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 constants:


Create entry if key doesn’t exist.


Fail if key exists.


Error if request must wait.


Private key.


Remove resource.


Set resource options.


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 */


ipc_perm structure that specifies the access permissions on the message queue.


Number of messages currently on the message queue.


Maximum number of bytes of message text allowed on the message queue.


ID of the process that performed the last msgsnd(2) system call.


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


Time of the last msgsnd(2) system call.


Time of the last msgrcv(2) system call.


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 */


ipc_perm structure that specifies the access permissions on the semaphore set.


Time of last semop(2) system call.


Time of last semctl(2) system call that changed a member of the above structure or of one semaphore belonging to the set.


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 for last operation */


Semaphore value: a nonnegative integer.


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 */


ipc_perm structure that specifies the access permissions on the shared memory segment.


Size in bytes of the shared memory segment.


ID of the process that created the shared memory segment.


ID of the last process that executed a shmat(2) or shmdt(2) system call.


Number of current alive attaches for this shared memory segment.


Time of the last shmat(2) system call.


Time of the last shmdt(2) system call.


Time of the last shmctl(2) system call that changed shmid_ds.


ipcmk(1), ipcrm(1), ipcs(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)


This page is part of release 3.69 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−pages/.


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