diff --git a/library/ssl_cli.c b/library/ssl_cli.c
index 01ce9d7..55a8e61 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -685,7 +685,7 @@
         return( 0 );
 
     MBEDTLS_SSL_DEBUG_MSG( 3,
-        ( "sending session ticket of length %zu", tlen ) );
+        ( "sending session ticket of length %" MBEDTLS_PRINTF_SIZET, tlen ) );
 
     memcpy( p, ssl->session_negotiate->ticket, tlen );
 
@@ -905,7 +905,8 @@
     *p++ = (unsigned char)( t >>  8 );
     *p++ = (unsigned char)( t       );
 
-    MBEDTLS_SSL_DEBUG_MSG( 3, ( "client hello, current time: %lld", (long long) t ) );
+    MBEDTLS_SSL_DEBUG_MSG( 3, ( "client hello, current time: %" MBEDTLS_PRINTF_LONGLONG,
+                                (long long) t ) );
 #else
     if( ( ret = ssl->conf->f_rng( ssl->conf->p_rng, p, 4 ) ) != 0 )
         return( ret );
@@ -1114,7 +1115,7 @@
     for( i = 0; i < n; i++ )
         *p++ = ssl->session_negotiate->id[i];
 
-    MBEDTLS_SSL_DEBUG_MSG( 3, ( "client hello, session id len.: %zu", n ) );
+    MBEDTLS_SSL_DEBUG_MSG( 3, ( "client hello, session id len.: %" MBEDTLS_PRINTF_SIZET, n ) );
     MBEDTLS_SSL_DEBUG_BUF( 3,   "client hello, session id", buf + 39, n );
 
     /*
@@ -1197,7 +1198,7 @@
     }
 
     MBEDTLS_SSL_DEBUG_MSG( 3,
-        ( "client hello, got %zu ciphersuites (excluding SCSVs)", n ) );
+        ( "client hello, got %" MBEDTLS_PRINTF_SIZET " ciphersuites (excluding SCSVs)", n ) );
 
     /*
      * Add TLS_EMPTY_RENEGOTIATION_INFO_SCSV
@@ -1420,7 +1421,7 @@
     /* olen unused if all extensions are disabled */
     ((void) olen);
 
-    MBEDTLS_SSL_DEBUG_MSG( 3, ( "client hello, total extension length: %zu",
+    MBEDTLS_SSL_DEBUG_MSG( 3, ( "client hello, total extension length: %" MBEDTLS_PRINTF_SIZET,
                                 ext_len ) );
 
     if( ext_len > 0 )
@@ -2261,7 +2262,7 @@
 
     mbedtls_ssl_optimize_checksum( ssl, ssl->handshake->ciphersuite_info );
 
-    MBEDTLS_SSL_DEBUG_MSG( 3, ( "server hello, session id len.: %zu", n ) );
+    MBEDTLS_SSL_DEBUG_MSG( 3, ( "server hello, session id len.: %" MBEDTLS_PRINTF_SIZET, n ) );
     MBEDTLS_SSL_DEBUG_BUF( 3,   "server hello, session id", buf + 35, n );
 
     /*
@@ -2373,7 +2374,7 @@
     ext = buf + 40 + n;
 
     MBEDTLS_SSL_DEBUG_MSG( 2,
-        ( "server hello, total extension length: %zu", ext_len ) );
+        ( "server hello, total extension length: %" MBEDTLS_PRINTF_SIZET, ext_len ) );
 
     while( ext_len )
     {
@@ -2628,7 +2629,7 @@
 
     if( ssl->handshake->dhm_ctx.len * 8 < ssl->conf->dhm_min_bitlen )
     {
-        MBEDTLS_SSL_DEBUG_MSG( 1, ( "DHM prime too short: %zu < %u",
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "DHM prime too short: %" MBEDTLS_PRINTF_SIZET " < %u",
                                     ssl->handshake->dhm_ctx.len * 8,
                                     ssl->conf->dhm_min_bitlen ) );
         return( MBEDTLS_ERR_SSL_BAD_HS_SERVER_KEY_EXCHANGE );
@@ -4347,7 +4348,7 @@
         return( MBEDTLS_ERR_SSL_BAD_HS_NEW_SESSION_TICKET );
     }
 
