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(ipc.info)shmget

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shmget
------

A shared memory segment is allocated by a shmget system call:

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

   * key : an integer usually got from `ftok' or IPC_PRIVATE

   * size : size of the segment in bytes (SHMMIN <= size <= SHMMAX).

   * shmflg :
          IPC_CREAT used to create a new resource

          IPC_EXCL used with IPC_CREAT to ensure failure if the
          resource exists.

          rwxrwxrwx  access permissions.

   * returns : shmid on success. -1 on failure.

   A descriptor for a shared memory segment is allocated if there isn't
one corresponding to the given key. The access permissions specified are
then copied into the `shm_perm' struct for the segment along with the
user-id etc. The user must use the IPC_CREAT flag or key = IPC_PRIVATE
to allocate a new segment.

   If the segment already exists, the access permissions are verified,
and a check is made to see that it is not marked for destruction.

   `size' is effectively rounded up to a multiple of PAGE_SIZE as shared
memory is allocated in pages.

Errors:
EINVAL : (allocate) Size not in range specified above.
(procure) Size greater than size of segment.
EEXIST : (allocate) IPC_CREAT | IPC_EXCL specified and resource exists.
EIDRM  : (procure) The resource is marked destroyed or was removed.
ENOSPC : (allocate) All id's are taken (max of SHMMNI id's system-wide).
Allocating a segment of the requested size would exceed the system wide
limit on total shared memory (SHMALL).
ENOENT : (procure) Resource does not exist and IPC_CREAT not specified.
EACCES : (procure) Do not have permission for specified access.
ENOMEM : (allocate) Could not allocate memory for shmid_ds or pg_table.