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IPC(5)			   Linux Programmer's Manual			IPC(5)

NAME
       ipc - System V interprocess communication mechanisms

SYNOPSIS
       # include <sys/types.h>
       # include <sys/ipc.h>
       # include <sys/msg.h>
       # include <sys/sem.h>
       # include <sys/shm.h>

DESCRIPTION
       The  manual  page  refers  to  the Linux	implementation of the System V
       interprocess communication mechanisms: message queues, semaphore	 sets,
       and  shared memory segments.  In	the following, the word	resource means
       an instantiation	of one among such mechanisms.

   Resource Access Permissions
       For each	resource, the system uses a common structure  of  type	struct
       ipc_perm	to store information needed in determining permissions to per-
       form  an	 ipc  operation.   The	ipc_perm  structure,  defined  by  the
       _sys/ipc.h_ system header file, includes	the following members:

	    ushort cuid;    /* creator user id */
	    ushort cgid;    /* creator group id	*/
	    ushort uid;	/* owner user id */
	    ushort gid;	/* owner group id */
	    ushort mode; /* r/w	permissions */

       The  mode  member  of  the ipc_perm structure defines, with its lower 9
       bits, the access	permissions to the resource for	a process executing an
       ipc system call.	 The permissions are interpreted as follows:

	    0400 Read by user.
	    0200 Write by user.
	    0040 Read by group.
	    0020 Write by group.
	    0004 Read by others.
	    0002 Write by others.

       Bits  0100, 0010, and 0001 (the execute bits) are unused	by the system.
       Furthermore, "write" effectively	means "alter" for a semaphore set.

       The same	system header file also	defines	the  following	symbolic  con-
       stants:

       IPC_CREAT     Create entry if key doesn't exist.

       IPC_EXCL	     Fail if key exists.

       IPC_NOWAIT    Error if request must wait.

       IPC_PRIVATE   Private key.

       IPC_RMID	     Remove resource.

       IPC_SET	     Set resource options.

       IPC_STAT	     Get resource options.

       Note  that  IPC_PRIVATE	is  a key_t type, while	all the	other symbolic
       constants are flag fields and can be OR'ed into an int type variable.

   Message Queues
       A message queue is uniquely  identified	by  a  positive	 integer  (its
       msqid)  and  has	an associated data structure of	type struct msquid_ds,
       defined in _sys/msg.h_, containing the following	members:

	    struct ipc_perm msg_perm;
	    ushort msg_qnum;	 /* no of messages on queue */
	    ushort msg_qbytes;	 /* bytes max on a queue */
	    ushort msg_lspid;	 /* pid	of last	msgsnd call */
	    ushort msg_lrpid;	 /* pid	of last	msgrcv call */
	    time_t msg_stime;	 /* last msgsnd	time */
	    time_t msg_rtime;	 /* last msgrcv	time */
	    time_t msg_ctime;	 /* last change	time */

       msg_perm	  ipc_perm structure that specifies the	access permissions  on
		  the message queue.

       msg_qnum	  Number of messages currently on the message queue.

       msg_qbytes Maximum  number of bytes of message text allowed on the mes-
		  sage queue.

       msg_lspid  ID of	the process that  performed  the  last	msgsnd	system
		  call.

       msg_lrpid  ID  of  the  process	that  performed	the last msgrcv	system
		  call.

       msg_stime  Time of the last msgsnd system call.

       msg_rtime  Time of the last msgcv system	call.

       msg_ctime  Time of the last system call that changed a  member  of  the
		  msqid_ds structure.

   Semaphore Sets
       A  semaphore  set  is  uniquely	identified  by a positive integer (its
       semid) and has an associated data structure of  type  struct  semid_ds,
       defined in _sys/sem.h_, containing the following	members:

	    struct ipc_perm sem_perm;
	    time_t sem_otime;	 /* last operation time	*/
	    time_t sem_ctime;	 /* last change	time */
	    ushort sem_nsems;	 /* count of sems in set */

       sem_perm	  ipc_perm  structure that specifies the access	permissions on
		  the semaphore	set.

       sem_otime  Time of last semop system call.

       sem_ctime  Time of last semctl system call that changed a member	of the
		  above	structure or of	one semaphore belonging	to the set.

       sem_nsems  Number  of semaphores	in the set.  Each semaphore of the set
		  is referenced	by a non-negative integer ranging  from	 0  to
		  sem_nsems-1.

       A  semaphore is a data structure	of type	struct sem containing the fol-
       lowing members:

	    ushort semval;  /* semaphore value */
	    short sempid;   /* pid for last operation */
	    ushort semncnt;	 /* no.	of awaiting semval to increase */
	    ushort semzcnt;	 /* no.	of awaiting semval = 0 */

       semval	  Semaphore value: a non-negative integer.

       sempid	  ID of	the last process that performed	a semaphore  operation
		  on this semaphore.

       semncnt	  Number   of  processes  suspended  awaiting  for  semval  to
		  increase.

       semznt	  Number of processes suspended	awaiting for semval to	become
		  zero.

   Shared Memory Segments
       A  shared  memory  segment is uniquely identified by a positive integer
       (its shmid) and	has  an	 associated  data  structure  of  type	struct
       shmid_ds, defined in _sys/shm.h_, containing the	following members:

	    struct ipc_perm shm_perm;
	    int	shm_segsz;  /* size of segment */
	    ushort shm_cpid;	 /* pid	of creator */
	    ushort shm_lpid;	 /* pid, last operation	*/
	    short shm_nattch;	 /* no.	of current attaches */
	    time_t shm_atime;	 /* time of last attach	*/
	    time_t shm_dtime;	 /* time of last detach	*/
	    time_t shm_ctime;	 /* time of last change	*/

       shm_perm	  ipc_perm  structure that specifies the access	permissions on
		  the shared memory segment.

       shm_segsz  Size in bytes	of the shared memory segment.

       shm_cpid	  ID of	the process that created the shared memory segment.

       shm_lpid	  ID of	the last process that executed a shmat or shmdt	system
		  call.

       shm_nattch Number of current alive attaches for this shared memory seg-
		  ment.

       shm_atime  Time of the last shmat system	call.

       shm_dtime  Time of the last shmdt system	call.

       shm_ctime  Time of the last shmctl system call that changed shmid_ds.

SEE ALSO
       ftok(3),	 msgctl(2),  msgget(2),	  msgrcv(2),   msgsnd(2),   semctl(2),
       semget(2), semop(2), shmat(2), shmctl(2), shmget(2), shmdt(2)

Linux 0.99.13			  1993-11-01				IPC(5)

NAME | SYNOPSIS | DESCRIPTION | SEE ALSO

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