Manual page for MSGGET(2)
msgget - get message queue
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int msgget(key, msgflg)
key_t key;
int msgflg;
DESCRIPTION
msgget()
returns the message queue identifier associated with
key.
A message queue identifier and associated message queue and data structure
(see
intro.2
are created for
key()
if one of the following is true:
- key
is equal to
IPC_PRIVATE.
- key
does not already have a message queue identifier associated with it, and
(msgflg &
IPC_CREAT)
is ``true''.
Upon creation, the data structure associated with the new message queue
identifier is initialized as follows:
- msg_perm.cuid, msg_perm.uid,
msg_perm.cgid, and msg_perm.gid
are set equal to the effective user
ID
and effective group
ID,
respectively, of the calling process.
- The low-order 9 bits of
msg_perm.mode
are set equal to the low-order 9 bits of
msgflg.
- msg_qnum, msg_lspid, msg_lrpid,
msg_stime, and msg_rtime
are set equal to 0.
- msg_ctime
is set equal to the current time.
- msg_qbytes
is set equal to the system-wide standard value of the maximum number of bytes
allowed on a message queue.
A message queue identifier
(msqid)
is a unique positive integer created by a
msgget.2
system call.
Each msqid has a message queue and a data structure associated with it.
The data structure is referred to as
msqid_ds()
and contains the following members:
-
struct ipc_perm msg_perm; /* operation permission struct */
ushort msg_qnum; /* number of msgs on q */
ushort msg_qbytes; /* max number of bytes on q */
ushort msg_lspid; /* pid of last msgsnd operation */
ushort msg_lrpid; /* pid of last msgrcv operation */
time_t msg_stime; /* last msgsnd time */
time_t msg_rtime; /* last msgrcv time */
time_t msg_ctime; /* last change time */
/* Times measured in secs since */
/* 00:00:00 GMT, Jan. 1, 1970 */
msg_perm()
is an ipc_perm structure that
specifies the message operation permission (see below).
This structure includes the following members:
-
ushort cuid; /* creator user id */
ushort cgid; /* creator group id */
ushort uid; /* user id */
ushort gid; /* group id */
ushort mode; /* r/w permission */
msg_qnum
is the number of messages currently on the queue.
msg_qbytes
is the maximum number of bytes allowed on the queue.
msg_lspid
is the process ID of the last process that performed a
msgsnd operation.
msg_lrpid
is the process ID of the last process that performed a
msgrcv operation.
msg_stime
is the time of the last
msgsnd
operation,
msg_rtime
is the time of the last
msgrcv
operation, and
msg_ctime
is the time of the last
msgctl.2
operation that changed a member of the above structure.
RETURN VALUES
msgget()
returns
A non-negative message queue identifier
on success.
On failure,
it returns
-1
and sets
errno
to indicate the error.
ERRORS
- EACCES
-
A message queue identifier exists for
key,
but operation permission (see
intro.2
as specified by the low-order 9 bits of
msgflg
would not be granted.
- EEXIST
-
A message queue identifier exists for
key()
but
( (msgflg &
IPC_CREAT) & (msgflg &
IPC_EXCL))
is ``true''.
- ENOENT
-
A message queue identifier does not exist for
key()
and
(msgflg &
IPC_CREAT)
is ``false''.
- ENOSPC
-
A message queue identifier is to be created but
the system-imposed limit on the maximum number of
allowed message queue identifiers system wide
would be exceeded.
SEE ALSO
intro.2
msgctl.2
msgop.2
Created by unroff & hp-tools.
© somebody (See intro for details). All Rights Reserved.
Last modified 11/5/97
