This is svmsg/svmsg_ls.c (Listing 46-6, page 953), 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_ls.c

   Display a list of all System V message queues on the system.

   This program is Linux-specific.
#define _GNU_SOURCE
#include <sys/types.h>
#include <sys/msg.h>
#include "tlpi_hdr.h"
main(int argc, char *argv[])
    int maxind, ind, msqid;
    struct msqid_ds ds;
    struct msginfo msginfo;

    /* Obtain size of kernel 'entries' array */

    maxind = msgctl(0, MSG_INFO, (struct msqid_ds *) &msginfo);
    if (maxind == -1)

    printf("maxind: %d\n\n", maxind);
    printf("index     id       key      messages\n");

    /* Retrieve and display information from each element of 'entries' array */

    for (ind = 0; ind <= maxind; ind++) {
        msqid = msgctl(ind, MSG_STAT, &ds);
        if (msqid == -1) {
            if (errno != EINVAL && errno != EACCES)
                errMsg("msgctl-MSG_STAT");              /* Unexpected error */
            continue;                                   /* Ignore this item */

        printf("%4d %8d  0x%08lx %7ld\n", ind, msqid,
                (unsigned long) ds.msg_perm.__key, (long) ds.msg_qnum);



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