This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       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  Section2.7, XSIInterprocessCommunication can be easily modified to use the alternative
       interfaces.

       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(3POSIX)

       The  semctl()  function  operates  on  XSI  semaphores  (see the Base Definitions volume of POSIX.1‐2017,
       Section4.17, Semaphore).  It is unspecified  whether  this  function  interoperates  with  the  realtime
       interprocess communication facilities defined in Section2.8, 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 unionsemun,
       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  Section2.7, XSIInterprocessCommunication.  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 Section2.7.1, IPCGeneralDescription.  Requires alter
                   permission; see Section2.7, XSIInterprocessCommunication.

       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 Section2.7.1, IPCGeneralDescription.  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 Section2.7.1,  IPCGeneralDescription  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 Section2.7.1, IPCGeneralDescription.

                   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.

       The semctl() function shall fail if:

       EACCES Operation permission is  denied  to  the  calling  process;  see  Section2.7,  XSIInterprocessCommunication.

       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.

       Thefollowingsectionsareinformative.

       Refer to semop().

       None.

       semctl — XSI semaphore control operations

       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.

       None.

       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.

Section2.7, XSIInterprocessCommunication, Section2.8, 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, Section4.17, Semaphore, <sys_sem.h>

       #include <sys/sem.h>

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