Re: Unexpected -Wformat-truncation warnings

[Mason <slash dot tmp at free dot fr>]

On 02/03/2018 15:38, Jonathan Wakely wrote:

> On 2 March 2018 at 14:27, Mason wrote:
>
>> I am confused by both of these warnings.
>> I am aware that adding two 60-char strings might overflow an 80-char buffer,
>> which is why I'm using snprintf.
> 
> In which case the output would be truncated instead of overflowing,
> and that's what the warning says.
> 
>> I'm not sure these warnings are very helpful. What am I missing?
>> (I may have oversimplified the testcase.)
> 
> I think the point is that although you avoid undefined behaviour, you
> might still not get the output you expected.

To provide some context, I am using the following struct:

struct expr
{
	int val;
	char txt[60];
};

to store symbolic arithmetic expressions.

I start with simple symbolic constants "a", "b", "c", etc
and build up more complex expressions, such as "(a + b)" and "((a + b) * c)"

Actual code, for example,

	struct expr bak = tab[i];
	snprintf(tab[i].txt, 60, "(%s + %s)", bak.txt, tab[j].txt);

Is there a different way to write this code to avoid the warning(s) or should
I just disable Wformat-truncation?
(I hate disabling warnings, but this one looks irrelevant, in my situation.)

Regards.