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 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 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
       A valid segment identifier, shmid, is returned on success, -1 on 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 non-portable Linux extension.

NOTES
       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).

       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 depen-
       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 3.23 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)
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