semctl(3p) - phpMan

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  Defini-
       tions volume of POSIX.1-2008, Section 4.16, Semaphore).  It is unspeci-
       fied 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 opera-
       tions 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 exe-
       cuted with respect to the semaphore specified by semid and semnum.  The
       level of permission required for each operation is shown with each com-
       mand;  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 Interpro-
                   cess 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  sema-
       phores:
       GETALL      Return  the value of semval for each semaphore in the sema-
                   phore 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  Descrip-
                   tion.  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 sem-
                   ctl().  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 exe-
                   cuted 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 fol-
       lows:
       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 Sec-
              tion 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  effec-
              tive  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  inter-
       process  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.1-2008, Section  4.16,  Semaphore,
       <sys_sem.h>
COPYRIGHT
       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 Electri-
       cal  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.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013                           SEMCTL(3P)