Fix gcc4 warning, idx may be used uninitialized

Quoted 9 lines · Jesse Millan

This patch eliminates the warning that is generated when passing an uninitialized variable to a function, and in that function it 'looks' like you may read the contents.

In this case, the address of a local variable 'idx' is passed to the function bvec_alloc_bs(). Inside bvec_alloc_bs(), it is possible that no value will be assigned to idx, in which case the function immediately returns null and does not go on to read it. Human eyes can see that its safe.

I think this should go straight to http://gcc.gnu.org/bugzilla/enter_bug.cgi

Quoted 8 lines · Jesse Millan

So, the compiler should be able to tell that either one of the two cases apply?

a: an assignment statement will be done on idx before its ever read

or

b: no assignment statement, control flow will never allow idx to be read

gcc is normally pretty good at detecting this. Since gcc-4.0, it appears to do more checking inside inline functions, where older versions just assumed that a variable is initialized after a reference to it is passed to an inline function.

I'd suggest fixing the inline function to always do this instead of fixing the caller. After all, this needs to be fixed in the kernel, even if future gcc versions become smarter about it gcc-4.0 will continue to be used.

	Arnd <><

fs/bio.c +1 -0

@@ -90,6 +90,7 @@
 		case  65 ... 128: *idx = 4; break;
 		case 129 ... BIO_MAX_PAGES: *idx = 5; break;
 		default:
+			*idx = 0;
 			return NULL;
 	}
 	/*