1 files changed, 20 insertions, 0 deletions
diff --git a/src/math/logb.c b/src/math/logb.c
new file mode 100644
index 00000000..f7cd7613
--- /dev/null
+++ b/src/math/logb.c
@@ -0,0 +1,20 @@
+#include <limits.h>
+#include "libm.h"
+
+/*
+special cases:
+	logb(+-0) = -inf
+	logb(+-inf) = +inf
+	logb(nan) = nan
+these are calculated at runtime to raise fp exceptions
+*/
+
+double logb(double x) {
+	int i = ilogb(x);
+
+	if (i == FP_ILOGB0)
+		return -1.0/fabs(x);
+	if (i == FP_ILOGBNAN || i == INT_MAX)
+		return x * x;
+	return i;
+}