diff --git a/ChangeLog b/ChangeLog
index e318bcb..c43be03 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -35,6 +35,10 @@
    * It is now possible to #include a user-provided configuration file at the
      end of the default config.h by defining MBEDTLS_USER_CONFIG_FILE on the
      compiler's command line.
+   * Prepend a "thread identifier" to debug messages (issue pointed out by
+     Hugo Leisink) (#210).
+   * Add mbedtls_ssl_get_max_frag_len() to query the current maximum fragment
+     length.
 
 = mbed TLS 2.0.0 released 2015-07-13
 
diff --git a/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index 288627b..d051035 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -2027,6 +2027,26 @@
  */
 int mbedtls_ssl_get_record_expansion( const mbedtls_ssl_context *ssl );
 
+#if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
+/**
+ * \brief          Return the maximum fragment length (payload, in bytes).
+ *                 This is the value negotiated with peer if any,
+ *                 or the locally configured value.
+ *
+ * \note           With DTLS, \c mbedtls_ssl_write() will return an error if
+ *                 called with a larger length value.
+ *                 With TLS, \c mbedtls_ssl_write() will fragment the input if
+ *                 necessary and return the number of bytes written; it is up
+ *                 to the caller to call \c mbedtls_ssl_write() again in
+ *                 order to send the remaining bytes if any.
+ *
+ * \param ssl      SSL context
+ *
+ * \return         Current maximum fragment length.
+ */
+size_t mbedtls_ssl_get_max_frag_len( const mbedtls_ssl_context *ssl );
+#endif /* MBEDTLS_SSL_MAX_FRAGMENT_LENGTH */
+
 #if defined(MBEDTLS_X509_CRT_PARSE_C)
 /**
  * \brief          Return the peer certificate from the current connection
@@ -2124,26 +2144,33 @@
 int mbedtls_ssl_read( mbedtls_ssl_context *ssl, unsigned char *buf, size_t len );
 
 /**
- * \brief          Write exactly 'len' application data bytes
+ * \brief          Try to write exactly 'len' application data bytes
+ *
+ * \warning        This function will do partial writes in some cases. If the
+ *                 return value is non-negative but less than length, the
+ *                 function must be called again with updated arguments:
+ *                 buf + ret, len - ret (if ret is the return value) until
+ *                 it returns a value equal to the last 'len' argument.
  *
  * \param ssl      SSL context
  * \param buf      buffer holding the data
  * \param len      how many bytes must be written
  *
- * \return         the number of bytes written,
- *                 or a negative error code.
+ * \return         the number of bytes actually written (may be less than len),
+ *                 or MBEDTLS_ERR_SSL_WANT_WRITE of MBEDTLS_ERR_SSL_WANT_READ,
+ *                 or another negative error code.
  *
- * \note           When this function returns MBEDTLS_ERR_SSL_WANT_WRITE,
+ * \note           When this function returns MBEDTLS_ERR_SSL_WANT_WRITE/READ,
  *                 it must be called later with the *same* arguments,
  *                 until it returns a positive value.
  *
  * \note           If the requested length is greater than the maximum
  *                 fragment length (either the built-in limit or the one set
  *                 or negotiated with the peer), then:
- *                 - with TLS, less bytes than requested are written. (In
- *                 order to write larger messages, this function should be
- *                 called in a loop.)
+ *                 - with TLS, less bytes than requested are written.
  *                 - with DTLS, MBEDTLS_ERR_SSL_BAD_INPUT_DATA is returned.
+ *                 \c mbedtls_ssl_get_max_frag_len() may be used to query the
+ *                 active maximum fragment length.
  */
 int mbedtls_ssl_write( mbedtls_ssl_context *ssl, const unsigned char *buf, size_t len );
 
diff --git a/library/debug.c b/library/debug.c
index 2220e33..f9b8229 100644
--- a/library/debug.c
+++ b/library/debug.c
@@ -43,6 +43,10 @@
 #define mbedtls_snprintf    snprintf
 #endif
 
