.\" the source, must acknowledge the copyright and authors of this work.
.\" %%%LICENSE_END
.\"
-.TH MQ_OVERVIEW 7 2014-09-21 "Linux" "Linux Programmer's Manual"
+.TH MQ_OVERVIEW 7 2017-09-15 "Linux" "Linux Programmer's Manual"
.SH NAME
mq_overview \- overview of POSIX message queues
.SH DESCRIPTION
.BR msgsnd (2),
.BR msgrcv (2),
etc.), but provides similar functionality.
-
+.PP
Message queues are created and opened using
.BR mq_open (3);
this function returns a
followed by one or more characters, none of which are slashes.
Two processes can operate on the same queue by passing the same name to
.BR mq_open (3).
-
+.PP
Messages are transferred to and from a queue using
.BR mq_send (3)
and
A process can request asynchronous notification
of the arrival of a message on a previously empty queue using
.BR mq_notify (3).
-
+.PP
A message queue descriptor is a reference to an
.I "open message queue description"
-(cf.
+(see
.BR open (2)).
After a
.BR fork (2),
a child inherits copies of its parent's message queue descriptors,
and these descriptors refer to the same open message queue descriptions
-as the corresponding descriptors in the parent.
-Corresponding descriptors in the two processes share the flags
+as the corresponding message queue descriptors in the parent.
+Corresponding message queue descriptors in the two processes share the flags
.RI ( mq_flags )
that are associated with the open message queue description.
-
+.PP
Each message has an associated
.IR priority ,
and messages are always delivered to the receiving process
(high).
On Linux,
.I sysconf(_SC_MQ_PRIO_MAX)
-returns 32768, but POSIX.1-2001 requires only that
+returns 32768, but POSIX.1 requires only that
an implementation support at least priorities in the range 0 to 31;
some implementations provide only this range.
.PP
.IR librt .
.SS /proc interfaces
The following interfaces can be used to limit the amount of
-kernel memory consumed by POSIX message queues:
+kernel memory consumed by POSIX message queues and to set
+the default attributes for new message queues:
+.TP
+.IR /proc/sys/fs/mqueue/msg_default " (since Linux 3.5)"
+This file defines the value used for a new queue's
+.I mq_maxmsg
+setting when the queue is created with a call to
+.BR mq_open (3)
+where
+.I attr
+is specified as NULL.
+The default value for this file is 10.
+The minimum and maximum are as for
+.IR /proc/sys/fs/mqueue/msg_max .
+A new queue's default
+.I mq_maxmsg
+value will be the smaller of
+.IR msg_default
+and
+.IR msg_max .
+Up until Linux 2.6.28, the default
+.I mq_maxmsg
+was 10;
+from Linux 2.6.28 to Linux 3.4, the default was the value defined for the
+.I msg_max
+limit.
.TP
.I /proc/sys/fs/mqueue/msg_max
This file can be used to view and change the ceiling value for the
but the
.BR HARD_MSGMAX
ceiling is nevertheless imposed.
+.IP
The definition of
.BR HARD_MSGMAX
has changed across kernel versions:
-before Linux 2.6.33, it was
-.IR "(131072\ /\ sizeof(void\ *))"
-(32768 on Linux/86);
-from Linux 2.6.33, it was
-.IR "(32768\ *\ sizeof(void\ *)/4)" ;
-since Linux 3.5,
+.RS
+.IP * 3
+Up to Linux 2.6.32:
+.IR "131072\ /\ sizeof(void\ *)"
+.IP *
+Linux 2.6.33 to 3.4:
+.IR "(32768\ *\ sizeof(void\ *) / 4)"
+.IP *
+Since Linux 3.5:
.\" commit 5b5c4d1a1440e94994c73dddbad7be0676cd8b9a
-it is 65,536.
+65,536
+.RE
+.TP
+.IR /proc/sys/fs/mqueue/msgsize_default " (since Linux 3.5)"
+This file defines the value used for a new queue's
+.I mq_msgsize
+setting when the queue is created with a call to
+.BR mq_open (3)
+where
+.I attr
+is specified as NULL.
+The default value for this file is 8192 (bytes).
+The minimum and maximum are as for
+.IR /proc/sys/fs/mqueue/msgsize_max .
+If
+.IR msgsize_default
+exceeds
+.IR msgsize_max ,
+a new queue's default
+.I mq_msgsize
+value is capped to the
+.I msgsize_max
+limit.
+Up until Linux 2.6.28, the default
+.I mq_msgsize
+was 8192;
+from Linux 2.6.28 to Linux 3.4, the default was the value defined for the
+.I msgsize_max
+limit.
.TP
.I /proc/sys/fs/mqueue/msgsize_max
This file can be used to view and change the ceiling on the
The minimum value is 128 (8192 in kernels before 2.6.28).
