diff --git a/ext/socket/extconf.rb b/ext/socket/extconf.rb
index c86cc8f8c0..b70a862414 100644
--- a/ext/socket/extconf.rb
+++ b/ext/socket/extconf.rb
@@ -484,6 +484,9 @@ EOF
   have_func("getpeerucred(0, (ucred_t **)NULL)", headers) # SunOS
 
   have_func_decl = proc do |name, headers|
+    # check if there is a declaration of <name> by trying to declare
+    # both "int <name>(void)" and "void <name>(void)"
+    # (at least one attempt should fail if there is a declaration)
     if !checking_for("declaration of #{name}()") {!%w[int void].all? {|ret| try_compile(<<EOF)}}
 #{cpp_include(headers)}
 #{ret} #{name}(void);
@@ -492,10 +495,10 @@ EOF
     end
   end
   if have_func('if_indextoname(0, "")', headers)
-    have_func_decl["if_indextoname"]
+    have_func_decl["if_indextoname", headers]
   end
   if have_func('if_nametoindex("")', headers)
-    have_func_decl["if_nametoindex"]
+    have_func_decl["if_nametoindex", headers]
   end
 
   have_func("hsterror", headers)