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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_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 configured 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 similarly named
                   constants in mmap(2).

       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).

       In  addition  to  the  above  flags,  the  least  significant 9 bits of shmflg specify the
       permissions 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  in  the  user  namespace that governs its IPC
              namespace.

       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 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

       POSIX.1-2001, POSIX.1-2008, SVr4.

       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 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/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
              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/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
              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
              (32 MB).

              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
              (currently 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

       memfd_create(2), shmat(2), shmctl(2), shmdt(2), ftok(3), capabilities(7), shm_overview(7),
       svipc(7)

COLOPHON

       This page is part of release 4.13 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 https://www.kernel.org/doc/man-pages/.