+#if ( defined(__ARMCC_VERSION) || defined(_MSC_VER) ) && !defined(inline)
+#define inline __inline
+#endif
+
 #define DEBUG_BUF_SIZE      512
 
 static int debug_threshold = 0;
@@ -52,6 +56,27 @@
     debug_threshold = threshold;
 }
 
+/*
+ * All calls to f_dbg must be made via this function
+ */
+static inline void debug_send_line( const mbedtls_ssl_context *ssl, int level,
+                                    const char *file, int line,
+                                    const char *str )
+{
+    /*
+     * If in a threaded environment, we need a thread identifier.
+     * Since there is no portable way to get one, use the address of the ssl
+     * context instead, as it shouldn't be shared between threads.
+     */
+#if defined(MBEDTLS_THREADING_C)
+    char idstr[20 + DEBUG_BUF_SIZE]; /* 0x + 16 nibbles + ': ' */
+    mbedtls_snprintf( idstr, sizeof( idstr ), "%p: %s", ssl, str );
+    ssl->conf->f_dbg( ssl->conf->p_dbg, level, file, line, idstr );
+#else
+    ssl->conf->f_dbg( ssl->conf->p_dbg, level, file, line, str );
+#endif
+}
+
 void mbedtls_debug_print_msg( const mbedtls_ssl_context *ssl, int level,
                               const char *file, int line,
                               const char *format, ... )
@@ -86,7 +111,7 @@
         str[ret + 1] = '\0';
     }
 
-    ssl->conf->f_dbg( ssl->conf->p_dbg, level, file, line, str );
+    debug_send_line( ssl, level, file, line, str );
 }
 
 void mbedtls_debug_print_ret( const mbedtls_ssl_context *ssl, int level,
@@ -109,7 +134,7 @@
     mbedtls_snprintf( str, sizeof( str ), "%s() returned %d (-0x%04x)\n",
               text, ret, -ret );
 
-    ssl->conf->f_dbg( ssl->conf->p_dbg, level, file, line, str );
+    debug_send_line( ssl, level, file, line, str );
 }
 
 void mbedtls_debug_print_buf( const mbedtls_ssl_context *ssl, int level,
@@ -126,7 +151,7 @@
     mbedtls_snprintf( str + idx, sizeof( str ) - idx, "dumping '%s' (%u bytes)\n",
               text, (unsigned int) len );
 
-    ssl->conf->f_dbg( ssl->conf->p_dbg, level, file, line, str );
+    debug_send_line( ssl, level, file, line, str );
 
     idx = 0;
     memset( txt, 0, sizeof( txt ) );
@@ -140,7 +165,7 @@
             if( i > 0 )
             {
                 mbedtls_snprintf( str + idx, sizeof( str ) - idx, "  %s\n", txt );
-                ssl->conf->f_dbg( ssl->conf->p_dbg, level, file, line, str );
+                debug_send_line( ssl, level, file, line, str );
 
                 idx = 0;
                 memset( txt, 0, sizeof( txt ) );
@@ -162,7 +187,7 @@
             idx += mbedtls_snprintf( str + idx, sizeof( str ) - idx, "   " );
 
         mbedtls_snprintf( str + idx, sizeof( str ) - idx, "  %s\n", txt );
-        ssl->conf->f_dbg( ssl->conf->p_dbg, level, file, line, str );
+        debug_send_line( ssl, level, file, line, str );
     }
 }
 
@@ -207,7 +232,7 @@
     mbedtls_snprintf( str + idx, sizeof( str ) - idx, "value of '%s' (%d bits) is:\n",
               text, (int) ( ( n * ( sizeof(mbedtls_mpi_uint) << 3 ) ) + j + 1 ) );
 
