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.

       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:
                  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)