */

    struct mq_attr attr;
    if (mq_getattr(mqd, &attr) == -1)

    void *buffer = malloc(attr.mq_msgsize);
    if (buffer == NULL)

    /* Block the signal that we'll accept using sigwaitinfo() */

    sigset_t blockMask;
    sigaddset(&blockMask, NOTIFY_SIG);
    if (sigprocmask(SIG_BLOCK, &blockMask, NULL) == -1)

    /* Set up message notification using the signal NOTIFY_SIG */

    struct sigevent sev;
    sev.sigev_notify = SIGEV_SIGNAL;
    sev.sigev_signo = NOTIFY_SIG;
    sev.sigev_value.sival_ptr = &mqd;
                /* This allows us to obtain a pointer to 'mqd' in the
                   siginfo_t structure returned by sigwaitinfo() */

    if (mq_notify(mqd, &sev) == -1)

    for (;;) {

        /* Wait for a signal; when it is received, display associated
           information */

        siginfo_t si;
        if (sigwaitinfo(&blockMask, &si) == -1)

        printf("Accepted signal:\n");
        printf("        si_signo   = %d\n", si.si_signo);
        printf("        si_pid     = %ld\n", (long) si.si_pid);
        printf("        si_uid     = %ld\n", (long) si.si_uid);
        printf("        si_code    = %d (%s)\n", si.si_code,
                (si.si_code == SI_MESGQ) ? "SI_MESGQ" : "???");
        printf("        *sival_ptr = %p\n\n", si.si_value.sival_ptr);

        /* Reestablish message notification */

        if (mq_notify(mqd, &sev) == -1)

        /* Although only one signal might have been queued (if NOTIFY_SIG
           is a standard signal) we might have received multiple messages,
           so use nonblocking mq_receive() calls inside a loop to read
           as many messages as possible. */

        ssize_t numRead;
        while ((numRead = mq_receive(mqd, buffer, attr.mq_msgsize, NULL)) >= 0)
            printf("Read %ld bytes\n", (long) numRead);

        if (errno != EAGAIN)            /* Unexpected error */


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