IPC (5)
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.
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
IPC_NOWAIT
Error if request must wait.
IPC_PRIVATE
IPC_RMID
IPC_SET
IPC_STAT
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.
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.
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.
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
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