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           #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 seg-
           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_NORE-
                       SERVE 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).
           When  a new shared memory segment is created, its contents are initial-
           ized to zero values, and its associated data structure,  shmid_ds  (see
           shmctl(2)), is initialized as follows:
                  shm_ctime is set to the current time.
           If the shared memory segment already exists, the permissions are  veri-
           fied, 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 > SHM-
                  MAX, or no new segment was to be created, a segment  with  given
                  key  existed, but size is greater than the size of that segment.
           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 spec-
           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 sys-
                  tem to exceed the system-wide limit on shared memory (SHMALL).
           EPERM  The SHM_HUGETLB flag was specified, but the caller was not priv-
                  ileged (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  sig-
           nificant  9  bits of shmflg and creates a new shared memory segment (on
           SHMMNI System wide maximum number of shared memory segments:  implemen-
                  tation  dependent  (currently 4096, was 128 before Linux 2.3.99;
                  on Linux, this limit can be read and modified via /proc/sys/ker-
           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)

    Linux 2013-04-19 SHMGET(2)


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