diff --git a/eval.c b/eval.c
index e6b6ea10..5bb1e465 100644
--- a/eval.c
+++ b/eval.c
@@ -1058,7 +1058,17 @@ sexp sexp_open_binary_output_file (sexp ctx, sexp self, sexp_sint_t n, sexp path
 }
 
 sexp sexp_close_port_op (sexp ctx, sexp self, sexp_sint_t n, sexp port) {
+  sexp res = SEXP_VOID;
   sexp_assert_type(ctx, sexp_portp, SEXP_OPORT, port);
+  /* we can't run arbitrary scheme code in the finalizer, so we need */
+  /* to flush and run the closer here */
+  if (sexp_port_customp(port)) {
+    if (sexp_oportp(port)) res = sexp_flush_output(ctx, port);
+    if (sexp_exceptionp(res)) return res;
+    if (sexp_applicablep(sexp_port_closer(port)))
+      res = sexp_apply1(ctx, sexp_port_closer(port), port);
+    if (sexp_exceptionp(res)) return res;
+  }
   return sexp_finalize_port(ctx, self, n, port);
 }
 
diff --git a/include/chibi/features.h b/include/chibi/features.h
index 7349a370..6e4b5fe4 100644
--- a/include/chibi/features.h
+++ b/include/chibi/features.h
@@ -179,10 +179,10 @@
 /*   Making them immutable allows for packed UTF-8 strings. */
 /* #define SEXP_USE_MUTABLE_STRINGS 0 */
 
-/* uncomment this to disable string ports */
-/*   If disabled some basic functionality such as number->string */
-/*   will not be available by default. */
-/* #define SEXP_USE_STRING_STREAMS 0 */
+/* uncomment this to base string ports on C streams */
+/*   This historic option enables string and custom ports backed */
+/*   by FILE* objects using memstreams and funopen/fopencookie. */
+/* #define SEXP_USE_STRING_STREAMS 1 */
 
 /* uncomment this to disable automatic closing of ports */
 /*   If enabled, the underlying FILE* for file ports will be */
@@ -525,11 +525,7 @@
 #endif
 
 #ifndef SEXP_USE_STRING_STREAMS
-#ifdef _WIN32
 #define SEXP_USE_STRING_STREAMS 0
-#else
-#define SEXP_USE_STRING_STREAMS ! SEXP_USE_NO_FEATURES
-#endif
 #endif
 
 #ifndef SEXP_USE_AUTOCLOSE_PORTS
@@ -544,6 +540,10 @@
 #define SEXP_USE_BIDIRECTIONAL_PORTS ! SEXP_USE_NO_FEATURES
 #endif
 
+#ifndef SEXP_PORT_BUFFER_SIZE
+#define SEXP_PORT_BUFFER_SIZE 4096
+#endif
+
 #ifndef SEXP_USE_2010_EPOCH
 #define SEXP_USE_2010_EPOCH ! SEXP_USE_NO_FEATURES
 #endif
diff --git a/include/chibi/sexp.h b/include/chibi/sexp.h
index f3d9f6b5..929bab38 100755
--- a/include/chibi/sexp.h
+++ b/include/chibi/sexp.h
@@ -681,6 +681,15 @@ sexp sexp_make_flonum(sexp ctx, double f);
 #define sexp_stream_portp(x) (sexp_port_stream(x) != NULL)
 #endif
 
+#define sexp_port_customp(x) (sexp_vectorp(sexp_port_cookie(x)) && sexp_vector_length(sexp_port_cookie(x)) == 6)
+
+/* only valid on custom ports */
+#define sexp_port_buffer(x)  (sexp_vector_ref(sexp_port_cookie(x), SEXP_ONE))
+#define sexp_port_reader(x)  (sexp_vector_ref(sexp_port_cookie(x), SEXP_TWO))
+#define sexp_port_writer(x)  (sexp_vector_ref(sexp_port_cookie(x), SEXP_THREE))
+#define sexp_port_seeker(x)  (sexp_vector_ref(sexp_port_cookie(x), SEXP_FOUR))
+#define sexp_port_closer(x)  (sexp_vector_ref(sexp_port_cookie(x), SEXP_FIVE))
+
 /***************************** constructors ****************************/
 
