shmget


SYNOPSIS
       #include <sys/ipc.h>
       #include <sys/shm.h>

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

DESCRIPTION
       shmget()  returns  the identifier of the System V shared memory segment
       associated with the value of the argument key.   A  new  shared  memory
       segment,  with size equal to the value of size rounded up to a multiple
       of PAGE_SIZE, is created if key has the value IPC_PRIVATE or key  isn't
       IPC_PRIVATE,  no shared memory segment corresponding to key exists, and
       IPC_CREAT is specified in shmflg.

       If shmflg specifies both IPC_CREAT and IPC_EXCL  and  a  shared  memory
       segment  already  exists for key, then shmget() fails with errno set to
       EEXIST.  (This is analogous to the effect of the combination O_CREAT  |
       O_EXCL for open(2).)

       The value shmflg is composed of:

       IPC_CREAT   to  create  a  new segment.  If this flag is not used, then
                   shmget() will find the  segment  associated  with  key  and
                   check  to see if the user has permission to access the seg-
                   ment.

       IPC_EXCL    used with  IPC_CREAT  to  ensure  failure  if  the  segment
                   already exists.

       mode_flags  (least  significant  9  bits)  specifying  the  permissions
                   granted to the owner, group, and world.   These  bits  have
                   the same format, and the same meaning, as the mode argument
                   of open(2).  Presently, the  execute  permissions  are  not
                   used by the system.

       SHM_HUGETLB (since Linux 2.6)
                   Allocate  the  segment  using  "huge pages."  See the Linux
                   kernel  source  file  Documentation/vm/hugetlbpage.txt  for
                   further information.

       SHM_NORESERVE (since Linux 2.6.15)
                   This  flag serves the same purpose as the mmap(2) MAP_NORE-
                   SERVE flag.  Do not reserve swap space  for  this  segment.
                   When  swap space is reserved, one has the guarantee that it
                   is possible to modify the segment.  When swap space is  not
                   reserved  one might get SIGSEGV upon a write if no physical
                   memory is available.  See also the discussion of  the  file
                   /proc/sys/vm/overcommit_memory in proc(5).

       When  a new shared memory segment is created, its contents are initial-
       ized to zero values, and its associated data structure,  shmid_ds  (see
       shmctl(2)), is initialized as follows:


              shm_ctime is set to the current time.

       If the shared memory segment already exists, the permissions are  veri-
       fied, and a check is made to see if it is marked for destruction.

RETURN VALUE
       On success, a valid shared memory identifier is returned.  On errir, -1
       is returned, and errno is set to indicate the error.

ERRORS
       On failure, errno is set to one of the following:

       EACCES The user does not have permission to access  the  shared  memory
              segment, and does not have the CAP_IPC_OWNER capability.

       EEXIST IPC_CREAT | IPC_EXCL was specified and the segment exists.

       EINVAL A new segment was to be created and size < SHMMIN or size > SHM-
              MAX, or no new segment was to be created, a segment  with  given
              key existed, but size is greater than the size of that segment.

       ENFILE The  system  limit  on  the  total number of open files has been
              reached.

       ENOENT No segment exists for the given key, and IPC_CREAT was not spec-
              ified.

       ENOMEM No memory could be allocated for segment overhead.

       ENOSPC All  possible  shared  memory  IDs  have been taken (SHMMNI), or
              allocating a segment of the requested size would cause the  sys-
              tem to exceed the system-wide limit on shared memory (SHMALL).

       EPERM  The SHM_HUGETLB flag was specified, but the caller was not priv-
              ileged (did not have the CAP_IPC_LOCK capability).

CONFORMING TO
       SVr4, POSIX.1-2001.

       SHM_HUGETLB is a nonportable Linux extension.

NOTES
       The inclusion of <sys/types.h> and <sys/ipc.h> isn't required on  Linux
       or by any version of POSIX.  However, some old implementations required
       the inclusion of these header files, and the SVID also documented their
       inclusion.   Applications  intended  to be portable to such old systems
       may need to include these header files.

       IPC_PRIVATE isn't a flag field but a key_t type.  If this special value
       is  used for key, the system call ignores everything but the least sig-
       nificant 9 bits of shmflg and creates a new shared memory  segment  (on
       success).

              tive minimum size).

       SHMMNI System  wide maximum number of shared memory segments: implemen-
              tation dependent (currently 4096, was 128 before  Linux  2.3.99;
              on Linux, this limit can be read and modified via /proc/sys/ker-
              nel/shmmni).

       The implementation has no specific limits for the  per-process  maximum
       number of shared memory segments (SHMSEG).

   Linux notes
       Until  version  2.3.30  Linux  would  return  EIDRM for a shmget() on a
       shared memory segment scheduled for deletion.

BUGS
       The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more
       clearly show its function.

SEE ALSO
       shmat(2),  shmctl(2),  shmdt(2),  ftok(3),  capabilities(7),  shm_over-
       view(7), svipc(7)

COLOPHON
       This page is part of release 3.54 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2013-04-19                         SHMGET(2)
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