posix man page for semget

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

PROLOG
       This  manual page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (consult the correspond-
       ing 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 unspec-
       ified 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  simpli-
       fies 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 Technol-
       ogy  --	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 Stan-
       dard 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.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE
/The Open Group						       2013								SEMGET(3P)