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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 Allocate  the  segment  using "huge pages."  See the kernel
                   source file  Documentation/vm/hugetlbpage.txt  for  further
                   information.

       If  a  new  shared  memory segment is created, then its associated data
       structure shmid_ds (see shmctl(2)) is initialised 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.

SYSTEM CALLS

       fork() After a fork() the child inherits  the  attached  shared  memory
              segments.

       exec() After an exec() all attached shared memory segments are detached
              (not destroyed).

       exit() Upon exit() all attached shared  memory  segments  are  detached
              (not destroyed).

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).

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).

BUGS

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

CONFORMING TO

       SVr4,  SVID.   SVr4  documents  an  additional  error condition EEXIST.
       Until version 2.3.30 Linux would return  EIDRM  for  a  shmget()  on  a
       shared memory segment scheduled for deletion.

       SHM_HUGETLB is a non-portable Linux extension.

SEE ALSO

       shmat(2), shmctl(2), shmdt(2), ftok(3), ipc(5), capabilities(7)