Created on 2024-04-04; last updated on 2024-05-13
If the type of a pointer has the volatile qualifier, then loads and stores with that pointer are guaranteed by the compiler to take place and to take place in the same order as in the source code relative to other volatile loads and stores.
The volatile qualifier on a pointer type communicates that the data referenced by pointers of that type may change outside the program's control flow. This has the effect of suppressing optimizations by the compiler (and only the compiler).
For example, the type *volatile T describes a single-item pointer to data that may change outside the program's control flow.
Pointers that do not have the volatile qualifier coerce to pointer types that do have the volatile qualifier. For example, pointers of type *T coerce to the type *volatile T.
The @volatileCast builtin function removes the volatile qualifier from a pointer.
Pointers having the volatile qualifier should be used for only memory-mapped input/output (MMIO).
There is discussion about introducing two builtin functions named @volatileLoad and @volatileStore that operate on pointers. These functions may or may not replace the volatile pointer attribute.
The volatile keyword was initially proposed in and implemented to close ziglang/zig issue 238.