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



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 per-
                   missions 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 seg-
                   ment.  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 call-
              ing 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  signifi-
              cant 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 depen-
              dent (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 3.22 of the Linux man-pages project.  A description of
       the  project, and information about reporting bugs, can be found at http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2006-05-02                         SHMGET(2)

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