 #define sexp_make_boolean(x) ((x) ? SEXP_TRUE : SEXP_FALSE)
diff --git a/lib/chibi/io.sld b/lib/chibi/io.sld
index ab53e3c2..80d0bd48 100644
--- a/lib/chibi/io.sld
+++ b/lib/chibi/io.sld
@@ -5,7 +5,8 @@
           port->list port->string-list port->sexp-list port->string
           file-position set-file-position! seek/set seek/cur seek/end
           make-custom-input-port make-custom-output-port
-          make-null-output-port make-broadcast-port make-concatenated-port
+          make-null-output-port make-null-input-port
+          make-broadcast-port make-concatenated-port
           make-generated-input-port make-filtered-output-port
           make-filtered-input-port string-count
           open-input-bytevector open-output-bytevector get-output-bytevector
diff --git a/lib/chibi/io/io.scm b/lib/chibi/io/io.scm
index 775d6180..12f84a7f 100644
--- a/lib/chibi/io/io.scm
+++ b/lib/chibi/io/io.scm
@@ -1,5 +1,5 @@
 ;; io.scm -- various input/output utilities
-;; Copyright (c) 2010-2011 Alex Shinn.  All rights reserved.
+;; Copyright (c) 2010-2012 Alex Shinn.  All rights reserved.
 ;; BSD-style license: http://synthcode.com/license.txt
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
@@ -37,7 +37,7 @@
  ((not string-streams)
   (define (write-string str n . o)
     (let ((out (if (pair? o) (car o) (current-output-port))))
-      (display (substring str 0 n out))))))
+      (display (substring str 0 n) out)))))
 
 ;;> @subsubsubsection{(read-line [in [n]])}
 
@@ -90,7 +90,7 @@
     (let ((out (open-output-string)))
       (do ((i 0 (+ i 1))
            (ch (read-char in) (read-char in)))
-          ((or (= i n) (eof-object? ch)) (get-output-string out))
+          ((or (= i n) (eof-object? ch)) (list i (get-output-string out)))
         (write-char ch out))))))
 
 (define (read-string n . o)
@@ -183,76 +183,84 @@
     (%make-custom-output-port write seek close)))
 
 (define (make-null-output-port)
-  (make-custom-output-port (lambda (str n) 0)))
+  (make-custom-output-port (lambda (str start end) 0)))
 
 (define (make-broadcast-port . ports)
   (make-custom-output-port
-   (lambda (str n)
-     (for-each (lambda (p) (write-string str n p)) ports)
-     n)))
+   (lambda (str start end)
+     (let ((str (if (zero? start) str (substring str start)))
+           (n (- end start)))
+       (for-each (lambda (p) (write-string str n p)) ports)
+       n))))
 
 (define (make-filtered-output-port filter out)
   (make-custom-output-port
-   (lambda (str n)
+   (lambda (str start end)
      (let* ((len (string-length str))
-            (s1 (if (= n len) str (substring str 0 n)))
+            (s1 (if (and (zero? start) (= end len)) str (substring str start end)))
             (s2 (filter s1)))
        (if (string? s2)
            (write-string s2 (string-length s2) out))))))
 
 (define (make-concatenated-port . ports)
   (make-custom-input-port
-   (lambda (str n)
+   (lambda (str start end)
      (if (null? ports)
          0
-         (let lp ((i (read-string! str n (car ports))))
-           (cond
-            ((>= i n)
-             i)
-            (else
-             (set! ports (cdr ports))
+         (let ((str (if (zero? start) str (substring str start)))
+               (n (- end start)))
+           (let lp ((i (read-string! str n (car ports))))
              (cond
-              ((null? ports)
+              ((>= i n)
                i)
               (else
-               (let* ((s (read-string (- n i) (car ports)))
-                      (len (if (string? s) (string-length s) 0)))
-                 (if (and (string? str) (> len 0))
-                     (string-copy! str i s 0 len))
-                 (lp (+ i len))))))))))))
+               (set! ports (cdr ports))
+               (cond
+                ((null? ports)
+                 i)
+                (else
+                 (let* ((s (read-string (- n i) (car ports)))
+                        (len (if (string? s) (string-length s) 0)))
+                   (if (and (string? str) (> len 0))
+                       (string-copy! str i s 0 len))
+                   (lp (+ i len)))))))))))))
+
+(define (make-null-input-port)
+  (make-concatenated-port))
 
