Re: format string is not a string literal

On Feb 25, 2015, at 4:05 PM, Simon Marchi <[email protected]> wrote:

>> I didn’t say it’s a bug, Andrew did.  But I agree with him.
>> 
>> My comment (“the code is legit”) simply meant that GDB uses variable formats for obvious valid reasons (so the format can vary, being user-supplied).  Given that it’s intentional, the warning is not wanted.
>> 
>> But that point is really applicable to printf, not vprintf.  Andrew’s point is that checking formats for vprintf is not possible because you can’t know the argument list; only in printf do you see the arguments so you can match the types.  So the bug is that format checking and complaining for non-literal formats should not be enabled at all for vprintf.  That may be a header issue rather than a compiler issue, but either way, it’s not the right thing to do.
>> 
>>        paul
> 
> I think the warning is relevant. If you instruct the compiler that
> inferior_debug takes a format string and format arguments (using a
> format attribute, as mentioned by Richard in the bug report), then it
> can check if the callers are doing something wrong.
> 
> In the case of inferior_debug, the attribute should be
>    __attribute__((format (printf, 2, 3)))
> 
> By adding the attribute, you get nice warnings of this kind:
> 
> test.c: In function ‘main’:
> test.c:17:2: warning: too many arguments for format [-Wformat-extra-args]
>  inferior_debug (1, "pouet %d", 2, "hello");
> 
> If the function is vprintf-style, it's similar but the last argument
> should be 0. It will push the argument check a level higher, where
> eventually they are explicitely defined printf-style. The doc is
> somewhere here [2] in the middle.

Then clang's warning should suggest putting the format attribute on that function rather than giving out a warning that seems like it is a bogus one. 

Gcc does that iirc why not clang. 

Thanks,
Andrew


> 
> The warning also has some value because it will tell you if the string
> originally comes from a non-literal, which should be avoided [1].
> 
> [1] http://en.wikipedia.org/wiki/Uncontrolled_format_string
> [2] https://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html
> 
> Simon