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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  cre-
       ated  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/hugetlb-
                   page.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 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  suc-
       cess).

       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 (SHM-
       SEG).

   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 informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2006-05-02                         SHMGET(2)