LOCAL static 变量何时初始化

Question

I keep on seeing the same sentence that static variables are initialized only once, and I also saw a sentence stating that "when the block is entered for the first time".

Are local static variables initialized like other global variables - at the start of program execution? Or do local static variables differ from normal globals, and only get initialized once their function/block is called/reached?

Answer 1

C17 6.2.4 (3)

An object whose identifier is declared without the storage-class specifier _Thread_local , and either with external or internal linkage or with the storage-class specifier static , has static storage duration. Its lifetime is the entire execution of the program and its stored value is initialized only once, prior to program startup.

However, remember the as-if rule . An implementation could wait to initialize the variable until the first call to the function, as a conforming program has no way to access its value before then, and so would not be able to tell the difference.

If you have an implementation with extensions or implementation-defined behavior that do provide a way to access the variable before the first call to the function, then such an implementation ought to document whether you would see the initialized value in such a case. In most cases I would expect the answer to be "yes".

The most common implementation I'm familiar with is to load the initial value from the executable, or to place it in a bss section that is zeroed at startup, just as is done for global or file-scope static variables.

Answer 2

Although implementation dependent, static variables - any scope - are initialized as the executable is loaded.