-    ssl->conf->f_dbg( ssl->conf->p_dbg, level, file, line, str );
+    debug_send_line( ssl, level, file, line, str );
 
     idx = 0;
     for( i = n + 1, j = 0; i > 0; i-- )
@@ -227,7 +252,7 @@
                 if( j > 0 )
                 {
                     mbedtls_snprintf( str + idx, sizeof( str ) - idx, "\n" );
-                    ssl->conf->f_dbg( ssl->conf->p_dbg, level, file, line, str );
+                    debug_send_line( ssl, level, file, line, str );
                     idx = 0;
                 }
             }
@@ -244,7 +269,7 @@
         idx += mbedtls_snprintf( str + idx, sizeof( str ) - idx, " 00" );
 
     mbedtls_snprintf( str + idx, sizeof( str ) - idx, "\n" );
-    ssl->conf->f_dbg( ssl->conf->p_dbg, level, file, line, str );
+    debug_send_line( ssl, level, file, line, str );
 }
 #endif /* MBEDTLS_BIGNUM_C */
 
@@ -261,7 +286,7 @@
 
     if( mbedtls_pk_debug( pk, items ) != 0 )
     {
-        ssl->conf->f_dbg( ssl->conf->p_dbg, level, file, line,
+        debug_send_line( ssl, level, file, line,
                           "invalid PK context\n" );
         return;
     }
@@ -282,7 +307,7 @@
             mbedtls_debug_print_ecp( ssl, level, file, line, name, items[i].value );
         else
 #endif
-            ssl->conf->f_dbg( ssl->conf->p_dbg, level, file, line,
+            debug_send_line( ssl, level, file, line,
                               "should not happen\n" );
     }
 }
@@ -305,7 +330,7 @@
             memcpy( str, start, len );
             str[len] = '\0';
 
-            ssl->conf->f_dbg( ssl->conf->p_dbg, level, file, line, str );
+            debug_send_line( ssl, level, file, line, str );
 
             start = cur + 1;
         }
@@ -327,7 +352,7 @@
         char buf[1024];
 
         mbedtls_snprintf( str, sizeof( str ), "%s #%d:\n", text, ++i );
-        ssl->conf->f_dbg( ssl->conf->p_dbg, level, file, line, str );
+        debug_send_line( ssl, level, file, line, str );
 
         mbedtls_x509_crt_info( buf, sizeof( buf ) - 1, "", crt );
         debug_print_line_by_line( ssl, level, file, line, buf );
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 9007562..552a099 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -3718,6 +3718,9 @@
 {
     int ret;
 
+    if( ssl == NULL || ssl->conf == NULL )
+        return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
+
     MBEDTLS_SSL_DEBUG_MSG( 2, ( "=> send alert message" ) );
 
     ssl->out_msgtype = MBEDTLS_SSL_MSG_ALERT;
@@ -5862,6 +5865,29 @@
     return( (int)( mbedtls_ssl_hdr_len( ssl ) + transform_expansion ) );
 }
 
+#if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
+size_t mbedtls_ssl_get_max_frag_len( const mbedtls_ssl_context *ssl )
+{
+    size_t max_len;
+
+    /*
+     * Assume mfl_code is correct since it was checked when set
+     */
+    max_len = mfl_code_to_length[ssl->conf->mfl_code];
+
+    /*
+     * Check if a smaller max length was negotiated
+     */
+    if( ssl->session_out != NULL &&
+        mfl_code_to_length[ssl->session_out->mfl_code] < max_len )
+    {
+        max_len = mfl_code_to_length[ssl->session_out->mfl_code];
+    }
+
+    return max_len;
+}
+#endif /* MBEDTLS_SSL_MAX_FRAGMENT_LENGTH */
+
 #if defined(MBEDTLS_X509_CRT_PARSE_C)
 const mbedtls_x509_crt *mbedtls_ssl_get_peer_cert( const mbedtls_ssl_context *ssl )
 {
@@ -5894,6 +5920,9 @@
 {
     int ret = MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE;
 
