diff --git a/configure.ac b/configure.ac
index aead3fe1..0a710435 100644
--- a/configure.ac
+++ b/configure.ac
@@ -140,6 +140,20 @@ else
   AC_SUBST([CYTHON])
 fi
 
+#
+# If we're running GCC or clang define _U_ to be "__attribute__((unused))"
+# so we can use _U_ to flag unused function parameters and not get warnings
+# about them. Otherwise, define _U_ to be an empty string so that _U_ used
+# to flag an unused function parameters will compile with other compilers.
+#
+# XXX - similar hints for other compilers?
+#
+if test "x$GCC" = "xyes" -o "x$CC" = "xclang" ; then
+  AC_DEFINE([_U_], [__attribute__((unused))], [Hint to the compiler that a function parameters is not used])
+else
+  AC_DEFINE([_U_], , [Hint to the compiler that a function parameters is not used])
+fi
+
 AX_CXX_COMPILE_STDCXX_11([noext], [optional])
 
 AC_LANG_PUSH(C++)
@@ -529,7 +543,6 @@ if test "x$werror" != "xno"; then
     AX_CHECK_COMPILE_FLAG([-Wcast-align], [CFLAGS="$CFLAGS -Wcast-align"])
     AX_CHECK_COMPILE_FLAG([-Wclobbered], [CFLAGS="$CFLAGS -Wclobbered"])
     AX_CHECK_COMPILE_FLAG([-Wvla], [CFLAGS="$CFLAGS -Wvla"])
-    AX_CHECK_COMPILE_FLAG([-Wno-unused-parameter], [CFLAGS="$CFLAGS -Wno-unused-parameter"])
     AX_CHECK_COMPILE_FLAG([-Wpragmas], [CFLAGS="$CFLAGS -Wpragmas"])
     AX_CHECK_COMPILE_FLAG([-Wunreachable-code], [CFLAGS="$CFLAGS -Wunreachable-code"])
     AX_CHECK_COMPILE_FLAG([-Waddress], [CFLAGS="$CFLAGS -Waddress"])
diff --git a/examples/client.c b/examples/client.c
index a08d943a..a008ba54 100644
--- a/examples/client.c
+++ b/examples/client.c
@@ -40,6 +40,7 @@
 #include <stdio.h>
 #include <assert.h>
 
+#include <config.h>
 #include <nghttp2/nghttp2.h>
 
 #include <openssl/ssl.h>
@@ -144,8 +145,8 @@ static void diec(const char *func, int error_code)
  * bytes actually written. See the documentation of
  * nghttp2_send_callback for the details.
  */
-static ssize_t send_callback(nghttp2_session *session,
-                             const uint8_t *data, size_t length, int flags,
+static ssize_t send_callback(nghttp2_session *session _U_,
+                             const uint8_t *data, size_t length, int flags _U_,
                              void *user_data)
 {
   struct Connection *connection;
@@ -173,8 +174,8 @@ static ssize_t send_callback(nghttp2_session *session,
  * |length| bytes. Returns the number of bytes stored in |buf|. See
  * the documentation of nghttp2_recv_callback for the details.
  */
-static ssize_t recv_callback(nghttp2_session *session,
-                             uint8_t *buf, size_t length, int flags,
+static ssize_t recv_callback(nghttp2_session *session _U_,
+                             uint8_t *buf, size_t length, int flags _U_,
                              void *user_data)
 {
   struct Connection *connection;
@@ -199,7 +200,7 @@ static ssize_t recv_callback(nghttp2_session *session,
 }
 
 static int on_frame_send_callback(nghttp2_session *session,
-                                  const nghttp2_frame *frame, void *user_data)
+                                  const nghttp2_frame *frame, void *user_data _U_)
 {
   size_t i;
   switch(frame->hd.type) {
@@ -226,7 +227,7 @@ static int on_frame_send_callback(nghttp2_session *session,
 }
 
 static int on_frame_recv_callback(nghttp2_session *session,
-                                  const nghttp2_frame *frame, void *user_data)
+                                  const nghttp2_frame *frame, void *user_data _U_)
 {
   size_t i;
   switch(frame->hd.type) {
@@ -264,8 +265,8 @@ static int on_frame_recv_callback(nghttp2_session *session,
  */
 static int on_stream_close_callback(nghttp2_session *session,
                                     int32_t stream_id,
-                                    nghttp2_error_code error_code,
-                                    void *user_data)
+                                    uint32_t error_code _U_,
+                                    void *user_data _U_)
 {
   struct Request *req;
   req = nghttp2_session_get_stream_user_data(session, stream_id);
@@ -286,10 +287,10 @@ static int on_stream_close_callback(nghttp2_session *session,
  * The implementation of nghttp2_on_data_chunk_recv_callback type. We
  * use this function to print the received response body.
  */
-static int on_data_chunk_recv_callback(nghttp2_session *session, uint8_t flags,
+static int on_data_chunk_recv_callback(nghttp2_session *session, uint8_t flags _U_,
                                        int32_t stream_id,
                                        const uint8_t *data, size_t len,
-                                       void *user_data)
+                                       void *user_data _U_)
 {
   struct Request *req;
   req = nghttp2_session_get_stream_user_data(session, stream_id);
@@ -332,10 +333,10 @@ static void setup_nghttp2_callbacks(nghttp2_session_callbacks *callbacks)
  * HTTP/2 protocol, if server does not offer HTTP/2 the nghttp2
  * library supports, we terminate program.
  */
-static int select_next_proto_cb(SSL* ssl,
+static int select_next_proto_cb(SSL* ssl _U_,
                                 unsigned char **out, unsigned char *outlen,
                                 const unsigned char *in, unsigned int inlen,
-                                void *arg)
+                                void *arg _U_)
 {
   int rv;
   /* nghttp2_select_next_protocol() selects HTTP/2 protocol the
diff --git a/examples/deflate.c b/examples/deflate.c
index b68f7ae9..2ca84f7f 100644
--- a/examples/deflate.c
+++ b/examples/deflate.c
@@ -22,6 +22,7 @@
  * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
  * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
  */
+#include <config.h>
 #include <stdio.h>
 #include <string.h>
 