 (define (make-generated-input-port generator)
   (let ((buf "")
         (len 0)
         (offset 0))
     (make-custom-input-port
-     (lambda (str n)
-       (cond
-        ((>= (- len offset) n)
-         (string-copy! str 0 buf offset (+ offset n))
-         (set! offset (+ offset n))
-         n)
-        (else
-         (string-copy! str 0 buf offset len)
-         (let lp ((i (- len offset)))
-           (set! buf (generator))
-           (cond
-            ((not (string? buf))
-             (set! buf "")
-             (set! len 0)
-             (set! offset 0)
-             (- n i))
-            (else
-             (set! len (string-length buf))
-             (set! offset 0)
+     (lambda (str start end)
+       (let ((n (- end start)))
+         (cond
+          ((>= (- len offset) n)
+           (string-copy! str start buf offset (+ offset n))
+           (set! offset (+ offset n))
+           n)
+          (else
+           (string-copy! str start buf offset len)
+           (let lp ((i (+ start (- len offset))))
+             (set! buf (generator))
              (cond
-              ((>= (- len offset) (- n i))
-               (string-copy! str i buf offset (+ offset (- n i)))
-               (set! offset (+ offset (- n i)))
-               n)
+              ((not (string? buf))
+               (set! buf "")
+               (set! len 0)
+               (set! offset 0)
+               (- i start))
               (else
-               (string-copy! str i buf offset len)
-               (lp (+ i (- len offset))))))))))))))
+               (set! len (string-length buf))
+               (set! offset 0)
+               (cond
+                ((>= (- len offset) (- n i))
+                 (string-copy! str i buf offset (+ offset (- n i)))
+                 (set! offset (+ offset (- n i)))
+                 n)
+                (else
+                 (string-copy! str i buf offset len)
+                 (lp (+ i (- len offset)))))))))))))))
 
 (define (make-filtered-input-port filter in)
   (make-generated-input-port
diff --git a/lib/chibi/io/io.stub b/lib/chibi/io/io.stub
index f26040d5..c1b962e3 100644
--- a/lib/chibi/io/io.stub
+++ b/lib/chibi/io/io.stub
@@ -9,7 +9,6 @@
 
   (define-c size_t (%read-string! "fread")
     (string (value 1 size_t) size_t (default (current-input-port) input-port)))
-
   ))
 
 (define-c-const int (seek/set "SEEK_SET"))
diff --git a/lib/chibi/io/port.c b/lib/chibi/io/port.c
index 981f633b..049f1ec6 100644
--- a/lib/chibi/io/port.c
+++ b/lib/chibi/io/port.c
@@ -2,7 +2,6 @@
 #include <stdio.h>
 #include <chibi/eval.h>
 
-#define SEXP_PORT_BUFFER_SIZE 1024
 #define SEXP_LAST_CONTEXT_CHECK_LIMIT 256
 
 #define sexp_cookie_ctx(vec) sexp_vector_ref((sexp)vec, SEXP_ZERO)
@@ -62,7 +61,8 @@ static ssize_t sexp_cookie_reader (void *cookie, char *buffer, size_t size)
   sexp_gc_preserve2(ctx, ctx2, args);
   if (size > sexp_string_length(sexp_cookie_buffer(vec)))
     sexp_cookie_buffer_set(vec, sexp_make_string(ctx, sexp_make_fixnum(size), SEXP_VOID));
-  args = sexp_list2(ctx, sexp_cookie_buffer(vec), sexp_make_fixnum(size));
+  args = sexp_list2(ctx, SEXP_ZERO, sexp_make_fixnum(size));
+  args = sexp_cons(ctx, sexp_cookie_buffer(vec), args);
   res = sexp_apply(ctx, sexp_cookie_read(vec), args);
   sexp_gc_release2(ctx);
   if (sexp_fixnump(res)) {
@@ -88,7 +88,8 @@ static ssize_t sexp_cookie_writer (void *cookie, const char *buffer, size_t size
   if (size > sexp_string_length(sexp_cookie_buffer(vec)))
     sexp_cookie_buffer_set(vec, sexp_make_string(ctx, sexp_make_fixnum(size), SEXP_VOID));
   memcpy(sexp_string_data(sexp_cookie_buffer(vec)), buffer, size);
-  args = sexp_list2(ctx, sexp_cookie_buffer(vec), sexp_make_fixnum(size));
+  args = sexp_list2(ctx, SEXP_ZERO, sexp_make_fixnum(size));
+  args = sexp_cons(ctx, sexp_cookie_buffer(vec), args);
   res = sexp_apply(ctx, sexp_cookie_write(vec), args);
   sexp_gc_release2(ctx);
   return (sexp_fixnump(res) ? sexp_unbox_fixnum(res) : -1);
@@ -189,21 +190,47 @@ static sexp sexp_make_custom_port (sexp ctx, sexp self, char *mode,
 static sexp sexp_make_custom_port (sexp ctx, sexp self,
                                    char *mode, sexp read, sexp write,
                                    sexp seek, sexp close) {
-  return sexp_user_exception(ctx, self, "custom ports not supported in this configuration", SEXP_NULL);
+  sexp vec;
+  sexp_gc_var2(res, str);
+  sexp_gc_preserve2(ctx, res, str);
+  str = sexp_make_string(ctx, sexp_make_fixnum(SEXP_PORT_BUFFER_SIZE), SEXP_VOID);
+  if (sexp_exceptionp(str)) return str;
+  res = sexp_make_input_string_port(ctx, str);
+  if (sexp_exceptionp(res)) return res;
+  if (mode && mode[0] == 'w') {
+    sexp_pointer_tag(res) = SEXP_OPORT;
+    sexp_port_cookie(res) = str;
+  } else {
+    sexp_port_offset(res) = 0;
+    sexp_port_size(res) = 0;
+  }
+  vec = sexp_make_vector(ctx, SEXP_SIX, SEXP_VOID);
+  if (sexp_exceptionp(vec)) return vec;
+  sexp_vector_set(vec, SEXP_ZERO, SEXP_FALSE);
+  sexp_vector_set(vec, SEXP_ONE, sexp_port_cookie(res));
+  sexp_vector_set(vec, SEXP_TWO, read);
+  sexp_vector_set(vec, SEXP_THREE, write);
+  sexp_vector_set(vec, SEXP_FOUR, seek);
+  sexp_vector_set(vec, SEXP_FIVE, close);
+  sexp_port_cookie(res) = vec;
+  sexp_gc_release2(ctx);
+  return res;
 }
 
