svsem/semun.h

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

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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;
 #endif
 };
 
 #endif
+
+#endif

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