How to declare IEEE mathematical functions like 'ilogbf' in MSVC++6?

Question

Could someone please help and tell me how to include IEEE mathematical functions in MSVC++6? I tried both and , but I still get these errors:

error C2065: 'ilogbf' : undeclared identifier

error C2065: 'scalbnf' : undeclared identifier

Answer 1

Edit 3: Hopefully this will be my final edit. I have come to realize that I haven't properly addressed this question at all. I am going to leave my answer in place as a cautionary tale, and because it may have some educational value. But I understand why I have zero upvotes, and in fact I am going to upvote Andy Ross' answer because I think his is much more relevant (although incomplete at least at the time of writing). It seems to me my mistake was to take the Man definitions I found for ilogbf() a little superficially. It's a function that takes the integer part of the log of a float, how hard can that be to implement ? It turns out what the function is really about is IEEE floating point representation, in particular the exponent (as opposed to the mantissa) part of that representation. I should definitely have realized that before attempting to answer the question! An interesting point to me is how a function can possibly find the exponent part of a float, as I thought a fundamental rule of C is that floats are promoted to doubles as part of a function call. But that's a whole separate discussion of course.

--- End of edit 3, start of cautionary tale ---

A little googling suggests these are defined in some flavors of Unix, but maybe are not in any Posix or ANSI standard and so not provided with the MSVC libraries. If the functions aren't in the library they won't be declared in math.h. Obviously if the compiler can't see declarations for these external symbols it won't be happy and you'll get errors like the ones you list.

The obvious work around is to create your own versions of these functions, using math functions that are provided. eg

#include <math.h>

int ilogbf( float f )
{
    double d1 = (double)f;
    double d2 = log(d1);
    int ret = (int)d2;
    return ret;
}

Edit: This isn't quite right. Apparently, this function should use log to the base 2, rather than natural logs, so that the returned value is actually a binary exponent. It should also take the absolute value of its parameter, so that it will work for negative numbers as well. I will work up an improved version, if you ask me in a comment, otherwise I'm tempted to leave that as an exercise for the reader :-)

The essence of my answer, i.e. that ANSI C doesn't require this function and that MSVC doesn't include it, is apparently correct.

Edit 2: Okay I've weakened and provided an improved version without being asked. Here it is;

#include <math.h>

int ilogbf( float f )
{
    double d1 = (double)f;
    if( d1 < 0 )
        d1 = -d1;
    double d2 = log(d1) / log(2);  // log2(x) = ln(x)/ln(2)
    int ret = (int)d2;
    return ret;
}

Answer 2

These are C99 functions, not IEEE754-1985. Microsoft seems to have decided that their market doesn't care about C99 support, so they haven't bothered to provide them. This is a shame, but unless more of you (developers) complain, there's no reason to expect that the situation will change.

The brand new 754 standard, IEEE754-2008, requires these functions (Clause 5.3.3, "logBFormat operations"), but that version of the standard won't be widely adopted for several more years; even if it does reach wide adoption, Microsoft hasn't seen fit to provide these functions for the ten years they've been in C99 so why would they bother to provide them just because they're in the IEEE754 standard?

edit: note that scalb and logb are defined in the IEEE754-1985 Appendix "Recommended Functions and Predicates", but said appendix is explicitly "not a part of" said standard.

Answer 3

If you know you're on an IEEE system (and these days, you do), these functions aren't needed: just inspect the bits directly by unioning the double with a uint64_t. Presumably you're using these functions in the interest of efficiency in the first place (otherwise you'd be using more natural operations like log() or exp()), so spending a little effort on matching your code to the floating point representation is probably worthwhile.