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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 nonportable 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), shm_overview(7), svipc(7)

COLOPHON

       This  page  is  part of release 3.35 of the Linux man-pages project.  A description of the
       project, and information about reporting bugs, can be found at  http://man7.org/linux/man-
       pages/.