@@ -38,7 +39,7 @@ static void deflate(nghttp2_hd_deflater *deflater,
 static int inflate_header_block(nghttp2_hd_inflater *inflater,
                                 uint8_t *in, size_t inlen, int final);
 
-int main(int argc, char **argv)
+int main(int argc _U_, char **argv _U_)
 {
   int rv;
   nghttp2_hd_deflater *deflater;
diff --git a/examples/libevent-client.c b/examples/libevent-client.c
index ef2dde8a..5dd8e356 100644
--- a/examples/libevent-client.c
+++ b/examples/libevent-client.c
@@ -23,6 +23,7 @@
  * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
  */
 #include <sys/types.h>
+#include <config.h>
 #include <unistd.h>
 #include <sys/socket.h>
 #include <netinet/in.h>
@@ -179,9 +180,9 @@ static void print_headers(FILE *f, nghttp2_nv *nva, size_t nvlen)
 /* nghttp2_send_callback. Here we transmit the |data|, |length| bytes,
    to the network. Because we are using libevent bufferevent, we just
    write those bytes into bufferevent buffer. */
-static ssize_t send_callback(nghttp2_session *session,
+static ssize_t send_callback(nghttp2_session *session _U_,
                              const uint8_t *data, size_t length,
-                             int flags, void *user_data)
+                             int flags _U_, void *user_data)
 {
   http2_session_data *session_data = (http2_session_data*)user_data;
   struct bufferevent *bev = session_data->bev;
@@ -191,11 +192,11 @@ static ssize_t send_callback(nghttp2_session *session,
 
 /* nghttp2_on_header_callback: Called when nghttp2 library emits
    single header name/value pair. */
-static int on_header_callback(nghttp2_session *session,
+static int on_header_callback(nghttp2_session *session _U_,
                               const nghttp2_frame *frame,
                               const uint8_t *name, size_t namelen,
                               const uint8_t *value, size_t valuelen,
-                              uint8_t flags,
+                              uint8_t flags _U_,
                               void *user_data)
 {
   http2_session_data *session_data = (http2_session_data*)user_data;
@@ -213,7 +214,7 @@ static int on_header_callback(nghttp2_session *session,
 
 /* nghttp2_on_begin_headers_callback: Called when nghttp2 library gets
    started to receive header block. */
-static int on_begin_headers_callback(nghttp2_session *session,
+static int on_begin_headers_callback(nghttp2_session *session _U_,
                                      const nghttp2_frame *frame,
                                      void *user_data)
 {
@@ -232,7 +233,7 @@ static int on_begin_headers_callback(nghttp2_session *session,
 
 /* nghttp2_on_frame_recv_callback: Called when nghttp2 library
    received a complete frame from the remote peer. */
-static int on_frame_recv_callback(nghttp2_session *session,
+static int on_frame_recv_callback(nghttp2_session *session _U_,
                                   const nghttp2_frame *frame, void *user_data)
 {
   http2_session_data *session_data = (http2_session_data*)user_data;
@@ -252,7 +253,7 @@ static int on_frame_recv_callback(nghttp2_session *session,
    is meant to the stream we initiated, print the received data in
    stdout, so that the user can redirect its output to the file
    easily. */
-static int on_data_chunk_recv_callback(nghttp2_session *session, uint8_t flags,
+static int on_data_chunk_recv_callback(nghttp2_session *session _U_, uint8_t flags _U_,
                                        int32_t stream_id,
                                        const uint8_t *data, size_t len,
                                        void *user_data)
@@ -290,10 +291,10 @@ static int on_stream_close_callback(nghttp2_session *session,
 /* NPN TLS extension client callback. We check that server advertised
    the HTTP/2 protocol the nghttp2 library supports. If not, exit
    the program. */
-static int select_next_proto_cb(SSL* ssl,
+static int select_next_proto_cb(SSL* ssl _U_,
                                 unsigned char **out, unsigned char *outlen,
                                 const unsigned char *in, unsigned int inlen,
-                                void *arg)
+                                void *arg _U_)
 {
   if(nghttp2_select_next_protocol(out, outlen, in, inlen) <= 0) {
     errx(1, "Server did not advertise " NGHTTP2_PROTO_VERSION_ID);
@@ -454,7 +455,7 @@ static void readcb(struct bufferevent *bev, void *ptr)
    receiving GOAWAY, we check the some conditions on the nghttp2
    library and output buffer of bufferevent. If it indicates we have
    no business to this session, tear down the connection. */
-static void writecb(struct bufferevent *bev, void *ptr)
+static void writecb(struct bufferevent *bev _U_, void *ptr)
 {
   http2_session_data *session_data = (http2_session_data*)ptr;
   if(nghttp2_session_want_read(session_data->session) == 0 &&
diff --git a/examples/libevent-server.c b/examples/libevent-server.c
index 2d073ed7..7d7ab311 100644
--- a/examples/libevent-server.c
+++ b/examples/libevent-server.c
@@ -22,6 +22,7 @@
  * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
  * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
  */
+#include <config.h>
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <netdb.h>
@@ -79,8 +80,8 @@ struct app_context {
 static unsigned char next_proto_list[256];
 static size_t next_proto_list_len;
 
-static int next_proto_cb(SSL *s, const unsigned char **data, unsigned int *len,
-                         void *arg)
+static int next_proto_cb(SSL *s _U_, const unsigned char **data, unsigned int *len,
+                         void *arg _U_)
 {
   *data = next_proto_list;
   *len = (unsigned int)next_proto_list_len;
@@ -140,7 +141,7 @@ static void add_stream(http2_session_data *session_data,
   }
 }
 
