diff --git a/liba/include/math.h b/liba/include/math.h
index 4ebb43229..a604c044d 100644
--- a/liba/include/math.h
+++ b/liba/include/math.h
@@ -59,6 +59,7 @@ double asinh(double x);
 double atan(double x);
 double atanh(double x);
 double ceil(double x);
+double copysign(double x, double y);
 double cos(double x);
 double cosh(double x);
 double exp(double x);