|MQ_OVERVIEW(7)||Linux Programmer's Manual||MQ_OVERVIEW(7)|
Message queues are created and opened using mq_open(3); 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).
Messages are transferred to and from a queue using mq_send(3) and mq_receive(3). When a process has finished using the queue, it closes it using mq_close(3), and when the queue is no longer required, it can be deleted using mq_unlink(3). Queue attributes can be retrieved and (in some cases) modified using mq_getattr(3) and mq_setattr(3). A process can request asynchronous notification of the arrival of a message on a previously empty queue using mq_notify(3).
A message queue descriptor is a reference to an open message queue description (see open(2)). After a 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 message queue descriptors in the parent. Corresponding message queue descriptors in the two processes share the flags (mq_flags) that are associated with the open message queue description.
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 interface||System call|
- /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) 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.
- 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). 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 ignored for privileged processes (CAP_SYS_RESOURCE), but the HARD_MSGMAX ceiling is nevertheless 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) 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.
- 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). 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 versions:
- 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.
- This file can be used to view and change the system-wide limit on the number of message 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).
# mkdir /dev/mqueue # mount -t mqueue none /dev/mqueue
The sticky bit is automatically enabled on the mount directory.
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:
- Number of bytes of data in all messages in the queue (but see BUGS).
- If this is nonzero, then the process with this PID has used mq_notify(3) to register for asynchronous message notification, and the remaining fields describe how notification occurs.
- Notification method: 0 is SIGEV_SIGNAL; 1 is SIGEV_NONE; and 2 is SIGEV_THREAD.
- Signal number to be used for SIGEV_SIGNAL.
Linux does not currently (2.6.26) support the use of access control lists (ACLs) for POSIX message queues.
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 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 (and earlier stable kernel series), so that the count once more included just the bytes of user data in messages in the queue.