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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   to  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    used with  IPC_CREAT  to  ensure  failure  if  the  segment
                   already exists.

       mode_flags  (least  significant  9  bits)  specifying  the  permissions
                   granted to the owner, group, and world.   These  bits  have
                   the same format, and the same meaning, as the mode argument
                   of open(2).  Presently, the  execute  permissions  are  not
                   used by the system.

       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  available.   See  also  the
                   discussion  of  the  file /proc/sys/vm/overcommit_memory in

       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 | IPC_EXCL was specified and the segment exists.

       EINVAL A new segment was to be created and size  <  SHMMIN  or  size  >
              SHMMAX,  or  no  new  segment  was to be created, a segment with
              given key existed, but size is greater than  the  size  of  that

       ENFILE The  system  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).


       SVr4, POSIX.1-2001.

       SHM_HUGETLB is a nonportable Linux extension.


       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  everything  but  the  least
       significant  9  bits  of shmflg and creates a new shared memory segment
       (on success).

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

       SHMALL System  wide  maximum  of shared memory pages.  Since Linux 2.2,
              the default value for this limit is

                  SHMMAX / PAGE_SIZE * (SHMMNI / 16)

              Assuming a 4kB page size, this formula yields (since Linux  2.4)
              the value 2^20 (2,097,152).

              On   Linux,   this   limit   can   be   read  and  modified  via

       SHMMAX Maximum size in bytes for a shared memory segment.  Since  Linux
              2.2, the default value of this limit is 0x2000000 (32MB).

              On   Linux,   this   limit   can   be   read  and  modified  via

       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 maximum 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.


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


       This  page  is  part of release 3.65 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

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