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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.  It may be used either to obtain the identifier of a previously
       created  shared  memory  segment  (when  shmflg  is  zero  and key does not have the value
       IPC_PRIVATE), or to create a new set.

       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/admin-guide/mm/hugetlbpage.rst 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),
       sysvipc(7)

COLOPHON

       This  page  is  part of release 5.02 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/.