 #endif
 
-static sexp sexp_make_custom_input_port (sexp ctx, sexp self,
-                                         sexp read, sexp seek, sexp close) {
+sexp sexp_make_custom_input_port (sexp ctx, sexp self,
+                                  sexp read, sexp seek, sexp close) {
   return sexp_make_custom_port(ctx, self, "r", read, SEXP_FALSE, seek, close);
 }
 
-static sexp sexp_make_custom_output_port (sexp ctx, sexp self,
-                                          sexp write, sexp seek, sexp close) {
+sexp sexp_make_custom_output_port (sexp ctx, sexp self,
+                                   sexp write, sexp seek, sexp close) {
   sexp res = sexp_make_custom_port(ctx, self, "w", SEXP_FALSE, write, seek, close);
+#if SEXP_USE_STRING_STREAMS
   if (!sexp_exceptionp(res))
     sexp_pointer_tag(res) = SEXP_OPORT;
+#endif
   return res;
 }
 
diff --git a/sexp.c b/sexp.c
index 7958edd0..823eb105 100644
--- a/sexp.c
+++ b/sexp.c
@@ -124,7 +124,11 @@ sexp sexp_finalize_port (sexp ctx, sexp self, sexp_sint_t n, sexp port) {
 #if !SEXP_USE_STRING_STREAMS
               || (sexp_iportp(port) && sexp_truep(sexp_port_cookie(port)))
 #endif
-              ))
+              )
+#if SEXP_USE_STRING_STREAMS
+          && !sexp_port_customp(port)
+#endif
+          )
 	free(sexp_port_buf(port));
     }
 #ifndef PLAN9
@@ -1354,9 +1358,8 @@ sexp sexp_open_output_file_descriptor (sexp ctx, sexp self, sexp_sint_t n, sexp
 
