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+.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
+.\"
+.\" Permission is granted to make and distribute verbatim copies of this
+.\" manual provided the copyright notice and this permission notice are
+.\" preserved on all copies.
+.\"
+.\" Permission is granted to copy and distribute modified versions of this
+.\" manual under the conditions for verbatim copying, provided that the
+.\" entire resulting derived work is distributed under the terms of a
+.\" permission notice identical to this one.
+.\" 
+.\" Since the Linux kernel and libraries are constantly changing, this
+.\" manual page may be incorrect or out-of-date.  The author(s) assume no
+.\" responsibility for errors or omissions, or for damages resulting from
+.\" the use of the information contained herein.  The author(s) may not
+.\" have taken the same level of care in the production of this manual,
+.\" which is licensed free of charge, as they might when working
+.\" professionally.
+.\" 
+.\" Formatted or processed versions of this manual, if unaccompanied by
+.\" the source, must acknowledge the copyright and authors of this work.
+.\"
+.TH IPC 5 1993-11-01 "Linux 0.99.13" "Linux Programmer's Manual" 
+.SH NAME
+ipc \- System V interprocess communication mechanisms
+.SH SYNOPSIS
+.nf
+.B
+# include <sys/types.h>
+.B
+# include <sys/ipc.h>
+.B
+# include <sys/msg.h>
+.B
+# include <sys/sem.h>
+.B
+# include <sys/shm.h>
+.SH DESCRIPTION
+This 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
+.B resource
+means an instantiation of one among such mechanisms.
+.SS Resource Access Permissions
+For each resource, the system uses a common structure of type
+.BR "struct ipc_perm"
+to store information needed in determining permissions to perform an
+ipc operation.
+The
+.B ipc_perm
+structure, defined by the
+.I <sys/ipc.h>
+system header file, includes the following members:
+.sp
+.B
+	ushort cuid;	 
+/* creator user id */
+.br
+.B
+	ushort cgid;	 
+/* creator group id */
+.br
+.B
+	ushort uid;	 
+/* owner user id */
+.br
+.B
+	ushort gid;	
+/* owner group id */
+.br
+.B
+	ushort mode;	
+/* r/w permissions */
+.PP
+The
+.B mode
+member of the
+.B 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:
+.sp
+.nf
+	0400	Read by user.
+	0200	Write by user.
+.sp .5
+	0040	Read by group.
+	0020	Write by group.
+.sp .5
+	0004	Read by others.
+	0002	Write by others.
+.fi
+.PP
+Bits 0100, 0010, and 0001 (the execute bits) are unused by the system.
+Furthermore,
+"write"
+effectively means
+"alter"
+for a semaphore set.
+.PP
+The same system header file also defines the following symbolic
+constants:
+.TP 14
+.B IPC_CREAT
+Create entry if key doesn't exist.
+.TP
+.B IPC_EXCL
+Fail if key exists.
+.TP
+.B IPC_NOWAIT
+Error if request must wait.
+.TP
+.B IPC_PRIVATE
+Private key.
+.TP
+.B IPC_RMID
+Remove resource.
+.TP
+.B IPC_SET
+Set resource options.
+.TP
+.B IPC_STAT
+Get resource options.
+.PP
+Note that
+.B IPC_PRIVATE
+is a
+.B key_t
+type, while all the other symbolic constants are flag fields and can
+be OR'ed into an
+.B int
+type variable.
+.SS Message Queues
+A message queue is uniquely identified by a positive integer
+.RI "(its " msqid )
+and has an associated data structure of type
+.BR "struct msqid_ds" ,
+defined in
+.IR <sys/msg.h> ,
+containing the following members:
+.sp
+.B
+	struct ipc_perm msg_perm;
+.br
+.B
+	ushort msg_qnum;	
+/* no of messages on queue */
+.br
+.B
+	ushort msg_qbytes;	
+/* bytes max on a queue */
+.br
+.B
+	ushort msg_lspid;	
+/* pid of last msgsnd call */
+.br
+.B
+	ushort msg_lrpid;	
+/* pid of last msgrcv call */
+.br
+.B
+	time_t msg_stime;	
+/* last msgsnd time */
+.br
+.B
+	time_t msg_rtime;	
+/* last msgrcv time */
+.br
+.B
+	time_t msg_ctime;	
+/* last change time */
+.TP 11
+.B msg_perm
+.B ipc_perm
+structure that specifies the access permissions on the message
+queue.
+.TP
+.B msg_qnum
+Number of messages currently on the message queue.
