IPC
Section: Linux Programmer's Manual (5)
Updated: November 1, 1993
NAME
ipc - System V interprocess communication mechanisms
SYNOPSIS
# include <sys/types.h>
# include <sys/ipc.h>
# include <sys/msg.h>
# include <sys/sem.h>
# include <sys/shm.h>
DESCRIPTION
The manual page refers to the Linux implementation of the System V
interprocess communication 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, defined by the
<sys/ipc.h>
system header file, includes the following members:
ushort cuid;
/* creator user id */
ushort cgid;
/* creator group id */
ushort uid;
/* owner user id */
ushort gid;
/* owner group id */
ushort 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 defines also the following symbolic
constants:
- IPC_CREAT
-
Create entry if key doesn't exists.
- 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 others symbolic constants are flag fields or-able
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 msquid_ds,
defined in
<sys/msg.h>,
containing the following members:
struct ipc_perm msg_perm;
ushort msg_qnum;
/* no of messages on queue */
ushort msg_qbytes;
/* bytes max on a queue */
ushort msg_lspid;
/* pid of last msgsnd call */
ushort msg_lrpid;
/* pid of last msgrcv call */
time_t msg_stime;
/* last msgsnd time */
time_t msg_rtime;
/* last msgrcv 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
system call.
- msg_lrpid
-
ID of the process that performed the last
msgrcv
system call.
- msg_stime
-
Time of the last
msgsnd
system call.
- msg_rtime
-
Time of the last
msgcv
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 ipc_perm sem_perm;
time_t sem_otime;
/* last operation time */
time_t sem_ctime;
/* last change time */
ushort 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
system call.
- sem_ctime
-
Time of last
semctl
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 non-negative integer
ranging from
0
to
sem_nsems-1.
A semaphore is a data structure of type
struct sem
containing the following members:
ushort semval;
/* semaphore value */
short sempid;
/* pid for last operation */
ushort semncnt;
/* no. of awaiting semval to increase */
ushort semzcnt;
/* no. of awaiting semval = 0 */
- semval
-
Semaphore value: a non-negative integer.
- sempid
-
ID of the last process that performed a semaphore operation
on this semaphore.
- semncnt
-
Number of processes suspended awaiting for
semval
to increase.
- semznt
-
Number of processes suspended awaiting for
semval
to become zero.
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 ipc_perm shm_perm;
int shm_segsz;
/* size of segment */
ushort shm_cpid;
/* pid of creator */
ushort shm_lpid;
/* pid, last operation */
short 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
or
shmdt
system call.
- shm_nattch
-
Number of current alive attaches for this shared memory segment.
- shm_atime
-
Time of the last
shmat
system call.
- shm_dtime
-
Time of the last
shmdt
system call.
- shm_ctime
-
Time of the last
shmctl
system call that changed
shmid_ds.
SEE ALSO
ftok(3),
msgctl(2),
msgget(2),
msgrcv(2),
msgsnd(2),
semctl(2),
semget(2),
semop(2),
shmat(2),
shmctl(2),
shmget(2),
shmdt (2).