Understanding the Linux Kernel, 3rd Edition

POSIX Message Queues

The POSIX standard (IEEE Std 1003.1-2001) defines an IPC mechanism based on message queues, which is usually known as POSIX message queues . They are much like the System V IPC’s message queues already examined in the section "IPC Messages" earlier in this chapter. However, POSIX message queues sport a number of advantages over the older queues:

• A much simpler file-based interface to the applications

• Native support for message priorities (the priority ultimately determines the position of the message in the queue)

• Native support for asynchronous notification of message arrivals, either by means of signals or thread creation

• Timeouts for blocking send and receive operations

POSIX message queues are handled by means of a set of library functions, which are shown in Table 19-15.

Table 19-15. Library functions for POSIX message queues

Function names	Description
mq_open( )	Open (optionally creating) a POSIX message queue
mq_close( )	Close a POSIX message queue (without destroying it)
mq_unlink( )	Destroy a POSIX message queue
mq_send( ) , mq_timedsend( )	Send a message to a POSIX message queue; the latter function defines a time limit for the operation
mq_receive( ) , mq_timedreceive( )	Fetch a message from a POSIX message queue; the latter function defines a time limit for the operation
mq_notify( )	Establish an asynchronous notification mechanism for the arrival of messages in an empty POSIX message queue
mq_getattr( ) , mq_setattr( )	Respectively ...