msgget(2) -- Linux man page
NAME
msgget - get a message queue identifier
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int msgget(key_t key,
int msgflg);
DESCRIPTION
The function returns the message queue identifier associated
with the value of the
key
argument.
A new message queue is created if
key
has the value
IPC_PRIVATE
or
key
isn't
IPC_PRIVATE,
no message queue with the given key
key
exists, and
IPC_CREAT
is asserted in
msgflg
(i.e.,
msgflg&IPC_CREAT
is nonzero).
The presence in
msgflg
of the fields
IPC_CREAT
and
IPC_EXCL
plays the same role, with respect to the existence
of the message queue, as the presence
of
O_CREAT
and
O_EXCL
in the mode argument of the
open(2)
system call: i.e. the
msgget
function fails if
msgflg
asserts both
IPC_CREAT
and
IPC_EXCL
and a message queue already exists for
key.
Upon creation, the lower 9 bits of the argument
msgflg
define the access permissions of the message queue.
These permission bits have the same format and semantics
as the access permissions parameter in
open(2)
or
creat(2)
system calls. (The execute permissions are not used.)
If a new message queue is created,
the system call initializes the system message queue data structure
msqid_ds
as follows:
-
msg_perm.cuid
and
msg_perm.uid
are set to the effective user-ID of the calling process.
-
msg_perm.cgid
and
msg_perm.gid
are set to the effective group-ID of the calling process.
-
The lowest order 9 bits of
msg_perm.mode
are set to the lowest order 9 bit of
msgflg.
-
msg_qnum,
msg_lspid,
msg_lrpid,
msg_stime
and
msg_rtime
are set to 0.
-
msg_ctime
is set to the current time.
-
msg_qbytes
is set to the system limit
MSGMNB.
If the message queue already exists the access permissions are
verified, and a check is made to see if it is marked for
destruction.
RETURN VALUE
If successful, the return value will be the message queue identifier (a
nonnegative integer), otherwise
-1
with
errno
indicating the error.
ERRORS
On failure,
errno
is set to one of the following values:
- EACCES
-
A message queue exists for
key,
but the calling process has no access permissions to the queue.
- EEXIST
-
A message queue exists for
key
and
msgflg
was asserting both
IPC_CREAT
and
IPC_EXCL.
- ENOENT
-
No message queue exists for
key
and
msgflg
wasn't asserting
IPC_CREAT.
- ENOMEM
-
A message queue has to be created but the system has not enough memory for
the new data structure.
- ENOSPC
-
A message queue has to be created but the system limit for the maximum
number of message queues
(MSGMNI)
would be exceeded.
NOTES
IPC_PRIVATE
isn't a flag field but a
key_t
type.
If this special value is used for
key,
the system call ignores everything but the lowest order 9 bits of
msgflg
and creates a new message queue (on success).
The following is a system limit on message queue resources affecting a
msgget
call:
- MSGMNI
-
System wide maximum number of message queues: policy
dependent.
BUGS
The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW
would more clearly show its function.
CONFORMING TO
SVr4, SVID.
Until version 2.3.20 Linux would return EIDRM for a
msgget
on a message queue scheduled for deletion.
SEE ALSO
ftok(3),
ipc(5),
msgctl(2),
msgsnd(2),
msgrcv(2)
|
Current Users : 22 (Member 0)
Write for us and get publicity, link to your website, and best of all get
paid. Please visit our 
