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

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
       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 Defi-
       nitions  volume  of  IEEE Std 1003.1-2001, Section 3.340, Shared Memory
       Object). It is unspecified whether this function interoperates with the
       realtime interprocess communication facilities defined in Realtime .
       The shmget() function shall return the shared memory identifier associ-
       ated with key.
       A shared memory identifier, associated data structure, and shared  mem-
       ory  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  equal 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 equal to  the  low-
          order nine bits of shmflg.
        * The value of shm_segsz is set equal to the value of size.
        * The values of shm_lpid, shm_nattch, shm_atime, and shm_dtime are set
          equal to 0.
        * The value of shm_ctime is set equal to the current time.
       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 inte-
       ger, 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 permis-
              sion as specified by the low-order nine bits of shmflg would not
              be granted; see XSI Interprocess Communication .
       EEXIST A shared memory identifier exists for the argument key but (shm-
              flg &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 sys-
              tem-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 and size is not 0.
       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
       None.
APPLICATION USAGE
       The  POSIX Realtime Extension defines alternative interfaces for inter-
       process communication. Application  developers  who  need  to  use  IPC
       should design their applications so that modules using the IPC routines
       described in XSI Interprocess Communication can be easily  modified  to
       use the alternative interfaces.
RATIONALE
       None.
FUTURE DIRECTIONS
       None.
SEE ALSO
       XSI  Interprocess  Communication, Realtime, shmat(), shmctl(), shmdt(),
       shm_open(),   shm_unlink(),   the   Base    Definitions    volume    of
       IEEE Std 1003.1-2001, <sys/shm.h>
COPYRIGHT
       Portions  of  this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
       --  Portable  Operating  System  Interface (POSIX), The Open Group Base
       Specifications Issue 6, Copyright (C) 2001-2003  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 .

IEEE/The Open Group                  2003                           SHMGET(3P)