diff --git a/headers/posix/math.h b/headers/posix/math.h
index 50f677790d..0b6bdfd6e6 100644
--- a/headers/posix/math.h
+++ b/headers/posix/math.h
@@ -138,6 +138,7 @@ extern double	sinh(double x);
 extern double	sqrt(double x);
 extern double	tan(double x);
 extern double	tanh(double x);
+extern double	trunc(double x);
 
 /* long double math functions */
 extern long double	roundl(long double x);