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 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

       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 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 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.1‐2008, Section 3.342, Shared Memory Object, <sys_shm.h>

COPYRIGHT

       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 .

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       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 .