diff --git a/lib/chibi/accept.c b/lib/chibi/accept.c
index e9642153..02ddd45a 100644
--- a/lib/chibi/accept.c
+++ b/lib/chibi/accept.c
@@ -13,8 +13,8 @@ sexp sexp_accept (sexp ctx, sexp self, int sock, struct sockaddr* addr, socklen_
 #if SEXP_USE_GREEN_THREADS
   if (res < 0 && errno == EWOULDBLOCK) {
     f = sexp_global(ctx, SEXP_G_THREADS_BLOCKER);
-    if (sexp_opcodep(f)) {
-      ((sexp_proc2)sexp_opcode_func(f))(ctx, f, 1, sexp_make_fixnum(sock));
+    if (sexp_applicablep(f)) {
+      sexp_apply2(ctx, f, sexp_make_fixnum(sock), SEXP_FALSE);
       return sexp_global(ctx, SEXP_G_IO_BLOCK_ERROR);
     }
   }
@@ -26,17 +26,19 @@ sexp sexp_accept (sexp ctx, sexp self, int sock, struct sockaddr* addr, socklen_
 
 /* likewise sendto and recvfrom should suspend the thread gracefully */
 
-sexp sexp_sendto (sexp ctx, sexp self, int sock, const void* buffer, size_t len, int flags, struct sockaddr* addr, socklen_t addr_len) {
+#define sexp_zerop(x) ((x) == SEXP_ZERO || (sexp_flonump(x) && sexp_flonum_value(x) == 0.0))
+
+sexp sexp_sendto (sexp ctx, sexp self, int sock, const void* buffer, size_t len, int flags, struct sockaddr* addr, socklen_t addr_len, sexp timeout) {
 #if SEXP_USE_GREEN_THREADS
   sexp f;
 #endif
   ssize_t res;
   res = sendto(sock, buffer, len, flags, addr, addr_len);
 #if SEXP_USE_GREEN_THREADS
-  if (res < 0 && errno == EWOULDBLOCK) {
+  if (res < 0 && errno == EWOULDBLOCK && !sexp_zerop(timeout)) {
     f = sexp_global(ctx, SEXP_G_THREADS_BLOCKER);
-    if (sexp_opcodep(f)) {
-      ((sexp_proc2)sexp_opcode_func(f))(ctx, f, 1, sexp_make_fixnum(sock));
+    if (sexp_applicablep(f)) {
+      sexp_apply2(ctx, f, sexp_make_fixnum(sock), timeout);
       return sexp_global(ctx, SEXP_G_IO_BLOCK_ERROR);
     }
   }
@@ -44,17 +46,17 @@ sexp sexp_sendto (sexp ctx, sexp self, int sock, const void* buffer, size_t len,
   return sexp_make_fixnum(res);
 }
 
