Add a "fork" of musl as //fusl.

fusl/src/math/modf.c

Index: fusl/src/math/modf.c
diff --git a/fusl/src/math/modf.c b/fusl/src/math/modf.c
new file mode 100644
index 0000000000000000000000000000000000000000..1c8a1db90db7b4b61c5ec4271f184308690704db
--- /dev/null
+++ b/fusl/src/math/modf.c
@@ -0,0 +1,34 @@
+#include "libm.h"
+
+double modf(double x, double *iptr)
+{
+	union {double f; uint64_t i;} u = {x};
+	uint64_t mask;
+	int e = (int)(u.i>>52 & 0x7ff) - 0x3ff;
+
+	/* no fractional part */
+	if (e >= 52) {
+		*iptr = x;
+		if (e == 0x400 && u.i<<12 != 0) /* nan */
+			return x;
+		u.i &= 1ULL<<63;
+		return u.f;
+	}
+
+	/* no integral part*/
+	if (e < 0) {
+		u.i &= 1ULL<<63;
+		*iptr = u.f;
+		return x;
+	}
+
+	mask = -1ULL>>12>>e;
+	if ((u.i & mask) == 0) {
+		*iptr = x;
+		u.i &= 1ULL<<63;
+		return u.f;
+	}
+	u.i &= ~mask;
+	*iptr = u.f;
+	return x - u.f;
+}