-static void remove_stream(http2_session_data *session_data,
+static void remove_stream(http2_session_data *session_data _U_,
                           http2_stream_data *stream_data)
 {
   stream_data->prev->next = stream_data->next;
@@ -259,9 +260,9 @@ static int session_recv(http2_session_data *session_data)
   return 0;
 }
 
-static ssize_t send_callback(nghttp2_session *session,
+static ssize_t send_callback(nghttp2_session *session _U_,
                              const uint8_t *data, size_t length,
-                             int flags, void *user_data)
+                             int flags _U_, void *user_data)
 {
   http2_session_data *session_data = (http2_session_data*)user_data;
   struct bufferevent *bev = session_data->bev;
@@ -330,9 +331,9 @@ static char* percent_decode(const uint8_t *value, size_t valuelen)
 }
 
 static ssize_t file_read_callback
-(nghttp2_session *session, int32_t stream_id,
+(nghttp2_session *session _U_, int32_t stream_id _U_,
  uint8_t *buf, size_t length, uint32_t *data_flags,
- nghttp2_data_source *source, void *user_data)
+ nghttp2_data_source *source, void *user_data _U_)
 {
   int fd = source->fd;
   ssize_t r;
@@ -412,8 +413,8 @@ static int on_header_callback(nghttp2_session *session,
                               const nghttp2_frame *frame,
                               const uint8_t *name, size_t namelen,
                               const uint8_t *value, size_t valuelen,
-                              uint8_t flags,
-                              void *user_data)
+                              uint8_t flags _U_,
+                              void *user_data _U_)
 {
   http2_stream_data *stream_data;
   const char PATH[] = ":path";
@@ -536,7 +537,7 @@ static int on_frame_recv_callback(nghttp2_session *session,
 
 static int on_stream_close_callback(nghttp2_session *session,
                                     int32_t stream_id,
-                                    nghttp2_error_code error_code,
+                                    uint32_t error_code _U_,
                                     void *user_data)
 {
   http2_session_data *session_data = (http2_session_data*)user_data;
@@ -607,7 +608,7 @@ static int send_server_connection_header(http2_session_data *session_data)
 
 /* readcb for bufferevent after client connection header was
    checked. */
-static void readcb(struct bufferevent *bev, void *ptr)
+static void readcb(struct bufferevent *bev _U_, void *ptr)
 {
   http2_session_data *session_data = (http2_session_data*)ptr;
   if(session_recv(session_data) != 0) {
@@ -642,7 +643,7 @@ static void writecb(struct bufferevent *bev, void *ptr)
 }
 
 /* eventcb for bufferevent */
-static void eventcb(struct bufferevent *bev, short events, void *ptr)
+static void eventcb(struct bufferevent *bev _U_, short events, void *ptr)
 {
   http2_session_data *session_data = (http2_session_data*)ptr;
   if(events & BEV_EVENT_CONNECTED) {
@@ -668,7 +669,7 @@ static void eventcb(struct bufferevent *bev, short events, void *ptr)
 }
 
 /* callback for evconnlistener */
-static void acceptcb(struct evconnlistener *listener, int fd,
+static void acceptcb(struct evconnlistener *listener _U_, int fd,
                      struct sockaddr *addr, int addrlen, void *arg)
 {
   app_context *app_ctx = (app_context*)arg;
diff --git a/examples/tiny-nghttpd.c b/examples/tiny-nghttpd.c
index 1bb86b63..d7d6dc27 100644
--- a/examples/tiny-nghttpd.c
+++ b/examples/tiny-nghttpd.c
@@ -27,6 +27,7 @@
  * overhead of underlying I/O library (e.g., libevent, Boost ASIO).
  */
 #define _GNU_SOURCE
+#include <config.h>
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <sys/stat.h>
@@ -532,7 +533,7 @@ static int io_loop_mod(io_loop *loop, int fd, uint32_t events, void *ptr)
   return io_loop_ctl(loop, EPOLL_CTL_MOD, fd, events, ptr);
 }
 
-static int io_loop_run(io_loop *loop, server *serv)
+static int io_loop_run(io_loop *loop, server *serv _U_)
 {
 #define NUM_EVENTS 1024
   struct epoll_event events[NUM_EVENTS];
@@ -557,7 +558,7 @@ static int io_loop_run(io_loop *loop, server *serv)
   }
 }
 
-static int handle_timer(io_loop *loop, uint32_t events, void *ptr)
+static int handle_timer(io_loop *loop _U_, uint32_t events _U_, void *ptr)
 {
   timer *tmr = ptr;
   int64_t buf;
@@ -572,7 +573,7 @@ static int handle_timer(io_loop *loop, uint32_t events, void *ptr)
   return 0;
 }
 
-static int handle_accept(io_loop *loop, uint32_t events, void *ptr)
+static int handle_accept(io_loop *loop, uint32_t events _U_, void *ptr)
 {
   int acfd;
   server *serv = ptr;
@@ -629,9 +630,9 @@ static void stream_error
 }
 
 static ssize_t fd_read_callback
-(nghttp2_session *session, int32_t stream_id,
+(nghttp2_session *session _U_, int32_t stream_id _U_,
  uint8_t *buf, size_t length, uint32_t *data_flags,
- nghttp2_data_source *source, void *user_data)
+ nghttp2_data_source *source, void *user_data _U_)
 {
   stream *strm = source->ptr;
   ssize_t nread;
@@ -651,9 +652,9 @@ static ssize_t fd_read_callback
 }
 
 static ssize_t resbuf_read_callback
-(nghttp2_session *session, int32_t stream_id,
+(nghttp2_session *session _U_, int32_t stream_id _U_,
  uint8_t *buf, size_t length, uint32_t *data_flags,
- nghttp2_data_source *source, void *user_data)
+ nghttp2_data_source *source, void *user_data _U_)
 {
   stream *strm = source->ptr;
   size_t left = strm->res_end - strm->res_begin;
@@ -715,7 +716,7 @@ static int check_path(const char *path, size_t len)
     (len < 2 || memcmp(path + len - 2, "/.", 2) != 0);
 }
 