-    MBEDTLS_SSL_DEBUG_MSG( 3, ( "ticket length: %zu", ticket_len ) );
+    MBEDTLS_SSL_DEBUG_MSG( 3, ( "ticket length: %" MBEDTLS_PRINTF_SIZET, ticket_len ) );
 
     /* We're not waiting for a NewSessionTicket message any more */
     ssl->handshake->new_session_ticket = 0;
diff --git a/library/ssl_msg.c b/library/ssl_msg.c
index 5f971e8..54a7be0 100644
--- a/library/ssl_msg.c
+++ b/library/ssl_msg.c
@@ -623,7 +623,8 @@
 
     if( rec->data_len > MBEDTLS_SSL_OUT_CONTENT_LEN )
     {
-        MBEDTLS_SSL_DEBUG_MSG( 1, ( "Record content %zu too large, maximum %zu",
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "Record content %" MBEDTLS_PRINTF_SIZET
+                                    " too large, maximum %" MBEDTLS_PRINTF_SIZET,
                                     rec->data_len,
                                     (size_t) MBEDTLS_SSL_OUT_CONTENT_LEN ) );
         return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
@@ -764,7 +765,7 @@
     {
         int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
         size_t olen;
-        MBEDTLS_SSL_DEBUG_MSG( 3, ( "before encrypt: msglen = %zu, "
+        MBEDTLS_SSL_DEBUG_MSG( 3, ( "before encrypt: msglen = %" MBEDTLS_PRINTF_SIZET ", "
                                     "including %d bytes of padding",
                                     rec->data_len, 0 ) );
 
@@ -842,7 +843,7 @@
                                dynamic_iv_is_explicit ? dynamic_iv_len : 0 );
         MBEDTLS_SSL_DEBUG_BUF( 4, "additional data used for AEAD",
                                add_data, add_data_len );
-        MBEDTLS_SSL_DEBUG_MSG( 3, ( "before encrypt: msglen = %zu, "
+        MBEDTLS_SSL_DEBUG_MSG( 3, ( "before encrypt: msglen = %" MBEDTLS_PRINTF_SIZET ", "
                                     "including 0 bytes of padding",
                                     rec->data_len ) );
 
@@ -945,8 +946,9 @@
         }
 #endif /* MBEDTLS_SSL_PROTO_TLS1_1 || MBEDTLS_SSL_PROTO_TLS1_2 */
 
-        MBEDTLS_SSL_DEBUG_MSG( 3, ( "before encrypt: msglen = %zu, "
-                            "including %zu bytes of IV and %zu bytes of padding",
+        MBEDTLS_SSL_DEBUG_MSG( 3, ( "before encrypt: msglen = %" MBEDTLS_PRINTF_SIZET ", "
+                            "including %" MBEDTLS_PRINTF_SIZET
+                            " bytes of IV and %" MBEDTLS_PRINTF_SIZET " bytes of padding",
                             rec->data_len, transform->ivlen,
                             padlen + 1 ) );
 
