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

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

       shmget — get an XSI shared memory segment

SYNOPSIS         top

       #include <sys/shm.h>

       int shmget(key_t key, size_t size, int shmflg);

DESCRIPTION         top

       The shmget() function operates on XSI shared memory (see the Base
       Definitions volume of POSIX.1‐2008, Section 3.342, Shared Memory
       Object).  It is unspecified whether this function interoperates with
       the realtime interprocess communication facilities defined in Section
       2.8, Realtime.

       The shmget() function shall return the shared memory identifier
       associated with key.

       A shared memory identifier, associated data structure, and shared
       memory segment of at least size bytes (see <sys/shm.h>) are 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 shared memory identifier
           associated with it and (shmflg &IPC_CREAT) is non-zero.

       Upon creation, the data structure associated with the new shared
       memory identifier shall be initialized as follows:

        *  The values of shm_perm.cuid, shm_perm.uid, shm_perm.cgid, and
           shm_perm.gid are set to the effective user ID and effective group
           ID, respectively, of the calling process.

        *  The low-order nine bits of shm_perm.mode are set to the low-order
           nine bits of shmflg.

        *  The value of shm_segsz is set to the value of size.

        *  The values of shm_lpid, shm_nattch, shm_atime, and shm_dtime are
           set to 0.

        *  The value of shm_ctime is set to the current time, as described
           in Section 2.7.1, IPC General Description.

       When the shared memory segment is created, it shall be initialized
       with all zero values.

RETURN VALUE         top

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

ERRORS         top

       The shmget() function shall fail if:

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

       EEXIST A shared memory identifier exists for the argument key but
              (shmflg &IPC_CREAT) &&(shmflg &IPC_EXCL) is non-zero.

       EINVAL A shared memory segment is to be created and the value of size
              is less than the system-imposed minimum or greater than the
              system-imposed maximum.

       EINVAL No shared memory segment is to be created and a shared memory
              segment exists for key but the size of the segment associated
              with it is less than size.

       ENOENT A shared memory identifier does not exist for the argument key
              and (shmflg &IPC_CREAT) is 0.

       ENOMEM A shared memory identifier and associated shared memory
              segment shall be created, but the amount of available physical
              memory is not sufficient to fill the request.

       ENOSPC A shared memory identifier is to be created, but the system-
              imposed limit on the maximum number of allowed shared memory
              identifiers system-wide would be exceeded.

       The following sections are informative.

EXAMPLES         top

       None.

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

       None.

SEE ALSO         top

       Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime,
       ftok(3p), shmat(3p), shmctl(3p), shmdt(3p), shm_open(3p),
       shm_unlink(3p)

       The Base Definitions volume of POSIX.1‐2008, Section 3.342, Shared
       Memory Object, sys_shm.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                          SHMGET(3P)

Pages that refer to this page: sys_shm.h(0p)ipcs(1p)_Exit(3p)ftok(3p)shmat(3p)shmctl(3p)shmdt(3p)