--- name: "ipc" section: "5" platform: "linux" package: "manpages" version: "4.16-2" release: "Debian 10 (Buster)" platforms: ["linux"] url: "https://new.carta.tech/svipc" --- # ipc(5) System V interprocess communication mechanisms Part of [svipc(7)](/svipc/7.md). ## NAME svipc - System V interprocess communication mechanisms ## SYNOPSIS ``` #include #include #include ``` ## 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 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 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 *\*, 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)](/msgsnd/2.md) system call. - *msg\_lrpid* ID of the process that performed the last [**msgrcv**(2)](/msgrcv/2.md) system call. - *msg\_stime* Time of the last [**msgsnd**(2)](/msgsnd/2.md) system call. - *msg\_rtime* Time of the last [**msgrcv**(2)](/msgrcv/2.md) 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 *\*, 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)](/semop/2.md) system call. - *sem\_ctime* Time of last [**semctl**(2)](/semctl/2.md) 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 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 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 *\*, 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)](/shmat/2.md) or [**shmdt**(2)](/shmdt/2.md) system call. - *shm\_nattch* Number of current alive attaches for this shared memory segment. - *shm\_atime* Time of the last [**shmat**(2)](/shmat/2.md) system call. - *shm\_dtime* Time of the last [**shmdt**(2)](/shmdt/2.md) system call. - *shm\_ctime* Time of the last [**shmctl**(2)](/shmctl/2.md) 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)](/namespaces/7.md). ## SEE ALSO [**ipcmk**(1)](/ipcmk/1.md), [**ipcrm**(1)](/ipcrm/1.md), [**ipcs**(1)](/ipcs/1.md), [**lsipc**(1)](/lsipc/1.md), [**ipc**(2)](/ipc/2.md), [**msgctl**(2)](/msgctl/2.md), [**msgget**(2)](/msgget/2.md), [**msgrcv**(2)](/msgrcv/2.md), [**msgsnd**(2)](/msgsnd/2.md), [**semctl**(2)](/semctl/2.md), [**semget**(2)](/semget/2.md), [**semop**(2)](/semop/2.md), [**shmat**(2)](/shmat/2.md), [**shmctl**(2)](/shmctl/2.md), [**shmdt**(2)](/shmdt/2.md), [**shmget**(2)](/shmget/2.md), [**ftok**(3)](/ftok/3.md), [**namespaces**(7)](/namespaces/7.md) ## 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/.