diff --git a/man3/sincos.3 b/man3/sincos.3
index b53a5b93e..5db36a227 100644
--- a/man3/sincos.3
+++ b/man3/sincos.3
@@ -6,6 +6,7 @@
 sincos, sincosf, sincosl \- calculate sin and cos simultaneously
 .SH SYNOPSIS
 .nf
+.B #define _GNU_SOURCE
 .B #include <math.h>
 .sp
 .BI "void sincos(double " x ", double *" sin ", double *" cos );