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

       shmget - allocates a 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 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

       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 Allocate the segment using "huge pages."  See the kernel
                   source file Documentation/vm/hugetlbpage.txt for further

       If a new shared memory segment is created, then its associated data
       structure shmid_ds (see shmctl(2)) is initialised 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.

       fork() After a fork() the child inherits the attached shared memory

       exec() After an exec() all attached shared memory segments are detached
              (not destroyed).

       exit() Upon exit() all attached shared memory segments are detached (not

       A valid segment identifier, shmid, is returned on success, -1 on 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 >
                   SHMMAX, or no new segment was to be created, a segment with
                   given key existed, but size is greater than the size of that

       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

       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

       EPERM       The SHM_HUGETLB flag was specified, but the caller was not
                   privileged (did not have the CAP_IPC_LOCK capability).

       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

       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

       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

       The implementation has no specific limits for the per process maximum
       number of shared memory segments (SHMSEG).

       The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more
       clearly show its function.

       SVr4, SVID.  SVr4 documents an additional error condition EEXIST.  Until
       version 2.3.30 Linux would return EIDRM for a shmget() on a shared memory
       segment scheduled for deletion.

       SHM_HUGETLB is a non-portable Linux extension.

       shmat(2), shmctl(2), shmdt(2), ftok(3), ipc(5), capabilities(7)

Linux 2.6.14                       2005-10-26                          SHMGET(2)