This is svsem/svsem_mon.c (Listing 47-3, page 973), 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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/* svsem_mon.c

   Display various information about the semaphores in a System V semaphore set.

   Since the information obtained by this program is not obtained atomically,
   it may not be consistent if another process makes changes to the semaphore
   at the moment this program is running.
#include <sys/types.h>
#include <sys/sem.h>
#include <time.h>
#include "semun.h"                      /* Definition of semun union */
#include "tlpi_hdr.h"
main(int argc, char *argv[])
    struct semid_ds ds;
    union semun arg, dummy;             /* Fourth argument for semctl() */
    int semid, j;

    if (argc != 2 || strcmp(argv[1], "--help") == 0)
        usageErr("%s semid\n", argv[0]);

    semid = getInt(argv[1], 0, "semid");

    arg.buf = &ds;
    if (semctl(semid, 0, IPC_STAT, arg) == -1)

    printf("Semaphore changed: %s", ctime(&ds.sem_ctime));
    printf("Last semop():      %s", ctime(&ds.sem_otime));

    /* Display per-semaphore information */

    arg.array = calloc(ds.sem_nsems, sizeof(arg.array[0]));
    if (arg.array == NULL)
    if (semctl(semid, 0, GETALL, arg) == -1)

    printf("Sem #  Value  SEMPID  SEMNCNT  SEMZCNT\n");

    for (j = 0; j < ds.sem_nsems; j++)
        printf("%3d   %5d   %5d  %5d    %5d\n", j, arg.array[j],
                semctl(semid, j, GETPID, dummy),
                semctl(semid, j, GETNCNT, dummy),
                semctl(semid, j, GETZCNT, dummy));



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