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 speci-
       fied 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 permis-
                   sions 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 seg-
                   ment.  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  struc-
       ture, 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  capa-
              bility.

       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 every-
       thing 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/ker-
              nel/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 3.25 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/.



Linux                                                      2006-05-02                                                  SHMGET(2)