Bug in handling of "1$", "2$" in printf format

[Corinna Vinschen]

On Dec  7 16:33, Jon Turney via Cygwin wrote:
> On 07/12/2024 05:26, Keith Thompson via Cygwin wrote:
> > Brian Inglis wrote:
> > > On 2024-12-06 19:16, Keith Thompson via Cygwin wrote:
> > > > The use of "1$", "2$" et al in printf format specifiers is a
> > > > POSIX-specific feature.
> > > > 
> > > > On Cygwin (newlib) this is handled correctly in most cases, but one
> > > > example I tried misbehaves.
> > > > The output is correct on other implementations, including glibc and
> > > > musl on Ubuntu.
> > > > 
> > > > This C program:
> > > > 
> > > > #include <stdio.h>
> > > > int main(void) {
> > > >       long long a = 123456789876543210;
> > > >       double b=1.0/3;
> > > >       printf("a:%2$8.8lld b:%1$10.2g\n", b, a);
> > > > }
> > > > 
> > > > should produce this output:
> > > > 
> > > > a:123456789876543210 b:      0.33
> > > > 
> > > > Under Cygwin (fully updated), with "gcc c.c -o c && ./c", the output is:
> > > > 
> > > > a:140732550844138 b:  7.1e-315
> > > > 
> > > 
> > > Confirmed with gcc 12.4 and minor tweaks to constant data types: printf is
> > > ignoring arg positions:
> > [SNIP]
> > 
> > It's not always ignoring arg positions. I think there's an interaction
> > between the "1$" / "2$" position specification and relatively complex
> > format specifiers.  The following case works correctly:
> > 
> > $ cat c2.c
> > #include <stdio.h>
> > int main(void) {
> >      int a = 42;
> >      double b = 1.0/3.0;
> >      printf("a:%2$d b:%1$g\n", b, a);
> >      printf("a:%1$d b:%2$g\n", a, b);
> >      printf("a:%d b:%g\n",     a, b);
> > }
> > $ gcc c2.c -o c2 && ./c2
> > a:42 b:0.333333
> > a:42 b:0.333333
> > a:42 b:0.333333
> > $
> > 
> > (And the version of gcc shouldn't matter.  printf is implemented in
> > newlib.  The code in question should be in newlib/libc/stdio/vfprintf.c.)
> 
> Not sure if this is the same/similar/different to the problem I reported at
> [1]
> 
> [1] https://sourceware.org/pipermail/newlib/2023/020374.html

Yeah, we might need a patch there...

Corinna