@@ -1366,7 +1368,8 @@
         {
             if( rec->data_len < dynamic_iv_len )
             {
-                MBEDTLS_SSL_DEBUG_MSG( 1, ( "msglen (%zu) < explicit_iv_len (%zu) ",
+                MBEDTLS_SSL_DEBUG_MSG( 1, ( "msglen (%" MBEDTLS_PRINTF_SIZET
+                                            " ) < explicit_iv_len (%" MBEDTLS_PRINTF_SIZET ") ",
                                             rec->data_len,
                                             dynamic_iv_len ) );
                 return( MBEDTLS_ERR_SSL_INVALID_MAC );
@@ -1385,7 +1388,8 @@
         /* Check that there's space for the authentication tag. */
         if( rec->data_len < transform->taglen )
         {
-            MBEDTLS_SSL_DEBUG_MSG( 1, ( "msglen (%zu) < taglen (%zu) ",
+            MBEDTLS_SSL_DEBUG_MSG( 1, ( "msglen (%" MBEDTLS_PRINTF_SIZET
+                                        ") < taglen (%" MBEDTLS_PRINTF_SIZET ") ",
                                         rec->data_len,
                                         transform->taglen ) );
             return( MBEDTLS_ERR_SSL_INVALID_MAC );
@@ -1488,7 +1492,9 @@
         if( rec->data_len < minlen + transform->ivlen ||
             rec->data_len < minlen + transform->maclen + 1 )
         {
-            MBEDTLS_SSL_DEBUG_MSG( 1, ( "msglen (%zu) < max( ivlen(%zu), maclen (%zu) "
+            MBEDTLS_SSL_DEBUG_MSG( 1, ( "msglen (%" MBEDTLS_PRINTF_SIZET
+                                        ") < max( ivlen(%" MBEDTLS_PRINTF_SIZET
+                                        "), maclen (%" MBEDTLS_PRINTF_SIZET ") "
                                 "+ 1 ) ( + expl IV )", rec->data_len,
                                 transform->ivlen,
                                 transform->maclen ) );
@@ -1554,7 +1560,8 @@
          * data_len >= minlen + ivlen ( = minlen or 2 * minlen ). */
         if( rec->data_len % transform->ivlen != 0 )
         {
-            MBEDTLS_SSL_DEBUG_MSG( 1, ( "msglen (%zu) %% ivlen (%zu) != 0",
+            MBEDTLS_SSL_DEBUG_MSG( 1, ( "msglen (%" MBEDTLS_PRINTF_SIZET
+                                        ") %% ivlen (%" MBEDTLS_PRINTF_SIZET ") != 0",
                                         rec->data_len, transform->ivlen ) );
             return( MBEDTLS_ERR_SSL_INVALID_MAC );
         }
@@ -1624,7 +1631,9 @@
 #if defined(MBEDTLS_SSL_DEBUG_ALL)
             if( rec->data_len < transform->maclen + padlen + 1 )
             {
-                MBEDTLS_SSL_DEBUG_MSG( 1, ( "msglen (%zu) < maclen (%zu) + padlen (%zu)",
+                MBEDTLS_SSL_DEBUG_MSG( 1, ( "msglen (%" MBEDTLS_PRINTF_SIZET
+                                            ") < maclen (%" MBEDTLS_PRINTF_SIZET
+                                            ") + padlen (%" MBEDTLS_PRINTF_SIZET ")",
                                             rec->data_len,
                                             transform->maclen,
                                             padlen + 1 ) );
@@ -1653,8 +1662,8 @@
             if( padlen > transform->ivlen )
             {
 #if defined(MBEDTLS_SSL_DEBUG_ALL)
-                MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad padding length: is %zu, "
-                                            "should be no more than %zu",
+                MBEDTLS_SSL_DEBUG_MSG( 1, ( "bad padding length: is %" MBEDTLS_PRINTF_SIZET ", "
+                                            "should be no more than %" MBEDTLS_PRINTF_SIZET,
                                             padlen, transform->ivlen ) );
 #endif
                 correct = 0;
@@ -1890,7 +1899,7 @@
 
     memcpy( msg_pre, ssl->out_msg, len_pre );
 
-    MBEDTLS_SSL_DEBUG_MSG( 3, ( "before compression: msglen = %zu, ",
+    MBEDTLS_SSL_DEBUG_MSG( 3, ( "before compression: msglen = %" MBEDTLS_PRINTF_SIZET ", ",
                    ssl->out_msglen ) );
 
     MBEDTLS_SSL_DEBUG_BUF( 4, "before compression: output payload",
@@ -1911,7 +1920,7 @@
     ssl->out_msglen = out_buf_len -
                       ssl->transform_out->ctx_deflate.avail_out - bytes_written;
 
-    MBEDTLS_SSL_DEBUG_MSG( 3, ( "after compression: msglen = %zu, ",
+    MBEDTLS_SSL_DEBUG_MSG( 3, ( "after compression: msglen = %" MBEDTLS_PRINTF_SIZET ", ",
                    ssl->out_msglen ) );
 
