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Thanks for the explanation. In case FILE was opaque in glibc, would the same test (including <stdio.h> and declaring a variable of type FILE) also fail with the

Thanks for the explanation. In case FILE was opaque in glibc, would the same test (including <stdio.h> and declaring a variable of type FILE) also fail with the unknown storage size error? If so, would linking again some library (-l) be necessary? EDIT: after some more thinking I assume the key is that we wouldn't be able to have a variable of type FILE, but a pointer, whose size is always known.

clappski 1y ago
You'd have an error about an incomplete type - see https://godbolt.org/z/G4Gsfn7MT https://godbolt.org/z/G4Gsfn7MT > a pointer, whose size is always known Yeah, this is exactly how it works. You work with a pointer that acts like a void* in your code, and the library with the definition is allowed to reach into the fields of that pointer. Normally you'd have a C API like struct Op; Op* init_op(); void free_op( Op* ); void do_something_with_op( Op* ); in the header provided by the library that you compile as part of your code, and the definition/implementation in some .a or .so/.dll that you'll link against.*