Implicit function delcaration error with Apple Clang

[p-linnane]

Hello 👋 . I'm a maintainer for the Homebrew project. We've noticed the following error with Apple Clang when trying to compile augeas. We are currently working around this by passing -Wno-implicit-function-declaration, but hoping to get an upstream fix. Thanks!

lens.c:979:9: error: call to undeclared function 'dump_lens_tree'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
        dump_lens_tree(lens);
        ^
1 error generated.

[igalic]

this code is a bit weird. both dump_lens_tree and the debugging function guarding its call are gated by # if ENABLE_DEBUG. but unlike debugging, there's no # else branch declaring a (useless) dump_lens_tree version to appease the compiler.

so, despite the fact that the compiler sees this:

if ((0))
      dump_lens_tree(lens);

it doesn't just delete the code from the AST, it tries to actually analyse it.

A possibly fix would be to declare the two dump functions regardless of whether Debugging is enabled. Ideally we could declare them static, so as to not pollute the exported functions

[georgehansper]

Thanks for pointing this out @igalic

The function dump_lens_tree() is defined within an #if ENABLE_DEBUG ... #endif clause, as is the corresponding function declaration in the header file lens.h

I was wondering why the compiler did not complain that the function didn't exist at all after complaining that it was an "undeclared function".

I expect it is because the of the if(0), which allows the compiler to optimise it out of existance in the code.

The "undeclared function" warning probably happens on a eariler pass of the compiler over the code, before it does the optimisation

PR#818 puts the code inside an #if ENABLE_DEBUG clause.

As an alternative, the lens.h could be updated along the lines of debugging() in internal.h, like this:

#if ENABLE_DEBUG
void dump_lens_tree(struct lens *lens);
void dump_lens(FILE *out, struct lens *lens);
#else
#define dump_lens_tree(lens) (void)0
#endif

I'm thinking that this is closer to the existing solution applied for the function debugging() which is in a similar situation

The net result is that when ENABLE_DEBUG is false, to compiler sees the dummy macros in place of the functions and stops complaining about them being "undeclared functions"

[p-linnane]

Confirming that https://github.com/hercules-team/augeas/pull/818 fixes the issue.

[georgehansper]

Thanks @p-linnane for confirming this

I have updated the Pull Request #818 to use #define dump_lens_tree as I mentioned above, instead of the previous solution

The end-result is the same (the compiler warning is gone), but it "fits" better with the existing code and is slightly more readable

Sorry for any inconvenience.