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

       semget — get set of XSI semaphores

SYNOPSIS

       #include <sys/sem.h>

       int semget(key_t key, int nsems, int semflg);

DESCRIPTION

       The  semget()  function  operates  on  XSI  semaphores  (see the Base Definitions volume of POSIX.1‐2008,
       Section 4.16, Semaphore).  It is unspecified  whether  this  function  interoperates  with  the  realtime
       interprocess communication facilities defined in Section 2.8, Realtime.

       The semget() function shall return the semaphore identifier associated with key.

       A  semaphore  identifier  with  its  associated  semid_ds  data structure and its associated set of nsems
       semaphores (see <sys/sem.h>) is created for key if one of the following is true:

        *  The argument key is equal to IPC_PRIVATE.

        *  The argument key does not already  have  a  semaphore  identifier  associated  with  it  and  (semflg
           &IPC_CREAT) is non-zero.

       Upon creation, the semid_ds data structure associated with the new semaphore identifier is initialized as
       follows:

        *  In the operation permissions structure sem_perm.cuid, sem_perm.uid, sem_perm.cgid,  and  sem_perm.gid
           shall be set to the effective user ID and effective group ID, respectively, of the calling process.

        *  The low-order 9 bits of sem_perm.mode shall be set to the low-order 9 bits of semflg.

        *  The variable sem_nsems shall be set to the value of nsems.

        *  The variable sem_otime shall be set to 0 and sem_ctime shall be set to the current time, as described
           in Section 2.7.1, IPC General Description.

        *  The data structure associated with each semaphore in the set need not be  initialized.  The  semctl()
           function with the command SETVAL or SETALL can be used to initialize each semaphore.

RETURN VALUE

       Upon  successful completion, semget() shall return a non-negative integer, namely a semaphore identifier;
       otherwise, it shall return −1 and set errno to indicate the error.

ERRORS

       The semget() function shall fail if:

       EACCES A semaphore identifier exists for key, but operation permission as specified by  the  low-order  9
              bits of semflg would not be granted; see Section 2.7, XSI Interprocess Communication.

       EEXIST A  semaphore identifier exists for the argument key but ((semflg &IPC_CREAT) &&(semflg &IPC_EXCL))
              is non-zero.

       EINVAL The value of nsems is either less than or equal to 0 or greater than the system-imposed limit,  or
              a  semaphore  identifier  exists  for  the  argument  key, but the number of semaphores in the set
              associated with it is less than nsems and nsems is not equal to 0.

       ENOENT A semaphore identifier does not exist for the argument key and (semflg &IPC_CREAT) is equal to 0.

       ENOSPC A semaphore identifier is to be created but the system-imposed limit  on  the  maximum  number  of
              allowed semaphores system-wide would be exceeded.

       The following sections are informative.

EXAMPLES

       Refer to semop().

APPLICATION USAGE

       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

       A future version may require that the value of the semval, sempid, semncnt, and semzcnt  members  of  all
       semaphores  in  a  semaphore  set be initialized to zero when a call to semget() creates a semaphore set.
       Many semaphore implementations already do this and it greatly simplifies what an application must  do  to
       initialize a semaphore set.

SEE ALSO

       Section   2.7,  XSI  Interprocess  Communication,  Section  2.8,  Realtime,  ftok(),  semctl(),  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 Electrical 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 .

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