Re: -Wconversion

[Zack Weinberg <zack at wolery dot cumb dot org>]

On Fri, Jul 14, 2000 at 07:27:28PM +0200, Patrik Hagglund wrote:
> I don't understand the output from -Wconversion.
> 
> The following program is stored in the file test.c:
> 
>   #include <stdio.h>
>   
>   static void foo(unsigned char ch) {
>       printf("%c\n", ch);
>   }
>   
>   int main(int argc, char **argv) {
>   
>       unsigned char bar = 'a';
>   
>       foo(bar);
>   
>       return 0;
>   }
>   
>   > gcc -Wconversion test.c
>   test.c: In function `main':
>   test.c:12: warning: passing arg 1 of `foo' with different width
>   due to prototype

Yep.  That's the expected behavior.

> I can't match this behavior with the documentation:
> 
>  `-Wconversion'
>      Warn if a prototype causes a type conversion that is different
>      from what would happen to the same argument in the absence of a
>      prototype.  This includes conversions of fixed point to floating
>      and vice versa, and conversions changing the width or signedness
>      of a fixed point argument except when the same as the default
>      promotion.

...
> The test program above don't causes any conversion and should
> therefore not give any warning?

If your code read

>   static void foo(); 
>
>   int main() {
>       unsigned char bar = 'a';
>   
>       foo(bar);
>       return 0;
>   }

then bar would be converted to an int before it was passed to foo,
and foo would have to be written to expect that, i.e.

>   static void foo(ch) 
>       int ch; 
>   {
>       printf("%c\n", ch);
>   }

That is known as a _default promotion_.  Adding a prototype prevents
this from happening.

The purpose of -Wconversion is to shake out bugs introduced when you
add prototypes to a program that did not have them.  So it warns you
whenever you write a prototype that causes different argument
conversion than the default.  That includes no conversion at all.

We don't put it in -Wtraditional because the warning triggers for
legitimate code and there's no way to get rid of it.  For instance, it
will complain about every function in <math.h> that takes float rather
than double arguments (sinf, cosf, etc.)  -Wconversion is not intended
for day-to-day use; -Wtraditional is.

zw