diff --git a/CHANGELOG b/CHANGELOG
index 2be486e1..66ccf057 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -17,6 +17,11 @@ HISTORY
 
  ++ Bugfixes:
 
+  2011-07-22: Simon Goldschmidt
+  * api_lib.c, api_msg.c, sockets.c, api.h: fixed bug #31084 (socket API returns
+    always EMSGSIZE on non-blocking sockets if data size > send buffers) -> now
+    lwip_send() sends as much as possible for non-blocking sockets
+
   2011-07-22: Simon Goldschmidt
   * pbuf.c/.h, timers.c: freeing ooseq pbufs when the pbuf pool is empty implemented
     for NO_SYS==1: when not using sys_check_timeouts(), call PBUF_CHECK_FREE_OOSEQ()
diff --git a/src/api/api_lib.c b/src/api/api_lib.c
index b1a9e525..3a7ad58c 100644
--- a/src/api/api_lib.c
+++ b/src/api/api_lib.c
@@ -582,22 +582,30 @@ netconn_send(struct netconn *conn, struct netbuf *buf)
  * - NETCONN_COPY: data will be copied into memory belonging to the stack
  * - NETCONN_MORE: for TCP connection, PSH flag will be set on last segment sent
  * - NETCONN_DONTBLOCK: only write the data if all dat can be written at once
+ * @param bytes_written pointer to a location that receives the number of written bytes
  * @return ERR_OK if data was sent, any other err_t on error
  */
 err_t
-netconn_write(struct netconn *conn, const void *dataptr, size_t size, u8_t apiflags)
+netconn_write_partly(struct netconn *conn, const void *dataptr, size_t size,
+                     u8_t apiflags, size_t *bytes_written)
 {
   struct api_msg msg;
   err_t err;
+  u8_t dontblock;
 
   LWIP_ERROR("netconn_write: invalid conn",  (conn != NULL), return ERR_ARG;);
   LWIP_ERROR("netconn_write: invalid conn->type",  (conn->type == NETCONN_TCP), return ERR_VAL;);
   if (size == 0) {
     return ERR_OK;
   }
+  dontblock = netconn_is_nonblocking(conn) || (apiflags & NETCONN_DONTBLOCK);
+  if (dontblock && !bytes_written) {
+    /* This implies netconn_write() cannot be used for non-blocking send, since
+       it has no way to return the number of bytes written. */
+    return ERR_VAL;
+  }
 
-  /* @todo: for non-blocking write, check if 'size' would ever fit into
-            snd_queue or snd_buf */
+  /* non-blocking write sends as much  */
   msg.function = do_write;
   msg.msg.conn = conn;
   msg.msg.msg.w.dataptr = dataptr;
@@ -607,6 +615,15 @@ netconn_write(struct netconn *conn, const void *dataptr, size_t size, u8_t apifl
      but if it is, this is done inside api_msg.c:do_write(), so we can use the
      non-blocking version here. */
   err = TCPIP_APIMSG(&msg);
+  if ((err == ERR_OK) && (bytes_written != NULL)) {
+    if (dontblock) {
+      /* nonblocking write: maybe the data has been sent partly */
+      *bytes_written = msg.msg.msg.w.len;
+    } else {
+      /* blocking call succeeded: all data has been sent if it */
+      *bytes_written = size;
+    }
+  }
 
