SVIPC(7)		   Linux Programmer's Manual		      SVIPC(7)



NAME
       svipc - System V interprocess communication mechanisms

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

DESCRIPTION
       This  manual  page  refers  to the Linux implementation of the System V
       interprocess communication (IPC) 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 includes	the  following
       members:

	   struct ipc_perm {
	       uid_t	      cuid;   /* creator user ID */
	       gid_t	      cgid;   /* creator group ID */
	       uid_t	      uid;    /* owner user ID */
	       gid_t	      gid;    /* owner group ID */
	       unsigned short 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 msqid_ds,
       defined in <sys/msg.h>, containing the following members:

	   struct msqid_ds {
	       struct ipc_perm msg_perm;
	       msgqnum_t       msg_qnum;    /* no of messages on queue */
	       msglen_t	       msg_qbytes;  /* bytes max on a queue */
	       pid_t	       msg_lspid;   /* PID of last msgsnd(2) call */
	       pid_t	       msg_lrpid;   /* PID of last msgrcv(2) call */
	       time_t	       msg_stime;   /* last msgsnd(2) time */
	       time_t	       msg_rtime;   /* last msgrcv(2) 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(2)	system
		  call.

       msg_lrpid  ID  of  the process that performed the last msgrcv(2) system
		  call.

       msg_stime  Time of the last msgsnd(2) system call.

       msg_rtime  Time of the last msgrcv(2) 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 semid_ds {
	       struct ipc_perm sem_perm;
	       time_t	       sem_otime;   /* last operation time */
	       time_t	       sem_ctime;   /* last change time */
	       unsigned long   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(2) system call.

       sem_ctime  Time of last semctl(2) 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	 nonnegative integer ranging from 0 to
		  sem_nsems-1.

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

	   struct sem {
	       int semval;  /* semaphore value */
	       int sempid;  /* PID of process that last modified */
	   };

       semval	  Semaphore value: a nonnegative integer.

       sempid	  PID of the last process that modified the value of this sem-
		  aphore.

   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 shmid_ds {
	       struct ipc_perm shm_perm;
	       size_t	       shm_segsz;   /* size of segment */
	       pid_t	       shm_cpid;    /* PID of creator */
	       pid_t	       shm_lpid;    /* PID, last operation */
	       shmatt_t	       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(2) or shmdt(2)
		  system call.

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

       shm_atime  Time of the last shmat(2) system call.

       shm_dtime  Time of the last shmdt(2) system call.

       shm_ctime  Time	 of  the  last	shmctl(2)  system  call	 that  changed
		  shmid_ds.

   IPC namespaces
       For a discussion of the interaction of System V	IPC  objects  and  IPC
       namespaces, see namespaces(7).

SEE ALSO
       ipcmk(1),  ipcrm(1),  ipcs(1),  lsipc(1), ipc(2), msgctl(2), msgget(2),
       msgrcv(2), msgsnd(2), semctl(2), semget(2),  semop(2),  shmat(2),  shm-
       ctl(2), shmdt(2), shmget(2), ftok(3), namespaces(7)

COLOPHON
       This  page  is  part of release 4.10 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at
       https://www.kernel.org/doc/man-pages/.



Linux				  2016-03-15			      SVIPC(7)