-static int make_path(io_buf *pathbuf, const char *req, size_t reqlen)
+static int make_path(io_buf *pathbuf, const char *req, size_t reqlen _U_)
 {
   uint8_t *p;
 
@@ -953,7 +954,7 @@ static int on_begin_headers_callback
 static int on_header_callback
 (nghttp2_session *session, const nghttp2_frame *frame,
  const uint8_t *name, size_t namelen, const uint8_t *value, size_t valuelen,
- uint8_t flags, void *user_data)
+ uint8_t flags _U_, void *user_data)
 {
   connection *conn = user_data;
   stream *strm;
@@ -1079,8 +1080,8 @@ static int on_frame_recv_callback
 }
 
 static int on_stream_close_callback
-(nghttp2_session *session, int32_t stream_id, uint32_t error_code,
- void *user_data)
+(nghttp2_session *session, int32_t stream_id, uint32_t error_code _U_,
+ void *user_data _U_)
 {
   stream *strm;
 
@@ -1096,7 +1097,7 @@ static int on_stream_close_callback
 }
 
 static int on_frame_not_send_callback
-(nghttp2_session *session, const nghttp2_frame *frame, int lib_error_code,
+(nghttp2_session *session _U_, const nghttp2_frame *frame, int lib_error_code _U_,
  void *user_data)
 {
   connection *conn = user_data;
diff --git a/lib/nghttp2_frame.c b/lib/nghttp2_frame.c
index 9a661a08..906053f0 100644
--- a/lib/nghttp2_frame.c
+++ b/lib/nghttp2_frame.c
@@ -94,7 +94,7 @@ void nghttp2_frame_priority_init(nghttp2_priority *frame, int32_t stream_id,
   frame->pri_spec = *pri_spec;
 }
 
-void nghttp2_frame_priority_free(nghttp2_priority *frame)
+void nghttp2_frame_priority_free(nghttp2_priority *frame _U_)
 {}
 
 void nghttp2_frame_rst_stream_init(nghttp2_rst_stream *frame,
@@ -106,7 +106,7 @@ void nghttp2_frame_rst_stream_init(nghttp2_rst_stream *frame,
   frame->error_code = error_code;
 }
 
-void nghttp2_frame_rst_stream_free(nghttp2_rst_stream *frame)
+void nghttp2_frame_rst_stream_free(nghttp2_rst_stream *frame _U_)
 {}
 
 
@@ -153,7 +153,7 @@ void nghttp2_frame_ping_init(nghttp2_ping *frame, uint8_t flags,
   }
 }
 
-void nghttp2_frame_ping_free(nghttp2_ping *frame)
+void nghttp2_frame_ping_free(nghttp2_ping *frame _U_)
 {}
 
 void nghttp2_frame_goaway_init(nghttp2_goaway *frame, int32_t last_stream_id,
@@ -185,7 +185,7 @@ void nghttp2_frame_window_update_init(nghttp2_window_update *frame,
   frame->reserved = 0;
 }
 
-void nghttp2_frame_window_update_free(nghttp2_window_update *frame)
+void nghttp2_frame_window_update_free(nghttp2_window_update *frame _U_)
 {}
 
 void nghttp2_frame_altsvc_init(nghttp2_extension *frame, int32_t stream_id,
@@ -242,7 +242,7 @@ void nghttp2_frame_data_init(nghttp2_data *frame, uint8_t flags,
   frame->padlen = 0;
 }
 
-void nghttp2_frame_data_free(nghttp2_data *frame)
+void nghttp2_frame_data_free(nghttp2_data *frame _U_)
 {}
 
 size_t nghttp2_frame_priority_len(uint8_t flags)
@@ -382,9 +382,9 @@ void nghttp2_frame_pack_priority_spec(uint8_t *buf,
 }
 
 void nghttp2_frame_unpack_priority_spec(nghttp2_priority_spec *pri_spec,
-                                        uint8_t flags,
+                                        uint8_t flags _U_,
                                         const uint8_t *payload,
-                                        size_t payloadlen)
+                                        size_t payloadlen _U_)
 {
   int32_t dep_stream_id;
   uint8_t exclusive;
@@ -466,7 +466,7 @@ int nghttp2_frame_pack_rst_stream(nghttp2_bufs *bufs,
 
 void nghttp2_frame_unpack_rst_stream_payload(nghttp2_rst_stream *frame,
                                              const uint8_t *payload,
-                                             size_t payloadlen)
+                                             size_t payloadlen _U_)
 {
   frame->error_code = nghttp2_get_uint32(payload);
 }
@@ -602,7 +602,7 @@ int nghttp2_frame_pack_push_promise(nghttp2_bufs *bufs,
 
 int nghttp2_frame_unpack_push_promise_payload(nghttp2_push_promise *frame,
                                               const uint8_t *payload,
-                                              size_t payloadlen)
+                                              size_t payloadlen _U_)
 {
   frame->promised_stream_id = nghttp2_get_uint32(payload) &
     NGHTTP2_STREAM_ID_MASK;
@@ -633,7 +633,7 @@ int nghttp2_frame_pack_ping(nghttp2_bufs *bufs, nghttp2_ping *frame)
 
 void nghttp2_frame_unpack_ping_payload(nghttp2_ping *frame,
                                        const uint8_t *payload,
-                                       size_t payloadlen)
+                                       size_t payloadlen _U_)
 {
   memcpy(frame->opaque_data, payload, sizeof(frame->opaque_data));
 }
@@ -672,7 +672,7 @@ int nghttp2_frame_pack_goaway(nghttp2_bufs *bufs, nghttp2_goaway *frame)
 
