1MQ_OVERVIEW(7)             Linux Programmer's Manual            MQ_OVERVIEW(7)
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NAME

6       mq_overview - overview of POSIX message queues
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DESCRIPTION

9       POSIX  message  queues  allow processes to exchange data in the form of
10       messages.  This API is distinct from that provided by System V  message
11       queues  (msgget(2),  msgsnd(2),  msgrcv(2), etc.), but provides similar
12       functionality.
13
14       Message queues are created and opened using mq_open(3);  this  function
15       returns  a  message queue descriptor (mqd_t), which is used to refer to
16       the open message queue in later calls.  Each message queue  is  identi‐
17       fied by a name of the form /somename; that is, a null-terminated string
18       of up to NAME_MAX (i.e.,  255)  characters  consisting  of  an  initial
19       slash,  followed  by one or more characters, none of which are slashes.
20       Two processes can operate on the same queue by passing the same name to
21       mq_open(3).
22
23       Messages  are  transferred  to  and  from  a queue using mq_send(3) and
24       mq_receive(3).  When a process has finished using the queue, it  closes
25       it  using mq_close(3), and when the queue is no longer required, it can
26       be deleted using mq_unlink(3).  Queue attributes can be  retrieved  and
27       (in  some  cases)  modified  using  mq_getattr(3) and mq_setattr(3).  A
28       process can request asynchronous notification of the arrival of a  mes‐
29       sage on a previously empty queue using mq_notify(3).
30
31       A  message queue descriptor is a reference to an open message queue de‐
32       scription (see open(2)).  After a fork(2), a child inherits  copies  of
33       its  parent's message queue descriptors, and these descriptors refer to
34       the same open message queue descriptions as the  corresponding  message
35       queue  descriptors in the parent.  Corresponding message queue descrip‐
36       tors in the two processes share the flags (mq_flags) that  are  associ‐
37       ated with the open message queue description.
38
39       Each message has an associated priority, and messages are always deliv‐
40       ered to the receiving process highest priority first.  Message  priori‐
41       ties  range  from  0  (low) to sysconf(_SC_MQ_PRIO_MAX) - 1 (high).  On
42       Linux, sysconf(_SC_MQ_PRIO_MAX) returns  32768,  but  POSIX.1  requires
43       only  that an implementation support at least priorities in the range 0
44       to 31; some implementations provide only this range.
45
46       The remainder of this section describes some specific  details  of  the
47       Linux implementation of POSIX message queues.
48
49   Library interfaces and system calls
50       In  most  cases  the  mq_*() library interfaces listed above are imple‐
51       mented on top of underlying system calls of the same name.   Deviations
52       from this scheme are indicated in the following table:
53
54              Library interface    System call
55              mq_close(3)          close(2)
56              mq_getattr(3)        mq_getsetattr(2)
57              mq_notify(3)         mq_notify(2)
58              mq_open(3)           mq_open(2)
59              mq_receive(3)        mq_timedreceive(2)
60              mq_send(3)           mq_timedsend(2)
61              mq_setattr(3)        mq_getsetattr(2)
62              mq_timedreceive(3)   mq_timedreceive(2)
63              mq_timedsend(3)      mq_timedsend(2)
64              mq_unlink(3)         mq_unlink(2)
65
66   Versions
67       POSIX  message  queues have been supported on Linux since kernel 2.6.6.
68       Glibc support has been provided since version 2.3.4.
69
70   Kernel configuration
71       Support  for  POSIX  message  queues  is  configurable  via  the   CON‐
72       FIG_POSIX_MQUEUE  kernel  configuration option.  This option is enabled
73       by default.
74
75   Persistence
76       POSIX message queues have kernel persistence: if not removed by  mq_un‐
77       link(3), a message queue will exist until the system is shut down.
78
79   Linking
80       Programs  using  the  POSIX  message queue API must be compiled with cc
81       -lrt to link against the real-time library, librt.
82
83   /proc interfaces
84       The following interfaces can be used to limit the amount of kernel mem‐
85       ory  consumed by POSIX message queues and to set the default attributes
86       for new message queues:
87
88       /proc/sys/fs/mqueue/msg_default (since Linux 3.5)
89              This file defines the value used for  a  new  queue's  mq_maxmsg
90              setting  when  the  queue  is  created with a call to mq_open(3)
91              where attr is specified as NULL.  The  default  value  for  this
92              file   is   10.    The   minimum   and   maximum   are   as  for
93              /proc/sys/fs/mqueue/msg_max.  A new  queue's  default  mq_maxmsg
94              value  will be the smaller of msg_default and msg_max.  Up until
95              Linux 2.6.28, the default mq_maxmsg was 10; from Linux 2.6.28 to
96              Linux  3.4,  the  default  was the value defined for the msg_max
97              limit.
98
99       /proc/sys/fs/mqueue/msg_max
100              This file can be used to view and change the ceiling  value  for
101              the maximum number of messages in a queue.  This value acts as a
