# mq_overview(7) - man - phpMan [MQ_OVERVIEW(7)](https://www.chedong.com/phpMan.php/man/MQOVERVIEW/7/markdown) Linux Programmer's Manual [MQ_OVERVIEW(7)](https://www.chedong.com/phpMan.php/man/MQOVERVIEW/7/markdown) ## NAME mq_overview - overview of POSIX message queues ## DESCRIPTION POSIX message queues allow processes to exchange data in the form of messages. This API is distinct from that provided by System V message queues ([**msgget**(2)](https://www.chedong.com/phpMan.php/man/msgget/2/markdown), [**msgsnd**(2)](https://www.chedong.com/phpMan.php/man/msgsnd/2/markdown), [**msgrcv**(2)](https://www.chedong.com/phpMan.php/man/msgrcv/2/markdown), etc.), but provides similar functionality. Message queues are created and opened using **mq**___**[open**(3)](https://www.chedong.com/phpMan.php/man/open/3/markdown); this function returns a _message_ _queue_ _descriptor_ (_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 _/somename_; that is, a null-terminated string of up to **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 **mq**___**[open**(3)](https://www.chedong.com/phpMan.php/man/open/3/markdown). Messages are transferred to and from a queue using **mq**___**[send**(3)](https://www.chedong.com/phpMan.php/man/send/3/markdown) and **mq**___**[receive**(3)](https://www.chedong.com/phpMan.php/man/receive/3/markdown). When a process has finished using the queue, it closes it using **mq**___**[close**(3)](https://www.chedong.com/phpMan.php/man/close/3/markdown), and when the queue is no longer required, it can be deleted using **mq**___**[unlink**(3)](https://www.chedong.com/phpMan.php/man/unlink/3/markdown). Queue attributes can be retrieved and (in some cases) modified using **mq**___**[getattr**(3)](https://www.chedong.com/phpMan.php/man/getattr/3/markdown) and **mq**___**[setattr**(3)](https://www.chedong.com/phpMan.php/man/setattr/3/markdown). A process can request asynchronous notification of the arrival of a message on a previously empty queue using **mq**___**[notify**(3)](https://www.chedong.com/phpMan.php/man/notify/3/markdown). A message queue descriptor is a reference to an _open_ _message_ _queue_ _description_ (see [**open**(2)](https://www.chedong.com/phpMan.php/man/open/2/markdown)). After a [**fork**(2)](https://www.chedong.com/phpMan.php/man/fork/2/markdown), 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 message queue descriptors in the parent. Corresponding message queue descriptors in the two pro‐ cesses share the flags (_mq_flags_) that are associated with the open message queue descrip‐ tion. Each message has an associated _priority_, and messages are always delivered to the receiving process highest priority first. Message priorities range from 0 (low) to _sysconf(_SC_MQ_PRIO_MAX)_ _-_ _1_ (high). On Linux, _sysconf(_SC_MQ_PRIO_MAX)_ 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. The remainder of this section describes some specific details of the Linux implementation of POSIX message queues. ### Library interfaces and system calls In most cases the **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: **Library** **interface** **System** **call** [mq_close(3)](https://www.chedong.com/phpMan.php/man/mqclose/3/markdown) [close(2)](https://www.chedong.com/phpMan.php/man/close/2/markdown) [mq_getattr(3)](https://www.chedong.com/phpMan.php/man/mqgetattr/3/markdown) [mq_getsetattr(2)](https://www.chedong.com/phpMan.php/man/mqgetsetattr/2/markdown) [mq_notify(3)](https://www.chedong.com/phpMan.php/man/mqnotify/3/markdown) [mq_notify(2)](https://www.chedong.com/phpMan.php/man/mqnotify/2/markdown) [mq_open(3)](https://www.chedong.com/phpMan.php/man/mqopen/3/markdown) [mq_open(2)](https://www.chedong.com/phpMan.php/man/mqopen/2/markdown) [mq_receive(3)](https://www.chedong.com/phpMan.php/man/mqreceive/3/markdown) [mq_timedreceive(2)](https://www.chedong.com/phpMan.php/man/mqtimedreceive/2/markdown) [mq_send(3)](https://www.chedong.com/phpMan.php/man/mqsend/3/markdown) [mq_timedsend(2)](https://www.chedong.com/phpMan.php/man/mqtimedsend/2/markdown) [mq_setattr(3)](https://www.chedong.com/phpMan.php/man/mqsetattr/3/markdown) [mq_getsetattr(2)](https://www.chedong.com/phpMan.php/man/mqgetsetattr/2/markdown) [mq_timedreceive(3)](https://www.chedong.com/phpMan.php/man/mqtimedreceive/3/markdown) [mq_timedreceive(2)](https://www.chedong.com/phpMan.php/man/mqtimedreceive/2/markdown) [mq_timedsend(3)](https://www.chedong.com/phpMan.php/man/mqtimedsend/3/markdown) [mq_timedsend(2)](https://www.chedong.com/phpMan.php/man/mqtimedsend/2/markdown) [mq_unlink(3)](https://www.chedong.com/phpMan.php/man/mqunlink/3/markdown) [mq_unlink(2)](https://www.chedong.com/phpMan.php/man/mqunlink/2/markdown) ### Versions