 void nghttp2_frame_unpack_goaway_payload(nghttp2_goaway *frame,
                                          const uint8_t *payload,
-                                         size_t payloadlen,
+                                         size_t payloadlen _U_,
                                          uint8_t *var_gift_payload,
                                          size_t var_gift_payloadlen)
 {
@@ -739,7 +739,7 @@ int nghttp2_frame_pack_window_update(nghttp2_bufs *bufs,
 
 void nghttp2_frame_unpack_window_update_payload(nghttp2_window_update *frame,
                                                 const uint8_t *payload,
-                                                size_t payloadlen)
+                                                size_t payloadlen _U_)
 {
   frame->window_size_increment = nghttp2_get_uint32(payload) &
     NGHTTP2_WINDOW_SIZE_INCREMENT_MASK;
@@ -803,7 +803,7 @@ int nghttp2_frame_pack_altsvc(nghttp2_bufs *bufs, nghttp2_extension *frame)
 
 int nghttp2_frame_unpack_altsvc_payload(nghttp2_extension *frame,
                                         const uint8_t *payload,
-                                        size_t payloadlen,
+                                        size_t payloadlen _U_,
                                         uint8_t *var_gift_payload,
                                         size_t var_gift_payloadlen)
 {
diff --git a/lib/nghttp2_hd.c b/lib/nghttp2_hd.c
index ff882790..b687d67e 100644
--- a/lib/nghttp2_hd.c
+++ b/lib/nghttp2_hd.c
@@ -1141,7 +1141,7 @@ ssize_t nghttp2_hd_deflate_hd(nghttp2_hd_deflater *deflater,
   return (ssize_t)buflen;
 }
 
-size_t nghttp2_hd_deflate_bound(nghttp2_hd_deflater *deflater,
+size_t nghttp2_hd_deflate_bound(nghttp2_hd_deflater *deflater _U_,
                                 const nghttp2_nv *nva, size_t nvlen)
 {
   size_t n = 0;
diff --git a/lib/nghttp2_session.c b/lib/nghttp2_session.c
index 3f23cfd3..b0dd9fcf 100644
--- a/lib/nghttp2_session.c
+++ b/lib/nghttp2_session.c
@@ -1233,7 +1233,7 @@ static int stream_predicate_for_send(nghttp2_stream *stream)
  *     received.
  */
 static int session_predicate_request_headers_send
-(nghttp2_session *session, nghttp2_headers *frame)
+(nghttp2_session *session, nghttp2_headers *frame _U_)
 {
   if(session->goaway_flags) {
     /* When GOAWAY is sent or received, peer must not send new request
@@ -1302,7 +1302,7 @@ static int session_predicate_response_headers_send
  *   RST_STREAM was queued for this stream.
  */
 static int session_predicate_push_response_headers_send
-(nghttp2_session *session, nghttp2_stream *stream)
+(nghttp2_session *session _U_, nghttp2_stream *stream)
 {
   int rv;
   /* TODO Should disallow HEADERS if GOAWAY has already been issued? */
@@ -1506,8 +1506,8 @@ static int session_predicate_altsvc_send
  *
  * Currently this function always returns 0.
  */
-static int nghttp2_session_predicate_settings_send(nghttp2_session *session,
-                                                   nghttp2_frame *frame)
+static int nghttp2_session_predicate_settings_send(nghttp2_session *session _U_,
+                                                   nghttp2_frame *frame _U_)
 {
   return 0;
 }
@@ -2815,7 +2815,7 @@ static int session_call_on_header(nghttp2_session *session,
  *   Out of memory.
  */
 static int session_handle_frame_size_error(nghttp2_session *session,
-                                           nghttp2_frame *frame)
+                                           nghttp2_frame *frame _U_)
 {
   /* TODO Currently no callback is called for this error, because we
      call this callback before reading any payload */
@@ -2998,7 +2998,7 @@ static int inflate_header_block(nghttp2_session *session,
  * NGHTTP2_ERR_NOMEM
  *     Out of memory.
  */
-int nghttp2_session_end_request_headers_received(nghttp2_session *session,
+int nghttp2_session_end_request_headers_received(nghttp2_session *session _U_,
                                                  nghttp2_frame *frame,
                                                  nghttp2_stream *stream)
 {
diff --git a/lib/nghttp2_stream.c b/lib/nghttp2_stream.c
index 368d9d2f..3ee4e069 100644
--- a/lib/nghttp2_stream.c
+++ b/lib/nghttp2_stream.c
@@ -74,7 +74,7 @@ void nghttp2_stream_init(nghttp2_stream *stream, int32_t stream_id,
   stream->root_next = NULL;
 }
 
-void nghttp2_stream_free(nghttp2_stream *stream)
+void nghttp2_stream_free(nghttp2_stream *stream _U_)
 {
   /* We don't free stream->data_item.  If it is assigned to aob, then
      active_outbound_item_reset() will delete it.  If it is queued,
@@ -1060,7 +1060,7 @@ void nghttp2_stream_roots_init(nghttp2_stream_roots *roots)
   roots->num_streams = 0;
 }
 
-void nghttp2_stream_roots_free(nghttp2_stream_roots *roots)
+void nghttp2_stream_roots_free(nghttp2_stream_roots *roots _U_)
 {}
 
 void nghttp2_stream_roots_add(nghttp2_stream_roots *roots,
diff --git a/lib/nghttp2_submit.c b/lib/nghttp2_submit.c
index 497e421a..0ed6fce2 100644
--- a/lib/nghttp2_submit.c
+++ b/lib/nghttp2_submit.c
@@ -178,13 +178,13 @@ int32_t nghttp2_submit_headers(nghttp2_session *session, uint8_t flags,
 }
 
 
-int nghttp2_submit_ping(nghttp2_session *session, uint8_t flags,
+int nghttp2_submit_ping(nghttp2_session *session, uint8_t flags _U_,
                         const uint8_t *opaque_data)
 {
   return nghttp2_session_add_ping(session, NGHTTP2_FLAG_NONE, opaque_data);
 }
 
