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



NAME
       shmget - allocates a System V 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 System V 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   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    This flag is used with IPC_CREAT to ensure that this call creates the segment.
                   If the segment already exists, the call fails.

       SHM_HUGETLB (since Linux 2.6)
                   Allocate the segment using "huge pages."  See the  Linux  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  avail‐
                   able.   See  also the discussion of the file /proc/sys/vm/overcommit_memory in
                   proc(5).

       In addition to the above flags, the least significant 9 bits of shmflg specify the permis‐
       sions  granted  to the owner, group, and others.  These bits have the same format, and the
       same meaning, as the mode argument of open(2).  Presently,  execute  permissions  are  not
       used by the system.

       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
       On success, a valid shared memory identifier is returned.  On error, -1 is  returned,  and
       errno is set to indicate the 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 and IPC_EXCL were specified  in  shmflg,  but  a  shared  memory  segment
              already exists for key.

       EINVAL A new segment was to be created and size is less than SHMIN or greater than SHMMAX.

       EINVAL A  segment for the given key exists, but size is greater than the size of that seg‐
              ment.

       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 and SHM_NORESERVE are Linux extensions.

NOTES
       The  inclusion  of <sys/types.h> and <sys/ipc.h> isn't required on Linux or by any version
       of POSIX.  However, some old implementations required the inclusion of these header files,
       and  the  SVID  also  documented their inclusion.  Applications intended to be portable to
       such old systems may need to include these header files.

       IPC_PRIVATE isn't a flag field but a key_t type.  If this special value is used  for  key,
       the  system  call ignores all but the least significant 9 bits of shmflg and creates a new
       shared memory segment.

   Shared memory limits
       The following limits on shared memory segment resources affect the shmget() call:

       SHMALL System-wide limit on the number of pages of shared memory.

              On Linux, this limit can be read and modified via  /proc/sys/kernel/shmall.   Since
              Linux 3.16, the default value for this limit is:

                  ULONG_MAX - 2^24

              The  effect of this value (which is suitable for both 32-bit and 64-bit systems) is
              to impose no limitation on allocations.  This value,  rather  than  ULONG_MAX,  was
              chosen  as  the  default to prevent some cases where historical applications simply
              raised the existing limit without first checking its current value.  Such  applica‐
              tions would cause the value to overflow if the limit was set at ULONG_MAX.

              From Linux 2.4 up to Linux 3.15, the default value for this limit was:

                  SHMMAX / PAGE_SIZE * (SHMMNI / 16)

              If SHMMAX and SHMMNI were not modified, then multiplying the result of this formula
              by the page size (to get a value in bytes) yielded a value of 8 GB as the limit  on
              the total memory used by all shared memory segments.

       SHMMAX Maximum size in bytes for a shared memory segment.

              On  Linux,  this limit can be read and modified via /proc/sys/kernel/shmmax.  Since
              Linux 3.16, the default value for this limit is:

                  ULONG_MAX - 2^24

              The effect of this value (which is suitable for both 32-bit and 64-bit systems)  is
              to  impose  no limitation on allocations.  See the description of SHMALL for a dis‐
              cussion of why this default value (rather than ULONG_MAX) is used.

              From Linux 2.2 up to Linux 3.15, the default value  of  this  limit  was  0x2000000
              (32MB).

              Because  it is not possible to map just part of a shared memory segment, the amount
              of virtual memory places another limit on the maximum size of a usable segment: for
              example,  on i386 the largest segments that can be mapped have a size of around 2.8
              GB, and on x86_64 the limit is around 127 TB.

       SHMMIN Minimum size in bytes for a shared memory segment: implementation  dependent  (cur‐
              rently 1 byte, though PAGE_SIZE is the effective minimum size).

       SHMMNI System-wide  limit  on  the  number  of  shared memory segments.  In Linux 2.2, the
              default value for this limit was 128; since Linux 2.4, the default value is 4096.

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



Linux                                       2014-10-02                                  SHMGET(2)


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