This is svmsg/svmsg_receive.c (Listing 46-3, page 945), 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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/* svmsg_receive.c

   Usage: svmsg_receive [-nex] [-t msg-type] msqid [max-bytes]

   Experiment with the msgrcv() system call to receive messages from a
   System V message queue.

   See also svmsg_send.c.
#define _GNU_SOURCE             /* Get definition of MSG_EXCEPT */
#include <sys/types.h>
#include <sys/msg.h>
#include "tlpi_hdr.h"

#define MAX_MTEXT 1024

struct mbuf {
    long mtype;                 /* Message type */
    char mtext[MAX_MTEXT];      /* Message body */
static void
usageError(const char *progName, const char *msg)
    if (msg != NULL)
        fprintf(stderr, "%s", msg);
    fprintf(stderr, "Usage: %s [options] msqid [max-bytes]\n", progName);
    fprintf(stderr, "Permitted options are:\n");
    fprintf(stderr, "    -e       Use MSG_NOERROR flag\n");
    fprintf(stderr, "    -t type  Select message of given type\n");
    fprintf(stderr, "    -n       Use IPC_NOWAIT flag\n");
    fprintf(stderr, "    -x       Use MSG_EXCEPT flag\n");
main(int argc, char *argv[])
    int msqid, flags, type;
    ssize_t msgLen;
    size_t maxBytes;
    struct mbuf msg;            /* Message buffer for msgrcv() */
    int opt;                    /* Option character from getopt() */

    /* Parse command-line options and arguments */

    flags = 0;
    type = 0;
    while ((opt = getopt(argc, argv, "ent:x")) != -1) {
        switch (opt) {
        case 'e':       flags |= MSG_NOERROR;   break;
        case 'n':       flags |= IPC_NOWAIT;    break;
        case 't':       type = atoi(optarg);    break;
        case 'x':       flags |= MSG_EXCEPT;    break;
        default:        usageError(argv[0], NULL);

    if (argc < optind + 1 || argc > optind + 2)
        usageError(argv[0], "Wrong number of arguments\n");

    msqid = getInt(argv[optind], 0, "msqid");
    maxBytes = (argc > optind + 1) ?
                getInt(argv[optind + 1], 0, "max-bytes") : MAX_MTEXT;

    /* Get message and display on stdout */

    msgLen = msgrcv(msqid, &msg, maxBytes, type, flags);
    if (msgLen == -1)

    printf("Received: type=%ld; length=%ld", msg.mtype, (long) msgLen);
    if (msgLen > 0)
        printf("; body=%s", msg.mtext);



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