diff --git a/contrib/05.r7rs/src/file.c b/contrib/05.r7rs/src/file.c
index 36e50e86..c354c179 100644
--- a/contrib/05.r7rs/src/file.c
+++ b/contrib/05.r7rs/src/file.c
@@ -20,7 +20,7 @@ generic_open_file(pic_state *pic, const char *fname, char *mode, short flags)
   struct pic_port *port;
   xFILE *file;
 
-  file = xfopen(fname, mode);
+  file = xfopen(pic, fname, mode);
   if (! file) {
     file_error(pic, "could not open file");
   }
diff --git a/contrib/05.r7rs/src/load.c b/contrib/05.r7rs/src/load.c
index 385767d8..8f519327 100644
--- a/contrib/05.r7rs/src/load.c
+++ b/contrib/05.r7rs/src/load.c
@@ -10,7 +10,7 @@ pic_load(pic_state *pic, const char *filename)
   struct pic_port *port;
   xFILE *file;
 
-  file = xfopen(filename, "r");
+  file = xfopen(pic, filename, "r");
   if (file == NULL) {
     pic_errorf(pic, "could not open file: %s", filename);
   }
diff --git a/extlib/benz/file.c b/extlib/benz/file.c
index 23a78f45..146f63e6 100644
--- a/extlib/benz/file.c
+++ b/extlib/benz/file.c
@@ -51,7 +51,7 @@ file_close(pic_state PIC_UNUSED(*pic), void *cookie) {
   return fclose(cookie);
 }
 
-xFILE *xfopen(const char *name, const char *mode) {
+xFILE *xfopen(pic_state *pic, const char *name, const char *mode) {
   FILE *fp;
 
   if ((fp = fopen(name, mode)) == NULL) {
@@ -60,28 +60,28 @@ xFILE *xfopen(const char *name, const char *mode) {
 
   switch (*mode) {
   case 'r':
-    return xfunopen(fp, file_read, NULL, file_seek, file_close);
+    return xfunopen(pic, fp, file_read, NULL, file_seek, file_close);
   default:
-    return xfunopen(fp, NULL, file_write, file_seek, file_close);
+    return xfunopen(pic, fp, NULL, file_write, file_seek, file_close);
   }
 }
 
 #define FILE_VTABLE { 0, file_read, file_write, file_seek, file_close }
 
-xFILE x_iob[XOPEN_MAX] = {
+const xFILE x_iob[XOPEN_MAX] = {
   { { 0 }, 0, NULL, NULL, FILE_VTABLE, X_READ },
   { { 0 }, 0, NULL, NULL, FILE_VTABLE, X_WRITE | X_LNBUF },
   { { 0 }, 0, NULL, NULL, FILE_VTABLE, X_WRITE | X_UNBUF }
 };
 
-xFILE *xfunopen(void *cookie, int (*read)(pic_state *, void *, char *, int), int (*write)(pic_state *, void *, const char *, int), long (*seek)(pic_state *, void *, long, int), int (*close)(pic_state *, void *)) {
+xFILE *xfunopen(pic_state *pic, void *cookie, int (*read)(pic_state *, void *, char *, int), int (*write)(pic_state *, void *, const char *, int), long (*seek)(pic_state *, void *, long, int), int (*close)(pic_state *, void *)) {
   xFILE *fp;
 
-  for (fp = x_iob; fp < x_iob + XOPEN_MAX; fp++)
+  for (fp = pic->files; fp < pic->files + XOPEN_MAX; fp++)
     if ((fp->flag & (X_READ | X_WRITE)) == 0)
       break;  /* found free slot */
 
-  if (fp >= x_iob + XOPEN_MAX)  /* no free slots */
+  if (fp >= pic->files + XOPEN_MAX)  /* no free slots */
     return NULL;
 
   fp->cnt = 0;
@@ -198,7 +198,7 @@ int xfflush(pic_state *pic, xFILE *f) {
   if (f == NULL) {
     /* flush all output streams */
     for (i = 0; i < XOPEN_MAX; i++) {
-      if ((x_iob[i].flag & X_WRITE) && (xfflush(pic, &x_iob[i]) == -1))
+      if ((pic->files[i].flag & X_WRITE) && (xfflush(pic, &pic->files[i]) == -1))
         retval = -1;
     }
   } else {
diff --git a/extlib/benz/include/picrin.h b/extlib/benz/include/picrin.h
index 8eb641fe..f250305a 100644
--- a/extlib/benz/include/picrin.h
+++ b/extlib/benz/include/picrin.h
@@ -46,6 +46,9 @@ extern "C" {
 #include "picrin/value.h"
 
 typedef struct pic_code pic_code;
+typedef struct pic_state pic_state;
+
+#include "picrin/file.h"
 
 typedef struct pic_jmpbuf {
   PIC_JMPBUF buf;
@@ -71,9 +74,7 @@ typedef struct {
 
 typedef void *(*pic_allocf)(void *, size_t);
 
