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 {
	val;
	struct semid_ds *buf;
	ushort *array;
} arg;

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

The following cmds 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 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 the 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. 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 /* 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.

In the semop(2) and semctl(2) system call descriptions, the permission required for an operation is given as "[token]", where “token” is the type of permission needed interpreted as follows:

    00400
        Read by user
    00200
        Alter by user
    00060
        Read, Alter by group
    00006
        Read, Alter by others

Read and Alter permissions on a semid are granted to a process if one or more of the following are true:

    The effective user ID of the process is super-user.

    The effective user ID of the process matches sem_perm.[c]uid in the data structure associated with semid and the appropriate bit of the “user” portion (0600) of sem_perm.mode is set.

    The effective user ID of the process does not match sem_perm.[c]uid and the effective group ID of the process matches sem_perm.[c]gid and the appropriate bit of the “group” portion (060) of sem_perm.mode is set.

    The effective user ID of the process does not match sem_perm.[c]uid and the effective group ID of the process does not match sem_perm.[c]gid and the appropriate bit of the “other” portion (06) of sem_perm.mode is set.

Otherwise, the corresponding permissions are denied.

RETURN VALUES

On success, the value returned by semctl() 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
    0.

On failure, semctl() returns -1 and sets errno to indicate the error.

ERRORS

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

EFAULT
    arg.buf points to an illegal address.

EINVAL
    semid is not a valid semaphore identifier.

    semnum is less than zero or greater than sem_nsems.

    cmd is not a valid command.

EPERM
    cmd is IPC_RMID or IPC_SET and the effective user ID of the calling process is not super-user.

    cmd is IPC_RMID or IPC_SET and the effective user ID of the calling process is not 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.

SEE ALSO
intro(2), semget(2), semop(2), ipcrm(1), ipcs(1)