Re: vprintf.c bug

[Rich Felker <dalias@libc.org>]

On Tue, Jul 26, 2016 at 03:25:40PM -0700, Jacob Abrams wrote:
> I believe there is a small bug in vfprintf.c
> 
> I had to change the if statement from
> 
>     ret = printf_core(f, fmt, &ap2, nl_arg, nl_type);
>     if (saved_buf) {
> 
> to
> 
>     ret = printf_core(f, fmt, &ap2, nl_arg, nl_type);
>     if (f->buf == internal_buf) {
> 
> Because the saved_buf may be NULL which will result the internal
> buffer being used but the file not being reset properly after the call
> to printf_core. This was discovered while using MUSL v1.1.4 on an OS
> other than Linux.

Where do you get a FILE with a null f->buf? While it's not written
down anywhere, that breaks a contract expected several other places in
the stdio implementation. If you're trying to use vfprintf.c outside
of musl I think you probably need to disable/remove this buffer
replacement code, which would be poking at the internals of another
stdio implementation in a likely-invalid way.

Rich