diff --git a/include/chibi/sexp.h b/include/chibi/sexp.h
index 24e94f9f..cba48fce 100755
--- a/include/chibi/sexp.h
+++ b/include/chibi/sexp.h
@@ -1205,10 +1205,10 @@ enum sexp_context_globals {
 #define sexp_flush(x, p) (sexp_port_buf(p) ? sexp_buffered_flush(x, p) : (fflush(sexp_port_stream(p)), SEXP_VOID))
 
 SEXP_API int sexp_buffered_read_char (sexp ctx, sexp p);
-SEXP_API sexp sexp_buffered_write_char (sexp ctx, int c, sexp p);
-SEXP_API sexp sexp_buffered_write_string_n (sexp ctx, const char *str, sexp_uint_t len, sexp p);
-SEXP_API sexp sexp_buffered_write_string (sexp ctx, const char *str, sexp p);
-SEXP_API sexp sexp_buffered_flush (sexp ctx, sexp p);
+SEXP_API int sexp_buffered_write_char (sexp ctx, int c, sexp p);
+SEXP_API int sexp_buffered_write_string_n (sexp ctx, const char *str, sexp_uint_t len, sexp p);
+SEXP_API int sexp_buffered_write_string (sexp ctx, const char *str, sexp p);
+SEXP_API int sexp_buffered_flush (sexp ctx, sexp p);
 
 #endif
 
diff --git a/sexp.c b/sexp.c
index 0f16a29e..9b40310c 100644
--- a/sexp.c
+++ b/sexp.c
@@ -1315,14 +1315,14 @@ int sexp_buffered_read_char (sexp ctx, sexp p) {
   }
 }
 
-sexp sexp_buffered_write_char (sexp ctx, int c, sexp p) {
+int sexp_buffered_write_char (sexp ctx, int c, sexp p) {
   if (sexp_port_offset(p)+1 >= sexp_port_size(p))
     sexp_buffered_flush(ctx, p);
   sexp_port_buf(p)[sexp_port_offset(p)++] = c;
-  return SEXP_VOID;
+  return 0;
 }
 
-sexp sexp_buffered_write_string_n (sexp ctx, const char *str,
+int sexp_buffered_write_string_n (sexp ctx, const char *str,
                                    sexp_uint_t len, sexp p) {
   int diff;
   while (sexp_port_offset(p)+len >= sexp_port_size(p)) {
@@ -1334,32 +1334,32 @@ sexp sexp_buffered_write_string_n (sexp ctx, const char *str,
   }
   memcpy(sexp_port_buf(p)+sexp_port_offset(p), str, len);
   sexp_port_offset(p) += len;
-  return SEXP_VOID;
+  return 0;
 }
 
-sexp sexp_buffered_write_string (sexp ctx, const char *str, sexp p) {
+int sexp_buffered_write_string (sexp ctx, const char *str, sexp p) {
   return sexp_buffered_write_string_n(ctx, str, strlen(str), p);
 }
 
-sexp sexp_buffered_flush (sexp ctx, sexp p) {
+int sexp_buffered_flush (sexp ctx, sexp p) {
+  int res;
   sexp_gc_var1(tmp);
-  if (! sexp_oportp(p))
-    return sexp_type_exception(ctx, NULL, SEXP_OPORT, p);
-  if (! sexp_port_openp(p))
-    return sexp_user_exception(ctx, SEXP_FALSE, "port is closed", p);
-  else {
-    if (sexp_port_stream(p)) {
-      fwrite(sexp_port_buf(p), 1, sexp_port_offset(p), sexp_port_stream(p));
-      fflush(sexp_port_stream(p));
-    } else if (sexp_port_offset(p) > 0) {
-      sexp_gc_preserve1(ctx, tmp);
-      tmp = sexp_c_string(ctx, sexp_port_buf(p), sexp_port_offset(p));
+  if (! (sexp_oportp(p) && sexp_port_openp(p)))
+    return -1;
+  if (sexp_port_stream(p)) {
+    fwrite(sexp_port_buf(p), 1, sexp_port_offset(p), sexp_port_stream(p));
+    res = fflush(sexp_port_stream(p));
+  } else if (sexp_port_offset(p) > 0) {
+    sexp_gc_preserve1(ctx, tmp);
+    tmp = sexp_c_string(ctx, sexp_port_buf(p), sexp_port_offset(p));
+    if (tmp && sexp_stringp(tmp))
       sexp_push(ctx, sexp_port_cookie(p), tmp);
-      sexp_gc_release1(ctx);
-    }
-    sexp_port_offset(p) = 0;
-    return SEXP_VOID;
+    else
+      res = -1;
+    sexp_gc_release1(ctx);
   }
+  sexp_port_offset(p) = 0;
+  return res;
 }
 
 sexp sexp_make_input_string_port_op (sexp ctx, sexp self, sexp_sint_t n, sexp str) {
@@ -1845,8 +1845,15 @@ int sexp_write_utf8_char (sexp ctx, int c, sexp out) {
 #endif
 
