diff --git a/extlib/benz/README.md b/extlib/benz/README.md
index 81722f44..67c7a32d 100644
--- a/extlib/benz/README.md
+++ b/extlib/benz/README.md
@@ -19,7 +19,7 @@ main(int argc, char *argv[])
   pic_state *pic;
   pic_value expr;
 
-  pic = pic_open(argc, argv, NULL);
+  pic = pic_open(pic_default_allocf, NULL);
 
   while (1) {
     printf("> ");
@@ -61,7 +61,7 @@ pic_value factorial(pic_state *pic) {
 int
 main(int argc, char *argv[])
 {
-  pic_state *pic = pic_open(argc, argv, NULL);
+  pic_state *pic = pic_open(pic_default_allocf, NULL);
 
   pic_defun(pic, "fact", factorial); /* define fact procedure */
 
diff --git a/extlib/benz/include/picrin.h b/extlib/benz/include/picrin.h
index e19ced8b..7479ab60 100644
--- a/extlib/benz/include/picrin.h
+++ b/extlib/benz/include/picrin.h
@@ -74,6 +74,7 @@ struct pic_state {
   char **argv, **envp;
 
   pic_allocf allocf;
+  void *userdata;
 
   pic_checkpoint *cp;
   struct pic_cont *cc;
@@ -169,9 +170,10 @@ void pic_gc_arena_restore(pic_state *, size_t);
     pic_gc_arena_restore(pic, ai);              \
   } while (0)
 
-pic_state *pic_open(int argc, char *argv[], char **envp, pic_allocf);
 void *pic_default_allocf(void *, size_t);
+pic_state *pic_open(pic_allocf, void *);
 void pic_close(pic_state *);
+void pic_set_argv(pic_state *, int argc, char *argv[], char **envp);
 
 void pic_add_feature(pic_state *, const char *);
 
diff --git a/extlib/benz/state.c b/extlib/benz/state.c
index 6c4fe6b8..7a419911 100644
--- a/extlib/benz/state.c
+++ b/extlib/benz/state.c
@@ -4,6 +4,14 @@
 
 #include "picrin.h"
 
+void
+pic_set_argv(pic_state *pic, int argc, char *argv[], char **envp)
+{
+  pic->argc = argc;
+  pic->argv = argv;
+  pic->envp = envp;
+}
+
 void
 pic_add_feature(pic_state *pic, const char *feature)
 {
@@ -152,7 +160,7 @@ pic_init_core(pic_state *pic)
 }
 
 pic_state *
-pic_open(int argc, char *argv[], char **envp, pic_allocf allocf)
+pic_open(pic_allocf allocf, void *userdata)
 {
   struct pic_port *pic_make_standard_port(pic_state *, xFILE *, short);
   char t;
@@ -169,6 +177,9 @@ pic_open(int argc, char *argv[], char **envp, pic_allocf allocf)
   /* allocator */
   pic->allocf = allocf;
 
+  /* user data */
+  pic->userdata = userdata;
+
   /* turn off GC */
   pic->gc_enable = false;
 
@@ -180,9 +191,9 @@ pic_open(int argc, char *argv[], char **envp, pic_allocf allocf)
   pic->cp = NULL;
 
   /* command line */
-  pic->argc = argc;
-  pic->argv = argv;
-  pic->envp = envp;
+  pic->argc = 0;
+  pic->argv = NULL;
+  pic->envp = NULL;
 
   /* prepare VM stack */
   pic->stbase = pic->sp = allocf(NULL, PIC_STACK_SIZE * sizeof(pic_value));
diff --git a/src/main.c b/src/main.c
index a4a330ef..5de70fdd 100644
--- a/src/main.c
+++ b/src/main.c
@@ -41,7 +41,8 @@ main(int argc, char *argv[], char **envp)
   struct pic_lib *PICRIN_MAIN;
   int status = 0;
 
-  pic = pic_open(argc, argv, envp, pic_default_allocf);
+  pic = pic_open(pic_default_allocf, NULL);
+  pic_set_argv(pic, argc, argv, envp);
 
   pic_init_picrin(pic);