This is svmsg/svmsg_send.c (Listing 46-2, page 941), 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_send.c

   Usage: svmsg_send [-n] msqid msg-type [msg-text]

   Experiment with the msgsnd() system call to send messages to a
   System V message queue.

   See also svmsg_receive.c.
#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             /* Print (optional) message, then usage description */
usageError(const char *progName, const char *msg)
    if (msg != NULL)
        fprintf(stderr, "%s", msg);
    fprintf(stderr, "Usage: %s [-n] msqid msg-type [msg-text]\n", progName);
    fprintf(stderr, "    -n       Use IPC_NOWAIT flag\n");
main(int argc, char *argv[])
    int msqid, flags, msgLen;
    struct mbuf msg;                    /* Message buffer for msgsnd() */
    int opt;                            /* Option character from getopt() */

    /* Parse command-line options and arguments */

    flags = 0;
    while ((opt = getopt(argc, argv, "n")) != -1) {
        if (opt == 'n')
            flags |= IPC_NOWAIT;
            usageError(argv[0], NULL);

    if (argc < optind + 2 || argc > optind + 3)
        usageError(argv[0], "Wrong number of arguments\n");

    msqid = getInt(argv[optind], 0, "msqid");
    msg.mtype = getInt(argv[optind + 1], 0, "msg-type");

    if (argc > optind + 2) {            /* 'msg-text' was supplied */
        msgLen = strlen(argv[optind + 2]) + 1;
        if (msgLen > MAX_MTEXT)
            cmdLineErr("msg-text too long (max: %d characters)\n", MAX_MTEXT);

        memcpy(msg.mtext, argv[optind + 2], msgLen);

    } else {                            /* No 'msg-text' ==> zero-length msg */
        msgLen = 0;

    /* Send message */

    if (msgsnd(msqid, &msg, msgLen, flags) == -1)



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