+    if( ssl == NULL || ssl->conf == NULL )
+        return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
+
 #if defined(MBEDTLS_SSL_CLI_C)
     if( ssl->conf->endpoint == MBEDTLS_SSL_IS_CLIENT )
         ret = mbedtls_ssl_handshake_client_step( ssl );
@@ -5913,6 +5942,9 @@
 {
     int ret = 0;
 
+    if( ssl == NULL || ssl->conf == NULL )
+        return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
+
     MBEDTLS_SSL_DEBUG_MSG( 2, ( "=> handshake" ) );
 
     while( ssl->state != MBEDTLS_SSL_HANDSHAKE_OVER )
@@ -6008,6 +6040,9 @@
 {
     int ret = MBEDTLS_ERR_SSL_FEATURE_UNAVAILABLE;
 
+    if( ssl == NULL || ssl->conf == NULL )
+        return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
+
 #if defined(MBEDTLS_SSL_SRV_C)
     /* On server, just send the request */
     if( ssl->conf->endpoint == MBEDTLS_SSL_IS_SERVER )
@@ -6085,6 +6120,9 @@
     int ret, record_read = 0;
     size_t n;
 
+    if( ssl == NULL || ssl->conf == NULL )
+        return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
+
     MBEDTLS_SSL_DEBUG_MSG( 2, ( "=> read" ) );
 
 #if defined(MBEDTLS_SSL_PROTO_DTLS)
@@ -6339,23 +6377,7 @@
 {
     int ret;
 #if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
-    unsigned int max_len;
-#endif
-
-#if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
-    /*
-     * Assume mfl_code is correct since it was checked when set
-     */
-    max_len = mfl_code_to_length[ssl->conf->mfl_code];
-
-    /*
-     * Check if a smaller max length was negotiated
-     */
-    if( ssl->session_out != NULL &&
-        mfl_code_to_length[ssl->session_out->mfl_code] < max_len )
-    {
-        max_len = mfl_code_to_length[ssl->session_out->mfl_code];
-    }
+    size_t max_len = mbedtls_ssl_get_max_frag_len( ssl );
 
     if( len > max_len )
     {
@@ -6444,6 +6466,9 @@
 
     MBEDTLS_SSL_DEBUG_MSG( 2, ( "=> write" ) );
 
+    if( ssl == NULL || ssl->conf == NULL )
+        return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
+
 #if defined(MBEDTLS_SSL_RENEGOTIATION)
     if( ( ret = ssl_check_ctr_renegotiate( ssl ) ) != 0 )
     {
@@ -6479,6 +6504,9 @@
 {
     int ret;
 
+    if( ssl == NULL || ssl->conf == NULL )
+        return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
+
     MBEDTLS_SSL_DEBUG_MSG( 2, ( "=> write close notify" ) );
 
     if( ssl->out_left != 0 )
diff --git a/programs/ssl/ssl_client2.c b/programs/ssl/ssl_client2.c
index 2e0ac1e..c60e710 100644
--- a/programs/ssl/ssl_client2.c
+++ b/programs/ssl/ssl_client2.c
@@ -1240,6 +1240,11 @@
     else
         mbedtls_printf( "    [ Record expansion is unknown (compression) ]\n" );
 
+#if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
+    mbedtls_printf( "    [ Maximum fragment length is %u ]\n",
+                    (unsigned int) mbedtls_ssl_get_max_frag_len( &ssl ) );
+#endif
+
 #if defined(MBEDTLS_SSL_ALPN)
     if( opt.alpn_string != NULL )
     {
diff --git a/programs/ssl/ssl_server2.c b/programs/ssl/ssl_server2.c
index 3aa05d5..e4940b8 100644
--- a/programs/ssl/ssl_server2.c
+++ b/programs/ssl/ssl_server2.c
@@ -1946,6 +1946,11 @@
     else
         mbedtls_printf( "    [ Record expansion is unknown (compression) ]\n" );
 
