Initializing a Semaphore Set

semget() initializes or gains access to a semaphore. When the call succeeds, it returns the semaphore ID (semid). The key argument is a value associated with the semaphore ID. The nsems argument specifies the number of elements in a semaphore array. The call fails when nsems is greater than the number of elements in an existing array. When the correct count is not known, supplying 0 for this argument ensures that it will succeed. The semflg argument specifies the initial access permissions and creation control flags. For more information, see the semget(2) man page.

The SEMMNI system configuration option determines the maximum number of semaphore arrays allowed. The SEMMNS option determines the maximum possible number of individual semaphores across all semaphore sets. Because of fragmentation between semaphore sets, allocating all available semaphores might not be possible.

The following code illustrates semget().

#include                        <sys/types.h>
#include                        <sys/ipc.h>
#include                        <sys/sem.h>
...
         key_t    key;       /* key to pass to semget() */
         int      semflg;    /* semflg to pass to semget() */
         int      nsems;     /* nsems to pass to semget() */
         int      semid;     /* return value from semget() */
         ...
         key = ...
         nsems = ...
         semflg = ...
         ...
         if ((semid = semget(key, nsems, semflg)) == -1) {
                 perror("semget: semget failed");
                 exit(1);
         } else
                 exit(0);
...