Volatile variables

miikalo wrote on Thursday, August 27, 2009:


Is it necessary or encouraged to use volatile variables when a global variable is shared by a lower priority and a higher priority task? Thanks for your replies.

spacewrench wrote on Thursday, August 27, 2009:

“Sometimes,” or “it depends.”  It’s a compiler question, not a FreeRTOS issue.  If a task that uses the variable does not call any subroutines (or if the compiler can prove that the subroutines it does call never alter the variable), then the optimizer may put the variable in a register temporary, or move the test outside a loop, or something like that.  The “volatile” keyword tells the compiler that it must read (and write) the variable every time the program says to do so.  (The precise requirements that the compiler must meet are quite a bit more complex – I think it may also have to ensure that volatile reads and writes happen in the same order as the program, and possibly that other memory accesses, even non-volatile ones, have completed before [or begin after] the volatile access.)

Task priority does not come into it at all.  Compilers usually don’t do anything different for threaded/tasked programs, and the RTOS ensures that each task can’t detect context switches (registers & stack never change in ways that the compiler doesn’t expect).