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SHMGET(2)                  Linux Programmer's Manual                 SHMGET(2)

       shmget - allocates a shared memory segment

       #include <&lt;sys/ipc.h>&gt;
       #include <&lt;sys/shm.h>&gt;

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

       shmget() returns the identifier of the 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-

       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 kernel
                   source file  Documentation/vm/hugetlbpage.txt  for  further

       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_perm.cuid  and shm_perm.uid are set to the effective user ID
              of the calling process.

              shm_perm.cgid and shm_perm.gid are set to the effective group ID
              of the calling process.

              The  least  significant  9  bits of shm_perm.mode are set to the
              least significant 9 bit of shmflg.

              shm_segsz is set to the value of size.

              shm_lpid, shm_nattch, shm_atime and shm_dtime are set to 0.

              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.

       A valid segment identifier, shmid, is returned on success, -1 on error.

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

       EACCES      The user does not have permission to access the shared mem-
                   ory segment, and does not have the  CAP_IPC_OWNER  capabil-

       EEXIST      IPC_CREAT | IPC_EXCL was specified and the segment exists.

       EINVAL      A new segment was to be created and size < SHMMIN or size >
                   SHMMAX, 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

       ENOENT      No  segment exists for the given key, and IPC_CREAT was not

       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
                   system to exceed the system-wide  limit  on  shared  memory

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

       SVr4, POSIX.1-2001.

       SHM_HUGETLB is a non-portable Linux extension.

       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

       The  following  limits  on  shared  memory segment resources affect the
       shmget() call:

       SHMALL     System wide maximum of shared memory pages (on  Linux,  this
                  limit can be read and modified via /proc/sys/kernel/shmall).

       SHMMAX     Maximum  size  in  bytes for a shared memory segment: policy
                  dependent (on Linux, this limit can be read and modified via

       SHMMIN     Minimum size in bytes for a shared memory segment: implemen-
                  tation dependent (currently 1 byte, though PAGE_SIZE is  the
                  effective minimum size).

       SHMMNI     System wide maximum number of shared memory segments: imple-
                  mentation dependent (currently 4096, was  128  before  Linux
                  2.3.99;  on  Linux,  this limit can be read and modified via

       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.

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

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

       This page is part of release 3.05 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                             2006-05-02                         SHMGET(2)