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NAME
sysvipc - 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 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 <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
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 <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.
IPC namespaces
For a discussion of the interaction of System V IPC objects and IPC namespaces, see
ipc_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), shmctl(2), shmdt(2), shmget(2), ftok(3),
ipc_namespaces(7)
COLOPHON
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project, information about reporting bugs, and the latest version of this page, can be
found at https://www.kernel.org/doc/man-pages/.