MSGGET
Section: Linux Programmer's Manual (2)
Updated: November 1, 1993
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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
to the value of the
key
argument.
A new message queue is created if
key
has value
IPC_PRIVATE
or
key
isn't
IPC_PRIVATE,
no existing message queue is associated to
key,
and
IPC_CREAT
is asserted in
msgflg
(i.e.
msgflg&IPC_CREAT
isn't zero).
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 (for owner, group and others)
to the message queue in the same format, and with the same
meaning, as for the access permissions parameter in the
open(2)
or
creat(2)
system calls (though the execute permissions are not used by the system).
Furthermore, while creating,
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
positive integer), otherwise
-1
with
errno
indicating the error.
ERRORS
For a failing return,
errno
will be set to one among 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.
- EIDRM
-
The message queue is marked as to be removed.
- 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
Use of
IPC_PRIVATE
don't inhibits to other processes the access to the allocated
message queue.
As for the files, there is currently no intrinsic way for a process to ensure
exclusive access to a message queue.
Asserting both
IPC_CREAT
and
IPC_EXCL
in
msgflg
only ensures (on success) that a new message queue will be created,
it doesn't imply exclusive access to the message queue.
SEE ALSO
ftok(3),
ipc(5),
msgctl(2),
msgsnd(2),
msgrcv(2).
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- NOTES
-
- BUGS
-
- SEE ALSO
-
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