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RedHat 9 (Linux i386) - man page for ipc (redhat section 5)

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 communica-
       tion mechanisms: message queues, semaphore sets, and shared memory segments.  In the  fol-
       lowing, 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 perform 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 per-
       missions 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 constants:

       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 asso-
       ciated 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 message 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 asso-
       ciated data structure of type struct semid_ds, defined in <sys/sem.h>, containing the fol-
       lowing 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 following 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 segment.

       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)


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