gcc: how to detect bad `bool` usage

Question

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?

Would lint catch these? Especially an updated tool such as PCLint? See for instance https://invisible-island.net/personal/lint-tools.html and see this about lint http://pubs.opengroup.org/onlinepubs/7908799/xcu/lint.html — Richard Chambers, Mar 16 '19 at 15:33
Looks like there is an updated version, splint, https://stackoverflow.com/questions/6881269/how-do-i-download-and-install-lint — Richard Chambers, Mar 16 '19 at 15:39
@RichardChambers thinks for the hint; but "splint" does not seem to understand C99+ at all and aborts when seeing variadic macros (e.g. `#define log_err(_fmt, ...)`). — ensc, Mar 16 '19 at 15:57

score 2 · Answer 1 · edited Mar 16 '19 at 18:14

2

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.

edited Mar 16 '19 at 18:14

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answered Mar 16 '19 at 15:37

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My code is some simplified variant of refactored code where functions previously returned a `bool` value and where converted to return richer values. The `return true` resp. `return false` was missed in the refactoring and stayed literally; I just want to detect them. – ensc Mar 16 '19 at 15:43
1

Just don't include stdbool.h (you don't need it), and you'll catch *all* the occurences. – wildplasser Mar 16 '19 at 15:47
@wildplasser or if stdbool.h is buried deeper, a temporary local redefinition to "break" `bool`, `true` and `false` could also help identify the places to change. (don't leave it like that, but...) – Alex Celeste Mar 16 '19 at 15:50
@wildplasser Not a good idea. In my experience, using `stdbool.h` can significantly increase code readability, simply because booleans are labeled as such. Of course, I would have preferred a definition like `((_Bool)0)`, but I've never run into the OP's problem (even though I'm making liberal use of `stdbool.h`). – cmaster - reinstate monica Mar 16 '19 at 16:12

score 2 · Answer 2 · answered Mar 16 '19 at 15:59

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.

gcc: how to detect bad `bool` usage

2 Answers2