     MBEDTLS_SSL_DEBUG_BUF( 4, "after compression: output payload",
@@ -1942,7 +1951,7 @@
 
     memcpy( msg_pre, ssl->in_msg, len_pre );
 
-    MBEDTLS_SSL_DEBUG_MSG( 3, ( "before decompression: msglen = %zu, ",
+    MBEDTLS_SSL_DEBUG_MSG( 3, ( "before decompression: msglen = %" MBEDTLS_PRINTF_SIZET ", ",
                    ssl->in_msglen ) );
 
     MBEDTLS_SSL_DEBUG_BUF( 4, "before decompression: input payload",
@@ -1963,7 +1972,7 @@
     ssl->in_msglen = in_buf_len -
                      ssl->transform_in->ctx_inflate.avail_out - header_bytes;
 
-    MBEDTLS_SSL_DEBUG_MSG( 3, ( "after decompression: msglen = %zu, ",
+    MBEDTLS_SSL_DEBUG_MSG( 3, ( "after decompression: msglen = %" MBEDTLS_PRINTF_SIZET ", ",
                    ssl->in_msglen ) );
 
     MBEDTLS_SSL_DEBUG_BUF( 4, "after decompression: input payload",
@@ -2042,7 +2051,8 @@
 
             if( ssl->in_left != 0 )
             {
-                MBEDTLS_SSL_DEBUG_MSG( 2, ( "next record in same datagram, offset: %zu",
+                MBEDTLS_SSL_DEBUG_MSG( 2, ( "next record in same datagram, offset: %"
+                                            MBEDTLS_PRINTF_SIZET,
                                     ssl->next_record_offset ) );
                 memmove( ssl->in_hdr,
                          ssl->in_hdr + ssl->next_record_offset,
@@ -2052,7 +2062,8 @@
             ssl->next_record_offset = 0;
         }
 
-        MBEDTLS_SSL_DEBUG_MSG( 2, ( "in_left: %zu, nb_want: %zu",
+        MBEDTLS_SSL_DEBUG_MSG( 2, ( "in_left: %" MBEDTLS_PRINTF_SIZET
+                                    ", nb_want: %" MBEDTLS_PRINTF_SIZET,
                        ssl->in_left, nb_want ) );
 
         /*
@@ -2153,7 +2164,8 @@
     else
 #endif
     {
-        MBEDTLS_SSL_DEBUG_MSG( 2, ( "in_left: %zu, nb_want: %zu",
+        MBEDTLS_SSL_DEBUG_MSG( 2, ( "in_left: %" MBEDTLS_PRINTF_SIZET
+                                    ", nb_want: %" MBEDTLS_PRINTF_SIZET,
                        ssl->in_left, nb_want ) );
 
         while( ssl->in_left < nb_want )
@@ -2177,7 +2189,8 @@
                 }
             }
 
-            MBEDTLS_SSL_DEBUG_MSG( 2, ( "in_left: %zu, nb_want: %zu",
+            MBEDTLS_SSL_DEBUG_MSG( 2, ( "in_left: %" MBEDTLS_PRINTF_SIZET
+                                        ", nb_want: %" MBEDTLS_PRINTF_SIZET,
                                         ssl->in_left, nb_want ) );
             MBEDTLS_SSL_DEBUG_RET( 2, "ssl->f_recv(_timeout)", ret );
 
@@ -2190,7 +2203,7 @@
             if ( (size_t)ret > len || ( INT_MAX > SIZE_MAX && ret > (int)SIZE_MAX ) )
             {
                 MBEDTLS_SSL_DEBUG_MSG( 1,
-                    ( "f_recv returned %d bytes but only %zu were requested",
+                    ( "f_recv returned %d bytes but only %" MBEDTLS_PRINTF_SIZET " were requested",
                     ret, len ) );
                 return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
             }
@@ -2230,7 +2243,8 @@
 
     while( ssl->out_left > 0 )
     {
-        MBEDTLS_SSL_DEBUG_MSG( 2, ( "message length: %zu, out_left: %zu",
+        MBEDTLS_SSL_DEBUG_MSG( 2, ( "message length: %" MBEDTLS_PRINTF_SIZET
+                                    ", out_left: %" MBEDTLS_PRINTF_SIZET,
                        mbedtls_ssl_out_hdr_len( ssl ) + ssl->out_msglen, ssl->out_left ) );
 
