The semctl() function provides a variety of semaphore control operations as specified by cmd. The fourth argument is optional, depending upon the operation requested. If required, it is of type union semun, which must be explicitly declared by the application program.

union semun {
        int             val;
        struct semid_ds *buf;
        ushort_t        *array;
} arg ;

The permission required for a semaphore operation is given as {token}, where token is the type of permission needed. The types of permission are interpreted as follows:

00400 READ by user 00200 ALTER by user 00040 READ by group 00020 ALTER by group 00004 READ by others 00002 ALTER by others

See the Semaphore Operation Permissions subsection of the DEFINITIONS section of intro(2) for more information. The following semaphore operations as specified by cmd are executed with respect to the semaphore specified by semid and semnum.

GETVAL

Return the value of semval (see intro(2)). {READ}

SETVAL

Set the value of semval to arg.val. {ALTER} When this command is successfully executed, the semadj value corresponding to the specified semaphore in all processes is cleared.

GETPID

Return the value of (int) sempid. {READ}

GETNCNT

Return the value of semncnt. {READ}

GETZCNT

Return the value of semzcnt. {READ}

The following operations return and set, respectively, every semval in the set of semaphores.

GETALL

Place semvals into array pointed to by arg.array. {READ}

SETALL

Set semvals according to the array pointed to by arg.array. {ALTER}. When this cmd is successfully executed, the semadj values corresponding to each specified semaphore in all processes are cleared.

The following operations are also available.

IPC_STAT

Place the current value of each member of the data structure associated with semid into the structure pointed to by arg.buf. The contents of this structure are defined in intro(2). {READ}

IPC_SET

Set the value of the following members of the data structure associated with semid to the corresponding value found in the structure pointed to by arg.buf:

sem_perm.uid sem_perm.gid sem_perm.mode /* access permission bits only */

This command can be executed only by a process that has an effective user ID equal to either that of super-user, or to the value of sem_perm.cuid or sem_perm.uid in the data structure associated with semid.

IPC_RMID

Remove the semaphore identifier specified by semid from the system and destroy the set of semaphores and data structure associated with it. This command can only be executed by a process that has an effective user ID equal to either that of super-user, or to the value of sem_perm.cuid or sem_perm.uid in the data structure associated with semid.

Upon successful completion, the value returned depends on cmd as follows:

GETVAL

the value of semval

GETPID

the value of (int) sempid

GETNCNT

the value of semncnt

GETZCNT

the value of semzcnt

All other successful completions return 0; otherwise, -1 is returned and errno is set to indicate the error.

The semctl() function will fail if:

EACCES

Operation permission is denied to the calling process (see intro(2)).

EFAULT

The source or target is not a valid address in the user process.

EINVAL

The semid argument is not a valid semaphore identifier; the semnum argument is less than 0 or greater than sem_nsems -1; or the cmd argument is not a valid command or is IPC_SET and sem_perm.uid or sem_perm.gid is not valid.

EPERM

The cmd argument is equal to IPC_RMID or IPC_SET and the effective user of the calling process is not super-user, or cmd is equal to the value of sem_perm.cuid or sem_perm.uid in the data structure associated with semid.

EOVERFLOW

The cmd argument is IPC_STAT and uid or gid is too large to be stored in the structure pointed to by arg.buf.

ERANGE

The cmd argument is SETVAL or SETALL and the value to which semval is to be set is greater than the system imposed maximum.

ipcs(1), intro(2), semget(2), semop(2)