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

       shmget - allocates a System V shared memory segment

       #include <sys/ipc.h>
       #include <sys/shm.h>

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

       shmget()	 returns  the identifier of the System V 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 Linux
		   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_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.

       On success, a valid shared memory identifier is returned.  On error, -1
       is returned, and errno is set to indicate the error.

       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

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

       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).

       SVr4, POSIX.1-2001.

       SHM_HUGETLB is a nonportable Linux extension.

       The inclusion of <sys/types.h> and <sys/ipc.h> isn't required on	 Linux
       or by any version of POSIX.  However, some old implementations required
       the inclusion of these header files, and the SVID also documented their
       inclusion.   Applications  intended  to be portable to such old systems
       may need to include these header files.

       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.   Since  Linux  2.2,
	      the default value for this limit is


	      Assuming	a 4kB page size, this formula yields (since Linux 2.4)
	      the value 2^20 (2,097,152).

	      On Linux, this limit can be read and modified via /proc/sys/ker‐

       SHMMAX Maximum  size in bytes for a shared memory segment.  Since Linux
	      2.2, the default value of this limit is 0x2000000 (32MB).

	      On Linux, this limit can be read and modified via /proc/sys/ker‐

       SHMMIN Minimum  size  in bytes for a shared memory segment: implementa‐
	      tion dependent (currently 1 byte, though PAGE_SIZE is the effec‐
	      tive minimum size).

       SHMMNI System  wide maximum number of shared memory segments.  In Linux
	      2.2, the default value for this limit was 128; since Linux  2.4,
	      the default value is 4096.

	      On Linux, this limit can be read and modified via /proc/sys/ker‐

       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 3.65 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

Linux				  2014-04-17			     SHMGET(2)

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