.\" the source, must acknowledge the copyright and authors of this work.
.\" %%%LICENSE_END
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-.TH MQ_OVERVIEW 7 2009-09-27 "Linux" "Linux Programmer's Manual"
+.TH MQ_OVERVIEW 7 2014-09-21 "Linux" "Linux Programmer's Manual"
.SH NAME
mq_overview \- overview of POSIX message queues
.SH DESCRIPTION
of the Linux implementation of POSIX message queues.
.SS Library interfaces and system calls
In most cases the
-.B mq_*()
+.BR mq_* ()
library interfaces listed above are implemented
on top of underlying system calls of the same name.
Deviations from this scheme are indicated in the following table:
.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
is 10.
The minimum value is 1 (10 in kernels before 2.6.28).
The upper limit is
-.BR HARD_MAX :
-.IR "(131072\ /\ sizeof(void\ *))"
-(32768 on Linux/86).
-This limit is ignored for privileged processes
+.BR HARD_MSGMAX .
+The
+.I msg_max
+limit is ignored for privileged processes
.RB ( CAP_SYS_RESOURCE ),
but the
-.BR HARD_MAX
+.BR HARD_MSGMAX
ceiling is nevertheless imposed.
+
+The definition of
+.BR HARD_MSGMAX
+has changed across kernel versions:
+.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
+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
or
.BR epoll (7).
This is not portable.
+.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
.SH EXAMPLE
An example of the use of various message queue functions is shown in
.BR mq_notify (3).
+.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.
.SH SEE ALSO
.BR getrlimit (2),
.BR mq_getsetattr (2),
.BR mq_receive (3),
.BR mq_send (3),
.BR mq_unlink (3),
-.BR epoll (7)
+.BR epoll (7),
+.BR namespaces (7)
+.SH COLOPHON
+This page is part of release 3.79 of the Linux
+.I man-pages
+project.
+A description of the project,
+information about reporting bugs,
+and the latest version of this page,
+can be found at
+\%http://www.kernel.org/doc/man\-pages/.
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