 #else  /* ! SEXP_USE_STRING_STREAMS */
 
-#define SEXP_PORT_BUFFER_SIZE 4096
-
 int sexp_buffered_read_char (sexp ctx, sexp p) {
+  sexp_gc_var1(tmp);
   int res = 0;
   if (sexp_port_offset(p) < sexp_port_size(p)) {
     return sexp_port_buf(p)[sexp_port_offset(p)++];
@@ -1376,6 +1379,21 @@ int sexp_buffered_read_char (sexp ctx, sexp p) {
       res = ((sexp_port_offset(p) < sexp_port_size(p))
              ? sexp_port_buf(p)[sexp_port_offset(p)++] : EOF);
     }
+  } else if (sexp_port_customp(p)) {
+    sexp_gc_preserve1(ctx, tmp);
+    tmp = sexp_list2(ctx, SEXP_ZERO, sexp_make_fixnum(SEXP_PORT_BUFFER_SIZE));
+    tmp = sexp_cons(ctx, sexp_port_buffer(p), tmp);
+    tmp = sexp_apply(ctx, sexp_port_reader(p), tmp);
+    if (sexp_fixnump(tmp) && sexp_unbox_fixnum(tmp) > 0) {
+      sexp_port_offset(p) = 0;
+      sexp_port_size(p) = sexp_unbox_fixnum(tmp);
+      res = ((sexp_port_offset(p) < sexp_port_size(p))
+             ? sexp_port_buf(p)[sexp_port_offset(p)++] : EOF);
+    } else {
+      res = EOF;
+      sexp_port_size(p) = 0;
+    }
+    sexp_gc_release1(ctx);
   } else {
     res = EOF;
   }
@@ -1434,15 +1452,22 @@ int sexp_buffered_flush (sexp ctx, sexp p) {
       sexp_port_offset(p) = 0;
       res = 0;
     }
-  } else if (sexp_port_offset(p) > 0) { /* string port */
+  } else if (sexp_port_offset(p) > 0) {
     sexp_gc_preserve1(ctx, tmp);
-    tmp = sexp_c_string(ctx, sexp_port_buf(p), off);
-    if (tmp && sexp_stringp(tmp)) {
-      sexp_push(ctx, sexp_port_cookie(p), tmp);
-      sexp_port_offset(p) = 0;
-      res = 0;
-    } else {
-      res = -1;
+    if (sexp_port_customp(p)) {   /* custom port */
+      tmp = sexp_list2(ctx, SEXP_ZERO, sexp_make_fixnum(sexp_port_offset(p)));
+      tmp = sexp_cons(ctx, sexp_port_buffer(p), tmp);
+      tmp = sexp_apply(ctx, sexp_port_writer(p), tmp);
+      res = (sexp_fixnump(tmp) && sexp_unbox_fixnum(tmp) > 0) ? 0 : -1;
+    } else {                      /* string port */
+      tmp = sexp_c_string(ctx, sexp_port_buf(p), off);
+      if (tmp && sexp_stringp(tmp)) {
+        sexp_push(ctx, sexp_port_cookie(p), tmp);
+        sexp_port_offset(p) = 0;
+        res = 0;
+      } else {
+        res = -1;
+      }
     }
     sexp_gc_release1(ctx);
   }
diff --git a/tests/io-tests.scm b/tests/io-tests.scm
index 1fd653d7..134fac16 100644
--- a/tests/io-tests.scm
+++ b/tests/io-tests.scm
@@ -23,40 +23,34 @@
   (call-with-input-string "abc\ndef"
     (lambda (in) (let ((line (read-line in))) (list line (read-line in))))))
 
-;; Custom ports are only supported with string streams (i.e. either
-;; GNU fopencookie or BSD funopen).
+(test "null-output-port" #t
+  (let ((out (make-null-output-port)))
+    (write 1 out)
+    (close-output-port out)
+    #t))
 
-(cond-expand
- (string-streams
+(test "null-input-port" #t
+  (let ((in (make-null-input-port)))
+    (let ((res (eof-object? (read-char in))))
+      (close-input-port in)
+      res)))
 
-  (test "null-output-port" #t
-    (let ((out (make-null-output-port)))
-      (write 1 out)
-      (close-output-port out)
-      #t))
+(define (string-upcase str)
+  (list->string (map char-upcase (string->list str))))
 
-  (test "null-input-port" #t
-    (let ((in (make-concatenated-port)))
-      (let ((res (eof-object? (read-char in))))
-        (close-input-port in)
-        res)))
+(test "upcase-input-port" "ABC"
+  (call-with-input-string "abc"
+    (lambda (in)
+      (let ((in (make-filtered-input-port string-upcase in)))
+        (let ((res (read-line in)))
+          (close-input-port in)
+          res)))))
 
-  (define (string-upcase str)
-    (list->string (map char-upcase (string->list str))))
-
-  (test "upcase-input-port" "ABC"
-    (call-with-input-string "abc"
-      (lambda (in)
-        (let ((in (make-filtered-input-port string-upcase in)))
-          (let ((res (read-line in)))
-            (close-input-port in)
-            res)))))
-
-  (test "upcase-output-port" "ABC"
-    (call-with-output-string
-      (lambda (out)
-        (let ((out (make-filtered-output-port string-upcase out)))
-          (display "abc" out)
-          (close-output-port out)))))))
+(test "upcase-output-port" "ABC"
+  (call-with-output-string
+    (lambda (out)
+      (let ((out (make-filtered-output-port string-upcase out)))
+        (display "abc" out)
+        (close-output-port out)))))
 
 (test-end)