   NETCONN_SET_SAFE_ERR(conn, err);
   return err;
diff --git a/src/api/api_msg.c b/src/api/api_msg.c
index 448f96dd..f9f709e7 100644
--- a/src/api/api_msg.c
+++ b/src/api/api_msg.c
@@ -1193,7 +1193,7 @@ do_recv(struct api_msg_msg *msg)
 static err_t
 do_writemore(struct netconn *conn)
 {
-  err_t err = ERR_OK;
+  err_t err;
   void *dataptr;
   u16_t len, available;
   u8_t write_finished = 0;
@@ -1224,62 +1224,62 @@ do_writemore(struct netconn *conn)
   if (available < len) {
     /* don't try to write more than sendbuf */
     len = available;
+    if (dontblock){ 
+      if (!len) {
+        err = ERR_WOULDBLOCK;
+        goto err_mem;
+      }
+    } else {
 #if LWIP_TCPIP_CORE_LOCKING
-    conn->flags |= NETCONN_FLAG_WRITE_DELAYED;
+      conn->flags |= NETCONN_FLAG_WRITE_DELAYED;
 #endif
-    apiflags |= TCP_WRITE_FLAG_MORE;
+      apiflags |= TCP_WRITE_FLAG_MORE;
+    }
   }
-  if (dontblock && (len < conn->current_msg->msg.w.len)) {
-    /* failed to send all data at once -> nonblocking write not possible */
-    err = ERR_MEM;
-  }
-  if (err == ERR_OK) {
-    LWIP_ASSERT("do_writemore: invalid length!", ((conn->write_offset + len) <= conn->current_msg->msg.w.len));
-    err = tcp_write(conn->pcb.tcp, dataptr, len, apiflags);
-  }
-  if (dontblock && (err == ERR_MEM)) {
-    /* nonblocking write failed */
-    write_finished = 1;
-    err = ERR_WOULDBLOCK;
-    /* let poll_tcp check writable space to mark the pcb
-       writable again */
-    conn->flags |= NETCONN_FLAG_CHECK_WRITESPACE;
-    /* let select mark this pcb as non-writable. */
-    API_EVENT(conn, NETCONN_EVT_SENDMINUS, len);
-  } else {
-    /* if OK or memory error, check available space */
-    if (((err == ERR_OK) || (err == ERR_MEM)) &&
-        ((tcp_sndbuf(conn->pcb.tcp) <= TCP_SNDLOWAT) ||
-         (tcp_sndqueuelen(conn->pcb.tcp) >= TCP_SNDQUEUELOWAT))) {
+  LWIP_ASSERT("do_writemore: invalid length!", ((conn->write_offset + len) <= conn->current_msg->msg.w.len));
+  err = tcp_write(conn->pcb.tcp, dataptr, len, apiflags);
+  /* if OK or memory error, check available space */
+  if ((err == ERR_OK) || (err == ERR_MEM)) {
+err_mem:
+    if (dontblock && (len < conn->current_msg->msg.w.len)) {
+      /* non-blocking write did not write everything: mark the pcb non-writable
+         and let poll_tcp check writable space to mark the pcb writable again */
+      API_EVENT(conn, NETCONN_EVT_SENDMINUS, len);
+      conn->flags |= NETCONN_FLAG_CHECK_WRITESPACE;
+    } else if ((tcp_sndbuf(conn->pcb.tcp) <= TCP_SNDLOWAT) ||
+               (tcp_sndqueuelen(conn->pcb.tcp) >= TCP_SNDQUEUELOWAT)) {
       /* The queued byte- or pbuf-count exceeds the configured low-water limit,
          let select mark this pcb as non-writable. */
       API_EVENT(conn, NETCONN_EVT_SENDMINUS, len);
     }
+  }
 
-    if (err == ERR_OK) {
-      conn->write_offset += len;
-      if (conn->write_offset == conn->current_msg->msg.w.len) {
-        /* everything was written */
-        write_finished = 1;
-        conn->write_offset = 0;
-      }
-      tcp_output(conn->pcb.tcp);
-    } else if (err == ERR_MEM) {
-      /* If ERR_MEM, we wait for sent_tcp or poll_tcp to be called
-         we do NOT return to the application thread, since ERR_MEM is
-         only a temporary error! */
-
-      /* tcp_write returned ERR_MEM, try tcp_output anyway */
-      tcp_output(conn->pcb.tcp);
-
-  #if LWIP_TCPIP_CORE_LOCKING
-      conn->flags |= NETCONN_FLAG_WRITE_DELAYED;
-  #endif
-    } else {
-      /* On errors != ERR_MEM, we don't try writing any more but return
-         the error to the application thread. */
+  if (err == ERR_OK) {
+    conn->write_offset += len;
+    if ((conn->write_offset == conn->current_msg->msg.w.len) || dontblock) {
+      /* return sent length */
+      conn->current_msg->msg.w.len = conn->write_offset;
+      /* everything was written */
       write_finished = 1;
+      conn->write_offset = 0;
     }
+    tcp_output(conn->pcb.tcp);
+  } else if ((err == ERR_MEM) && !dontblock) {
+    /* If ERR_MEM, we wait for sent_tcp or poll_tcp to be called
+       we do NOT return to the application thread, since ERR_MEM is
+       only a temporary error! */
+
+    /* tcp_write returned ERR_MEM, try tcp_output anyway */
+    tcp_output(conn->pcb.tcp);
+
+#if LWIP_TCPIP_CORE_LOCKING
+    conn->flags |= NETCONN_FLAG_WRITE_DELAYED;
+#endif
+  } else {
+    /* On errors != ERR_MEM, we don't try writing any more but return
+       the error to the application thread. */
+    write_finished = 1;
+    conn->current_msg->msg.w.len = 0;
   }
 