+.TP
+.B msg_qbytes
+Maximum number of bytes of message text allowed on the message
+queue.
+.TP
+.B msg_lspid
+ID of the process that performed the last
+.B msgsnd
+system call.
+.TP
+.B msg_lrpid
+ID of the process that performed the last
+.B msgrcv
+system call.
+.TP
+.B msg_stime
+Time of the last
+.B msgsnd
+system call.
+.TP
+.B msg_rtime
+Time of the last
+.B msgcv
+system call.
+.TP
+.B msg_ctime
+Time of the last
+system call that changed a member of the
+.B msqid_ds
+structure.
+.SS Semaphore Sets
+A semaphore set is uniquely identified by a positive integer
+.RI "(its " semid )
+and has an associated data structure of type
+.BR "struct semid_ds" ,
+defined in
+.IR <sys/sem.h> ,
+containing the following members:
+.sp
+.B
+	struct ipc_perm sem_perm;
+.br
+.B
+	time_t sem_otime;	
+/* last operation time */
+.br
+.B
+	time_t sem_ctime;	
+/* last change time */
+.br
+.B
+	ushort sem_nsems;	
+/* count of sems in set */
+.TP 11
+.B sem_perm
+.B ipc_perm
+structure that specifies the access permissions on the semaphore
+set.
+.TP
+.B sem_otime
+Time of last
+.B semop
+system call.
+.TP
+.B sem_ctime
+Time of last
+.B semctl
+system call that changed a member of the above structure or of one
+semaphore belonging to the set.
+.TP
+.B sem_nsems
+Number of semaphores in the set.
+Each semaphore of the set is referenced by a non-negative integer
+ranging from
+.B 0
+to
+.BR sem_nsems\-1 .
+.PP
+A semaphore is a data structure of type
+.B "struct sem"
+containing the following members:
+.sp
+.B
+	ushort semval;	
+/* semaphore value */
+.br
+.B
+	short sempid;	
+/* pid for last operation */
+.br
+.B
+	ushort semncnt;
+/* nr awaiting semval to increase */
+.br
+.B
+	ushort semzcnt;
+/* nr awaiting semval = 0 */
+.TP 11
+.B semval
+Semaphore value: a non-negative integer.
+.TP
+.B sempid
+ID of the last process that performed a semaphore operation
+on this semaphore.
+.TP
+.B semncnt
+Number of processes suspended awaiting for
+.B semval
+to increase.
+.TP
+.B semznt
+Number of processes suspended awaiting for
+.B semval
+to become zero.
+.SS Shared Memory Segments
+A shared memory segment is uniquely identified by a positive integer
+.RI "(its " shmid )
+and has an associated data structure of type
+.BR "struct shmid_ds" ,
+defined in
+.IR <sys/shm.h> ,
+containing the following members:
+.sp
+.B
+	struct ipc_perm shm_perm;
+.br
+.B
+	int shm_segsz; 	
+/* size of segment */
+.br
+.B
+	ushort shm_cpid;	
+/* pid of creator */
+.br
+.B
+	ushort shm_lpid;	
+/* pid, last operation */
+.br
+.B
+	short shm_nattch;	
+/* no. of current attaches */
+.br
+.B
+	time_t shm_atime;	
+/* time of last attach */
+.br
+.B
+	time_t shm_dtime;	
+/* time of last detach */
+.br
+.B
+	time_t shm_ctime;	
+/* time of last change */
+.TP 11
+.B shm_perm
+.B ipc_perm
+structure that specifies the access permissions on the shared memory
+segment.
+.TP
+.B shm_segsz
+Size in bytes of the shared memory segment.
+.TP
+.B shm_cpid
+ID of the process that created the shared memory segment.
+.TP
+.B shm_lpid
+ID of the last process that executed a
+.B shmat
+or
+.B shmdt
+system call.
+.TP
+.B shm_nattch
+Number of current alive attaches for this shared memory segment.
+.TP
+.B shm_atime
+Time of the last
+.B shmat
+system call.
+.TP
+.B shm_dtime
+Time of the last
+.B shmdt
+system call.
+.TP
+.B shm_ctime
+Time of the last
+.B shmctl
+system call that changed
+.BR shmid_ds .
+.SH "SEE ALSO"
+.BR msgctl (2),
+.BR msgget (2),
+.BR msgrcv (2),
+.BR msgsnd (2),
+.BR semctl (2),
+.BR semget (2),
+.BR semop (2),
+.BR shmat (2),
+.BR shmctl (2),
+.BR shmdt (2),
+.BR shmget (2),
+.BR ftok (3)