On 14 October 2016 at 20:34, Martin Sebor <msebor@xxxxxxxxx> wrote: > On 10/13/2016 12:32 PM, Ben Burns wrote: >> >> Hello gcc contributors. >> >> I had an issue which was ultimately caused by not using the return >> value of a function and I'm trying to find a compile-time warning that >> could have alerted me to the issue. >> >> I've been looking through the documentation but I'm not seeing a >> compile-time warning flag about ignoring function return values. Sure, >> I can add the attribute "warn_unused_result" but I'm wondering if the >> same affect is possible (i.e., for all functions) without changing any >> code. I feel like this must have come up before and there's a reason >> this warning doesn't exist -- perhaps this is better handled by a >> static analysis tool, or perhaps it generates too much noise -- but if >> there was a previous discussion about this I'd appreciate being >> pointed in that direction. Or perhaps I'm just not searching for the >> right terms, in which case I apologize for taking your time. > > > I don't know if the idea has ever been discussed but because I had submitted a patch to warn for unused return value of pure/const functions, but it didn't make it into trunk because it gave false positives for conditionally defined macros: https://gcc.gnu.org/ml/gcc-patches/2016-07/msg01672.html Thanks, Prathamesh > the return value of many common functions is rarely used (all > the string manipulation functions in the C library and many stdio > functions, for instance) issuing a warning for it would generate > far too much noise to be useful. It's quite easy to add this > warning as an experiment. With the patch below GCC emits the > following warnings for your example: > > a.c: In function ‘main’: > a.c:16:5: warning: ignoring return value of ‘scanf’ [-Wany-unused-result] > scanf("%d", &b); > ^~~~~~~~~~~~~~~ > a.c:17:5: warning: ignoring return value of ‘func’ [-Wany-unused-result] > func(b); > ^~~~~~~ > a.c:18:5: warning: ignoring return value of ‘inline_func’ > [-Wany-unused-result] > inline_func(b); > ^~~~~~~~~~~~~~ > > Martin > > > diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt > index e146781..21e0daa 100644 > --- a/gcc/c-family/c.opt > +++ b/gcc/c-family/c.opt > @@ -998,6 +998,10 @@ Wunused-result > C ObjC C++ ObjC++ Var(warn_unused_result) Init(1) Warning > Warn if a caller of a function, marked with attribute warn_unused_result, > does not use its return value. > > +Wany-unused-result > +C ObjC C++ ObjC++ Var(warn_any_unused_result) Init(0) Warning > +Warn if a caller of a function does not use its return value. > + > Wunused-variable > C ObjC C++ ObjC++ LangEnabledBy(C ObjC C++ ObjC++,Wunused) > ; documented in common.opt > diff --git a/gcc/tree-cfg.c b/gcc/tree-cfg.c > index 2ed450c..462c440 100644 > --- a/gcc/tree-cfg.c > +++ b/gcc/tree-cfg.c > @@ -8920,6 +8920,17 @@ do_warn_unused_result (gimple_seq seq) > "ignoring return value of function " > "declared with attribute warn_unused_result"); > } > + else if (warn_any_unused_result) > + { > + location_t loc = gimple_location (g); > + > + if (fdecl) > + warning_at (loc, OPT_Wany_unused_result, > + "ignoring return value of %qD", fdecl); > + else > + warning_at (loc, OPT_Wany_unused_result, > + "ignoring return value of function"); > + } > break; > > default: >
