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



NAME
       svipc - 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
       This  manual  page  refers  to the Linux implementation of the System V interprocess communication (IPC) mecha-
       nisms: 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 perform an IPC operation.  The ipc_perm structure, defined by the <sys/ipc.h> system
       header file, 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 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 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 message 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 non-negative integer
                  ranging from 0 to sem_nsems-1.

       A semaphore is a data structure of type struct sem containing the following members:

           struct sem {
               int semval;  /* semaphore value */
               int sempid;  /* PID for last operation */
           };

       semval     Semaphore value: a non-negative integer.

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

   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 segment.

       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.

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

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-01-26                          SVIPC(7)