-sexp sexp_recvfrom (sexp ctx, sexp self, int sock, void* buffer, size_t len, int flags, struct sockaddr* addr, socklen_t addr_len) {
+sexp sexp_recvfrom (sexp ctx, sexp self, int sock, void* buffer, size_t len, int flags, struct sockaddr* addr, socklen_t addr_len, sexp timeout) {
 #if SEXP_USE_GREEN_THREADS
   sexp f;
 #endif
   ssize_t res;
   res = recvfrom(sock, buffer, len, flags, addr, &addr_len);
 #if SEXP_USE_GREEN_THREADS
-  if (res < 0 && errno == EWOULDBLOCK) {
+  if (res < 0 && errno == EWOULDBLOCK && !sexp_zerop(timeout)) {
     f = sexp_global(ctx, SEXP_G_THREADS_BLOCKER);
-    if (sexp_opcodep(f)) {
-      ((sexp_proc2)sexp_opcode_func(f))(ctx, f, 1, sexp_make_fixnum(sock));
+    if (sexp_applicablep(f)) {
+      sexp_apply2(ctx, f, sexp_make_fixnum(sock), timeout);
       return sexp_global(ctx, SEXP_G_IO_BLOCK_ERROR);
     }
   }
diff --git a/lib/chibi/io/port.c b/lib/chibi/io/port.c
index f6ab8640..5d53bab2 100644
--- a/lib/chibi/io/port.c
+++ b/lib/chibi/io/port.c
@@ -358,7 +358,7 @@ sexp sexp_write_u8 (sexp ctx, sexp self, sexp u8, sexp out) {
 #if SEXP_USE_GREEN_THREADS
     if (errno == EAGAIN) {
       if (sexp_applicablep(sexp_global(ctx, SEXP_G_THREADS_BLOCKER)))
-        sexp_apply1(ctx, sexp_global(ctx, SEXP_G_THREADS_BLOCKER), out);
+        sexp_apply2(ctx, sexp_global(ctx, SEXP_G_THREADS_BLOCKER), out, SEXP_FALSE);
       return sexp_global(ctx, SEXP_G_IO_BLOCK_ERROR);
     }
 #endif
@@ -381,7 +381,7 @@ sexp sexp_read_u8 (sexp ctx, sexp self, sexp in) {
     if (sexp_port_stream(in))
       clearerr(sexp_port_stream(in));
     if (sexp_applicablep(sexp_global(ctx, SEXP_G_THREADS_BLOCKER)))
-      sexp_apply1(ctx, sexp_global(ctx, SEXP_G_THREADS_BLOCKER), in);
+      sexp_apply2(ctx, sexp_global(ctx, SEXP_G_THREADS_BLOCKER), in, SEXP_FALSE);
     return sexp_global(ctx, SEXP_G_IO_BLOCK_ERROR);
   }
 #endif
diff --git a/lib/chibi/net.scm b/lib/chibi/net.scm
index e3ef3436..7f1d3947 100644
--- a/lib/chibi/net.scm
+++ b/lib/chibi/net.scm
@@ -113,11 +113,17 @@
 ;;> have had a default address specified with \scheme{connect}.
 
 (define (send socket bv . o)
+  (apply send/non-blocking socket bv #f o))
+
+;;> Equivalent to \scheme{send} but gives up and returns false if the
+;;> packet can't be sent within \var{timeout} seconds.
+
+(define (send/non-blocking socket bv timeout . o)
   (let* ((flags (if (pair? o) (car o) 0))
          (addrinfo (and (pair? o) (pair? (cdr o)) (cadr o)))
          (sockaddr (and addrinfo (address-info-address addrinfo)))
          (sockaddr-len (if addrinfo (address-info-address-length addrinfo) 0)))
-    (%send socket bv flags sockaddr sockaddr-len)))
+    (%send socket bv flags sockaddr sockaddr-len timeout)))
 
 ;;> Recieves data from \var{socket} to fill the bytevector \var{bv} by
 ;;> calling recvfrom.  Returns the number of bytes read, or a negative
@@ -126,11 +132,17 @@
 ;;> \scheme{connect}.
 
 (define (receive! socket bv . o)
+  (apply receive!/non-blocking socket bv #f o))
+
+;;> Equivalent to \scheme{receive!} but gives up and returns false if
+;;> no packets are received within \var{timeout} seconds.
+
+(define (receive!/non-blocking socket bv timeout . o)
   (let* ((flags (if (pair? o) (car o) 0))
          (addrinfo (and (pair? o) (pair? (cdr o)) (cadr o)))
          (sockaddr (and addrinfo (address-info-address addrinfo)))
          (sockaddr-len (if addrinfo (address-info-address-length addrinfo) 0)))
-    (%receive! socket bv flags sockaddr sockaddr-len)))
+    (%receive! socket bv flags sockaddr sockaddr-len timeout)))
 
