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



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

NAME
       shmget -- get an XSI shared memory segment

SYNOPSIS
       #include <sys/shm.h>

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

DESCRIPTION
       The shmget() function operates on XSI shared memory (see the Base
       Definitions volume of POSIX.1-2017, Section 3.346, 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
       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
       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
              are to 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
       None.

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
       None.

SEE ALSO
       Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime,
       ftok(), shmat(), shmctl(), shmdt(), shm__open(), shm__unlink()

       The Base Definitions volume of POSIX.1-2017, Section 3.346, Shared
       Memory Object, <sys_shm.h>

COPYRIGHT
       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                            SHMGET(3)
