msgget
Section: System Calls (2)
Updated: 202-0-11
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NAME
msgget - get a System V 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 System V message queue identifier associated
with the value of the
key
argument.
It may be used either to obtain the identifier of a previously created
message queue (when
msgflg
is zero and
key
does not have the value
IPC_PRIVATE),
or to create a new set.
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 specified in
msgflg.
If
msgflg
specifies both
IPC_CREAT
and
IPC_EXCL
and a message queue already exists for
key,
then
msgget()
fails with
errno
set to
EEXIST.
(This is analogous to the effect of the combination
O_CREAT | O_EXCL
for
open(2).)
Upon creation, the least significant bits of the argument
msgflg
define the permissions of the message queue.
These permission bits have the same format and semantics
as the permissions specified for the
mode
argument of
open(2).
(The execute permissions are not used.)
If a new message queue is created,
then its associated data structure
msqid_ds
(see
msgctl(2))
is initialized as follows:
- [bu]
-
msg_perm.cuid
and
msg_perm.uid
are set to the effective user ID of the calling process.
- [bu]
-
msg_perm.cgid
and
msg_perm.gid
are set to the effective group ID of the calling process.
- [bu]
-
The least significant 9 bits of
msg_perm.mode
are set to the least significant 9 bits of
msgflg.
- [bu]
-
msg_qnum,
msg_lspid,
msg_lrpid,
msg_stime,
and
msg_rtime
are set to 0.
- [bu]
-
msg_ctime
is set to the current time.
- [bu]
-
msg_qbytes
is set to the system limit
MSGMNB.
If the message queue already exists the permissions are
verified, and a check is made to see if it is marked for
destruction.
RETURN VALUE
On success,
msgget()
returns the message queue identifier (a nonnegative integer).
On failure, -1 is returned, and
errno
is set to indicate the error.
ERRORS
- EACCES
-
A message queue exists for
key,
but the calling process does not have permission to access the queue,
and does not have the
CAP_IPC_OWNER
capability in the user namespace that governs its IPC namespace.
- EEXIST
-
IPC_CREAT
and
IPC_EXCL
were specified in
msgflg,
but a message queue already exists for
key.
- ENOENT
-
No message queue exists for
key
and
msgflg
did not specify
IPC_CREAT.
- ENOMEM
-
A message queue has to be created but the system does not have 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.
STANDARDS
POSIX.-2024 XSI.
HISTORY
SVr4,
SUSv1,
POSIX.-2001 XSI.
Linux
Until Linux 2.3.20, Linux would return
EIDRM
for a
msgget()
on a message queue scheduled for deletion.
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 least significant 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
-
Syste-wide limit on the number of message queues.
Before Linux 3.19,
the default value for this limit was calculated using a formula
based on available system memory.
Since Linux 3.19, the default value is 32,000.
On Linux, this limit can be read and modified via
/proc/sys/kernel/msgmni.
BUGS
The name choice
IPC_PRIVATE
was perhaps unfortunate,
IPC_NEW
would more clearly show its function.
SEE ALSO
msgctl(2),
msgrcv(2),
msgsnd(2),
ftok(3),
capabilities(7),
mq_overview(7),
sysvipc(7)
Index
- NAME
-
- LIBRARY
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- STANDARDS
-
- HISTORY
-
- Linux
-
- NOTES
-
- BUGS
-
- SEE ALSO
-
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