         buf = ssl->out_hdr - ssl->out_left;
@@ -2244,7 +2258,7 @@
         if( (size_t)ret > ssl->out_left || ( INT_MAX > SIZE_MAX && ret > (int)SIZE_MAX ) )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1,
-                ( "f_send returned %d bytes but only %zu bytes were sent",
+                ( "f_send returned %d bytes but only %" MBEDTLS_PRINTF_SIZET " bytes were sent",
                 ret, ssl->out_left ) );
             return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
         }
@@ -2286,14 +2300,15 @@
     /* Allocate space for current message */
     if( ( msg = mbedtls_calloc( 1, sizeof(  mbedtls_ssl_flight_item ) ) ) == NULL )
     {
-        MBEDTLS_SSL_DEBUG_MSG( 1, ( "alloc %zu bytes failed",
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "alloc %" MBEDTLS_PRINTF_SIZET " bytes failed",
                             sizeof( mbedtls_ssl_flight_item ) ) );
         return( MBEDTLS_ERR_SSL_ALLOC_FAILED );
     }
 
     if( ( msg->p = mbedtls_calloc( 1, ssl->out_msglen ) ) == NULL )
     {
-        MBEDTLS_SSL_DEBUG_MSG( 1, ( "alloc %zu bytes failed", ssl->out_msglen ) );
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "alloc %" MBEDTLS_PRINTF_SIZET " bytes failed",
+                                    ssl->out_msglen ) );
         mbedtls_free( msg );
         return( MBEDTLS_ERR_SSL_ALLOC_FAILED );
     }
@@ -2699,7 +2714,8 @@
     if( ssl->out_msglen > MBEDTLS_SSL_OUT_CONTENT_LEN )
     {
         MBEDTLS_SSL_DEBUG_MSG( 1, ( "Record too large: "
-                                    "size %zu, maximum %zu",
+                                    "size %" MBEDTLS_PRINTF_SIZET
+                                    ", maximum %" MBEDTLS_PRINTF_SIZET,
                                     ssl->out_msglen,
                                     (size_t) MBEDTLS_SSL_OUT_CONTENT_LEN ) );
         return( MBEDTLS_ERR_SSL_INTERNAL_ERROR );
@@ -2728,7 +2744,7 @@
             if( MBEDTLS_SSL_OUT_CONTENT_LEN - ssl->out_msglen < 8 )
             {
                 MBEDTLS_SSL_DEBUG_MSG( 1, ( "DTLS handshake message too large: "
-                              "size %zu, maximum %zu",
+                              "size %" MBEDTLS_PRINTF_SIZET ", maximum %" MBEDTLS_PRINTF_SIZET,
                                hs_len,
                                (size_t) ( MBEDTLS_SSL_OUT_CONTENT_LEN - 12 ) ) );
                 return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
@@ -2923,7 +2939,7 @@
         ssl->out_hdr[0] = (unsigned char) ssl->out_msgtype;
 
         MBEDTLS_SSL_DEBUG_MSG( 3, ( "output record: msgtype = %u, "
-                                    "version = [%u:%u], msglen = %zu",
+                                    "version = [%u:%u], msglen = %" MBEDTLS_PRINTF_SIZET,
                                     ssl->out_hdr[0], ssl->out_hdr[1],
                                     ssl->out_hdr[2], len ) );
 
@@ -3119,7 +3135,7 @@
 {
     if( ssl->in_msglen < mbedtls_ssl_hs_hdr_len( ssl ) )
     {
-        MBEDTLS_SSL_DEBUG_MSG( 1, ( "handshake message too short: %zu",
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "handshake message too short: %" MBEDTLS_PRINTF_SIZET,
                             ssl->in_msglen ) );
         return( MBEDTLS_ERR_SSL_INVALID_RECORD );
     }
@@ -3127,7 +3143,7 @@
     ssl->in_hslen = mbedtls_ssl_hs_hdr_len( ssl ) + ssl_get_hs_total_len( ssl );
 