   if (write_finished) {
diff --git a/src/api/sockets.c b/src/api/sockets.c
index e36012ce..a9a85c37 100644
--- a/src/api/sockets.c
+++ b/src/api/sockets.c
@@ -748,6 +748,7 @@ lwip_send(int s, const void *data, size_t size, int flags)
   struct lwip_sock *sock;
   err_t err;
   u8_t write_flags;
+  size_t written;
 
   LWIP_DEBUGF(SOCKETS_DEBUG, ("lwip_send(%d, data=%p, size=%"SZT_F", flags=0x%x)\n",
                               s, data, size, flags));
@@ -766,22 +767,15 @@ lwip_send(int s, const void *data, size_t size, int flags)
 #endif /* (LWIP_UDP || LWIP_RAW) */
   }
 
-  if ((flags & MSG_DONTWAIT) || netconn_is_nonblocking(sock->conn)) {
-    if ((size > TCP_SND_BUF) || ((size / TCP_MSS) > TCP_SND_QUEUELEN)) {
-      /* too much data to ever send nonblocking! */
-      sock_set_errno(sock, EMSGSIZE);
-      return -1;
-    }
-  }
-
   write_flags = NETCONN_COPY |
     ((flags & MSG_MORE)     ? NETCONN_MORE      : 0) |
     ((flags & MSG_DONTWAIT) ? NETCONN_DONTBLOCK : 0);
-  err = netconn_write(sock->conn, data, size, write_flags);
+  written = 0;
+  err = netconn_write_partly(sock->conn, data, size, write_flags, &written);
 
-  LWIP_DEBUGF(SOCKETS_DEBUG, ("lwip_send(%d) err=%d size=%"SZT_F"\n", s, err, size));
+  LWIP_DEBUGF(SOCKETS_DEBUG, ("lwip_send(%d) err=%d written=%"SZT_F"\n", s, err, written));
   sock_set_errno(sock, err_to_errno(err));
-  return (err == ERR_OK ? (int)size : -1);
+  return (err == ERR_OK ? (int)written : -1);
 }
 
 int
diff --git a/src/include/lwip/api.h b/src/include/lwip/api.h
index 91b9e5d2..6071f6d7 100644
--- a/src/include/lwip/api.h
+++ b/src/include/lwip/api.h
@@ -230,8 +230,10 @@ void    netconn_recved(struct netconn *conn, u32_t length);
 err_t   netconn_sendto(struct netconn *conn, struct netbuf *buf,
                        ip_addr_t *addr, u16_t port);
 err_t   netconn_send(struct netconn *conn, struct netbuf *buf);
-err_t   netconn_write(struct netconn *conn, const void *dataptr, size_t size,
-                      u8_t apiflags);
+err_t   netconn_write_partly(struct netconn *conn, const void *dataptr, size_t size,
+                             u8_t apiflags, size_t *bytes_written);
+#define netconn_write(conn, dataptr, size, apiflags) \
+          netconn_write_partly(conn, dataptr, size, apiflags, NULL)
 err_t   netconn_close(struct netconn *conn);
 err_t   netconn_shutdown(struct netconn *conn, u8_t shut_rx, u8_t shut_tx);