NAME
msgget —
get message queue
identifier
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/msg.h>
int
msgget(
key_t
key,
int msgflg);
DESCRIPTION
The
msgget() system call returns the message queue identifier
associated with
key. A message queue identifier is a
unique integer greater than zero.
A message queue is created if either
key is equal to
IPC_PRIVATE
, or
key does not
have a message queue identifier associated with it and the
IPC_CREAT
bit is set in
msgflg.
If both the
IPC_CREAT
bit and the
IPC_EXCL
bit are set in
msgflg,
and
key has a message queue identifier associated with
it already, the operation will fail.
If a new message queue is created, the data structure associated with it (the
msqid_ds structure, see
msgctl(2)) is initialized as
follows:
- msg_perm.cuid and
msg_perm.uid are set to the effective uid of the
calling process.
- msg_perm.gid and
msg_perm.cgid are set to the effective gid of the
calling process.
- msg_perm.mode is set to the lower
9 bits of msgflg.
- msg_qnum,
msg_lspid, msg_lrpid,
msg_rtime, and msg_stime are
set to 0.
- msg_qbytes is set to the system
wide maximum value for the number of bytes in a queue
(
MSGMNB
).
- msg_ctime is set to the current
time.
RETURN VALUES
Upon successful completion a positive message queue identifier is returned.
Otherwise, -1 is returned and the global variable
errno
is set to indicate the error.
ERRORS
-
-
- [
EACCES
]
- A message queue is already associated with
key and the caller has no permission to access
it.
-
-
- [
EEXIST
]
- Both
IPC_CREAT
and
IPC_EXCL
are set in msgflg,
and a message queue is already associated with
key.
-
-
- [
ENOSPC
]
- A new message queue could not be created because the system
limit for the number of message queues has been reached.
-
-
- [
ENOENT
]
IPC_CREAT
is not set in
msgflg and no message queue associated with
key was found.
SEE ALSO
msgctl(2),
msgrcv(2),
msgsnd(2),
ftok(3)
STANDARDS
The
msgget system call conforms to
X/Open
System Interfaces and Headers Issue 5 (“XSH5”).
HISTORY
Message queues appeared in the first release of
AT&T
System V UNIX.