The upper limit for
.I msgsize_max
-is 1,048,576 (in kernels before 2.6.28, the upper limit was
-.BR INT_MAX ;
-that is, 2,147,483,647 on Linux/86).
-This limit is ignored for privileged processes
-.RB ( CAP_SYS_RESOURCE ).
+has varied across kernel versions:
+.RS
+.IP * 3
+Before Linux 2.6.28, the upper limit is
+.BR INT_MAX .
+.IP *
+From Linux 2.6.28 to 3.4, the limit is 1,048,576.
+.IP *
+Since Linux 3.5, the limit is 16,777,216
+.RB ( HARD_MSGSIZEMAX ).
+.RE
+.IP
+The
+.I msgsize_max
+limit is ignored for privileged process
+.RB ( CAP_SYS_RESOURCE ),
+but, since Linux 3.5, the
+.BR HARD_MSGSIZEMAX
+ceiling is enforced for privileged processes.
.TP
.I /proc/sys/fs/mqueue/queues_max
This file can be used to view and change the system-wide limit on the
number of message queues that can be created.
-Only privileged processes
-.RB ( CAP_SYS_RESOURCE )
-can create new message queues once this limit has been reached.
The default value for
.I queues_max
-is 256; it can be changed to any value in the range 0 to INT_MAX.
+is 256.
+No ceiling is imposed on the
+.I queues_max
+limit; privileged processes
+.RB ( CAP_SYS_RESOURCE )
+can exceed the limit (but see BUGS).
.SS Resource limit
The
.B RLIMIT_MSGQUEUE
but the details are likely to differ.)
This filesystem can be mounted (by the superuser) using the following
commands:
+.PP
.in +4n
-.nf
-
+.EX
.RB "#" " mkdir /dev/mqueue"
.RB "#" " mount \-t mqueue none /dev/mqueue"
-
-.fi
+.EE
.in
+.PP
The sticky bit is automatically enabled on the mount directory.
-
+.PP
After the filesystem has been mounted, the message queues on the system
can be viewed and manipulated using the commands usually used for files
(e.g.,
.BR ls (1)
and
.BR rm (1)).
-
+.PP
The contents of each file in the directory consist of a single line
containing information about the queue:
+.PP
.in +4n
-.nf
-
+.EX
.RB "$" " cat /dev/mqueue/mymq"
QSIZE:129 NOTIFY:2 SIGNO:0 NOTIFY_PID:8260
-
-.fi
+.EE
.in
+.PP
These fields are as follows:
.TP
.B QSIZE
-Number of bytes of data in all messages in the queue.
+Number of bytes of data in all messages in the queue (but see BUGS).
.TP
.B NOTIFY_PID
If this is nonzero, then the process with this PID has used
.B SIGNO
Signal number to be used for
.BR SIGEV_SIGNAL .
-.SS Polling message queue descriptors
-On Linux, a message queue descriptor is actually a file descriptor,
-and can be monitored using
+.SS Linux implementation of message queue descriptors
+On Linux, a message queue descriptor is actually a file descriptor.
+(POSIX does not require such an implementation.)
+This means that a message queue descriptor can be monitored using
.BR select (2),
.BR poll (2),
or
.BR epoll (7).
This is not portable.
+.PP
+The close-on-exec flag (see
+.BR open (2))
+is automatically set on the file descriptor returned by
+.BR mq_open (2).
.SS IPC namespaces
For a discussion of the interaction of System V IPC objects and
IPC namespaces, see
.BR namespaces (7).
-.SH CONFORMING TO
-POSIX.1-2001.
.SH NOTES
System V message queues
.RB ( msgget (2),
System V message queues;
on the other hand POSIX message queues are less widely available
(especially on older systems) than System V message queues.
-
+.PP
Linux does not currently (2.6.26) support the use of access control
lists (ACLs) for POSIX message queues.
+.SH BUGS
+In Linux versions 3.5 to 3.14, the kernel imposed a ceiling of 1024
+.RB ( HARD_QUEUESMAX )
+on the value to which the
+.I queues_max
+limit could be raised,
+and the ceiling was enforced even for privileged processes.
+This ceiling value was removed in Linux 3.14,
+and patches to stable kernels 3.5.x to 3.13.x also removed the ceiling.
+.PP
+As originally implemented (and documented),
+the QSIZE field displayed the total number of (user-supplied)
+bytes in all messages in the message queue.
+Some changes in Linux 3.5
+.\" commit d6629859b36d
+inadvertently changed the behavior,
+so that this field also included a count of kernel overhead bytes
+used to store the messages in the queue.
+This behavioral regression was rectified in Linux 4.2
+.\" commit de54b9ac253787c366bbfb28d901a31954eb3511
+(and earlier stable kernel series),
+so that the count once more included just the bytes of user data
+in messages in the queue.
.SH EXAMPLE
An example of the use of various message queue functions is shown in
.BR mq_notify (3).
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