semctl

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



PROLOG
       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
       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(3P)