This is altio/select_mq.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 63-3 (page 1374).

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 altio/select_mq.c

  Cover of The Linux Programming Interface

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

/* select_mq.c

   Usage: select_mq msqid...

   Demonstrate how we can use a child process in conjunction with
   select() in order to wait for input on a file descriptor (in this
   case the terminal) and on a message queue.

   This program allows us to monitor multiple message queues by
   creating a separate child for each queue named on its command line.

   For experimenting, you may find it useful to use the msg_create.c
   and msg_send.c programs from the System V IPC chapter.
#include <sys/time.h>
#if ! defined(__hpux)
/* HP-UX 11 doesn't have this header file */
#include <sys/select.h>
#include <sys/msg.h>
#include <signal.h>
#include <stddef.h>
#include "tlpi_hdr.h"

#define BUF_SIZE 200

/* Total size of the 'pbuf' struct must not exceed PIPE_BUF, otherwise
   writes by multiple children may not be atomic, with the result that
   messages are broken up and interleaved. */

#define MAX_MTEXT 400

struct pbuf {
    int msqid;                  /* Origin of message */
    int len;                    /* Number of bytes used in mtext */
    long mtype;                 /* Message type */
    char mtext[MAX_MTEXT];      /* Message body */
/* Function called by child: monitors message queue identified by
   'msqid', copying every message to the pipe identified by 'fd'. */

static void
childMon(int msqid, int fd)
    for (;;) {
        struct pbuf pmsg;
        ssize_t msgLen = msgrcv(msqid, &pmsg.mtype, MAX_MTEXT, 0, 0);
        if (msgLen == -1)

        /* We add some info to the message read by msgrcv() before
           writing to the pipe. */

        pmsg.msqid = msqid;
        pmsg.len = msgLen;      /* So parent knows how much to read from pipe */

        size_t wlen = offsetof(struct pbuf, mtext) + msgLen;
        /* Or: wlen = &pmsg.mtext - &pmsg + msgLen */

        if (write(fd, &pmsg, wlen) != wlen)
            fatal("partial/failed write to pipe");
main(int argc, char *argv[])
    if (argc < 2 || strcmp(argv[1], "--help") == 0)
        usageErr("%s msqid...\n", argv[0]);

    /* Create pipe used to transfer messages from children to parent */

    int pfd[2];
    if (pipe(pfd) == -1)

    /* Create one child for each message queue being monitored */

    for (int j = 1; j < argc; j++) {
        switch (fork()) {
        case -1:
            killpg(0, SIGTERM);
            exit(EXIT_FAILURE);         /* NOTREACHED */

        case 0:
            childMon(getInt(argv[j], 0, "msqid"), pfd[1]);
            exit(EXIT_FAILURE);         /* NOTREACHED */


    /* Parent falls through to here */

    for (;;) {
        fd_set readfds;
        FD_SET(STDIN_FILENO, &readfds);
        FD_SET(pfd[0], &readfds);
        int nfds = max(STDIN_FILENO, pfd[0]) + 1;

        int ready = select(nfds, &readfds, NULL, NULL, NULL);
        if (ready == -1)

        /* Check if terminal fd is ready */

        if (FD_ISSET(STDIN_FILENO, &readfds)) {
            char buf[BUF_SIZE];
            ssize_t numRead = read(STDIN_FILENO, buf, BUF_SIZE - 1);
            if (numRead == -1)
                errExit("read stdin");

            buf[numRead] = '\0';
            printf("Read from terminal: %s", buf);
            if (numRead > 0 && buf[numRead - 1] != '\n')

        /* Check if pipe fd is ready */

        if (FD_ISSET(pfd[0], &readfds)) {
            struct pbuf pmsg;
            ssize_t numRead = read(pfd[0], &pmsg, offsetof(struct pbuf, mtext));
            if (numRead == -1)
                errExit("read pipe");
            if (numRead == 0)
                fatal("EOF on pipe");

            numRead = read(pfd[0], &pmsg.mtext, pmsg.len);
            if (numRead == -1)
                errExit("read pipe");
            if (numRead == 0)
                fatal("EOF on pipe");

            printf("MQ %d: type=%ld length=%d <%.*s>\n", pmsg.msqid,
                    pmsg.mtype, pmsg.len, pmsg.len, pmsg.mtext);


Download altio/select_mq.c

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