--- ocaml-3.11.0.orig/otherlibs/unix/unixsupport.h
+++ ocaml-3.11.0/otherlibs/unix/unixsupport.h
@@ -20,6 +20,7 @@
 #define Nothing ((value) 0)
 
 extern value unix_error_of_code (int errcode);
+extern int code_of_unix_error (value error);
 extern void unix_error (int errcode, char * cmdname, value arg) Noreturn;
 extern void uerror (char * cmdname, value arg) Noreturn;
 
--- ocaml-3.11.0.orig/otherlibs/unix/unixsupport.c
+++ ocaml-3.11.0/otherlibs/unix/unixsupport.c
@@ -263,6 +263,15 @@
   return err;
 }
 
+extern int code_of_unix_error (value error)
+{
+  if (Is_block(error)) {
+    return Int_val(Field(error, 0));
+  } else {
+    return error_table[Int_val(error)];
+  }
+}
+
 void unix_error(int errcode, char *cmdname, value cmdarg)
 {
   value res;