Why is volatile needed in C?

Question

Why is volatile needed in C? What is it used for? What will it do?

Answer 1

In the language designed by Dennis Ritchie, every access to any object, other than automatic objects whose address had not been taken, would behave as though it computed the address of the object and then read or wrote the storage at that address. This made the language very powerful, but severely limited optimization opportunities.

While it might have been possible to add a qualifier that would invite a compiler to assume that a particular object wouldn't be changed in weird ways, such an assumption would be appropriate for the vast majority of objects in C programs, and it would have been impractical to add a qualifier to all the objects for which such assumption would be appropriate. On the other hand, some programs need to use some objects for which such an assumption would not hold. To resolve this issue, the Standard says that compilers may assume that objects which are not declared volatile will not have their value observed or changed in ways that are outside the compiler's control, or would be outside a reasonable compiler's understanding.

Because various platforms may have different ways in which objects could be observed or modified outside a compiler's control, it is appropriate that quality compilers for those platforms should differ in their exact handling of volatile semantics. Unfortunately, because the Standard failed to suggest that quality compilers intended for low-level programming on a platform should handle volatile in a way that will recognize any and all relevant effects of a particular read/write operation on that platform, many compilers fall short of doing so in ways that make it harder to process things like background I/O in a way which is efficient but can't be broken by compiler "optimizations".