warn on unused result without attribute warn_unused_result

[Martin Sebor]

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
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: