bogus `-Wmissing-prototypes` warning with `extern "C"` function (fuzzer)

[firewave]

If you are writing a fuzzer the code will basically like this:

#include <cstddef>
#include <cstdint>

extern "C" int LLVMFuzzerTestOneInput(const uint8_t*, size_t)
{
    return 0;
}

But this code will produce a warning:

clang-13 -fsanitize=fuzzer -Weverything -o main main.cpp
main.cpp:4:16: warning: no previous prototype for function 'LLVMFuzzerTestOneInput' [-Wmissing-prototypes]
extern "C" int LLVMFuzzerTestOneInput(const uint8_t*, size_t)
               ^
main.cpp:4:12: note: declare 'static' if the function is not intended to be used outside of this translation unit
extern "C" int LLVMFuzzerTestOneInput(const uint8_t*, size_t)
           ^
           static
1 warning generated.

If you fix that warning it will obviously no longer link.

[EugeneZelenko]

I think it's reasonable warning. Prototypes belong to header files.

[firewave]

I think it's reasonable warning. Prototypes belong to header files.

But in this case there is no header - no even an external one. It's just a symbol expected by the fuzzer.

The example code from the libFuzzer documentation looks the same: https://llvm.org/docs/LibFuzzer.html#toy-example.

[EugeneZelenko]

If symbol is expected, so it should be defined in header and this header should be included. Same as in with any other C code.

[firewave]

I did not design libFuzzer and there's no header AFAIK. So this issue should be tagged with the fuzzer as well so someone from there can chime in.

[MaskRay]

The diagnostic is emitted as expected. It may not be desired in many scenarios: that is why -Wmissing-prototypes is in neither -Wall nor -Wextra. You enable -Weverything and are expected to see various warnings:)

Adding a declaration can suppress the warning, but I think most people will agree with me that libFuzzer probably should not advice this:

extern "C" int LLVMFuzzerTestOneInput(const uint8_t*, size_t);
extern "C" int LLVMFuzzerTestOneInput(const uint8_t*, size_t)
{
    return 0;
}

So I don't think this issue provides any action item worth addressing.

[ehem]

One word added and two emphasized:

If symbol is expected, so it should usually be defined in header and this header should be included. Same as in with any other C code.

Trick is if one is interfacing with source files not written in C and the program which processes those doesn't understand C headers (but can interface with C), then it is pointless to have the declaration in a header file. Notably a function which is called from assembly-language source.

This is a troublesome inconsistency that:

static void foo(void)
{
	printf("Hello, world!\n");
}
/* something which calls foo */

Is acceptable, yet:

extern void foo(void)
{
	printf("Hello, world!\n");
}

Is not. A better excuse would be GCC does it this way, but the inconsistency is Bad.