semctl(3p) — Linux manual page

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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.1‐2017, 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         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.1‐2017, Section 4.17,
       Semaphore, sys_sem.h(0p)

COPYRIGHT         top

       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)

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)