I think some change will have to be made to the XS code: for example, (a) make it output the newlines itself (directly where it outputs the numbers)

That does fix the problem - ie in MPFR.xs replace

SV * Rmpfr_out_str(SV * p, SV * base, SV * dig, SV * round) {
     if(SvIV(base) < 2 || SvIV(base) > 36) croak("2nd argument supplie
+d to Rmpfr_out_str is out of allowable range (must be between 2 and 3
+6 inclusive)");
     return newSVuv(mpfr_out_str(NULL, SvUV(base), SvUV(dig), *(INT2PT
+R(mpfr_t *, SvIV(SvRV(p)))), SvUV(round)));
}
[download]

with

SV * Rmpfr_out_str(SV * p, SV * base, SV * dig, SV * round) {
     unsigned long ret;
     if(SvIV(base) < 2 || SvIV(base) > 36) croak("2nd argument supplie
+d to Rmpfr_out_str is out of allowable range (must be between 2 and 3
+6 inclusive)");
     ret = mpfr_out_str(NULL, SvUV(base), SvUV(dig), *(INT2PTR(mpfr_t 
+*, SvIV(SvRV(p)))), SvUV(round));
     printf("\n");
     return newSVuv(ret);
}
[download]

On reflection, it probably doesn't make a lot of sense to be wrapping mpfr_out_str. I'll probably just leave Rmpfr_out_str as it is, and document the fact that calling it in a loop can lead to garbled output (and point out that it's safer to use Rmpfr_get_str).

I mentioned elsewhere in this thread that Rmpfr_get_str currently can sometimes output one less digit than it ought (due to programmer error). The fix for that is to replace, in MPFR.xs (in the Rmpfr_deref2 function)

mpfr_get_str(out, expptr, b, n_dig, *(INT2PTR(mpfr_t *, SvIV(SvRV(p)))
+), SvUV(round));
[download]

with

mpfr_get_str(out, expptr, b, SvUV(n_digits), *(INT2PTR(mpfr_t *, SvIV(
+SvRV(p)))), SvUV(round));
[download]

That fix will find its way into the next version of Math::MPFR.

Cheers,
Rob