-int nghttp2_submit_priority(nghttp2_session *session, uint8_t flags,
+int nghttp2_submit_priority(nghttp2_session *session, uint8_t flags _U_,
                             int32_t stream_id,
                             const nghttp2_priority_spec *pri_spec)
 {
@@ -229,7 +229,7 @@ int nghttp2_submit_priority(nghttp2_session *session, uint8_t flags,
   return 0;
 }
 
-int nghttp2_submit_rst_stream(nghttp2_session *session, uint8_t flags,
+int nghttp2_submit_rst_stream(nghttp2_session *session, uint8_t flags _U_,
                               int32_t stream_id,
                               uint32_t error_code)
 {
@@ -240,7 +240,7 @@ int nghttp2_submit_rst_stream(nghttp2_session *session, uint8_t flags,
   return nghttp2_session_add_rst_stream(session, stream_id, error_code);
 }
 
-int nghttp2_submit_goaway(nghttp2_session *session, uint8_t flags,
+int nghttp2_submit_goaway(nghttp2_session *session, uint8_t flags _U_,
                           int32_t last_stream_id,
                           uint32_t error_code,
                           const uint8_t *opaque_data, size_t opaque_data_len)
@@ -249,13 +249,13 @@ int nghttp2_submit_goaway(nghttp2_session *session, uint8_t flags,
                                     error_code, opaque_data, opaque_data_len);
 }
 
-int nghttp2_submit_settings(nghttp2_session *session, uint8_t flags,
+int nghttp2_submit_settings(nghttp2_session *session, uint8_t flags _U_,
                             const nghttp2_settings_entry *iv, size_t niv)
 {
   return nghttp2_session_add_settings(session, NGHTTP2_FLAG_NONE, iv, niv);
 }
 
-int32_t nghttp2_submit_push_promise(nghttp2_session *session, uint8_t flags,
+int32_t nghttp2_submit_push_promise(nghttp2_session *session, uint8_t flags _U_,
                                     int32_t stream_id,
                                     const nghttp2_nv *nva, size_t nvlen,
                                     void *promised_stream_user_data)
@@ -366,7 +366,7 @@ int nghttp2_submit_window_update(nghttp2_session *session, uint8_t flags,
   return 0;
 }
 
-int nghttp2_submit_altsvc(nghttp2_session *session, uint8_t flags,
+int nghttp2_submit_altsvc(nghttp2_session *session, uint8_t flags _U_,
                           int32_t stream_id,
                           uint32_t max_age, uint16_t port,
                           const uint8_t *protocol_id, size_t protocol_id_len,
diff --git a/tests/main.c b/tests/main.c
index 276abe60..95e3e988 100644
--- a/tests/main.c
+++ b/tests/main.c
@@ -54,7 +54,7 @@ static int clean_suite1(void)
 }
 
 
-int main(int argc, char* argv[])
+int main(int argc _U_, char* argv[] _U_)
 {
    CU_pSuite pSuite = NULL;
    unsigned int num_tests_failed;
diff --git a/tests/nghttp2_map_test.c b/tests/nghttp2_map_test.c
index 4103640e..a2ee2763 100644
--- a/tests/nghttp2_map_test.c
+++ b/tests/nghttp2_map_test.c
@@ -103,7 +103,7 @@ static void shuffle(int *a, int n)
   }
 }
 
-static int eachfun(nghttp2_map_entry *entry, void *ptr)
+static int eachfun(nghttp2_map_entry *entry _U_, void *ptr _U_)
 {
   return 0;
 }
@@ -152,7 +152,7 @@ void test_nghttp2_map_functional(void)
   nghttp2_map_free(&map);
 }
 
-static int entry_free(nghttp2_map_entry *entry, void *ptr)
+static int entry_free(nghttp2_map_entry *entry, void *ptr _U_)
 {
   free(entry);
   return 0;
diff --git a/tests/nghttp2_pq_test.c b/tests/nghttp2_pq_test.c
index 32ee893c..e7f698e5 100644
--- a/tests/nghttp2_pq_test.c
+++ b/tests/nghttp2_pq_test.c
@@ -89,7 +89,7 @@ static int node_compar(const void *lhs, const void *rhs)
   return ln->key - rn->key;
 }
 
-static int node_update(void *item, void *arg)
+static int node_update(void *item, void *arg _U_)
 {
   node *nd = (node*)item;
   if((nd->key % 2) == 0) {
diff --git a/tests/nghttp2_session_test.c b/tests/nghttp2_session_test.c
index 37904012..4dd76324 100644
--- a/tests/nghttp2_session_test.c
+++ b/tests/nghttp2_session_test.c
@@ -101,31 +101,31 @@ static void scripted_data_feed_init2(scripted_data_feed *df,
   df->feedseq[0] = len;
 }
 
-static ssize_t null_send_callback(nghttp2_session *session,
-                                  const uint8_t* data, size_t len, int flags,
-                                  void *user_data)
+static ssize_t null_send_callback(nghttp2_session *session _U_,
+                                  const uint8_t* data _U_, size_t len, int flags _U_,
+                                  void *user_data _U_)
 {
   return len;
 }
 
-static ssize_t fail_send_callback(nghttp2_session *session,
-                                  const uint8_t *data, size_t len, int flags,
-                                  void *user_data)
+static ssize_t fail_send_callback(nghttp2_session *session _U_,
+                                  const uint8_t *data _U_, size_t len _U_, int flags _U_,
+                                  void *user_data _U_)
 {
   return NGHTTP2_ERR_CALLBACK_FAILURE;
 }
 
-static ssize_t fixed_bytes_send_callback(nghttp2_session *session,
-                                         const uint8_t *data, size_t len,
-                                         int flags, void *user_data)
+static ssize_t fixed_bytes_send_callback(nghttp2_session *session _U_,
+                                         const uint8_t *data _U_, size_t len,
+                                         int flags _U_, void *user_data)
 {
   size_t fixed_sendlen = ((my_user_data*)user_data)->fixed_sendlen;
   return fixed_sendlen < len ? fixed_sendlen : len;
 }
 
