diff --git a/libc/spec/posix.td b/libc/spec/posix.td
index 26f41b61dc001..3b793ea90ffd3 100644
--- a/libc/spec/posix.td
+++ b/libc/spec/posix.td
@@ -1169,6 +1169,11 @@ def POSIX : StandardSpec<"POSIX"> {
               RetValSpec<IntType>,
               [ArgSpec<VoidType>]
           >,
+          FunctionSpec<
+            "fileno",
+            RetValSpec<IntType>,
+            [ArgSpec<FILEPtr>]
+          >,
       ]
   >;