The '3' is the manual page section ("library calls") where the function is documented... :)   (an old unix convention — typically, I avoid using it for just that reason... sorry I didn't in this case).

I guess that's likely to work on other operating systems as well ?

Yes I'd think so, it's a pretty standard library call (ANSI C - X3.159-1989, the man page says).

Yes I'd think so, it's a pretty standard library call

Yep ... thanks again almut.

Still, it's something that should be tested in an extension's test script ... so ... one has a module that contains an xsub that doesn't return anything:

void foo(mpfr_t * x) {
     mpfr_out_str(*x, .......);
     fflush(stdout);
}
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How does one verify (in a .t test script) that foo is producing the desired output ?

Heavens forbid ... I'm now straying into on-topic areas, aren't I :-)

Actually ... I'm about to answer the question I just posed, but I'll send this post anyway. As it turns out mpfr_out_str() is not a void function, but returns the number of bytes written (as a size_t). So, to make foo testable, I guess one could just rewrite it as:

SV * foo(mpfr_t * x) {
     size_t s = mpfr_out_str(*x, .......);
     fflush(stdout);
     return newSViv(s);
}
[download]

And then the test script would simply check that foo($var) returned the expected number of bytes.

If there are any comments on that, please don't be shy :-)

Cheers,
Rob