 ;;> Shortcut for \scheme{receive}, returning a newly created
 ;;> bytevector of length \var{n} on success and \scheme{#f} on
@@ -141,3 +153,12 @@
          (m (apply receive! socket bv o)))
     (and (>= m 0)
          (subbytes bv 0 m))))
+
+;;> Equivalent to \scheme{receive} but gives up and returns false if
+;;> no packets are received within \var{timeout} seconds.
+
+(define (receive/non-blocking socket n timeout . o)
+  (let* ((bv (make-bytevector n))
+         (m (apply receive!/non-blocking socket bv timeout o)))
+    (and (>= m 0)
+         (subbytes bv 0 m))))
diff --git a/lib/chibi/net.sld b/lib/chibi/net.sld
index 1522a5b2..09542fe7 100644
--- a/lib/chibi/net.sld
+++ b/lib/chibi/net.sld
@@ -5,6 +5,7 @@
           sockaddr-name sockaddr-port make-sockaddr
           with-net-io open-net-io make-listener-socket
           send receive! receive
+          send/non-blocking receive!/non-blocking receive/non-blocking
           address-info-family address-info-socket-type address-info-protocol
           address-info-address address-info-address-length address-info-next
           address-family/unix address-family/inet address-family/inet6
diff --git a/lib/chibi/net.stub b/lib/chibi/net.stub
index e319ba16..43578758 100644
--- a/lib/chibi/net.stub
+++ b/lib/chibi/net.stub
@@ -55,12 +55,12 @@
 (define-c sexp (%send "sexp_sendto")
   ((value ctx sexp) (value self sexp)
    fileno bytevector (value (bytevector-length arg3) size_t) int
-   (maybe-null sockaddr) socklen_t))
+   (maybe-null sockaddr) socklen_t sexp))
 
 (define-c sexp (%receive! "sexp_recvfrom")
   ((value ctx sexp) (value self sexp)
    fileno bytevector (value (bytevector-length arg3) size_t) int
-   (maybe-null sockaddr) socklen_t))
+   (maybe-null sockaddr) socklen_t sexp))
 
 ;;> Returns a list of 2 new sockets, the input and output end of a new
 ;;> pipe, respectively.
diff --git a/lib/srfi/18/threads.c b/lib/srfi/18/threads.c
index e5e11503..b5f5beb8 100644
--- a/lib/srfi/18/threads.c
+++ b/lib/srfi/18/threads.c
@@ -390,7 +390,7 @@ static sexp sexp_insert_pollfd (sexp ctx, int fd, int events) {
 }
 
 /* block the current thread on the specified port */
-static sexp sexp_blocker (sexp ctx, sexp self, sexp_sint_t n, sexp portorfd) {
+static sexp sexp_blocker (sexp ctx, sexp self, sexp_sint_t n, sexp portorfd, sexp timeout) {
   int fd;
   /* register the fd */
   if (sexp_portp(portorfd))
@@ -406,7 +406,7 @@ static sexp sexp_blocker (sexp ctx, sexp self, sexp_sint_t n, sexp portorfd) {
   /* pause the current thread */
   sexp_context_waitp(ctx) = 1;
   sexp_context_event(ctx) = portorfd;
-  sexp_insert_timed(ctx, ctx, SEXP_FALSE);
+  sexp_insert_timed(ctx, ctx, timeout);
   return SEXP_VOID;
 }
 
@@ -676,7 +676,7 @@ sexp sexp_init_library (sexp ctx, sexp self, sexp_sint_t n, sexp env, const char
   sexp_global(ctx, SEXP_G_THREADS_SCHEDULER)
     = sexp_make_foreign(ctx, "scheduler", 1, 0, (sexp_proc1)sexp_scheduler, SEXP_FALSE);
   sexp_global(ctx, SEXP_G_THREADS_BLOCKER)
-    = sexp_make_foreign(ctx, "blocker", 1, 0, (sexp_proc1)sexp_blocker, SEXP_FALSE);
+    = sexp_make_foreign(ctx, "blocker", 2, 0, (sexp_proc1)sexp_blocker, SEXP_FALSE);
 
