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



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

NAME
       semctl -- XSI semaphore control operations

SYNOPSIS
       #include <sys/sem.h>

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

DESCRIPTION
       The semctl() function operates on XSI semaphores (see the Base
       Definitions volume of POSIX.1-2017, Section 4.17, 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;

       Each operation shall be performed atomically.

       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
       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
       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
       Refer to semop().

APPLICATION USAGE
       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
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime,
       semget(), semop(), sem__close(), sem__destroy(), sem__getvalue(),
       sem__init(), sem__open(), sem__post(), sem__trywait(), sem__unlink()

       The Base Definitions volume of POSIX.1-2017, Section 4.17, Semaphore,
       <sys_sem.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  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.opengroup.org/unix/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                  2017                            SEMCTL(3)