     MBEDTLS_SSL_DEBUG_MSG( 3, ( "handshake message: msglen ="
-                        " %zu, type = %u, hslen = %zu",
+                        " %" MBEDTLS_PRINTF_SIZET ", type = %u, hslen = %" MBEDTLS_PRINTF_SIZET,
                         ssl->in_msglen, ssl->in_msg[0], ssl->in_hslen ) );
 
 #if defined(MBEDTLS_SSL_PROTO_DTLS)
@@ -3747,7 +3763,7 @@
     MBEDTLS_SSL_DEBUG_BUF( 4, "input record header", buf, rec->data_offset );
 
     MBEDTLS_SSL_DEBUG_MSG( 3, ( "input record: msgtype = %u, "
-                                "version = [%d:%d], msglen = %zu",
+                                "version = [%d:%d], msglen = %" MBEDTLS_PRINTF_SIZET,
                                 rec->type,
                                 major_ver, minor_ver, rec->data_len ) );
 
@@ -4325,21 +4341,31 @@
                     {
                         /* If we can't buffer a future message because
                          * of space limitations -- ignore. */
-                        MBEDTLS_SSL_DEBUG_MSG( 2, ( "Buffering of future message of size %zu would exceed the compile-time limit %zu (already %zu bytes buffered) -- ignore\n",
+                        MBEDTLS_SSL_DEBUG_MSG( 2, ( "Buffering of future message of size %" MBEDTLS_PRINTF_SIZET
+                                                    " would exceed the compile-time limit %" MBEDTLS_PRINTF_SIZET
+                                                    " (already %" MBEDTLS_PRINTF_SIZET
+                                                    " bytes buffered) -- ignore\n",
                              msg_len, (size_t) MBEDTLS_SSL_DTLS_MAX_BUFFERING,
                              hs->buffering.total_bytes_buffered ) );
                         goto exit;
                     }
                     else
                     {
-                        MBEDTLS_SSL_DEBUG_MSG( 2, ( "Buffering of future message of size %zu would exceed the compile-time limit %zu (already %zu bytes buffered) -- attempt to make space by freeing buffered future messages\n",
+                        MBEDTLS_SSL_DEBUG_MSG( 2, ( "Buffering of future message of size %" MBEDTLS_PRINTF_SIZET
+                                                    " would exceed the compile-time limit %" MBEDTLS_PRINTF_SIZET
+                                                    " (already %" MBEDTLS_PRINTF_SIZET
+                                                    " bytes buffered) -- attempt to make space by freeing buffered future messages\n",
                              msg_len, (size_t) MBEDTLS_SSL_DTLS_MAX_BUFFERING,
                              hs->buffering.total_bytes_buffered ) );
                     }
 
                     if( ssl_buffer_make_space( ssl, reassembly_buf_sz ) != 0 )
                     {
-                        MBEDTLS_SSL_DEBUG_MSG( 2, ( "Reassembly of next message of size %zu (%zu with bitmap) would exceed the compile-time limit %zu (already %zu bytes buffered) -- fail\n",
+                        MBEDTLS_SSL_DEBUG_MSG( 2, ( "Reassembly of next message of size %" MBEDTLS_PRINTF_SIZET
+                                                    " (%" MBEDTLS_PRINTF_SIZET " with bitmap) would exceed"
+                                                    " the compile-time limit %" MBEDTLS_PRINTF_SIZET
+                                                    " (already %" MBEDTLS_PRINTF_SIZET
+                                                    " bytes buffered) -- fail\n",
                              msg_len,
                              reassembly_buf_sz,
                              (size_t) MBEDTLS_SSL_DTLS_MAX_BUFFERING,
@@ -4349,7 +4375,7 @@
                     }
                 }
 
-                MBEDTLS_SSL_DEBUG_MSG( 2, ( "initialize reassembly, total length = %zu",
+                MBEDTLS_SSL_DEBUG_MSG( 2, ( "initialize reassembly, total length = %" MBEDTLS_PRINTF_SIZET,
                                             msg_len ) );
 
                 hs_buf->data = mbedtls_calloc( 1, reassembly_buf_sz );
@@ -4395,7 +4421,8 @@
                 frag_off = ssl_get_hs_frag_off( ssl );
                 frag_len = ssl_get_hs_frag_len( ssl );
 
