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man:svipc

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 4.16 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)

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