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SHMGET(2)			    Linux Programmer's Manual				SHMGET(2)

       shmget - allocates a shared memory segment

       #include <sys/ipc.h>

       #include <sys/shm.h>

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

       shmget()  returns  the  identifier of the shared memory segment associated to the value of
       the argument key.  A new shared memory segment, with size equal to the round up of size to
       a multiple of PAGE_SIZE, is created if key has value IPC_PRIVATE or key isn't IPC_PRIVATE,
       no shared memory segment is associated to key, and IPC_CREAT is asserted in  shmflg  (i.e.
       shmflg&IPC_CREAT isn't zero).

       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, check to  see  if  the	user  has  permission  to
		   receive  the  shmid associated with the segment, and ensure the segment is not
		   marked for destruction.

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

       mode_flags (lowest 9 bits)
		   specifying the permissions granted to the owner, group, and world.  Presently,
		   the execute permissions are not used by the system.

       If  a  new  segment  is	created,  the  access permissions from shmflg are copied into the
       shm_perm member of the shmid_ds structure that defines the segment.  The  shmid_ds  struc-

	    struct shmid_ds {
		 struct    ipc_perm shm_perm;  /* operation perms */
		 int  shm_segsz;	  /* size of segment (bytes) */
		 time_t    shm_atime;	       /* last attach time */
		 time_t    shm_dtime;	       /* last detach time */
		 time_t    shm_ctime;	       /* last change time */
		 unsigned short shm_cpid; /* pid of creator */
		 unsigned short shm_lpid; /* pid of last operator */
		 short	   shm_nattch;	       /* no. of current attaches */

	    struct ipc_perm {
	      key_t  key;
	      ushort uid;   /* owner euid and egid */
	      ushort gid;
	      ushort cuid;  /* creator euid and egid */
	      ushort cgid;
	      ushort mode;  /* lower 9 bits of shmflg */
	      ushort seq;   /* sequence number */

       Furthermore,  while creating, the system call initializes the system shared memory segment
       data structure shmid_ds as follows:

	      shm_perm.cuid and shm_perm.uid are set to the  effective	user-ID  of  the  calling

	      shm_perm.cgid  and  shm_perm.gid	are  set to the effective group-ID of the calling

	      The lowest order 9 bits of shm_perm.mode are set to the lowest order 9 bit of  shm-

	      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 access permissions are verified, and a
       check is made to see if it is marked for destruction.

       fork() After a fork() the child inherits the attached shared memory segments.

       exec() After an exec() all attached shared memory segments are detached (not destroyed).

       exit() Upon exit() all attached shared memory segments are detached (not destroyed).

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

       On failure, errno is set to one of the following:

       EINVAL	   is returned if 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 segment.

       EEXIST	   is returned if IPC_CREAT | IPC_EXCL was specified and the segment exists.

       EIDRM	   is returned if the segment is marked as destroyed, or was removed.

       ENOSPC	   is returned if all possible shared memory id's have been taken (SHMMNI) or  if
		   allocating  a  segment  of the requested size would cause the system to exceed
		   the system-wide limit on shared memory (SHMALL).

       ENOENT	   is returned if no segment exists for the given  key,  and  IPC_CREAT  was  not

       EACCES	   is  returned  if the user does not have permission to access the shared memory

       ENOMEM	   is returned if no memory could be allocated for segment overhead.

       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 lowest order 9 bits of shmflg and creates a new
       shared memory segment (on success).

       The followings are limits on shared memory segment resources affecting a shmget call:

       SHMALL	  System wide maximum of shared memory pages: policy dependent.

       SHMMAX	  Maximum size in bytes for a shared  memory  segment:	implementation	dependent
		  (currently 4M).

       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).

       The  implementation  has  no  specific limits for the per process maximum number of shared
       memory segments (SHMSEG).

       Use of IPC_PRIVATE doesn't inhibit to other processes the access to the	allocated  shared
       memory segment.

       There  is  currently no intrinsic way for a process to ensure exclusive access to a shared
       memory segment.	Asserting both IPC_CREAT and IPC_EXCL in shmflg only ensures (on success)
       that a new shared memory segment will be created, it doesn't imply exclusive access to the

       SVr4, SVID.  SVr4 documents an additional error condition EEXIST.  Neither SVr4	nor  SVID
       documents an EIDRM condition.

       ftok(3), ipc(5), shmctl(2), shmat(2), shmdt(2)

Linux 0.99.11				    1993-11-28					SHMGET(2)
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