-                MBEDTLS_SSL_DEBUG_MSG( 2, ( "adding fragment, offset = %zu, length = %zu",
+                MBEDTLS_SSL_DEBUG_MSG( 2, ( "adding fragment, offset = %" MBEDTLS_PRINTF_SIZET
+                                            ", length = %" MBEDTLS_PRINTF_SIZET,
                                             frag_off, frag_len ) );
                 memcpy( msg + frag_off, ssl->in_msg + 12, frag_len );
 
@@ -4622,7 +4649,10 @@
     if( rec->buf_len > ( MBEDTLS_SSL_DTLS_MAX_BUFFERING -
                          hs->buffering.total_bytes_buffered ) )
     {
-        MBEDTLS_SSL_DEBUG_MSG( 2, ( "Buffering of future epoch record of size %zu would exceed the compile-time limit %zu (already %zu bytes buffered) -- ignore\n",
+        MBEDTLS_SSL_DEBUG_MSG( 2, ( "Buffering of future epoch record of size %" MBEDTLS_PRINTF_SIZET
+                                    " would exceed the compile-time limit %" MBEDTLS_PRINTF_SIZET
+                                    " (already %" MBEDTLS_PRINTF_SIZET
+                                    " bytes buffered) -- ignore\n",
                         rec->buf_len, (size_t) MBEDTLS_SSL_DTLS_MAX_BUFFERING,
                         hs->buffering.total_bytes_buffered ) );
         return( 0 );
@@ -4903,7 +4933,7 @@
     {
         if( ssl->in_msglen != 1 )
         {
-            MBEDTLS_SSL_DEBUG_MSG( 1, ( "invalid CCS message, len: %zu",
+            MBEDTLS_SSL_DEBUG_MSG( 1, ( "invalid CCS message, len: %" MBEDTLS_PRINTF_SIZET,
                            ssl->in_msglen ) );
             return( MBEDTLS_ERR_SSL_INVALID_RECORD );
         }
@@ -4939,7 +4969,7 @@
             /* Note: Standard allows for more than one 2 byte alert
                to be packed in a single message, but Mbed TLS doesn't
                currently support this. */
-            MBEDTLS_SSL_DEBUG_MSG( 1, ( "invalid alert message, len: %zu",
+            MBEDTLS_SSL_DEBUG_MSG( 1, ( "invalid alert message, len: %" MBEDTLS_PRINTF_SIZET,
                            ssl->in_msglen ) );
             return( MBEDTLS_ERR_SSL_INVALID_RECORD );
         }
@@ -5771,7 +5801,8 @@
         if( ssl->conf->transport == MBEDTLS_SSL_TRANSPORT_DATAGRAM )
         {
             MBEDTLS_SSL_DEBUG_MSG( 1, ( "fragment larger than the (negotiated) "
-                                "maximum fragment length: %zu > %zu",
+                                "maximum fragment length: %" MBEDTLS_PRINTF_SIZET
+                                " > %" MBEDTLS_PRINTF_SIZET,
                                 len, max_len ) );
             return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
         }
diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index b41d385..807fb18 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -633,7 +633,7 @@
     /* Remember the client asked us to send a new ticket */
     ssl->handshake->new_session_ticket = 1;
 
-    MBEDTLS_SSL_DEBUG_MSG( 3, ( "ticket length: %zu", len ) );
+    MBEDTLS_SSL_DEBUG_MSG( 3, ( "ticket length: %" MBEDTLS_PRINTF_SIZET, len ) );
 
     if( len == 0 )
         return( 0 );
@@ -2826,7 +2826,8 @@
     *p++ = (unsigned char)( t >>  8 );
     *p++ = (unsigned char)( t       );
 
