diff --git a/extlib/benz/codegen.c b/extlib/benz/codegen.c
index 68149a05..062fc20c 100644
--- a/extlib/benz/codegen.c
+++ b/extlib/benz/codegen.c
@@ -555,7 +555,6 @@ analyze_quote(analyze_state *state, pic_value obj)
   return pic_list2(pic, pic_obj_value(pic->sQUOTE), pic_list_ref(pic, obj, 1));
 }
 
-
 #define ARGC_ASSERT_GE(n, name) do {                                    \
     if (pic_length(pic, obj) < (n) + 1) {                               \
       pic_errorf(pic,                                                   \
@@ -573,7 +572,6 @@ analyze_quote(analyze_state *state, pic_value obj)
     }                                                   \
   } while (0)
 
-
 static pic_value
 analyze_add(analyze_state *state, pic_value obj, bool tailpos)
 {
@@ -730,7 +728,6 @@ analyze_call_with_values(analyze_state *state, pic_value obj, bool tailpos)
             analyze(state, pic_list_ref(pic, obj, 1), false),   \
             analyze(state, pic_list_ref(pic, obj, 2), false))
 
-
 static pic_value
 analyze_node(analyze_state *state, pic_value obj, bool tailpos)
 {
diff --git a/extlib/benz/debug.c b/extlib/benz/debug.c
index eb969c1c..65b8e548 100644
--- a/extlib/benz/debug.c
+++ b/extlib/benz/debug.c
@@ -36,34 +36,27 @@ pic_get_backtrace(pic_state *pic)
 }
 
 void
-pic_print_backtrace(pic_state *pic)
+pic_print_backtrace(pic_state *pic, xFILE *file)
 {
-  size_t ai = pic_gc_arena_preserve(pic);
-  pic_str *trace;
-
   assert(! pic_undef_p(pic->err));
 
   if (! pic_error_p(pic->err)) {
-    trace = pic_format(pic, "raised: ~s", pic->err);
+    xfprintf(file, "raise: ");
+    pic_fwrite(pic, pic->err, file);
   } else {
     struct pic_error *e;
 
     e = pic_error_ptr(pic->err);
     if (e->type != pic_intern_cstr(pic, "")) {
-      trace = pic_format(pic, "~s ", pic_obj_value(e->type));
-    } else {
-      trace = pic_make_str(pic, NULL, 0);
+      pic_fwrite(pic, pic_obj_value(e->type), file);
+      xfprintf(file, " ");
     }
-    trace = pic_str_cat(pic, trace, pic_format(pic, "error: ~s", pic_obj_value(e->msg)));
+    xfprintf(file, "error: ");
+    pic_fwrite(pic, pic_obj_value(e->msg), file);
+    xfprintf(file, "\n");
 
     /* TODO: print error irritants */
 
-    trace = pic_str_cat(pic, trace, pic_make_str(pic, "\n", 1));
-    trace = pic_str_cat(pic, trace, e->stack);
+    xfputs(pic_str_cstr(pic, e->stack), file);
   }
-
-  /* print! */
-  xfprintf(xstderr, "%s", pic_str_cstr(pic, trace));
-
-  pic_gc_arena_restore(pic, ai);
 }
diff --git a/extlib/benz/error.c b/extlib/benz/error.c
index 3fd6035b..705f0055 100644
--- a/extlib/benz/error.c
+++ b/extlib/benz/error.c
@@ -9,6 +9,7 @@
 #include "picrin/data.h"
 #include "picrin/string.h"
 #include "picrin/error.h"
+#include "picrin/port.h"
 
 void
 pic_panic(pic_state *pic, const char *msg)
@@ -29,7 +30,7 @@ pic_warnf(pic_state *pic, const char *fmt, ...)
   err_line = pic_xvformat(pic, fmt, ap);
   va_end(ap);
 
-  fprintf(stderr, "warn: %s\n", pic_str_cstr(pic, pic_str_ptr(pic_car(pic, err_line))));
+  xfprintf(pic_stderr(pic)->file, "warn: %s\n", pic_str_cstr(pic, pic_str_ptr(pic_car(pic, err_line))));
 }
 
 void
diff --git a/extlib/benz/include/picrin.h b/extlib/benz/include/picrin.h
index 6ae0107e..8965af8d 100644
--- a/extlib/benz/include/picrin.h
+++ b/extlib/benz/include/picrin.h
@@ -224,7 +224,7 @@ PIC_NORETURN void pic_errorf(pic_state *, const char *, ...);
 void pic_warnf(pic_state *, const char *, ...);
 const char *pic_errmsg(pic_state *);
 pic_str *pic_get_backtrace(pic_state *);
-void pic_print_backtrace(pic_state *);
+void pic_print_backtrace(pic_state *, xFILE *);
 
 /* obsoleted */
 PIC_INLINE void pic_warn(pic_state *pic, const char *msg)
diff --git a/extlib/benz/port.c b/extlib/benz/port.c
index be574938..9f725f7f 100644
--- a/extlib/benz/port.c
+++ b/extlib/benz/port.c
@@ -38,6 +38,16 @@ pic_stdout(pic_state *pic)
   return pic_port_ptr(obj);
 }
 
+struct pic_port *
+pic_stderr(pic_state *pic)
+{
+  pic_value obj;
+
+  obj = pic_funcall(pic, pic->PICRIN_BASE, "current-error-port", pic_nil_value());
+
+  return pic_port_ptr(obj);
+}
+
 struct pic_port *
 pic_make_standard_port(pic_state *pic, xFILE *file, short dir)
 {
diff --git a/extlib/benz/write.c b/extlib/benz/write.c
index 29b4792f..f89e0703 100644
--- a/extlib/benz/write.c
+++ b/extlib/benz/write.c
@@ -406,7 +406,7 @@ display(pic_state *pic, pic_value obj, xFILE *file)
 pic_value
 pic_write(pic_state *pic, pic_value obj)
 {
-  return pic_fwrite(pic, obj, xstdout);
+  return pic_fwrite(pic, obj, pic_stdout(pic)->file);
 }
 
 pic_value
@@ -420,7 +420,7 @@ pic_fwrite(pic_state *pic, pic_value obj, xFILE *file)
 pic_value
 pic_display(pic_state *pic, pic_value obj)
 {
-  return pic_fdisplay(pic, obj, xstdout);
+  return pic_fdisplay(pic, obj, pic_stdout(pic)->file);
 }
 
 pic_value
@@ -434,6 +434,7 @@ pic_fdisplay(pic_state *pic, pic_value obj, xFILE *file)
 void
 pic_printf(pic_state *pic, const char *fmt, ...)
 {
+  xFILE *file = pic_stdout(pic)->file;
   va_list ap;
   pic_str *str;
 
@@ -443,8 +444,8 @@ pic_printf(pic_state *pic, const char *fmt, ...)
 
   va_end(ap);
 
-  xprintf("%s", pic_str_cstr(pic, str));
-  xfflush(xstdout);
+  xfprintf(file, "%s", pic_str_cstr(pic, str));
+  xfflush(file);
 }
 
 static pic_value
diff --git a/src/main.c b/src/main.c
index f43419f1..72bbaebb 100644
--- a/src/main.c
+++ b/src/main.c
@@ -65,7 +65,7 @@ main(int argc, char *argv[], char **envp)
     pic_funcall(pic, PICRIN_MAIN, "main", pic_nil_value());
   }
   pic_catch {
-    pic_print_backtrace(pic);
+    pic_print_backtrace(pic, xstderr);
     status = 1;
   }