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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 3.27 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.