 
-static ssize_t scripted_recv_callback(nghttp2_session *session,
-                                      uint8_t* data, size_t len, int flags,
+static ssize_t scripted_recv_callback(nghttp2_session *session _U_,
+                                      uint8_t* data, size_t len, int flags _U_,
                                       void *user_data)
 {
   scripted_data_feed *df = ((my_user_data*)user_data)->df;
@@ -139,16 +139,16 @@ static ssize_t scripted_recv_callback(nghttp2_session *session,
   return wlen;
 }
 
-static ssize_t eof_recv_callback(nghttp2_session *session,
-                                 uint8_t* data, size_t len, int flags,
-                                 void *user_data)
+static ssize_t eof_recv_callback(nghttp2_session *session _U_,
+                                 uint8_t* data _U_, size_t len _U_, int flags _U_,
+                                 void *user_data _U_)
 {
   return NGHTTP2_ERR_EOF;
 }
 
-static ssize_t accumulator_send_callback(nghttp2_session *session,
+static ssize_t accumulator_send_callback(nghttp2_session *session _U_,
                                          const uint8_t *buf, size_t len,
-                                         int flags, void* user_data)
+                                         int flags _U_, void* user_data)
 {
   accumulator *acc = ((my_user_data*)user_data)->acc;
   assert(acc->length+len < sizeof(acc->buf));
@@ -157,8 +157,8 @@ static ssize_t accumulator_send_callback(nghttp2_session *session,
   return len;
 }
 
-static int on_begin_frame_callback(nghttp2_session *session,
-                                   const nghttp2_frame_hd *hd,
+static int on_begin_frame_callback(nghttp2_session *session  _U_,
+                                   const nghttp2_frame_hd *hd _U_,
                                    void *user_data)
 {
   my_user_data *ud = (my_user_data*)user_data;
@@ -166,7 +166,7 @@ static int on_begin_frame_callback(nghttp2_session *session,
   return 0;
 }
 
-static int on_frame_recv_callback(nghttp2_session *session,
+static int on_frame_recv_callback(nghttp2_session *session _U_,
                                   const nghttp2_frame *frame,
                                   void *user_data)
 {
@@ -176,9 +176,9 @@ static int on_frame_recv_callback(nghttp2_session *session,
   return 0;
 }
 
-static int on_invalid_frame_recv_callback(nghttp2_session *session,
-                                          const nghttp2_frame *frame,
-                                          nghttp2_error_code error_code,
+static int on_invalid_frame_recv_callback(nghttp2_session *session _U_,
+                                          const nghttp2_frame *frame _U_,
+                                          nghttp2_error_code error_code _U_,
                                           void *user_data)
 {
   my_user_data *ud = (my_user_data*)user_data;
@@ -186,7 +186,7 @@ static int on_invalid_frame_recv_callback(nghttp2_session *session,
   return 0;
 }
 
-static int on_frame_send_callback(nghttp2_session *session,
+static int on_frame_send_callback(nghttp2_session *session _U_,
                                   const nghttp2_frame *frame,
                                   void *user_data)
 {
@@ -196,7 +196,7 @@ static int on_frame_send_callback(nghttp2_session *session,
   return 0;
 }
 
-static int on_frame_not_send_callback(nghttp2_session *session,
+static int on_frame_not_send_callback(nghttp2_session *session _U_,
                                       const nghttp2_frame *frame,
                                       int lib_error,
                                       void *user_data)
@@ -208,9 +208,9 @@ static int on_frame_not_send_callback(nghttp2_session *session,
   return 0;
 }
 
-static int on_data_chunk_recv_callback(nghttp2_session *session,
-                                       uint8_t flags, int32_t stream_id,
-                                       const uint8_t *data, size_t len,
+static int on_data_chunk_recv_callback(nghttp2_session *session _U_,
+                                       uint8_t flags _U_, int32_t stream_id _U_,
+                                       const uint8_t *data _U_, size_t len,
                                        void *user_data)
 {
   my_user_data *ud = (my_user_data*)user_data;
@@ -219,9 +219,9 @@ static int on_data_chunk_recv_callback(nghttp2_session *session,
   return 0;
 }
 
-static int pause_on_data_chunk_recv_callback(nghttp2_session *session,
-                                             uint8_t flags, int32_t stream_id,
-                                             const uint8_t *data, size_t len,
+static int pause_on_data_chunk_recv_callback(nghttp2_session *session _U_,
+                                             uint8_t flags _U_, int32_t stream_id _U_,
+                                             const uint8_t *data _U_, size_t len _U_,
                                              void *user_data)
 {
   my_user_data *ud = (my_user_data*)user_data;
@@ -229,7 +229,7 @@ static int pause_on_data_chunk_recv_callback(nghttp2_session *session,
   return NGHTTP2_ERR_PAUSE;
 }
 
