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-.TH MQ_OVERVIEW 7 2006-02-25 "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
+mq_overview \- overview of POSIX message queues
.SH DESCRIPTION
POSIX message queues allow processes to exchange data in
the form of messages.
.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
.RI ( mqd_t ),
which is used to refer to the open message queue in later calls.
Each message queue is identified by a name of the form
-.IR /somename .
+.IR /somename ;
+that is, a null-terminated string of up to
+.BI NAME_MAX
+(i.e., 255) characters consisting of an initial slash,
+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
-.IR "open message queue description"
-(cf.
+.I "open message queue description"
+(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 only requires
-an implementation to support priorities in the range 0 to 31;
-some implementations only provide this range.
+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
The remainder of this section describes some specific details
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:
-.in +0.25i
+.RS
.TS
lB lB
l l.
Library interface System call
mq_close(3) close(2)
mq_getattr(3) mq_getsetattr(2)
+mq_notify(3) mq_notify(2)
mq_open(3) mq_open(2)
mq_receive(3) mq_timedreceive(2)
mq_send(3) mq_timedsend(2)
mq_timedsend(3) mq_timedsend(2)
mq_unlink(3) mq_unlink(2)
.TE
-.in -0.25i
+.RE
.SS Versions
POSIX message queues have been supported on Linux since kernel 2.6.6.
Glibc support has been provided since version 2.3.4.
.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
maximum number of messages in a queue.
This value acts as a ceiling on the
-.I attr->mq_maxmsg
+.I attr\->mq_maxmsg
argument given to
.BR mq_open (3).
-The default and minimum value for
+The default value for
.I msg_max
-is 10; the upper limit is HARD_MAX:
-.IR "(131072\ /\ sizeof(void\ *))"
-(32768 on Linux/86).
-This limit is ignored for privileged processes
+is 10.
+The minimum value is 1 (10 in kernels before 2.6.28).
+The upper limit is
+.BR HARD_MSGMAX .
+The
+.I msg_max
+limit is ignored for privileged processes
.RB ( CAP_SYS_RESOURCE ),
-but the HARD_MAX ceiling is nevertheless imposed.
+but the
+.BR HARD_MSGMAX
+ceiling is nevertheless imposed.
+.IP
+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
maximum message size.
This value acts as a ceiling on the
-.I attr->mq_msgsize
+.I attr\->mq_msgsize
argument given to
.BR mq_open (3).
-The default and minimum value for
+The default value for
+.I msgsize_max
+is 8192 bytes.
+The minimum value is 128 (8192 in kernels before 2.6.28).
+The upper limit for
+.I msgsize_max
+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
-is 8192 bytes; the upper limit is INT_MAX
-(2147483647 on Linux/86).
-This limit is ignored for privileged processes
-.RB ( CAP_SYS_RESOURCE ).
+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
-.BR RLIMIT_MSGQUEUE
+.B RLIMIT_MSGQUEUE
resource limit, which places a limit on the amount of space
that can be consumed by all of the message queues
belonging to a process's real user ID, is described in
.BR getrlimit (2).
-.SS Mounting the message queue file system
-On Linux, message queues are created in a virtual file system.
+.SS Mounting the message queue filesystem
+On Linux, message queues are created in a virtual filesystem.
(Other implementations may also provide such a feature,
but the details are likely to differ.)
-This file system can be mounted using the following commands:
-.in +0.25i
-.nf
-
-$ mkdir /dev/mqueue
-$ mount -t mqueue none /dev/mqueue
-
-.fi
-.in -0.25i
+This filesystem can be mounted (by the superuser) using the following
+commands:
+.PP
+.in +4n
+.EX
+.RB "#" " mkdir /dev/mqueue"
+.RB "#" " mount \-t mqueue none /dev/mqueue"
+.EE
+.in
+.PP
The sticky bit is automatically enabled on the mount directory.
-
-After the file system has been mounted, the message queues on the system
+.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:
-.in +0.25i
-.nf
-
-$ ls /dev/mqueue/mymq
+.PP
+.in +4n
+.EX
+.RB "$" " cat /dev/mqueue/mymq"
QSIZE:129 NOTIFY:2 SIGNO:0 NOTIFY_PID:8260
-$ mount -t mqueue none /dev/mqueue
-
-.fi
-.in -0.25i
+.EE
+.in
+.PP
These fields are as follows:
.TP
-.B
-QSIZE
-Number of bytes of data in all messages in the queue.
+.B QSIZE
+Number of bytes of data in all messages in the queue (but see BUGS).
.TP
.B NOTIFY_PID
-If this is non-zero, then the process with this PID has used
+If this is nonzero, then the process with this PID has used
.BR mq_notify (3)
to register for asynchronous message notification,
and the remaining fields describe how notification occurs.
.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.
-.SH "CONFORMING TO"
-POSIX.1-2001.
+.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 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).
-.SH "SEE ALSO"
+.SH SEE ALSO
.BR getrlimit (2),
.BR mq_getsetattr (2),
+.BR poll (2),
+.BR select (2),
.BR mq_close (3),
.BR mq_getattr (3),
.BR mq_notify (3),
.BR mq_receive (3),
.BR mq_send (3),
.BR mq_unlink (3),
-.BR poll (2),
-.BR select (2),
-.BR epoll (4)
+.BR epoll (7),
+.BR namespaces (7)
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