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, cmd;
int semnum;
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 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 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.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.

    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 intro(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.

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.

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