Programming Embedded Systems in C and C++

Real-Time Characteristics 119

state = Available;

}

}

exitCS(); ////// Critical Section End

} /* release() */

Critical sections

The primary use of mutexes is for the protection of shared resources. Shared

resources are global variables, memory buffers, or device registers that are

accessed by multiple tasks. A mutex can be used to limit access to such a resource

to one task at a time. It is like the stoplight that controls access to an intersection.

Remember that in a multitasking environment you generally don’t know in which

order the tasks will be executed at runtime. One task might be writing some data

into a memory buffer when it is suddenly interrupted by a higher-priority task. If

the higher-priority task were to modify that same region of memory, then bad

things could happen. At the very least, some of the lower-priority task’s data

would be overwritten.

Pieces of code that access shared resources contain critical sections. We’ve already

seen something similar inside the operating system. There, we simply disabled

interrupts during the critical section. But tasks cannot (wisely) disable interrupts. If

they were allowed to do so, other tasks—even higher-priority tasks that didn’t

share the same resource—would not be able to execute during that interval. So we

want and need a mechanism to protect critical sections within tasks without dis-

abling interrupts. And mutexes provide that mechanism. ...