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       shmget - allocates a System V shared memory segment


       #include <sys/ipc.h>
       #include <sys/shm.h>

       int shmget(key_t key, size_t size, int shmflg);


       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

       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

       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.


       On success, a valid shared memory identifier is returned.  On error, -1
       is returned, and errno is set to indicate the error.


       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

       ENOENT No segment exists for the  given  key,  and  IPC_CREAT  was  not

       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

       EPERM  The SHM_HUGETLB flag was  specified,  but  the  caller  was  not
              privileged (did not have the CAP_IPC_LOCK capability).


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

       SHM_HUGETLB and SHM_NORESERVE are Linux extensions.


       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 (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 (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

       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.


       The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW would more
       clearly show its function.


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


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