Книга: Real-Time Concepts for Embedded Systems

7.4 Message Queue Content

7.4 Message Queue Content

Message queues can be used to send and receive a variety of data. Some examples include:

· a temperature value from a sensor,

· a bitmap to draw on a display,

· a text message to print to an LCD,

· a keyboard event, and

· a data packet to send over the network.

Some of these messages can be quite long and may exceed the maximum message length, which is determined when the queue is created. (Maximum message length should not be confused with total queue length, which is the total number of messages the queue can hold.) One way to overcome the limit on message length is to send a pointer to the data, rather than the data itself. Even if a long message might fit into the queue, it is sometimes better to send a pointer instead in order to improve both performance and memory utilization.

When a task sends a message to another task, the message normally is copied twice, as shown in Figure 7.3 The first time, the message is copied when the message is sent from the sending task’s memory area to the message queue’s memory area. The second copy occurs when the message is copied from the message queue’s memory area to the receiving task’s memory area.

An exception to this situation is if the receiving task is already blocked waiting at the message queue. Depending on a kernel’s implementation, the message might be copied just once in this case-from the sending task’s memory area to the receiving task’s memory area, bypassing the copy to the message queue’s memory area.

Because copying data can be expensive in terms of performance and memory requirements, keep copying to a minimum in a real-time embedded system by keeping messages small or, if that is not feasible, by using a pointer instead.

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