 sexp sexp_flush_output_op (sexp ctx, sexp self, sexp_sint_t n, sexp out) {
-  sexp_flush(ctx, out);
-  return SEXP_VOID;
+  int res;
+  sexp_assert_type(ctx, sexp_oportp, SEXP_OPORT, out);
+  res = sexp_flush(ctx, out);
+  if (res == EOF) {
+    if (sexp_port_stream(out) && ferror(sexp_port_stream(out)) && (errno == EAGAIN))
+      return sexp_global(ctx, SEXP_G_IO_BLOCK_ERROR);
+    return SEXP_FALSE;
+  }
+  return SEXP_TRUE;
 }
 
 #define INIT_STRING_BUFFER_SIZE 128
diff --git a/vm.c b/vm.c
index e142f0d1..402143f6 100644
--- a/vm.c
+++ b/vm.c
@@ -409,7 +409,7 @@ static void generate_tail_jump (sexp ctx, sexp name, sexp loc, sexp lam, sexp ap
     emit_word(ctx, sexp_param_index(lam, sexp_car(ls1)));
   }
 
-  /* jump */
+  /* drop the current result and jump */
   emit(ctx, SEXP_OP_JUMP);
   emit_word(ctx, (sexp_uint_t) (-sexp_context_pos(ctx) +
                                 (sexp_pairp(sexp_lambda_locals(lam))
@@ -840,6 +840,8 @@ static sexp sexp_print_vm_profile (sexp ctx, sexp self, sexp_sint_t n) {
 #define sexp_ensure_stack(n)
 #endif
 
+#include <unistd.h>
+
 sexp sexp_apply (sexp ctx, sexp proc, sexp args) {
   unsigned char *ip;
   sexp bc, cp, *stack = sexp_stack_data(sexp_context_stack(ctx));
@@ -1804,13 +1806,22 @@ sexp sexp_apply (sexp ctx, sexp proc, sexp args) {
     if (! sexp_oportp(_ARG3))
       sexp_raise("write-string: not an output-port", sexp_list1(ctx, _ARG3));
     sexp_context_top(ctx) = top;
-    if (sexp_port_stream(_ARG3)) {
-      i = fwrite(sexp_bytes_data(tmp1), 1, sexp_unbox_fixnum(_ARG2), sexp_port_stream(_ARG3));
+    if (sexp_port_stream(_ARG3) && sexp_port_fileno(_ARG3) >= 0) {
+      /* first flush anything pending */
+      i = fflush(sexp_port_stream(_ARG3));
 #if SEXP_USE_GREEN_THREADS
-      if ((i < sexp_unbox_fixnum(_ARG2))
-          && ferror(sexp_port_stream(_ARG3))
-          && (errno == EAGAIN)) {
+      if (i) {
+        i = 0;
         clearerr(sexp_port_stream(_ARG3));
+        if (errno == EAGAIN)
+          goto write_string_yield;
+      }
+      errno = 0;
+#endif
+      /* fwrite doesn't give reliable counts, use write(2) directly */
+      i = write(sexp_port_fileno(_ARG3), sexp_bytes_data(tmp1), sexp_unbox_fixnum(_ARG2));
+#if SEXP_USE_GREEN_THREADS
+      if (i < sexp_unbox_fixnum(_ARG2)) {
         /* modify stack in-place so we continue where we left off next time */
         if (i > 0) {
           if (sexp_stringp(_ARG1))
@@ -1819,7 +1830,8 @@ sexp sexp_apply (sexp ctx, sexp proc, sexp args) {
             _ARG1 = sexp_subbytes(ctx, _ARG1, sexp_make_fixnum(i), SEXP_FALSE);
           _ARG2 = sexp_make_fixnum(sexp_unbox_fixnum(_ARG2) - i);
         }
-        /* yield if threads are enabled */
+        /* yield if threads are enabled (otherwise busy loop) */
+      write_string_yield:
         if (sexp_applicablep(sexp_global(ctx, SEXP_G_THREADS_BLOCKER))) {
           sexp_apply1(ctx, sexp_global(ctx, SEXP_G_THREADS_BLOCKER), _ARG3);
           fuel = 0;
@@ -1832,8 +1844,9 @@ sexp sexp_apply (sexp ctx, sexp proc, sexp args) {
       if (sexp_bytes_length(tmp1) != sexp_unbox_fixnum(_ARG2))
         tmp1 = sexp_subbytes(ctx, tmp1, SEXP_ZERO, _ARG2);
       sexp_write_string(ctx, sexp_bytes_data(tmp1), _ARG3);
+      i = sexp_unbox_fixnum(_ARG2);
     }
-    tmp1 = _ARG2;     /* return the number of bytes written */
+    tmp1 = sexp_make_fixnum(i);     /* return the number of bytes written */
     top-=2;
     _ARG1 = tmp1;
     break;