I don't know why this is so hard to find, but it is. I ended up following the suggestion of a google answers question and converting the javascript formula at http://www.csgnetwork.com/csgcolorsel4.html to C++. (Ok, it is just plain C).

// RGB are from 0..1, H is from 0..360, SV from 0..1
double maxC = b;
if (maxC < g) maxC = g;
if (maxC < r) maxC = r;
double minC = b;
if (minC > g) minC = g;
if (minC > r) minC = r;

double delta = maxC - minC;

double V = maxC;
double S = 0;
double H = 0;

if (delta == 0)
{
	H = 0;
	S = 0;
}
else
{
	S = delta / maxC;
	double dR = 60*(maxC - r)/delta + 180;
	double dG = 60*(maxC - g)/delta + 180;
	double dB = 60*(maxC - b)/delta + 180;
	if (r == maxC)
		H = dB - dG;
	else if (g == maxC)
		H = 120 + dR - dB;
	else
		H = 240 + dG - dR;
}

if (H<0)
	H+=360;
if (H>=360)
	H-=360;