POSIX message queues have been supported on Linux since kernel 2.6.6. Glibc support has been provided since version 2.3.4. ### Kernel configuration Support for POSIX message queues is configurable via the **CONFIG**___**POSIX**___**MQUEUE** kernel configu‐ ration option. This option is enabled by default. ### Persistence POSIX message queues have kernel persistence: if not removed by **mq**___**[unlink**(3)](https://www.chedong.com/phpMan.php/man/unlink/3/markdown), a message queue will exist until the system is shut down. ### Linking Programs using the POSIX message queue API must be compiled with _cc_ _-lrt_ to link against the real-time library, _librt_. ### /proc interfaces The following interfaces can be used to limit the amount of kernel memory consumed by POSIX message queues and to set the default attributes for new message queues: _/proc/sys/fs/mqueue/msg_default_ (since Linux 3.5) This file defines the value used for a new queue's _mq_maxmsg_ setting when the queue is created with a call to **mq**___**[open**(3)](https://www.chedong.com/phpMan.php/man/open/3/markdown) where _attr_ is specified as NULL. The default value for this file is 10. The minimum and maximum are as for _/proc/sys/fs/mqueue/msg_max_. A new queue's default _mq_maxmsg_ value will be the smaller of _msg_default_ and _msg_max_. Up until Linux 2.6.28, the default _mq_maxmsg_ was 10; from Linux 2.6.28 to Linux 3.4, the default was the value defined for the _msg_max_ limit. _/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 _attr->mq_maxmsg_ argument given to **mq**___**[open**(3)](https://www.chedong.com/phpMan.php/man/open/3/markdown). The default value for _msg_max_ is 10. The minimum value is 1 (10 in kernels before 2.6.28). The upper limit is **HARD**___**MSGMAX**. The _msg_max_ limit is ig‐ nored for privileged processes (**CAP**___**SYS**___**RESOURCE**), but the **HARD**___**MSGMAX** ceiling is nev‐ ertheless imposed. The definition of **HARD**___**MSGMAX** has changed across kernel versions: * Up to Linux 2.6.32: _131072_ _/_ _sizeof(void_ _*)_ * Linux 2.6.33 to 3.4: _(32768_ _*_ _sizeof(void_ _*)_ _/_ _4)_ * Since Linux 3.5: 65,536 _/proc/sys/fs/mqueue/msgsize_default_ (since Linux 3.5) This file defines the value used for a new queue's _mq_msgsize_ setting when the queue is created with a call to **mq**___**[open**(3)](https://www.chedong.com/phpMan.php/man/open/3/markdown) where _attr_ is specified as NULL. The default value for this file is 8192 (bytes). The minimum and maximum are as for _/proc/sys/fs/mqueue/msgsize_max_. If _msgsize_default_ exceeds _msgsize_max_, a new queue's default _mq_msgsize_ value is capped to the _msgsize_max_ limit. Up until Linux 2.6.28, the default _mq_msgsize_ was 8192; from Linux 2.6.28 to Linux 3.4, the default was the value defined for the _msgsize_max_ limit. _/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 _attr->mq_msgsize_ argument given to **mq**___**[open**(3)](https://www.chedong.com/phpMan.php/man/open/3/markdown). The default value for _msgsize_max_ is 8192 bytes. The minimum value is 128 (8192 in kernels before 2.6.28). The upper limit for _msgsize_max_ has varied across kernel ver‐ sions: * Before Linux 2.6.28, the upper limit is **INT**___**MAX**. * From Linux 2.6.28 to 3.4, the limit is 1,048,576. * Since Linux 3.5, the limit is 16,777,216 (**HARD**___**MSGSIZEMAX**). The _msgsize_max_ limit is ignored for privileged process (**CAP**___**SYS**___**RESOURCE**), but, since Linux 3.5, the **HARD**___**MSGSIZEMAX** ceiling is enforced for privileged processes. _/proc/sys/fs/mqueue/queues_max_ This file can be used to view and change the system-wide limit on the number of mes‐ sage queues that can be created. The default value for _queues_max_ is 256. No ceiling is imposed on the _queues_max_ limit; privileged processes (**CAP**___**SYS**___**RESOURCE**) can exceed the limit (but see BUGS). ### Resource limit The **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 [**getrlimit**(2)](https://www.chedong.com/phpMan.php/man/getrlimit/2/markdown). ### 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 filesystem can be mounted (by the superuser) using the following commands: # **mkdir** **/dev/mqueue** # **mount** **-t** **mqueue** **none** **/dev/mqueue** The sticky bit is automatically enabled on the mount directory. After the filesystem has been mounted, the message queues on the system can be viewed and ma‐ nipulated using the commands usually used for files (e.g., [**ls**(1)](https://www.chedong.com/phpMan.php/man/ls/1/markdown) and [**rm**(1)](https://www.chedong.com/phpMan.php/man/rm/1/markdown)). The contents of each file in the directory consist of a single line containing information about the queue: $ **cat** **/dev/mqueue/mymq** QSIZE:129 NOTIFY:2 SIGNO:0 NOTIFY_PID:8260 These fields are as follows: **QSIZE** Number of bytes of data in all messages in the queue (but see BUGS). **NOTIFY**___**PID** If this is nonzero, then the process with this PID has used **mq**___**[notify**(3)](https://www.chedong.com/phpMan.php/man/notify/3/markdown) to register for asynchronous message notification, and the remaining fields describe how notifica‐ tion occurs. **NOTIFY** Notification method: 0 is **SIGEV**___**SIGNAL**; 1 is **SIGEV**___**NONE**; and 2 is **SIGEV**___**THREAD**. **SIGNO** Signal number to be used for **SIGEV**___**SIGNAL**. ### 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 [**select**(2)](https://www.chedong.com/phpMan.php/man/select/2/markdown), [**poll**(2)](https://www.chedong.com/phpMan.php/man/poll/2/markdown), or [**epoll**(7)](https://www.chedong.com/phpMan.php/man/epoll/7/markdown). This is not portable. The close-on-exec flag (see [**open**(2)](https://www.chedong.com/phpMan.php/man/open/2/markdown)) is automatically set on the file descriptor returned by **mq**___**[open**(2)](https://www.chedong.com/phpMan.php/man/open/2/markdown). ### IPC namespaces For a discussion of the interaction of POSIX message queue objects and IPC namespaces, see **ipc**___**[namespaces**(7)](https://www.chedong.com/phpMan.php/man/namespaces/7/markdown). ## NOTES System V message queues ([**msgget**(2)](https://www.chedong.com/phpMan.php/man/msgget/2/markdown), [**msgsnd**(2)](https://www.chedong.com/phpMan.php/man/msgsnd/2/markdown), [**msgrcv**(2)](https://www.chedong.com/phpMan.php/man/msgrcv/2/markdown), etc.) are an older API for exchang‐ ing messages between processes. POSIX message queues provide a better designed interface than System V message queues; on the other hand POSIX message queues are less widely avail‐ able (especially on older systems) than System V message queues. Linux does not currently (2.6.26) support the use of access control lists (ACLs) for POSIX message queues. ## BUGS In Linux versions 3.5 to 3.14, the kernel imposed a ceiling of 1024 (**HARD**___**QUEUESMAX**) on the value to which the _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. 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 inad‐ vertently 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 (and earlier stable kernel series), so that the count once more included just the bytes of user data in messages in the queue. ## EXAMPLES An example of the use of various message queue functions is shown in **mq**___**[notify**(3)](https://www.chedong.com/phpMan.php/man/notify/3/markdown). ## SEE ALSO [**getrlimit**(2)](https://www.chedong.com/phpMan.php/man/getrlimit/2/markdown), **mq**___**[getsetattr**(2)](https://www.chedong.com/phpMan.php/man/getsetattr/2/markdown), [**poll**(2)](https://www.chedong.com/phpMan.php/man/poll/2/markdown), [**select**(2)](https://www.chedong.com/phpMan.php/man/select/2/markdown), **mq**___**[close**(3)](https://www.chedong.com/phpMan.php/man/close/3/markdown), **mq**___**[getattr**(3)](https://www.chedong.com/phpMan.php/man/getattr/3/markdown), **mq**___**[notify**(3)](https://www.chedong.com/phpMan.php/man/notify/3/markdown), **mq**___**[open**(3)](https://www.chedong.com/phpMan.php/man/open/3/markdown), **mq**___**[receive**(3)](https://www.chedong.com/phpMan.php/man/receive/3/markdown), **mq**___**[send**(3)](https://www.chedong.com/phpMan.php/man/send/3/markdown), **mq**___**[unlink**(3)](https://www.chedong.com/phpMan.php/man/unlink/3/markdown), [**epoll**(7)](https://www.chedong.com/phpMan.php/man/epoll/7/markdown), [**namespaces**(7)](https://www.chedong.com/phpMan.php/man/namespaces/7/markdown) ## COLOPHON This page is part of release 5.10 of the Linux _man-pages_ project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at . Linux 2020-06-09 [MQ_OVERVIEW(7)](https://www.chedong.com/phpMan.php/man/MQOVERVIEW/7/markdown)