semctl(S)

Syntax

#include <sys/types.h>
#include  <sys/ipc.h>
#include  <sys/sem.h>

int semctl (semid, semnum, cmd, arg)
int semid, cmd;
int semnum;
union semun {
     int val;
     struct semid_ds *buf;
     ushort *array;
} arg;

Description

The following cmds are executed with respect to the semaphore specified by semid and semnum:

GETVAL

Return the value of semval. {READ}

SETVAL

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

GETPID

Return the value of sempid. {READ}

GETNCNT

Return the value of semncnt. {READ}

GETZCNT

Return the value of semzcnt. {READ}

The following cmds 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 cmds 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. {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 /* only low 9 bits */

This cmd 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.

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 cmd 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.

The semctl system call fails if one or more of the following is true:

[EACCES]

Operation permission is denied to the calling process.

[EINVAL]

semid is not a valid semaphore identifier.

[EINVAL]

semnum is less than zero or greater than sem_nsems.

[EINVAL]

cmd is not a valid command.

[EPERM]

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

[ERANGE]

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

Diagnostics

GETVAL	The value of semval.
GETPID	The value of sempid.
GETNCNT	The value of semncnt.
GETZCNT	The value of semzcnt.
All others	A value of 0.

 GETVAL        The value of semval.
 GETPID        The value of sempid.
 GETNCNT       The value of semncnt.
 GETZCNT       The value of semzcnt.
 All others    A value of 0.

Otherwise, a value of -1 is returned, and errno is set to indicate the error.

See also

Standards conformance

X/Open Portability Guide, Issue 3, 1989 .