shmget

       #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 seg-
                   ment.

       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_HUGE_2MB, SHM_HUGE_1GB (since Linux 3.8)
                   Used in conjunction with SHM_HUGETLB to select  alternative
                   hugetlb page sizes (respectively, 2 MB and 1 GB) on systems
                   that support multiple hugetlb page sizes.

                   More generally, the desired huge page size can  be  config-
                   ured  by  encoding the base-2 logarithm of the desired page
                   size in the six bits at the offset  SHM_HUGE_SHIFT.   Thus,
                   the above two constants are defined as:

                       #define SHM_HUGE_2MB    (21 << SHM_HUGE_SHIFT)
                       #define SHM_HUGE_1GB    (30 << SHM_HUGE_SHIFT)

                   For some additional details, see the discussion of the sim-
                   ilarly named constants in mmap(2).

       SHM_NORESERVE (since Linux 2.6.15)
                   This flag serves the same purpose as the mmap(2)  MAP_NORE-
                   SERVE  flag.   Do  not reserve swap space for this segment.
                   When swap space is reserved, one has the guarantee that  it

       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  veri-
       fied, 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 SHMMIN  or
              greater than SHMMAX.

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

       ENFILE The system-wide limit on the total number of open files has been
              reached.

       ENOENT No segment exists for the given key, and IPC_CREAT was not spec-
              ified.

       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 sys-
              tem to exceed the system-wide limit on shared memory (SHMALL).

       EPERM  The SHM_HUGETLB flag was specified, but the caller was not priv-
              ileged (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 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 total amount of shared memory, measured
              in units of the system page size.

              On Linux, this limit can be read and modified via /proc/sys/ker-
              nel/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 pre-
              vent some cases where historical applications simply raised  the
              existing  limit  without first checking its current value.  Such
              applications 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/ker-
              nel/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 discussion 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
              nel/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
       memfd_create(2),  shmat(2),  shmctl(2),  shmdt(2),  ftok(3),  capabili-
       ties(7), shm_overview(7), svipc(7)

COLOPHON
       This page is part of release 4.04 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                             2015-12-28                         SHMGET(2)
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