PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

SEMCTL(3P)                POSIX Programmer's Manual               SEMCTL(3P)

PROLOG         top

       This manual page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior), or
       the interface may not be implemented on Linux.

NAME         top

       semctl — XSI semaphore control operations

SYNOPSIS         top

       #include <sys/sem.h>

       int semctl(int semid, int semnum, int cmd, ...);

DESCRIPTION         top

       The semctl() function operates on XSI semaphores (see the Base
       Definitions volume of POSIX.1‐2008, Section 4.16, Semaphore).  It is
       unspecified whether this function interoperates with the realtime
       interprocess communication facilities defined in Section 2.8,
       Realtime.

       The semctl() function provides a variety of semaphore control
       operations as specified by cmd.  The fourth argument is optional and
       depends upon the operation requested. If required, it is of type
       union semun, which the application shall explicitly declare:

           union semun {
               int val;
               struct semid_ds *buf;
               unsigned short  *array;
           } arg;

       The following semaphore control operations as specified by cmd are
       executed with respect to the semaphore specified by semid and semnum.
       The level of permission required for each operation is shown with
       each command; see Section 2.7, XSI Interprocess Communication.  The
       symbolic names for the values of cmd are defined in the <sys/sem.h>
       header:

       GETVAL      Return the value of semval; see <sys/sem.h>.  Requires
                   read permission.

       SETVAL      Set the value of semval to arg.val, where arg is the
                   value of the fourth argument to semctl().  When this
                   command is successfully executed, the semadj value
                   corresponding to the specified semaphore in all processes
                   is cleared. Also, the sem_ctime timestamp shall be set to
                   the current time, as described in Section 2.7.1, IPC
                   General Description.  Requires alter permission; see
                   Section 2.7, XSI Interprocess Communication.

       GETPID      Return the value of sempid.  Requires read permission.

       GETNCNT     Return the value of semncnt.  Requires read permission.

       GETZCNT     Return the value of semzcnt.  Requires read permission.

       The following values of cmd operate on each semval in the set of
       semaphores:

       GETALL      Return the value of semval for each semaphore in the
                   semaphore set and place into the array pointed to by
                   arg.array, where arg is the fourth argument to semctl().
                   Requires read permission.

       SETALL      Set the value of semval for each semaphore in the
                   semaphore set according to the array pointed to by
                   arg.array, where arg is the fourth argument to semctl().
                   When this command is successfully executed, the semadj
                   values corresponding to each specified semaphore in all
                   processes are cleared. Also, the sem_ctime timestamp
                   shall be set to the current time, as described in Section
                   2.7.1, IPC General Description.  Requires alter
                   permission.

       The following values of cmd are also available:

       IPC_STAT    Place the current value of each member of the semid_ds
                   data structure associated with semid into the structure
                   pointed to by arg.buf, where arg is the fourth argument
                   to semctl().  The contents of this structure are defined
                   in <sys/sem.h>.  Requires read permission.

       IPC_SET     Set the value of the following members of the semid_ds
                   data structure associated with semid to the corresponding
                   value found in the structure pointed to by arg.buf, where
                   arg is the fourth argument to semctl():

                       sem_perm.uid
                       sem_perm.gid
                       sem_perm.mode

                   The mode bits specified in Section 2.7.1, IPC General
                   Description are copied into the corresponding bits of the
                   sem_perm.mode associated with semid.  The stored values
                   of any other bits are unspecified. The sem_ctime
                   timestamp shall be set to the current time, as described
                   in Section 2.7.1, IPC General Description.

                   This command can only be executed by a process that has
                   an effective user ID equal to either that of a process
                   with appropriate privileges or to the value of
                   sem_perm.cuid or sem_perm.uid in the semid_ds data
                   structure associated with semid.

       IPC_RMID    Remove the semaphore identifier specified by semid from
                   the system and destroy the set of semaphores and semid_ds
                   data structure associated with it. This command can only
                   be executed by a process that has an effective user ID
                   equal to either that of a process with appropriate
                   privileges or to the value of sem_perm.cuid or
                   sem_perm.uid in the semid_ds data structure associated
                   with semid.

RETURN VALUE         top

       If successful, 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.

       Otherwise, semctl() shall return −1 and set errno to indicate the
       error.

ERRORS         top

       The semctl() function shall fail if:

       EACCES Operation permission is denied to the calling process; see
              Section 2.7, XSI Interprocess Communication.

       EINVAL The value of semid is not a valid semaphore identifier, or the
              value of semnum is less than 0 or greater than or equal to
              sem_nsems, or the value of cmd is not a valid command.

       EPERM  The argument cmd is equal to IPC_RMID or IPC_SET and the
              effective user ID of the calling process is not equal to that
              of a process with appropriate privileges and it is not equal
              to the value of sem_perm.cuid or sem_perm.uid in the data
              structure associated with semid.

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

       The following sections are informative.

EXAMPLES         top

       Refer to semop(3p).

APPLICATION USAGE         top

       The fourth parameter in the SYNOPSIS section is now specified as
       "..." in order to avoid a clash with the ISO C standard when
       referring to the union semun (as defined in Issue 3) and for
       backwards-compatibility.

       The POSIX Realtime Extension defines alternative interfaces for
       interprocess communication. Application developers who need to use
       IPC should design their applications so that modules using the IPC
       routines described in Section 2.7, XSI Interprocess Communication can
       be easily modified to use the alternative interfaces.

RATIONALE         top

       None.

FUTURE DIRECTIONS         top

       None.

SEE ALSO         top

       Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime,
       semget(3p), semop(3p), sem_close(3p), sem_destroy(3p),
       sem_getvalue(3p), sem_init(3p), sem_open(3p), sem_post(3p),
       sem_trywait(3p), sem_unlink(3p)

       The Base Definitions volume of POSIX.1‐2008, Section 4.16, Semaphore,
       sys_sem.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) In the event of any discrepancy between this version and
       the original IEEE and The Open Group Standard, the original IEEE and
       The Open Group Standard is the referee document. The original
       Standard can be obtained online at http://www.unix.org/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the
       source files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                 2013                          SEMCTL(3P)

Pages that refer to this page: sys_sem.h(0p)ipcrm(1p)sem_close(3p)sem_destroy(3p)semget(3p)sem_getvalue(3p)semop(3p)sem_open(3p)sem_post(3p)sem_timedwait(3p)sem_trywait(3p)sem_unlink(3p)