Question

The following file foo.c is a simplified version of a subtler bug I found in my code.

int b;

void bar(int a);

void foo(int a)
{
  bar(a);
  a = 42;
}

The line a = 42 is in fact a typo in my code: I meant b = 42. I don’t expect the compiler to detect that I made a typo, but I would like the get a warning that I am assigning to a local variable (or a function parameter) that is not going to be used anymore. If I compile this file with

% gcc-4.6 -Wall -Wextra -pedantic -O3 -c foo.c

I get absolutely no warning. Inspecting the generated code shows that the assignment a = 42 is not performed, so gcc is perfectly well aware that this instruction is useless (hence potentially bogus). Commenting the call to bar(a); does produce a warning warning: parameter ‘a’ set but not used [-Wunused-but-set-parameter], so it seems like gcc will not warn as long as a is used somewhere in the function, even if it is before the assignment.

My questions:

1. Is there a way to tell GCC or Clang to produce a warning for such case? (I could not get clang 3.0 to produce any warning, even with the call to bar removed.)
2. Is there a reason for the actual behavior? I.e, some cases were it is actually desirable to assign to local variables that will be thrown away by the optimizer?

Answer 1

There is no gcc or clang option to my knowledge that can warn about this useless assignment.

PC-Lint on the other hand is able to warn in this situation.

Warning 438 Last value assigned to variable ‘Symbol’ not used — A value had
been assigned to a variable that was not subsequently used. The
message is issued either at a return statement or at the end of a
block when the variable goes out of scope.