diff --git a/cfg/posix.cfg b/cfg/posix.cfg
index 7adb4ff38..f54ab0070 100644
--- a/cfg/posix.cfg
+++ b/cfg/posix.cfg
@@ -180,6 +180,18 @@
       <not-uninit/>
     </arg>
   </function>
+  <!-- int isascii(int c) -->
+  <function name="isascii">
+    <use-retval/>
+    <pure/>
+    <returnValue type="int">arg1&ge;=0 &amp;&amp; arg1&le;=0x7F</returnValue>
+    <noreturn>false</noreturn>
+    <leak-ignore/>
+    <arg nr="1" direction="in">
+      <not-uninit/>
+      <valid>0:255</valid>
+    </arg>
+  </function>
   <!-- int isupper_l(int c, locale_t locale) -->
   <function name="isupper_l">
     <use-retval/>