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NAME

       shmget - allocates a shared memory segment

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
                   segment.

       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_NORESERVE 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
       initialized 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
       verified, 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 >
                   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
                   reached.

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

       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
                   (SHMALL).

       EPERM       The SHM_HUGETLB flag was specified, but the caller was  not
                   privileged (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
       significant  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
                  dependent (on Linux, this limit can be read and modified via
                  /proc/sys/kernel/shmmax).

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

       SHMMNI     System  wide  maximum  number  of  shared  memory  segments:
                  implementation dependent (currently  4096,  was  128  before
                  Linux  2.3.99; on Linux, this limit can be read and modified
                  via /proc/sys/kernel/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), svipc(7)

COLOPHON

       This  page  is  part of release 2.77 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/.