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shmget(3posix)


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.12008,  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.12008, 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 .

   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 .



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