diff --git a/lib/chibi/ast.c b/lib/chibi/ast.c
index 8e48aa3d..18def0d8 100644
--- a/lib/chibi/ast.c
+++ b/lib/chibi/ast.c
@@ -444,6 +444,14 @@ static sexp sexp_string_contains (sexp ctx, sexp self, sexp_sint_t n, sexp x, se
   return res ? sexp_make_fixnum(res-sexp_string_data(x)) : SEXP_FALSE;
 }
 
+static sexp sexp_errno (sexp ctx, sexp self, sexp_sint_t n) {
+#ifdef PLAN9
+  return SEXP_FALSE;
+#else
+  return sexp_make_fixnum(errno);
+#endif
+}
+
 static sexp sexp_error_string (sexp ctx, sexp self, sexp_sint_t n, sexp x) {
 #ifdef PLAN9
   return SEXP_FALSE;
@@ -612,6 +620,7 @@ sexp sexp_init_library (sexp ctx, sexp self, sexp_sint_t n, sexp env, const char
 #endif
   sexp_define_foreign(ctx, env, "thread-list", 0, sexp_thread_list);
   sexp_define_foreign(ctx, env, "string-contains", 2, sexp_string_contains);
+  sexp_define_foreign(ctx, env, "errno", 0, sexp_errno);
   sexp_define_foreign_opt(ctx, env, "integer->error-string", 1, sexp_error_string, SEXP_FALSE);
   sexp_define_foreign(ctx, env, "update-free-vars!", 1, sexp_update_free_vars);
   sexp_define_foreign(ctx, env, "setenv", 2, sexp_setenv);
diff --git a/lib/chibi/ast.sld b/lib/chibi/ast.sld
index a50730f2..c9356be1 100644
--- a/lib/chibi/ast.sld
+++ b/lib/chibi/ast.sld
@@ -35,7 +35,7 @@
    env-define! env-push! env-syntactic? env-syntactic?-set! core-code
    type-name type-cpl type-parent type-slots type-num-slots type-printer
    object-size integer->immediate gc atomically thread-list
-   string-contains integer->error-string
+   string-contains errno integer->error-string
    flatten-dot update-free-vars! setenv unsetenv)
   (import (chibi))
   (include-shared "ast")