SEMCTL(2)

NAME
semctl - semaphore control operations

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

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

DESCRIPTION
semctl provides a variety of semaphore control operations as specified by cmd.

The following operations are executed with respect to the semaphore specified by semid and semnum. The permission required for each operation is shown (see kedef(2)). The symbols are defined in <sys/sem.h>.

GETVAL
    Return the value of semval (see kedef(2)). Read permission required.

SETVAL
    Set the value of semval to arg.val. When this command is successfully executed, the semadj value corresponding to the specified semaphore in all processes is cleared. Alter permission is required.

GETPID
    Return the value of sempid. Read permission required.

GETNCNT
    Return the value of semncnt. Read permission.

GETZCNT
    Return the value of semzcnt. Read permission.

The following cmds operate on every semval in the set of semaphores.

GETALL
    Place semvals into array pointed to by arg.array. Read permission is needed.

SETALL
    Set semvals according to the array pointed to by arg.array. When this cmd is successfully executed the semadj values corresponding to each specified semaphore in all processes are cleared. Alter access is needed.

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. The contents of this structure are defined in kedef(2). Read permission is required.

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.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.uid in the data structure associated with semid.

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

ERRORS
semctl will fail if one or more of the following are true:

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.

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

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

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 and it is not equal to the value of sem_perm.uid in the data structure associated with semid.

EFAULT
    arg.buf points to an illegal address.

SEE ALSO
kedef(2), semget(2), semop(2).