Re: vasnprintf's "%n in writable segment" chokes with _FORTIFY_SOURCE == 2

[Jim Meyering]

Bruno Haible <address@hidden> wrote:

> Jim Meyering wrote:
>> When I build a coreutils snapshot with -D_FORTIFY_SOURCE=2 on a
>> relatively recent fedora-based system, seq always aborts like this:
>>
>>     $ ./seq 1
>>     *** %n in writable segment detected ***
>>     1zsh: abort      ./seq 1
>>     [Exit 134 (ABRT)]
>>
>> That is due to the fact that vasnprintf writes %n into a format
>> string that is subsequently used by snprintf.
>
> Use of %n is valid in ISO C99 and in POSIX.

As you know I don't dispute its validity.
However, vasnprintf.c should resort to incurring the
expense of that portability kludge only when necessary.
How about if it does that only when a configure-time test
has detected that snprintf's return value is not conforming?

Then, vasnprintf.c will be more efficient, and won't use the offending %n,
on the vast majority of systems that have an snprintf implementation
with a conforming return value.