   /* remember the env to lookup the runner later */
   sexp_global(ctx, SEXP_G_THREADS_SIGNAL_RUNNER) = env;
diff --git a/sexp.c b/sexp.c
index c850611d..6f2bb6b9 100644
--- a/sexp.c
+++ b/sexp.c
@@ -1840,8 +1840,8 @@ int sexp_maybe_block_port (sexp ctx, sexp in, int forcep) {
         if (sexp_port_stream(in))
           clearerr(sexp_port_stream(in));
         f = sexp_global(ctx, SEXP_G_THREADS_BLOCKER);
-        if (sexp_opcodep(f)) {
-          ((sexp_proc2)sexp_opcode_func(f))(ctx, f, 1, in);
+        if (sexp_applicablep(f)) {
+          sexp_apply2(ctx, f, in, SEXP_FALSE);
           return 1;
         }
       }
diff --git a/vm.c b/vm.c
index 963b73fc..48e2172a 100644
--- a/vm.c
+++ b/vm.c
@@ -1906,7 +1906,7 @@ sexp sexp_apply (sexp ctx, sexp proc, sexp args) {
           && (errno == EAGAIN)) {
         if (sexp_port_stream(_ARG2)) clearerr(sexp_port_stream(_ARG2));
         if (sexp_applicablep(sexp_global(ctx, SEXP_G_THREADS_BLOCKER)))
-          sexp_apply1(ctx, sexp_global(ctx, SEXP_G_THREADS_BLOCKER), _ARG2);
+          sexp_apply2(ctx, sexp_global(ctx, SEXP_G_THREADS_BLOCKER), _ARG2, SEXP_FALSE);
         else               /* no scheduler but output full, wait 5ms */
           usleep(5*1000);
         fuel = 0;
@@ -1959,7 +1959,7 @@ sexp sexp_apply (sexp ctx, sexp proc, sexp args) {
       /* yield if threads are enabled (otherwise busy loop) */
       /* TODO: the wait seems necessary on OS X to stop a print loop to ptys */
       if (sexp_applicablep(sexp_global(ctx, SEXP_G_THREADS_BLOCKER)))
-        sexp_apply1(ctx, sexp_global(ctx, SEXP_G_THREADS_BLOCKER), _ARG3);
+        sexp_apply2(ctx, sexp_global(ctx, SEXP_G_THREADS_BLOCKER), _ARG3, SEXP_FALSE);
       else               /* no scheduler but output full, wait 5ms */
         usleep(5*1000);
       fuel = 0;
@@ -1991,7 +1991,7 @@ sexp sexp_apply (sexp ctx, sexp proc, sexp args) {
         if (sexp_port_stream(_ARG1)) clearerr(sexp_port_stream(_ARG1));
         /* TODO: block and unblock */
         if (sexp_applicablep(sexp_global(ctx, SEXP_G_THREADS_BLOCKER)))
-          sexp_apply1(ctx, sexp_global(ctx, SEXP_G_THREADS_BLOCKER), _ARG1);
+          sexp_apply2(ctx, sexp_global(ctx, SEXP_G_THREADS_BLOCKER), _ARG1, SEXP_FALSE);
         fuel = 0;
         ip--;      /* try again */
       } else
@@ -2017,7 +2017,7 @@ sexp sexp_apply (sexp ctx, sexp proc, sexp args) {
           && (errno == EAGAIN)) {
         if (sexp_port_stream(_ARG1)) clearerr(sexp_port_stream(_ARG1));
         if (sexp_applicablep(sexp_global(ctx, SEXP_G_THREADS_BLOCKER)))
-          sexp_apply1(ctx, sexp_global(ctx, SEXP_G_THREADS_BLOCKER), _ARG1);
+          sexp_apply2(ctx, sexp_global(ctx, SEXP_G_THREADS_BLOCKER), _ARG1, SEXP_FALSE);
         fuel = 0;
         ip--;      /* try again */
       } else
@@ -2120,4 +2120,17 @@ sexp sexp_apply1 (sexp ctx, sexp f, sexp x) {
   return res;
 }
 
+sexp sexp_apply2 (sexp ctx, sexp f, sexp x, sexp y) {
+  sexp res;
+  sexp_gc_var1(args);
+  if (sexp_opcodep(f) && sexp_opcode_func(f)) {
+    res = ((sexp_proc3)sexp_opcode_func(f))(ctx, f, 2, x, y);
+  } else {
+    sexp_gc_preserve1(ctx, args);
+    res = sexp_apply(ctx, f, args=sexp_list2(ctx, x, y));
+    sexp_gc_release1(ctx);
+  }
+  return res;
+}
+
 #endif