I know that there are many questions like this, but this question is not about what static and volatile means from C standard's point of view. I'm interested in what is happening a bit lower - on assembly level.

static keyword for variables makes those variables to be visible statically (static storage duration), like global variables. To make it real a compiler should write those variables to .bss section or somewhere else? Also, static keyword prevents the variable/function to be used outside the file, does it happen only during compilation or there are some runtime-checks?

volatile keyword for variables makes those variables to be read from memory to make sure that if something else (like Peripheral Devices) wants to modify that variable it'll see exactly the value from that memory. Here, what does exactly mean "to be read from memory"? What is memory location used? .bss, .data, or something else?

Thanks!

CodePudding user response：

The static keyword has two meanings: (a) it conveys static storage class and (b) it conveys internal linkage. These two meanings have to be strictly distinguished.

An object having static storage class means that it is allocated at the start of the program and lives until the end of the program. This is usually achieved by placing the object into the data segment (for initialised objects) or into the bss segment (for uninitialised objects). Details may vary depending on the toolchain in question.

An identifier having internal linkage means that each identifier in the same translation unit with the same name and some linkage (i.e. the linkage is not “none”) refers to the same object as that identifier. This is usually realised by not making the symbol corresponding to the identifier a global symbol. The linker will then not recognise references of the same symbol from different translation units as referring to the same symbol.

The volatile keyword indicates that all operations performed on the volatile-qualified object in the abstract machine must be performed in the code generated. The compiler is not permitted to perform any optimisations that would discard any such operations performed on the volatile-qualified object as it usually would for non-volatile-qualified objects.

This keyword is purely an instruction to the compiler to suppress certain optimisations. It does not affect the storage class of the objects qualified such. See also my previous answer on this topic.