-static ssize_t select_padding_callback(nghttp2_session *session,
+static ssize_t select_padding_callback(nghttp2_session *session _U_,
                                        const nghttp2_frame *frame,
                                        size_t max_payloadlen,
                                        void *user_data)
@@ -239,23 +239,23 @@ static ssize_t select_padding_callback(nghttp2_session *session,
 }
 
 static ssize_t too_large_data_source_length_callback
-(nghttp2_session *session, uint8_t frame_type, int32_t stream_id,
- int32_t session_remote_window_size, int32_t stream_remote_window_size,
- uint32_t remote_max_frame_size, void *user_data) {
+(nghttp2_session *session _U_, uint8_t frame_type _U_, int32_t stream_id _U_,
+ int32_t session_remote_window_size _U_, int32_t stream_remote_window_size _U_,
+ uint32_t remote_max_frame_size _U_, void *user_data _U_) {
     return NGHTTP2_MAX_FRAME_SIZE_MAX + 1;
 }
 
 static ssize_t smallest_length_data_source_length_callback
-(nghttp2_session *session, uint8_t frame_type, int32_t stream_id,
- int32_t session_remote_window_size, int32_t stream_remote_window_size,
- uint32_t remote_max_frame_size, void *user_data) {
+(nghttp2_session *session _U_, uint8_t frame_type _U_, int32_t stream_id _U_,
+ int32_t session_remote_window_size _U_, int32_t stream_remote_window_size _U_,
+ uint32_t remote_max_frame_size _U_, void *user_data _U_) {
     return 1;
 }
 
 static ssize_t fixed_length_data_source_read_callback
-(nghttp2_session *session, int32_t stream_id,
- uint8_t *buf, size_t len, uint32_t *data_flags,
- nghttp2_data_source *source, void *user_data)
+(nghttp2_session *session _U_, int32_t stream_id _U_,
+ uint8_t *buf _U_, size_t len, uint32_t *data_flags,
+ nghttp2_data_source *source _U_, void *user_data)
 {
   my_user_data *ud = (my_user_data*)user_data;
   size_t wlen;
@@ -272,17 +272,17 @@ static ssize_t fixed_length_data_source_read_callback
 }
 
 static ssize_t temporal_failure_data_source_read_callback
-(nghttp2_session *session, int32_t stream_id,
- uint8_t *buf, size_t len, uint32_t *data_flags,
- nghttp2_data_source *source, void *user_data)
+(nghttp2_session *session _U_, int32_t stream_id _U_,
+ uint8_t *buf _U_, size_t len _U_, uint32_t *data_flags _U_,
+ nghttp2_data_source *source _U_, void *user_data _U_)
 {
   return NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE;
 }
 
 static ssize_t fail_data_source_read_callback
-(nghttp2_session *session, int32_t stream_id,
- uint8_t *buf, size_t len, uint32_t *data_flags,
- nghttp2_data_source *source, void *user_data)
+(nghttp2_session *session _U_, int32_t stream_id _U_,
+ uint8_t *buf _U_, size_t len _U_, uint32_t *data_flags _U_,
+ nghttp2_data_source *source _U_, void *user_data _U_)
 {
   return NGHTTP2_ERR_CALLBACK_FAILURE;
 }
@@ -297,9 +297,9 @@ static ssize_t fail_data_source_read_callback
 /*   ++my_data->stream_close_cb_called; */
 /* } */
 
-static ssize_t block_count_send_callback(nghttp2_session* session,
-                                         const uint8_t *data, size_t len,
-                                         int flags,
+static ssize_t block_count_send_callback(nghttp2_session* session _U_,
+                                         const uint8_t *data _U_, size_t len,
+                                         int flags _U_,
                                          void *user_data)
 {
   my_user_data *ud = (my_user_data*)user_data;
@@ -313,11 +313,11 @@ static ssize_t block_count_send_callback(nghttp2_session* session,
   return r;
 }
 
-static int on_header_callback(nghttp2_session *session,
+static int on_header_callback(nghttp2_session *session _U_,
                               const nghttp2_frame *frame,
                               const uint8_t *name, size_t namelen,
                               const uint8_t *value, size_t valuelen,
-                              uint8_t flags,
+                              uint8_t flags _U_,
                               void *user_data)
 {
   my_user_data *ud = (my_user_data*)user_data;
@@ -356,8 +356,8 @@ static int temporal_failure_on_header_callback
   return NGHTTP2_ERR_TEMPORAL_CALLBACK_FAILURE;
 }
 
-static int on_begin_headers_callback(nghttp2_session *session,
-                                     const nghttp2_frame *frame,
+static int on_begin_headers_callback(nghttp2_session *session _U_,
+                                     const nghttp2_frame *frame _U_,
                                      void *user_data)
 {
   my_user_data *ud = (my_user_data*)user_data;
@@ -366,15 +366,15 @@ static int on_begin_headers_callback(nghttp2_session *session,
 }
 
 static ssize_t defer_data_source_read_callback
-(nghttp2_session *session, int32_t stream_id,
- uint8_t *buf, size_t len, uint32_t *data_flags,
- nghttp2_data_source *source, void *user_data)
+(nghttp2_session *session _U_, int32_t stream_id _U_,
+ uint8_t *buf _U_, size_t len _U_, uint32_t *data_flags _U_,
+ nghttp2_data_source *source _U_, void *user_data _U_)
 {
   return NGHTTP2_ERR_DEFERRED;
 }
 
 static int stream_close_callback(nghttp2_session *session, int32_t stream_id,
-                                 nghttp2_error_code error_code,
+                                 nghttp2_error_code error_code _U_,
                                  void *user_data)
 {
   my_user_data* my_data = (my_user_data*)user_data;
@@ -3307,9 +3307,9 @@ void test_nghttp2_submit_data_read_length_smallest(void)
 }
 
 static ssize_t submit_data_twice_data_source_read_callback
-(nghttp2_session *session, int32_t stream_id,
- uint8_t *buf, size_t len, uint32_t *data_flags,
- nghttp2_data_source *source, void *user_data)
+(nghttp2_session *session _U_, int32_t stream_id _U_,
+ uint8_t *buf _U_, size_t len, uint32_t *data_flags,
+ nghttp2_data_source *source _U_, void *user_data _U_)
 {
   *data_flags |= NGHTTP2_DATA_FLAG_EOF;
   return nghttp2_min(len, 16);
@@ -3317,7 +3317,7 @@ static ssize_t submit_data_twice_data_source_read_callback
 
 static int submit_data_twice_on_frame_send_callback(nghttp2_session *session,
                                                     const nghttp2_frame *frame,
-                                                    void *user_data)
+                                                    void *user_data _U_)
 {
   static int called = 0;
   int rv;