PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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

       semget — get set of XSI semaphores

SYNOPSIS         top

       #include <sys/sem.h>

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

DESCRIPTION         top

       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         top

       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         top

       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         top

       Refer to semop(3p).

APPLICATION USAGE         top

       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

       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         top

       Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime,
       ftok(3p), semctl(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.1‐2008, Section 4.16, Semaphore,
       sys_sem.h(0p)

COPYRIGHT         top

       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 .

       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)

Pages that refer to this page: sys_sem.h(0p)ipcs(1p)ftok(3p)sem_close(3p)semctl(3p)sem_destroy(3p)sem_getvalue(3p)semop(3p)sem_open(3p)sem_post(3p)sem_timedwait(3p)sem_trywait(3p)sem_unlink(3p)sigaction(3p)