1 files changed, 3 insertions, 0 deletions
diff --git a/complex.c b/complex.c
index 78f0902a83..a21249681f 100644
--- a/complex.c
+++ b/complex.c
@@ -1173,6 +1173,9 @@ nucomp_eql_p(VALUE self, VALUE other)
 inline static VALUE
 f_signbit(VALUE x)
 {
+#if defined(HAVE_SIGNBIT) && defined(__GNUC__) && defined(__sun__)
+    extern int signbit(double x);
+#endif
     switch (TYPE(x)) {
       case T_FLOAT: {
 	double f = RFLOAT_VALUE(x);