Crthuky

Is there some way to detect the bad usage of bool values in code like

#include <stdbool.h>

void *foo(void)

 return false;


int bar(void)

 return true;

Both functions are accepted by gcc (8.3.1) and clang (7.0.1) without any warnings

$ gcc -Wall -W -pedantic -c x.c
$ clang -Xclang -analyzer-checker=alpha --analyze -Wall -W -pedantic -c x.c
$ clang -Wall -W -pedantic -c x.c
$

Compiling as C++ code would detect the problem in foo() but is not an option but rest of code is C, not C++.

Are there other (-W) options or switches which would create diagnostics for these cases?

Make the example less trivial:

bool x;

void *foo(void)

 return x;


int bar(void)

 return x;

and it want compile at all.

usually true and false are just definitions and have value 1 and 0

From the stdbool.h header file

#ifndef _STDBOOL_H
#define _STDBOOL_H

#ifndef __cplusplus

#define bool _Bool
#define true 1
#define false 0

#else /* __cplusplus */

in your first example you just return zero and most compilers will not warn as they treat it as NULL. Try to return true and you will get the warning.

C defines the <stdbool.h> macros true and false as expanding to integer constant expressions of value 1 and 0 respectively. Since they're ints and bool (_Bool) in itself is an integer type, any such usage is equally valid. Even the value of the boolean expressions in C is an int and not a bool, so there is not much help for you with the bar function.

However, foo is a different beast - if the return value were true then it would be caught right away because 1 is not convertible to a pointer. false, having the integer constant value 0 is a null-pointer constant and will be converted to null pointer. You could perhaps catch the incorrect use by replacing the #include <stdbool.h> with something that does the inclusion but defines false as say 0.0 which is a falsy value but not an integer constant expression.

The interresting part here is that you call it "bad usage". But the code is perfectly valid.

_Bool is an integer type.

From the standard:

7.18 The macro bool expands to _Bool.

and

6.2.4 An object declared as type _Bool is large enough to store the values 0 and 1.

and

6.3.2.3 An integer may be converted to any pointer type.

So there is nothing in your code that violates the standard.

That said, you may get a warning for a missing cast to void* but besides that there is no reason for any warnings.

The code may look strange and in theory a compiler could detect that and give a warning. To my knowledge there is no compiler that will do that.