Is it legal C to declare a non-const variable 'const' externally?

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Say I have a file, window.h, which defines:

extern const int window_width, window_height;

I don't want anyone to change these variables, so they're const for all includers of window.h. However, is it legal to declare them non-const in the source file?

// window.c
int window_width, window_height;

void onResizeWindow(int w, int h) {
    window_width = w;
    window_height = h;
}

This compiles without linker errors for me in Apple clang version 12.0.0 (clang-1200.0.22.7). But is it legal and well-defined C?

constants linker c
1 Answers

No, it is undefined behaviour. Two declarations for the same object must have compatible types, and a non-const-qualified type is not compatible with the const-qualified version of the same type. (Like other issues which involve two or more translation units, this undefined behaviour does not require a diagnostic. But the absence of a diagnostic message doesn't mean that it's OK. It just means that the compiler only looks at one translation unit at a time, so it can't see inconsistencies.)

It is not an error to use a pointer to a const-qualified type to access a non-const object of that type, or even to use a pointer to a non-const qualified type to read from (but not mutate) a const-qualified object. But be aware that as long as the actual definition of the variable is not const-qualified, it is also not an error to "cast away const" from a pointer and use it to modify the variable.

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