-typedef struct xFILE xFILE;
-
-typedef struct {
+struct pic_state {
   int argc;
   char **argv, **envp;
 
@@ -137,6 +138,7 @@ typedef struct {
   struct pic_reg *attrs;
 
   struct pic_reader *reader;
+  xFILE files[XOPEN_MAX];
 
   bool gc_enable;
   struct pic_heap *heap;
@@ -148,7 +150,7 @@ typedef struct {
 
   pic_code *iseq;               /* for pic_apply_trampoline */
   char *native_stack_start;
-} pic_state;
+};
 
 typedef pic_value (*pic_func_t)(pic_state *);
 
@@ -283,7 +285,6 @@ pic_value pic_fdisplay(pic_state *, pic_value, xFILE *);
 #include "picrin/read.h"
 #include "picrin/vector.h"
 #include "picrin/reg.h"
-#include "picrin/file.h"
 
 #if defined(__cplusplus)
 }
diff --git a/extlib/benz/include/picrin/file.h b/extlib/benz/include/picrin/file.h
index 2b761184..22af7f8e 100644
--- a/extlib/benz/include/picrin/file.h
+++ b/extlib/benz/include/picrin/file.h
@@ -14,7 +14,7 @@ extern "C" {
 #define XBUFSIZ 1024
 #define XOPEN_MAX 1024
 
-struct xFILE {
+typedef struct {
   /* buffer */
   char buf[1];                  /* fallback buffer */
   long cnt;                     /* characters left */
@@ -29,13 +29,13 @@ struct xFILE {
     int (*close)(pic_state *, void *);
   } vtable;
   int flag;                     /* mode of the file access */
-};
+} xFILE;
 
-extern xFILE x_iob[XOPEN_MAX];
+#define xstdin  (&pic->files[0])
+#define xstdout (&pic->files[1])
+#define xstderr (&pic->files[2])
 
-#define xstdin  (x_iob[0].vtable.cookie || (x_iob[0].vtable.cookie = stdin ), &x_iob[0])
-#define xstdout (x_iob[1].vtable.cookie || (x_iob[1].vtable.cookie = stdout), &x_iob[1])
-#define xstderr (x_iob[2].vtable.cookie || (x_iob[2].vtable.cookie = stderr), &x_iob[2])
+extern const xFILE x_iob[XOPEN_MAX];
 
 enum _flags {
   X_READ  = 01,
@@ -63,8 +63,8 @@ enum _flags {
 #define xputchar(pic, x)  xputc((pic), (x), xstdout)
 
 /* resource aquisition */
-xFILE *xfunopen(void *cookie, int (*read)(pic_state *, void *, char *, int), int (*write)(pic_state *, void *, const char *, int), long (*seek)(pic_state *, void *, long, int), int (*close)(pic_state *, void *));
-xFILE *xfopen(const char *, const char *);
+xFILE *xfunopen(pic_state *, void *cookie, int (*read)(pic_state *, void *, char *, int), int (*write)(pic_state *, void *, const char *, int), long (*seek)(pic_state *, void *, long, int), int (*close)(pic_state *, void *));
+xFILE *xfopen(pic_state *, const char *, const char *);
 int xfclose(pic_state *, xFILE *);
 
 /* buffer management */
diff --git a/extlib/benz/port.c b/extlib/benz/port.c
index 867626f8..c14ce632 100644
--- a/extlib/benz/port.c
+++ b/extlib/benz/port.c
@@ -131,9 +131,9 @@ string_open(pic_state *pic, const char *data, size_t size)
 
   if (data != NULL) {
     memcpy(m->buf, data, size);
-    file = xfunopen(m, string_read, NULL, string_seek, string_close);
+    file = xfunopen(pic, m, string_read, NULL, string_seek, string_close);
   } else {
-    file = xfunopen(m, NULL, string_write, string_seek, string_close);
+    file = xfunopen(pic, m, NULL, string_write, string_seek, string_close);
   }
 
   if (file == NULL) {
diff --git a/extlib/benz/state.c b/extlib/benz/state.c
index 7a355e47..fc0877ff 100644
--- a/extlib/benz/state.c
+++ b/extlib/benz/state.c
@@ -254,6 +254,12 @@ pic_open(int argc, char *argv[], char **envp, pic_allocf allocf)
   /* raised error object */
   pic->err = pic_invalid_value();
 
+  /* file pool */
+  memcpy(pic->files, x_iob, sizeof pic->files);
+  pic->files[0].vtable.cookie = stdin;
+  pic->files[1].vtable.cookie = stdout;
+  pic->files[2].vtable.cookie = stderr;
+
   /* parameter table */
   pic->ptable = pic_nil_value();