semctl(S)                      6 January 1993                      semctl(S)


 Name

    semctl - semaphore control operations

 Syntax


    cc  . . .  -lc


    #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 semctl system call 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. {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 cor-
                responding to each specified semaphore in all processes are
                cleared.

    The following cmds are also available:

    IPCSTAT    Place the current value of each member of the data structure
                associated with semid into the structure pointed to by
                arg.buf.  {READ}

    IPCSET     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 effec-
                tive user ID equal to either that of super-user, or to the
                value of semperm.cuid or semperm.uid in the data structure
                associated with semid.

    IPCRMID    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 pro-
                cess that has an effective user ID equal to either that of
                super-user, or to the value of semperm.cuid or semperm.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 semnsems.

    [EINVAL]    cmd is not a valid command.

    [EPERM]     cmd is equal to IPCRMID or IPCSET and the effective user ID
                of the calling process is not equal to that of super-user or
                to the value of semperm.cuid or semperm.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

    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

    semget(S), semop(S)

 Standards conformance

    semctl is conformant with:
    AT&T SVID Issue 2;
    and X/Open Portability Guide, Issue 3, 1989.