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



NAME
       shmget - allocates a System V 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 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   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	   This flag is used with IPC_CREAT to ensure that  this  call
		   creates  the	 segment.   If the segment already exists, the
		   call fails.

       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_HUGE_2MB, SHM_HUGE_1GB (since Linux 3.8)
		   Used in conjunction with SHM_HUGETLB to select  alternative
		   hugetlb page sizes (respectively, 2 MB and 1 GB) on systems
		   that support multiple hugetlb page sizes.

		   More generally, the desired huge page size can  be  config-
		   ured	 by  encoding the base-2 logarithm of the desired page
		   size in the six bits at the offset  SHM_HUGE_SHIFT.	 Thus,
		   the above two constants are defined as:

		       #define SHM_HUGE_2MB    (21 << SHM_HUGE_SHIFT)
		       #define SHM_HUGE_1GB    (30 << SHM_HUGE_SHIFT)

		   For some additional details, see the discussion of the sim-
		   ilarly named constants in mmap(2).

       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).

       In addition to the above flags, the least significant 9 bits of	shmflg
       specify the permissions granted to the owner, group, and others.	 These
       bits have the same format, and the same meaning, as the	mode  argument
       of open(2).  Presently, execute permissions are not used by the system.

       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.

RETURN VALUE
       On success, a valid shared memory identifier is returned.  On error, -1
       is returned, and errno is set to indicate the 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 in the
	      user namespace that governs its IPC namespace.

       EEXIST IPC_CREAT and IPC_EXCL were specified in shmflg,	but  a	shared
	      memory segment already exists for key.

       EINVAL A	 new segment was to be created and size is less than SHMMIN or
	      greater than SHMMAX.

       EINVAL A segment for the given key exists, but size is greater than the
	      size of that segment.

       ENFILE The system-wide 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
       POSIX.1-2001, POSIX.1-2008, SVr4.

       SHM_HUGETLB and SHM_NORESERVE are Linux extensions.

NOTES
       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 all but the least significant
       9 bits of shmflg and creates a new shared memory segment.

   Shared memory limits
       The following limits on shared  memory  segment	resources  affect  the
       shmget() call:

       SHMALL System-wide limit on the total amount of shared memory, measured
	      in units of the system page size.

	      On Linux, this limit can be read and modified via /proc/sys/ker-
	      nel/shmall.   Since Linux 3.16, the default value for this limit
	      is:

		  ULONG_MAX - 2^24

	      The effect of this value (which is suitable for both 32-bit  and
	      64-bit systems) is to impose no limitation on allocations.  This
	      value, rather than ULONG_MAX, was chosen as the default to  pre-
	      vent  some cases where historical applications simply raised the
	      existing limit without first checking its current	 value.	  Such
	      applications  would cause the value to overflow if the limit was
	      set at ULONG_MAX.

	      From Linux 2.4 up to Linux 3.15,	the  default  value  for  this
	      limit was:

		  SHMMAX / PAGE_SIZE * (SHMMNI / 16)

	      If  SHMMAX  and  SHMMNI  were not modified, then multiplying the
	      result of this formula by the page  size	(to  get  a  value  in
	      bytes)  yielded a value of 8 GB as the limit on the total memory
	      used by all shared memory segments.

       SHMMAX Maximum size in bytes for a shared memory segment.

	      On Linux, this limit can be read and modified via /proc/sys/ker-
	      nel/shmmax.   Since Linux 3.16, the default value for this limit
	      is:

		  ULONG_MAX - 2^24

	      The effect of this value (which is suitable for both 32-bit  and
	      64-bit  systems) is to impose no limitation on allocations.  See
	      the description of SHMALL for a discussion of why	 this  default
	      value (rather than ULONG_MAX) is used.

	      From Linux 2.2 up to Linux 3.15, the default value of this limit
	      was 0x2000000 (32MB).

	      Because it is not possible to map just part of a	shared	memory
	      segment,	the  amount  of virtual memory places another limit on
	      the maximum size of a usable segment: for example, on  i386  the
	      largest  segments	 that  can be mapped have a size of around 2.8
	      GB, and on x86_64 the limit is around 127 TB.

       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 limit on the 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-
	      nel/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
       memfd_create(2),	 shmat(2),  shmctl(2),	shmdt(2),  ftok(3),  capabili-
       ties(7), shm_overview(7), svipc(7)

COLOPHON
       This  page  is  part of release 4.10 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at
       https://www.kernel.org/doc/man-pages/.



Linux				  2016-10-08			     SHMGET(2)