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
       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 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
          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 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  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  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   interprocess
       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 .