+#if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)
+    mbedtls_printf( "    [ Maximum fragment length is %u ]\n",
+                    (unsigned int) mbedtls_ssl_get_max_frag_len( &ssl ) );
+#endif
+
 #if defined(MBEDTLS_SSL_ALPN)
     if( opt.alpn_string != NULL )
     {
diff --git a/tests/ssl-opt.sh b/tests/ssl-opt.sh
index bd36e22..77db588 100755
--- a/tests/ssl-opt.sh
+++ b/tests/ssl-opt.sh
@@ -1085,6 +1085,8 @@
             "$P_SRV debug_level=3" \
             "$P_CLI debug_level=3" \
             0 \
+            -c "Maximum fragment length is 16384" \
+            -s "Maximum fragment length is 16384" \
             -C "client hello, adding max_fragment_length extension" \
             -S "found max fragment length extension" \
             -S "server hello, max_fragment_length extension" \
@@ -1094,6 +1096,8 @@
             "$P_SRV debug_level=3" \
             "$P_CLI debug_level=3 max_frag_len=4096" \
             0 \
+            -c "Maximum fragment length is 4096" \
+            -s "Maximum fragment length is 4096" \
             -c "client hello, adding max_fragment_length extension" \
             -s "found max fragment length extension" \
             -s "server hello, max_fragment_length extension" \
@@ -1103,6 +1107,8 @@
             "$P_SRV debug_level=3 max_frag_len=4096" \
             "$P_CLI debug_level=3" \
             0 \
+            -c "Maximum fragment length is 16384" \
+            -s "Maximum fragment length is 4096" \
             -C "client hello, adding max_fragment_length extension" \
             -S "found max fragment length extension" \
             -S "server hello, max_fragment_length extension" \
@@ -1113,6 +1119,7 @@
             "$G_SRV" \
             "$P_CLI debug_level=3 max_frag_len=4096" \
             0 \
+            -c "Maximum fragment length is 4096" \
             -c "client hello, adding max_fragment_length extension" \
             -c "found max_fragment_length extension"
 
@@ -1120,6 +1127,8 @@
             "$P_SRV debug_level=3" \
             "$P_CLI debug_level=3 max_frag_len=2048 request_size=2048" \
             0 \
+            -c "Maximum fragment length is 2048" \
+            -s "Maximum fragment length is 2048" \
             -c "client hello, adding max_fragment_length extension" \
             -s "found max fragment length extension" \
             -s "server hello, max_fragment_length extension" \
@@ -1131,6 +1140,8 @@
             "$P_SRV debug_level=3" \
             "$P_CLI debug_level=3 max_frag_len=2048 request_size=2345" \
             0 \
+            -c "Maximum fragment length is 2048" \
+            -s "Maximum fragment length is 2048" \
             -c "client hello, adding max_fragment_length extension" \
             -s "found max fragment length extension" \
             -s "server hello, max_fragment_length extension" \
@@ -1143,6 +1154,8 @@
             "$P_SRV debug_level=3 dtls=1" \
             "$P_CLI debug_level=3 dtls=1 max_frag_len=2048 request_size=2345" \
             1 \
+            -c "Maximum fragment length is 2048" \
+            -s "Maximum fragment length is 2048" \
             -c "client hello, adding max_fragment_length extension" \
             -s "found max fragment length extension" \
             -s "server hello, max_fragment_length extension" \
diff --git a/tests/suites/test_suite_debug.function b/tests/suites/test_suite_debug.function
index 3c77ca5..98f98b0 100644
--- a/tests/suites/test_suite_debug.function
+++ b/tests/suites/test_suite_debug.function
@@ -25,6 +25,11 @@
     *p++ = ':';
     *p++ = ' ';
 
+#if defined(MBEDTLS_THREADING_C)
+    /* Skip "thread ID" (up to the first space) as it is not predictable */
+    while( *str++ != ' ' );
+#endif
+
     memcpy( p, str, strlen( str ) );
     p += strlen( str );