diff --git a/extlib/benz/file.c b/extlib/benz/file.c
index 7a4908f3..23a78f45 100644
--- a/extlib/benz/file.c
+++ b/extlib/benz/file.c
@@ -1,6 +1,7 @@
 #include "picrin.h"
 
-static int file_read(void *cookie, char *ptr, int size) {
+static int
+file_read(pic_state PIC_UNUSED(*pic), void *cookie, char *ptr, int size) {
   FILE *file = cookie;
   int r;
 
@@ -16,7 +17,8 @@ static int file_read(void *cookie, char *ptr, int size) {
   return r;
 }
 
-static int file_write(void *cookie, const char *ptr, int size) {
+static int
+file_write(pic_state PIC_UNUSED(*pic), void *cookie, const char *ptr, int size) {
   FILE *file = cookie;
   int r;
 
@@ -28,7 +30,8 @@ static int file_write(void *cookie, const char *ptr, int size) {
   return r;
 }
 
-static long file_seek(void *cookie, long pos, int whence) {
+static long
+file_seek(pic_state PIC_UNUSED(*pic), void *cookie, long pos, int whence) {
   switch (whence) {
   case XSEEK_CUR:
     whence = SEEK_CUR;
@@ -43,7 +46,8 @@ static long file_seek(void *cookie, long pos, int whence) {
   return fseek(cookie, pos, whence);
 }
 
-static int file_close(void *cookie) {
+static int
+file_close(pic_state PIC_UNUSED(*pic), void *cookie) {
   return fclose(cookie);
 }
 
@@ -70,7 +74,7 @@ xFILE x_iob[XOPEN_MAX] = {
   { { 0 }, 0, NULL, NULL, FILE_VTABLE, X_WRITE | X_UNBUF }
 };
 
-xFILE *xfunopen(void *cookie, int (*read)(void *, char *, int), int (*write)(void *, const char *, int), long (*seek)(void *, long, int), int (*close)(void *)) {
+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 *fp;
 
   for (fp = x_iob; fp < x_iob + XOPEN_MAX; fp++)
@@ -98,7 +102,7 @@ int xfclose(pic_state *pic, xFILE *fp) {
   fp->flag = 0;
   if (fp->base != fp->buf)
     pic_free(pic, fp->base);
-  return fp->vtable.close(fp->vtable.cookie);
+  return fp->vtable.close(pic, fp->vtable.cookie);
 }
 
 int x_fillbuf(pic_state *pic, xFILE *fp) {
@@ -121,7 +125,7 @@ int x_fillbuf(pic_state *pic, xFILE *fp) {
   bufsize = (fp->flag & X_UNBUF) ? sizeof(fp->buf) : XBUFSIZ;
 
   fp->ptr = fp->base;
-  fp->cnt = fp->vtable.read(fp->vtable.cookie, fp->ptr, bufsize);
+  fp->cnt = fp->vtable.read(pic, fp->vtable.cookie, fp->ptr, bufsize);
 
   if (--fp->cnt < 0) {
     if (fp->cnt == -1)
@@ -157,7 +161,7 @@ int x_flushbuf(pic_state *pic, int x, xFILE *fp) {
     fp->cnt = 0;
     if (x == EOF)
       return EOF;
-    num_written = fp->vtable.write(fp->vtable.cookie, (const char *) &c, 1);
+    num_written = fp->vtable.write(pic, fp->vtable.cookie, (const char *) &c, 1);
     bufsize = 1;
   } else {
     /* buffered write */
@@ -168,7 +172,7 @@ int x_flushbuf(pic_state *pic, int x, xFILE *fp) {
     bufsize = (int)(fp->ptr - fp->base);
     while(bufsize - num_written > 0) {
       int t;
-      t = fp->vtable.write(fp->vtable.cookie, fp->base + num_written, bufsize - num_written);
+      t = fp->vtable.write(pic, fp->vtable.cookie, fp->base + num_written, bufsize - num_written);
       if (t < 0)
         break;
       num_written += t;
@@ -334,7 +338,7 @@ long xfseek(pic_state *pic, xFILE *fp, long offset, int whence) {
   fp->ptr = fp->base;
   fp->cnt = 0;
 
