The variables are updated only in the interrupt. I know the returned value of a function is an rval. The fact is that I read C declarations left to right just like everything else. The const keyword specifies that the pointer cannot be modified after initialization; the pointer is protected from modification thereafter. When you store to a global variable in C, what computations running on the machine are guaranteed to see the store? If we take this view all optimizing compilers I am aware of do then the text I wrote in this post applies. However, some older compilers used that as a trick to tell the compiler that the function does not return.
Misplaced qualification At the level of C syntax, volatile is a type qualifier. The volatile declaration belongs in the structure: it is the device registers which are volatile and that is where the information should be kept; it reinforces the fact for a human reader. When a name is declared as volatile, the compiler reloads the value from memory each time it is accessed by the program. Then you have unavoidable duplication and you may choose to optimise against different goals anyway. Unfortunately, most books about the C programming language dismiss volatile in a sentence or two.
The volatile keyword used to restrain the compiler from making the assumption about the object value. It should be understood that Standard C takes no special precautions to avoid these problems, and that the usual techniques should be employed. However, the reverse is not true; no implicit conversion exists to remove the const attribute from an object or pointer. Another bad solution is to insert a call to an external function where you would put the barrier. The DoWork method may optimize the member access, resulting in reading stale data.
. In below section, I am describing some situation where pointer and volatile keyword come together. That means that an alternative rewrite of the last example would be possible. This whole class of problem is what is known as a critical region, and is well understood by those who regularly work in asynchronous environments. Consider below code, we are changing value of const object using pointer and we are compiling code without optimization option.
You have processor registers, various levels of caches, and main memory shared by multiple processors. Moreover, code motion is not permitted around the barrier in either direction. Therefore, the volatile keyword is useful for declaring objects in shared memory that can be accessed by multiple processes or global data areas used for communication with interrupt service routines. The C standard is unambiguous that volatile side effects must not move past sequence points, so this will work. Detailed explanation link - How to Answer for What is Volatile? This article will teach you the proper way to do it.
The issue here is one of visibility. I won't ask you about people, but I could ask you why you use the exceptional, non-canonical form. Make a volatile function pointer to func, and call the function via the function pointer. If you are unlucky or you haven't yet learned to take compiler warnings seriously , your code will fail miserably. A const pointer of a given type can be assigned to a pointer of the same type. I'm not sure how you can declare a function to be volatile, only a pointer to a function, the same as any variable type as opposed to a function. That way, all of the registers in the struct will be treated, by the compiler, as volatile and yet it is possible to have other e.
When you are reading data that is non-volatile, the executing thread may or may not always get the latest value. With this placement, only the specific registers within the struct that are declared volatile will be treated, by the compiler, as subject to asynchronous change. The correct analysis of a declarator is a syntactic one, similar to the analysis of an expression. This article was published in the July 2001 issue of. Maybe a peripheral version code register doesn't need volatile as that value should never change, but status, interrupt mask, receive buffer, etc.
Resist this temptation, since it is essentially a substitute for thought. However, it is far more common that memory-mapped peripherals have at least a half dozen registers. Unless you defeat the intent by doing naughty things with pointers, a compiler is able to check that const objects are not modified explicitly by the user. There are languages where an attribute like 'constant' appears after the object that is constant, just because your language puts it before the object doesn't mean C should. Volatile keywords can be used with pointers in following ways We can use the pointer with a volatile keyword in many ways.
} MyReg ; If you want to learn more about the c language, here 10 Free days up to 200 minutes for you. To discuss this is any depth would take us beyond the scope of this book, but we can at least outline some of the issues. Therefore, the volatile keyword is useful for declaring objects in shared memory that can be accessed by multiple processes or global data areas used for communication with interrupt service routines. When we declare a variable of volatile then all member of the structure are volatile but sometime in the program, we need only some specific member as volatile so in that situation, we have to declare this member implicitly volatile. The compiler is not allowed to reorder volatile variable accesses for any reason. For instance, a lot of peripherals contain registers that are cleared simply by reading them. This makes no sense to me.