102              ceiling on the attr->mq_maxmsg  argument  given  to  mq_open(3).
103              The default value for msg_max is 10.  The minimum value is 1 (10
104              in kernels before 2.6.28).  The upper limit is HARD_MSGMAX.  The
105              msg_max  limit  is ignored for privileged processes (CAP_SYS_RE‐
106              SOURCE), but the HARD_MSGMAX ceiling is nevertheless imposed.
107
108              The definition of HARD_MSGMAX has  changed  across  kernel  ver‐
109              sions:
110
111              *  Up to Linux 2.6.32: 131072 / sizeof(void *)
112
113              *  Linux 2.6.33 to 3.4: (32768 * sizeof(void *) / 4)
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115              *  Since Linux 3.5: 65,536
116
117       /proc/sys/fs/mqueue/msgsize_default (since Linux 3.5)
118              This  file  defines  the value used for a new queue's mq_msgsize
119              setting when the queue is created  with  a  call  to  mq_open(3)
120              where  attr  is  specified  as NULL.  The default value for this
121              file is 8192 (bytes).   The  minimum  and  maximum  are  as  for
122              /proc/sys/fs/mqueue/msgsize_max.  If msgsize_default exceeds ms‐
123              gsize_max, a new queue's default mq_msgsize value is  capped  to
124              the  msgsize_max  limit.   Up  until  Linux  2.6.28, the default
125              mq_msgsize was 8192; from Linux 2.6.28 to Linux 3.4, the default
126              was the value defined for the msgsize_max limit.
127
128       /proc/sys/fs/mqueue/msgsize_max
129              This file can be used to view and change the ceiling on the max‐
130              imum message  size.   This  value  acts  as  a  ceiling  on  the
131              attr->mq_msgsize  argument  given  to  mq_open(3).   The default
132              value for msgsize_max is 8192 bytes.  The minimum value  is  128
133              (8192  in  kernels  before  2.6.28).   The  upper limit for msg‐
134              size_max has varied across kernel versions:
135
136              *  Before Linux 2.6.28, the upper limit is INT_MAX.
137
138              *  From Linux 2.6.28 to 3.4, the limit is 1,048,576.
139
140              *  Since Linux 3.5, the limit is 16,777,216 (HARD_MSGSIZEMAX).
141
142              The  msgsize_max  limit  is  ignored  for   privileged   process
143              (CAP_SYS_RESOURCE),  but,  since  Linux 3.5, the HARD_MSGSIZEMAX
144              ceiling is enforced for privileged processes.
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146       /proc/sys/fs/mqueue/queues_max
147              This file can be used to view and change the  system-wide  limit
148              on  the  number  of message queues that can be created.  The de‐
149              fault value for queues_max is 256.  No ceiling is imposed on the
150              queues_max  limit;  privileged  processes (CAP_SYS_RESOURCE) can
151              exceed the limit (but see BUGS).
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153   Resource limit
154       The RLIMIT_MSGQUEUE resource limit, which places a limit on the  amount
155       of space that can be consumed by all of the message queues belonging to
156       a process's real user ID, is described in getrlimit(2).
157
158   Mounting the message queue filesystem
159       On Linux, message queues are created in a virtual  filesystem.   (Other
160       implementations  may  also  provide such a feature, but the details are
161       likely to differ.)  This filesystem can be mounted (by  the  superuser)
162       using the following commands:
163
164           # mkdir /dev/mqueue
165           # mount -t mqueue none /dev/mqueue
166
167       The sticky bit is automatically enabled on the mount directory.
168
169       After the filesystem has been mounted, the message queues on the system
170       can be viewed and manipulated using the commands usually used for files
171       (e.g., ls(1) and rm(1)).
172
173       The  contents  of  each  file in the directory consist of a single line
174       containing information about the queue:
175
176           $ cat /dev/mqueue/mymq
177           QSIZE:129     NOTIFY:2    SIGNO:0    NOTIFY_PID:8260
178
179       These fields are as follows:
180
181       QSIZE  Number of bytes of data in all messages in the  queue  (but  see
182              BUGS).
183
184       NOTIFY_PID
185              If  this  is  nonzero,  then  the process with this PID has used
186              mq_notify(3) to register for asynchronous message  notification,
187              and the remaining fields describe how notification occurs.
188
189       NOTIFY Notification  method:  0 is SIGEV_SIGNAL; 1 is SIGEV_NONE; and 2
190              is SIGEV_THREAD.
191
192       SIGNO  Signal number to be used for SIGEV_SIGNAL.
193
194   Linux implementation of message queue descriptors
195       On Linux, a message queue descriptor is  actually  a  file  descriptor.
196       (POSIX  does  not  require  such an implementation.)  This means that a
197       message queue descriptor can be monitored using select(2), poll(2),  or
198       epoll(7).  This is not portable.
199
200       The  close-on-exec  flag (see open(2)) is automatically set on the file
201       descriptor returned by mq_open(2).
202
203   IPC namespaces
204       For a discussion of the interaction of POSIX message queue objects  and
205       IPC namespaces, see ipc_namespaces(7).
206