-  if ((s = fp->vtable.seek(fp->vtable.cookie, offset, whence)) != 0)
+  if ((s = fp->vtable.seek(pic, fp->vtable.cookie, offset, whence)) != 0)
     return s;
   fp->flag &= ~X_EOF;
   return 0;
diff --git a/extlib/benz/include/picrin/file.h b/extlib/benz/include/picrin/file.h
index 189bd3de..2b761184 100644
--- a/extlib/benz/include/picrin/file.h
+++ b/extlib/benz/include/picrin/file.h
@@ -23,10 +23,10 @@ struct xFILE {
   /* operators */
   struct {
     void *cookie;
-    int (*read)(void *, char *, int);
-    int (*write)(void *, const char *, int);
-    long (*seek)(void *, long, int);
-    int (*close)(void *);
+    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 *);
   } vtable;
   int flag;                     /* mode of the file access */
 };
@@ -63,7 +63,7 @@ enum _flags {
 #define xputchar(pic, x)  xputc((pic), (x), xstdout)
 
 /* resource aquisition */
-xFILE *xfunopen(void *cookie, int (*read)(void *, char *, int), int (*write)(void *, const char *, int), long (*seek)(void *, long, int), int (*close)(void *));
+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 *);
 int xfclose(pic_state *, xFILE *);
 
diff --git a/extlib/benz/port.c b/extlib/benz/port.c
index 8a0d7df4..867626f8 100644
--- a/extlib/benz/port.c
+++ b/extlib/benz/port.c
@@ -54,13 +54,12 @@ DEFINE_STANDARD_PORT_ACCESSOR(pic_stdout, "current-output-port")
 DEFINE_STANDARD_PORT_ACCESSOR(pic_stderr, "current-error-port")
 
 struct strfile {
-  pic_state *pic;
   char *buf;
   long pos, end, capa;
 };
 
 static int
-string_read(void *cookie, char *ptr, int size)
+string_read(pic_state PIC_UNUSED(*pic), void *cookie, char *ptr, int size)
 {
   struct strfile *m = cookie;
 
@@ -72,13 +71,13 @@ string_read(void *cookie, char *ptr, int size)
 }
 
 static int
-string_write(void *cookie, const char *ptr, int size)
+string_write(pic_state *pic, void *cookie, const char *ptr, int size)
 {
   struct strfile *m = cookie;
 
   if (m->pos + size >= m->capa) {
     m->capa = (m->pos + size) * 2;
-    m->buf = pic_realloc(m->pic, m->buf, (size_t)m->capa);
+    m->buf = pic_realloc(pic, m->buf, (size_t)m->capa);
   }
   memcpy(m->buf + m->pos, ptr, size);
   m->pos += size;
@@ -88,7 +87,7 @@ string_write(void *cookie, const char *ptr, int size)
 }
 
 static long
-string_seek(void *cookie, long pos, int whence)
+string_seek(pic_state PIC_UNUSED(*pic), void *cookie, long pos, int whence)
 {
   struct strfile *m = cookie;
 
@@ -108,12 +107,12 @@ string_seek(void *cookie, long pos, int whence)
 }
 
 static int
-string_close(void *cookie)
+string_close(pic_state *pic, void *cookie)
 {
   struct strfile *m = cookie;
 
-  pic_free(m->pic, m->buf);
-  pic_free(m->pic, m);
+  pic_free(pic, m->buf);
+  pic_free(pic, m);
   return 0;
 }
 
@@ -124,7 +123,6 @@ string_open(pic_state *pic, const char *data, size_t size)
   xFILE *file;
 
   m = pic_malloc(pic, sizeof(struct strfile));
-  m->pic = pic;
   m->buf = pic_malloc(pic, size);
   m->pos = 0;
   m->end = size;
@@ -139,7 +137,7 @@ string_open(pic_state *pic, const char *data, size_t size)
   }
 
   if (file == NULL) {
-    string_close(m);
+    string_close(pic, m);
     pic_error(pic, "could not open new output string/bytevector port", pic_nil_value());
   }
   return file;