Programming Embedded Systems, 2nd Edition

Task Synchronization

Though we frequently talk about the tasks in a multitasking operating system as completely independent entities, that’s not completely accurate. All of the tasks are working together to solve a larger problem and must occasionally communicate with one another to synchronize their activities. For example, in the print-server device, the printer task doesn’t have any work to do until new data is supplied to it by one of the computer tasks. So the printer and computer tasks must communicate with one another to coordinate their access to common data buffers.

Operating systems contain various mechanisms that aid in synchronization between two tasks. Each of the mechanisms is useful for different scenarios. A particular operating system may offer additional or slightly different types of methods for synchronization, so we will cover the ones commonly found in real-time operating systems.

In Chapter 8, we introduced the concept of a critical section, which is simply a segment of code that must be executed in its entirety before the operating system is allowed to run anything else. Often a critical section consists of a single C-language statement that sets or reads a variable. We saw some examples using interrupts in Chapter 8; another example could be a status variable that is shared between a printer task and other tasks in the system.

Remember that in a multitasking environment, you generally don’t know in which order the tasks will be executed at runtime. One task ...