NOTES

208       System  V message queues (msgget(2), msgsnd(2), msgrcv(2), etc.) are an
209       older API for exchanging messages  between  processes.   POSIX  message
210       queues  provide  a  better  designed  interface  than  System V message
211       queues; on the other hand POSIX message queues are less  widely  avail‐
212       able (especially on older systems) than System V message queues.
213
214       Linux  does  not  currently  (2.6.26) support the use of access control
215       lists (ACLs) for POSIX message queues.
216

BUGS

218       In Linux versions 3.5 to 3.14, the kernel imposed  a  ceiling  of  1024
219       (HARD_QUEUESMAX)  on  the  value to which the queues_max limit could be
220       raised, and the ceiling was enforced  even  for  privileged  processes.
221       This  ceiling  value  was  removed in Linux 3.14, and patches to stable
222       kernels 3.5.x to 3.13.x also removed the ceiling.
223
224       As originally implemented (and documented), the QSIZE  field  displayed
225       the  total  number of (user-supplied) bytes in all messages in the mes‐
226       sage queue.  Some changes in Linux 3.5 inadvertently changed the behav‐
227       ior,  so that this field also included a count of kernel overhead bytes
228       used to store the messages in the queue.   This  behavioral  regression
229       was  rectified in Linux 4.2 (and earlier stable kernel series), so that
230       the count once more included just the bytes of user data in messages in
231       the queue.
232

EXAMPLES

234       An  example  of  the use of various message queue functions is shown in
235       mq_notify(3).
236

SEE ALSO

238       getrlimit(2),  mq_getsetattr(2),   poll(2),   select(2),   mq_close(3),
239       mq_getattr(3),  mq_notify(3),  mq_open(3),  mq_receive(3),  mq_send(3),
240       mq_unlink(3), epoll(7), namespaces(7)
241

COLOPHON

243       This page is part of release 5.12 of the Linux  man-pages  project.   A
244       description  of  the project, information about reporting bugs, and the
245       latest    version    of    this    page,    can     be     found     at
246       https://www.kernel.org/doc/man-pages/.
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250Linux                             2020-06-09                    MQ_OVERVIEW(7)
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