musl/src/math/logbl.c

#include "libm.h"
#if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
long double logbl(long double x)
{
	return logb(x);
}
#else
long double logbl(long double x)
{
	if (!isfinite(x))
		return x * x;
	if (x == 0)
		return -1/(x+0);
	return ilogbl(x);
}
#endif