diff --git a/lib/chibi/io/io.stub b/lib/chibi/io/io.stub
index e1b084bd..5c423820 100644
--- a/lib/chibi/io/io.stub
+++ b/lib/chibi/io/io.stub
@@ -11,14 +11,19 @@
     (string (value 1 size_t) size_t (default (current-input-port) input-port)))
   ))
 
+(c-include "port.c")
+
 (define-c-const int (seek/set "SEEK_SET"))
 (define-c-const int (seek/cur "SEEK_CUR"))
 (define-c-const int (seek/end "SEEK_END"))
 
-(define-c long (file-position "ftell") (port))
-(define-c long (set-file-position! "fseek") (port long int))
+;; (define-c off_t (file-position "ftell") (port))
+;; (define-c off_t (set-file-position! "fseek") (port off_t int))
 
-(c-include "port.c")
+(define-c sexp (file-position "sexp_tell")
+  ((value ctx sexp) (value self sexp) sexp))
+(define-c sexp (set-file-position! "sexp_seek")
+  ((value ctx sexp) (value self sexp) sexp off_t int))
 
 (define-c boolean (is-a-socket? "sexp_is_a_socket_p") (fileno))
 
diff --git a/lib/chibi/io/port.c b/lib/chibi/io/port.c
index a4639e12..55173aa1 100644
--- a/lib/chibi/io/port.c
+++ b/lib/chibi/io/port.c
@@ -402,6 +402,22 @@ int sexp_is_a_socket_p (int fd) {
 #endif
 }
 
+sexp sexp_seek (sexp ctx, sexp self, sexp x, off_t offset, int whence) {
+  if (! (sexp_portp(x) || sexp_filenop(x)))
+    return sexp_type_exception(ctx, self, SEXP_IPORT, x);
+  if (sexp_filenop(x))
+    return sexp_make_integer(ctx, lseek(sexp_fileno_fd(x), offset, whence));
+  if (sexp_filenop(sexp_port_fd(x)))
+    return sexp_make_integer(ctx, lseek(sexp_fileno_fd(sexp_port_fd(x)), offset, whence));
+  if (sexp_stream_portp(x))
+    return sexp_make_integer(ctx, fseek(sexp_port_stream(x), offset, whence));
+  return sexp_xtype_exception(ctx, self, "not a seekable port", x);
+}
+
+sexp sexp_tell (sexp ctx, sexp self, sexp x) {
+  return sexp_seek(ctx, self, x, 0, SEEK_CUR);
+}
+
 int sexp_send_file (int fd, int s, off_t offset, off_t len, off_t* res) {
 #if SEXP_USE_SEND_FILE
   *res = len;