Allow debug prints without __VA_ARGS__ in non-MSVC

[hemmick]

The terminating comma does not get removed in non-MSVC compilers. Use ## to consume the comma in this case, allowing zero-arg prints

[geky]

Hi @hemmick, thanks for creating a PR. Unfortunately this doesn't work in standard c99 which means if we start adopting zero-arg prints we'll start writing code that can't be ported to other standard compilers.

So the current solution is to avoid zero-arg prints. It's not a good solution but it's the only solution the standard offers us.

---

That was the current state of things, but it's been getting more and more annoying in a few other projects I've been working on so I looked around for more info and did find one portable hack that would get around this. It's not the greatest, but it does have the expected semantics:

#ifndef LFS_NO_DEBUG
#define LFS_DEBUG_(fmt, ...) \
    printf("waka_debug:%d: " fmt "%s\n", __LINE__, __VA_ARGS__)
#define LFS_DEBUG(...) \
    LFS_DEBUG_(__VA_ARGS__, "")
#else
#define LFS_DEBUG(fmt, ...)
#endif

I think that's what we should go with. And if the ##__VA_ARGS__ is available users can override the LFS_DEBUG declaration with their own locally.

We could detect some compilers and use ##__VA_ARGS__ where available, but messing around with this in GCC I kept getting a "ISO C99 requires at least one argument" warning. Searching around as far as I can tell this is impossible to disable with -pedantic, which is too useful to turn off.

It's still possible for users to override LFS_DEBUG locally if they're compiling without -pedantic, so I think this is the best solution available to us.

[dpgeorge]

FWIW, the way we deal with this kind of problem in MicroPython is to have 2 macros, one that takes no variable arguments and one that does (and must take at least one): https://github.com/micropython/micropython/blob/master/py/compile.c#L67-L68

[geky]

@dpgeorge, that is a good way to handle it! It hadn't even crossed my mind to just make a separate macro for single args. I'll have to pocket this for the future.

I still want to consider the printf("debug: " fmt "%s\n", args, "") option above for the ergonomics.

As far as I can tell the only downside is the runtime overhead of parsing another argument. But the logging functions are exactly expected to be performance sensitivity or lightweight. And if the overhead is a concern, the macros can be locally overwritten to use toolchain specific tricks like @hemmick's PR.

Interested in other thoughts though.

[hemmick]

@geky Thanks! Excellent write-up -- I will use your new method

[geky]

Hi @hemmick, did you have a chance to try the LFS_DEBUG indirection? Is it possible to update this PR? If not I can bring it in and make the suggested change in the next release.

[geky]

@hemmick Thanks for the original PR! The improved debug statements should now be available on master.