Debug mode output broken

[Paolo Carlini]

Jonathan Wakely wrote:

> What's supposed to happen is that __n chars are copied to __buf,
> followed by '\0'.

*at most* __n chars are copied to __buf: that's the semantics of snprintf.

>My first patch (to insert '\0' at __buf[__n-1]) gives the right answer,
>but doesn't prevent buffer overflow if strlen(__s) >= __bufsize.
>  
>
It's not only that doesn't prevent the overflow.

You have not explained *why* you have to add by hand that '\0'. This is 
never
supposed to happen with functions that always add a '\0' automatically.

The real fix involves understanding *why* there is junk between 
__buf[__n - 1]
and the position in the allocated buffer where sprintf actually places 
the '\0'.

You have not explained that.

Paolo.