How does C compute sin() and other math functions?

Question

I've been poring through .NET disassemblies and the GCC source code, but can't seem to find anywhere the actual implementation of `sin()` and other math functions... they always seem to be referencing something else.Can anyone help me find them? I feel like it's unlikely that ALL hardware that C will run on supports trig functions in hardware, so there must be a software algorithm *somewhere*, right? <hr>I'm aware of several ways that functions *can* be calculated, and have written my own routines to compute functions using taylor series for fun. I'm curious about how real, production languages do it, since all of my implementations are always several orders of magnitude slower, even though I think my algorithms are pretty clever (obviously they're not).

Answer 1

So the trigonometric functions, when you need them, are compiled into a program when you use the -lm flag on the compiler, for example,

gcc triggered triggered.c -lm

where triggered.c is:

#include <math.h>

#include <stdio.h>

#define PI 3.14159

int main ()

{

printf ("sine of PI/7 is %f\n", sin((double) (PI/7.0)));

}

you can run

./triggered

to produce:

sine of PI/7 is 0.433883

But where is the source for the math library you link in statically with -lm?

It is in glibc. The source for glibc lives in the gnu git repository (today) which you can obtain with

git clone git://sourceware.org/git/glibc.git

you might want to go to glibc/ieee754/dbl-64/dosincos.c

to see a fairly straightforward implementation of the sine function as you're likely to experience it when you actually do the calculation. It's only about a hundred LOC, and worthy of study.