This is svsem/semun.h (Listing 47-2, page 970), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU Lesser General Public License, version 3.

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/* semun.h

   Definition of the semun union used by the System V semaphore semop()
   system call.
#ifndef SEMUN_H
#define SEMUN_H                 /* Prevent accidental double inclusion */

#include <sys/types.h>          /* For portability */
#include <sys/sem.h>

#if ! defined(__FreeBSD__) && ! defined(__OpenBSD__) && \
                ! defined(__sgi) && ! defined(__APPLE__)
                /* Some implementations already declare this union */

union semun {                   /* Used in calls to semctl() */
    int                 val;
    struct semid_ds *   buf;
    unsigned short *    array;
#if defined(__linux__)
    struct seminfo *    __buf;




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