svipc - System V interprocess communication mechanisms


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


   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
   perform  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

   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

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

   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

   sem_nsems  Number of semaphores in the set.  Each semaphore of the  set
              is  referenced  by  a  nonnegative integer ranging from 0 to

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

   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

   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

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


   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),
   shmctl(2), shmdt(2), shmget(2), ftok(3), namespaces(7)


   This  page  is  part of release 4.09 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


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