3 ms·

Well, you can do: int a; int a; static int b; static int b; And it's a valid way to define a single global/static symbol called `a`/`b`.

• arcticbull 7y ago
  They have different semantics. The static one can only be referenced in the current translation unit, whereas the non-static one is a global which can be referenced from any translation unit. The problem is that in the header-only case defining "int a" means you can only import the header from a single source file. With "static int a" the visibility is wrong. And if you did "extern int a" linking would fail because the symbol is never assigned to a translation unit.
  • tomjakubowski 7y ago
    Header-only libraries sometimes require you to define those externs in exactly one "impl" file, which you compile and link to your artifact. Something like this: // libfoo_impl.c // or in some other translation unit, such as main.c #define LIBFOO_IMPL #include "vendor/libfoo.h" // main.c #include "vendor/libfoo.h" int main() { foo_inc(); printf("%d\n", foo_count); return 0; } // vendor/libfoo.h #pragma once extern int foo_count; void foo_inc() { foo_count += 1; } #ifdef LIBFOO_IMPL int foo_count = 0; #endif main.c and libfoo_impl.c both include libfoo.h, which declares foo_count with the right linkage, but the variable is only defined once, in whichever translation unit defines LIBFOO_IMPL. Occasionally you'll find a header library which supports this _IMPL paradigm as an option to avoid inlining its functions in every translation unit that calls them.
  • megous 7y ago
    You're misunderstanding. I'm saying you can have multiple identical `int a;` across multiple translation units and it will point by default to a single global variable across the program. So you can include a single header from multiple places, and you just get a single shared global variable, if the header contains `int a`.