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


6       mq_overview - overview of POSIX message queues


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.
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).
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).
31       A  message  queue  descriptor  is  a reference to an open message queue
32       description (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.
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.
46       The remainder of this section describes some specific  details  of  the
47       Linux implementation of POSIX message queues.
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:
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)
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.
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.
75   Persistence
76       POSIX message  queues  have  kernel  persistence:  if  not  removed  by
77       mq_unlink(3), a message queue will exist until the system is shut down.
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.
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:
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.
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
106              (CAP_SYS_RESOURCE), but the HARD_MSGMAX ceiling is  nevertheless
107              imposed.
109              The  definition  of  HARD_MSGMAX  has changed across kernel ver‐
110              sions:
112              *  Up to Linux 2.6.32: 131072 / sizeof(void *)
114              *  Linux 2.6.33 to 3.4: (32768 * sizeof(void *) / 4)
116              *  Since Linux 3.5: 65,536
118       /proc/sys/fs/mqueue/msgsize_default (since Linux 3.5)
119              This file defines the value used for a  new  queue's  mq_msgsize
120              setting  when  the  queue  is  created with a call to mq_open(3)
121              where attr is specified as NULL.  The  default  value  for  this
122              file  is  8192  (bytes).   The  minimum  and  maximum are as for
123              /proc/sys/fs/mqueue/msgsize_max.   If  msgsize_default   exceeds
124              msgsize_max, a new queue's default mq_msgsize value is capped to
125              the msgsize_max limit.   Up  until  Linux  2.6.28,  the  default
126              mq_msgsize was 8192; from Linux 2.6.28 to Linux 3.4, the default
127              was the value defined for the msgsize_max limit.
129       /proc/sys/fs/mqueue/msgsize_max
130              This file can be used to view and change the ceiling on the max‐
131              imum  message  size.   This  value  acts  as  a  ceiling  on the
132              attr->mq_msgsize argument  given  to  mq_open(3).   The  default
133              value  for  msgsize_max is 8192 bytes.  The minimum value is 128
134              (8192 in kernels before  2.6.28).   The  upper  limit  for  msg‐
135              size_max has varied across kernel versions:
137              *  Before Linux 2.6.28, the upper limit is INT_MAX.
139              *  From Linux 2.6.28 to 3.4, the limit is 1,048,576.
141              *  Since Linux 3.5, the limit is 16,777,216 (HARD_MSGSIZEMAX).
143              The   msgsize_max   limit  is  ignored  for  privileged  process
144              (CAP_SYS_RESOURCE), but, since Linux  3.5,  the  HARD_MSGSIZEMAX
145              ceiling is enforced for privileged processes.
147       /proc/sys/fs/mqueue/queues_max
148              This  file  can be used to view and change the system-wide limit
149              on the number of  message  queues  that  can  be  created.   The
150              default  value  for queues_max is 256.  No ceiling is imposed on
151              the queues_max limit;  privileged  processes  (CAP_SYS_RESOURCE)
152              can exceed the limit (but see BUGS).
154   Resource limit
155       The  RLIMIT_MSGQUEUE resource limit, which places a limit on the amount
156       of space that can be consumed by all of the message queues belonging to
157       a process's real user ID, is described in getrlimit(2).
159   Mounting the message queue filesystem
160       On  Linux,  message queues are created in a virtual filesystem.  (Other
161       implementations may also provide such a feature, but  the  details  are
162       likely  to  differ.)  This filesystem can be mounted (by the superuser)
163       using the following commands:
165           # mkdir /dev/mqueue
166           # mount -t mqueue none /dev/mqueue
168       The sticky bit is automatically enabled on the mount directory.
170       After the filesystem has been mounted, the message queues on the system
171       can be viewed and manipulated using the commands usually used for files
172       (e.g., ls(1) and rm(1)).
174       The contents of each file in the directory consist  of  a  single  line
175       containing information about the queue:
177           $ cat /dev/mqueue/mymq
178           QSIZE:129     NOTIFY:2    SIGNO:0    NOTIFY_PID:8260
180       These fields are as follows:
182       QSIZE  Number  of  bytes  of data in all messages in the queue (but see
183              BUGS).
185       NOTIFY_PID
186              If this is nonzero, then the process  with  this  PID  has  used
187              mq_notify(3)  to register for asynchronous message notification,
188              and the remaining fields describe how notification occurs.
190       NOTIFY Notification method: 0 is SIGEV_SIGNAL; 1 is SIGEV_NONE;  and  2
191              is SIGEV_THREAD.
193       SIGNO  Signal number to be used for SIGEV_SIGNAL.
195   Linux implementation of message queue descriptors
196       On  Linux,  a  message  queue descriptor is actually a file descriptor.
197       (POSIX does not require such an implementation.)   This  means  that  a
198       message  queue descriptor can be monitored using select(2), poll(2), or
199       epoll(7).  This is not portable.
201       The close-on-exec flag (see open(2)) is automatically set on  the  file
202       descriptor returned by mq_open(2).
204   IPC namespaces
205       For  a  discussion  of  the interaction of System V IPC objects and IPC
206       namespaces, see namespaces(7).


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


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


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


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


244       This  page  is  part of release 4.15 of the Linux man-pages project.  A
245       description of the project, information about reporting bugs,  and  the
246       latest     version     of     this    page,    can    be    found    at
247       https://www.kernel.org/doc/man-pages/.
251Linux                             2017-09-15                    MQ_OVERVIEW(7)