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       shmget - allocates a 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 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 kernel
                   source file  Documentation/vm/hugetlbpage.txt  for  further

       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.


       A valid segment identifier, shmid, is returned on success, -1 on 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.


       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 (on Linux, this limit
              can be read and modified via /proc/sys/kernel/shmall).

       SHMMAX Maximum size in  bytes  for  a  shared  memory  segment:  policy
              dependent  (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:
              implementation  dependent  (currently 4096, was 128 before Linux
              2.3.99; 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)


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