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26 .TH MQ_OVERVIEW 7 2009-09-27 "Linux" "Linux Programmer's Manual"
28 mq_overview \- Overview of POSIX message queues
30 POSIX message queues allow processes to exchange data in
32 This API is distinct from that provided by System V message queues
36 etc.), but provides similar functionality.
38 Message queues are created and opened using
40 this function returns a
41 .I message queue descriptor
43 which is used to refer to the open message queue in later calls.
44 Each message queue is identified by a name of the form
46 that is, a null-terminated string of up to
48 (i.e., 255) characters consisting of an initial slash,
49 followed by one or more characters, none of which are slashes.
50 Two processes can operate on the same queue by passing the same name to
53 Messages are transferred to and from a queue using
57 When a process has finished using the queue, it closes it using
59 and when the queue is no longer required, it can be deleted using
61 Queue attributes can be retrieved and (in some cases) modified using
65 A process can request asynchronous notification
66 of the arrival of a message on a previously empty queue using
69 A message queue descriptor is a reference to an
70 .I "open message queue description"
75 a child inherits copies of its parent's message queue descriptors,
76 and these descriptors refer to the same open message queue descriptions
77 as the corresponding descriptors in the parent.
78 Corresponding descriptors in the two processes share the flags
80 that are associated with the open message queue description.
82 Each message has an associated
84 and messages are always delivered to the receiving process
85 highest priority first.
86 Message priorities range from 0 (low) to
87 .I sysconf(_SC_MQ_PRIO_MAX)\ -\ 1
90 .I sysconf(_SC_MQ_PRIO_MAX)
91 returns 32768, but POSIX.1-2001 only requires
92 an implementation to support priorities in the range 0 to 31;
93 some implementations only provide this range.
95 The remainder of this section describes some specific details
96 of the Linux implementation of POSIX message queues.
97 .SS Library interfaces and system calls
100 library interfaces listed above are implemented
101 on top of underlying system calls of the same name.
102 Deviations from this scheme are indicated in the following table:
107 Library interface System call
109 mq_getattr(3) mq_getsetattr(2)
110 mq_notify(3) mq_notify(2)
111 mq_open(3) mq_open(2)
112 mq_receive(3) mq_timedreceive(2)
113 mq_send(3) mq_timedsend(2)
114 mq_setattr(3) mq_getsetattr(2)
115 mq_timedreceive(3) mq_timedreceive(2)
116 mq_timedsend(3) mq_timedsend(2)
117 mq_unlink(3) mq_unlink(2)
121 POSIX message queues have been supported on Linux since kernel 2.6.6.
122 Glibc support has been provided since version 2.3.4.
123 .SS Kernel configuration
124 Support for POSIX message queues is configurable via the
125 .B CONFIG_POSIX_MQUEUE
126 kernel configuration option.
127 This option is enabled by default.
129 POSIX message queues have kernel persistence:
132 a message queue will exist until the system is shut down.
134 Programs using the POSIX message queue API must be compiled with
136 to link against the real-time library,
139 The following interfaces can be used to limit the amount of
140 kernel memory consumed by POSIX message queues:
142 .I /proc/sys/fs/mqueue/msg_max
143 This file can be used to view and change the ceiling value for the
144 maximum number of messages in a queue.
145 This value acts as a ceiling on the
149 The default value for
152 The minimum value is 1 (10 in kernels before 2.6.28).
155 .IR "(131072\ /\ sizeof(void\ *))"
157 This limit is ignored for privileged processes
158 .RB ( CAP_SYS_RESOURCE ),
161 ceiling is nevertheless imposed.
163 .I /proc/sys/fs/mqueue/msgsize_max
164 This file can be used to view and change the ceiling on the
165 maximum message size.
166 This value acts as a ceiling on the
170 The default value for
173 The minimum value is 128 (8192 in kernels before 2.6.28).
176 is 1,048,576 (in kernels before 2.6.28, the upper limit was
178 that is, 2,147,483,647 on Linux/86).
179 This limit is ignored for privileged processes
180 .RB ( CAP_SYS_RESOURCE ).
182 .I /proc/sys/fs/mqueue/queues_max
183 This file can be used to view and change the system-wide limit on the
184 number of message queues that can be created.
185 Only privileged processes
186 .RB ( CAP_SYS_RESOURCE )
187 can create new message queues once this limit has been reached.
188 The default value for
190 is 256; it can be changed to any value in the range 0 to INT_MAX.
194 resource limit, which places a limit on the amount of space
195 that can be consumed by all of the message queues
196 belonging to a process's real user ID, is described in
198 .SS Mounting the message queue file system
199 On Linux, message queues are created in a virtual file system.
200 (Other implementations may also provide such a feature,
201 but the details are likely to differ.)
202 This file system can be mounted (by the superuser) using the following
207 .RB "#" " mkdir /dev/mqueue"
208 .RB "#" " mount \-t mqueue none /dev/mqueue"
212 The sticky bit is automatically enabled on the mount directory.
214 After the file system has been mounted, the message queues on the system
215 can be viewed and manipulated using the commands usually used for files
221 The contents of each file in the directory consist of a single line
222 containing information about the queue:
226 .RB "$" " cat /dev/mqueue/mymq"
227 QSIZE:129 NOTIFY:2 SIGNO:0 NOTIFY_PID:8260
231 These fields are as follows:
234 Number of bytes of data in all messages in the queue.
237 If this is nonzero, then the process with this PID has used
239 to register for asynchronous message notification,
240 and the remaining fields describe how notification occurs.
253 Signal number to be used for
255 .SS Polling message queue descriptors
256 On Linux, a message queue descriptor is actually a file descriptor,
257 and can be monitored using
262 This is not portable.
266 System V message queues
270 etc.) are an older API for exchanging messages between processes.
271 POSIX message queues provide a better designed interface than
272 System V message queues;
273 on the other hand POSIX message queues are less widely available
274 (especially on older systems) than System V message queues.
276 Linux does not currently (2.6.26) support the use of access control
277 lists (ACLs) for POSIX message queues.
279 An example of the use of various message queue functions is shown in
283 .BR mq_getsetattr (2),