diff --git a/cfg/posix.cfg b/cfg/posix.cfg
index 7c0f76528..ed6b58d09 100644
--- a/cfg/posix.cfg
+++ b/cfg/posix.cfg
@@ -370,12 +370,31 @@
     <returnValue type="int"/>
     <arg nr="1" direction="in">
       <not-uninit/>
+      <valid>0:</valid>
     </arg>
     <arg nr="2" direction="in">
       <not-uninit/>
       <not-bool/>
     </arg>
   </function>
+  <!-- int lockf(int fd, int cmd, off_t len); -->
+  <function name="lockf">
+    <noreturn>false</noreturn>
+    <leak-ignore/>
+    <returnValue type="int"/>
+    <arg nr="1" direction="in">
+      <not-uninit/>
+      <valid>0:</valid>
+    </arg>
+    <arg nr="2" direction="in">
+      <not-uninit/>
+      <not-bool/>
+    </arg>
+    <arg nr="3" direction="in">
+      <not-uninit/>
+      <not-bool/>
+    </arg>
+  </function>
   <!-- int symlink(const char *oldpath, const char *newpath); -->
   <function name="symlink">
     <noreturn>false</noreturn>