-    MBEDTLS_SSL_DEBUG_MSG( 3, ( "server hello, current time: %lld", (long long) t ) );
+    MBEDTLS_SSL_DEBUG_MSG( 3, ( "server hello, current time: %" MBEDTLS_PRINTF_LONGLONG,
+                                (long long) t ) );
 #else
     if( ( ret = ssl->conf->f_rng( ssl->conf->p_rng, p, 4 ) ) != 0 )
         return( ret );
@@ -2914,7 +2915,7 @@
     memcpy( p, ssl->session_negotiate->id, ssl->session_negotiate->id_len );
     p += ssl->session_negotiate->id_len;
 
-    MBEDTLS_SSL_DEBUG_MSG( 3, ( "server hello, session id len.: %zu", n ) );
+    MBEDTLS_SSL_DEBUG_MSG( 3, ( "server hello, session id len.: %" MBEDTLS_PRINTF_SIZET, n ) );
     MBEDTLS_SSL_DEBUG_BUF( 3,   "server hello, session id", buf + 39, n );
     MBEDTLS_SSL_DEBUG_MSG( 3, ( "%s session has been resumed",
                    ssl->handshake->resume ? "a" : "no" ) );
@@ -2995,7 +2996,8 @@
     ext_len += olen;
 #endif
 
-    MBEDTLS_SSL_DEBUG_MSG( 3, ( "server hello, total extension length: %zu", ext_len ) );
+    MBEDTLS_SSL_DEBUG_MSG( 3, ( "server hello, total extension length: %" MBEDTLS_PRINTF_SIZET,
+                                ext_len ) );
 
     if( ext_len > 0 )
     {
diff --git a/library/ssl_tls.c b/library/ssl_tls.c
index 747025a..ef031d2 100644
--- a/library/ssl_tls.c
+++ b/library/ssl_tls.c
@@ -2215,7 +2215,8 @@
         n = crt->raw.len;
         if( n > MBEDTLS_SSL_OUT_CONTENT_LEN - 3 - i )
         {
-            MBEDTLS_SSL_DEBUG_MSG( 1, ( "certificate too large, %zu > %zu",
+            MBEDTLS_SSL_DEBUG_MSG( 1, ( "certificate too large, %" MBEDTLS_PRINTF_SIZET
+                                        " > %" MBEDTLS_PRINTF_SIZET,
                            i + 3 + n, (size_t) MBEDTLS_SSL_OUT_CONTENT_LEN ) );
             return( MBEDTLS_ERR_SSL_CERTIFICATE_TOO_LARGE );
         }
@@ -2831,7 +2832,7 @@
     chain = mbedtls_calloc( 1, sizeof( mbedtls_x509_crt ) );
     if( chain == NULL )
     {
-        MBEDTLS_SSL_DEBUG_MSG( 1, ( "alloc(%zu bytes) failed",
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "alloc(%" MBEDTLS_PRINTF_SIZET " bytes) failed",
                                     sizeof( mbedtls_x509_crt ) ) );
         mbedtls_ssl_send_alert_message( ssl,
                                         MBEDTLS_SSL_ALERT_LEVEL_FATAL,
@@ -3858,7 +3859,7 @@
     ssl->in_buf = mbedtls_calloc( 1, in_buf_len );
     if( ssl->in_buf == NULL )
     {
-        MBEDTLS_SSL_DEBUG_MSG( 1, ( "alloc(%zu bytes) failed", in_buf_len ) );
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "alloc(%" MBEDTLS_PRINTF_SIZET " bytes) failed", in_buf_len ) );
         ret = MBEDTLS_ERR_SSL_ALLOC_FAILED;
         goto error;
     }
@@ -3869,7 +3870,7 @@
     ssl->out_buf = mbedtls_calloc( 1, out_buf_len );
     if( ssl->out_buf == NULL )
     {
-        MBEDTLS_SSL_DEBUG_MSG( 1, ( "alloc(%zu bytes) failed", out_buf_len ) );
+        MBEDTLS_SSL_DEBUG_MSG( 1, ( "alloc(%" MBEDTLS_PRINTF_SIZET " bytes) failed", out_buf_len ) );
         ret = MBEDTLS_ERR_SSL_ALLOC_FAILED;
         goto error;
     }