diff --git a/CMakeLists.txt b/CMakeLists.txt
index d97f19424b..faa463d0c6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -427,10 +427,11 @@ if( CMAKE_C_COMPILER_ID MATCHES "MSVC")
 	# Additional compiler warnings to be treated as "Level 3"
 	# when compiling Wireshark sources. (Selected from "level 4" warnings).
 	## 4295: array is too small to include a terminating null character
+	## 4100: unreferenced formal parameter
 	## 4189: local variable is initialized but not referenced
 	# Disable warnings about about use of flexible array members:
 	## 4200: nonstandard extension used : zero-sized array in struct/union
-	list(APPEND LOCAL_CFLAGS /w34295 /w34189 /wd4200)
+	list(APPEND LOCAL_CFLAGS /w34295 /w34100 /w34189 /wd4200)
 
 	# We've matched these to specific compiler versions using the
 	# checks above. There's no need to pass them to check_c_compiler_flag
diff --git a/caputils/capture-pcap-util.c b/caputils/capture-pcap-util.c
index 80304011d1..4a743558bc 100644
--- a/caputils/capture-pcap-util.c
+++ b/caputils/capture-pcap-util.c
@@ -823,12 +823,13 @@ linktype_name_to_val(const char *linktype)
  * rest-of-the-universe libpcap.
  */
 int
-get_pcap_datalink(pcap_t *pch, const char *devicename
-#ifndef _AIX
-    _U_)
+get_pcap_datalink(pcap_t *pch,
+#ifdef _AIX
+    const char* devicename
 #else
-    )
+    const char* devicename _U_
 #endif
+    )
 {
 	int datalink;
 #ifdef _AIX
@@ -1257,11 +1258,11 @@ get_if_capabilities_pcap_create(interface_options *interface_opts,
 }
 
 pcap_t *
-open_capture_device_pcap_create(capture_options *capture_opts
+open_capture_device_pcap_create(
 #if defined(HAVE_PCAP_SET_TSTAMP_PRECISION) || defined (HAVE_PCAP_SET_TSTAMP_TYPE)
-    ,
+    capture_options* capture_opts,
 #else
-    _U_,
+    capture_options* capture_opts _U_,
 #endif
     interface_options *interface_opts, int timeout,
     cap_device_open_err *open_err,
diff --git a/dumpcap.c b/dumpcap.c
index 31a6ab8b05..fa20b402b8 100644
--- a/dumpcap.c
+++ b/dumpcap.c
@@ -591,9 +591,10 @@ relinquish_all_capabilities(void)
 
 static const char *
 get_pcap_failure_secondary_error_message(cap_device_open_err open_err,
+#ifdef __hpux
                                          const char *open_err_str
-#ifndef __hpux
-                                                                  _U_
+#else
+                                         const char* open_err_str _U_
 #endif
                                          )
 {
diff --git a/ui/win32/file_dlg_win32.c b/ui/win32/file_dlg_win32.c
index 6b82e8b7f2..6a8a9201e2 100644
--- a/ui/win32/file_dlg_win32.c
+++ b/ui/win32/file_dlg_win32.c
@@ -1477,7 +1477,7 @@ save_as_file_hook_proc(HWND sf_hwnd, UINT msg, WPARAM w_param, LPARAM l_param) {
 }
 
 static UINT_PTR CALLBACK
-save_as_statstree_hook_proc(HWND sf_hwnd, UINT msg, WPARAM w_param, LPARAM l_param) {
+save_as_statstree_hook_proc(HWND sf_hwnd, UINT msg, WPARAM w_param _U_, LPARAM l_param _U_) {
 
     switch(msg) {
         case WM_INITDIALOG:
diff --git a/ws_attributes.h b/ws_attributes.h
index 7e2245b9c3..d7c4445323 100644
--- a/ws_attributes.h
+++ b/ws_attributes.h
@@ -27,6 +27,8 @@ extern "C" {
 #if defined(__GNUC__)
   /* This includes clang */
   #define _U_ __attribute__((unused))
+#elif defined(_MSC_VER)
+  #define _U_ __pragma(warning(suppress:4100))
 #else
   #define _U_
 #endif
diff --git a/wsutil/file_util.c b/wsutil/file_util.c
index 745e2f3b43..a8a2a6bd2a 100644
--- a/wsutil/file_util.c
+++ b/wsutil/file_util.c
@@ -57,6 +57,7 @@
 #include <stdlib.h>
 
 #include "file_util.h"
+#include "ws_attributes.h"
 
 static gchar *program_path = NULL;
 static gchar *system_path = NULL;
@@ -189,7 +190,7 @@ ws_stdio_rename (const gchar *oldfilename, const gchar *newfilename)
  * Since: 2.6
  */
 int
-ws_stdio_mkdir (const gchar *filename, int mode)
+ws_stdio_mkdir (const gchar *filename, int mode _U_)
 {
     wchar_t *wfilename = g_utf8_to_utf16 (filename, -1, NULL, NULL, NULL);
     int retval;
diff --git a/wsutil/filesystem.c b/wsutil/filesystem.c
index 72b3c9bf94..f56b4f0c48 100644
--- a/wsutil/filesystem.c
+++ b/wsutil/filesystem.c
@@ -469,9 +469,11 @@ get_executable_path(void)
  * g_mallocated string containing an error on failure.
  */
 char *
-init_progfile_dir(const char *arg0
+init_progfile_dir(
 #ifdef _WIN32
-    _U_
+    const char* arg0 _U_
+#else
+    const char* arg0
 #endif
 )
 {
diff --git a/wsutil/getopt_long.c b/wsutil/getopt_long.c
index c1f2b2a4cd..6e4d0070a8 100644
--- a/wsutil/getopt_long.c
+++ b/wsutil/getopt_long.c
@@ -19,6 +19,7 @@
 #endif
 
 #include "config.h"
+#include "ws_attributes.h"
 
 #include <stdio.h>
 
@@ -271,7 +272,13 @@ exchange (char **argv, struct _getopt_data *d)
 /* Initialize the internal data when the first call is made.  */
 
 static const char *
-_getopt_initialize (int argc, char *const *argv, const char *optstring,
+_getopt_initialize (
+#if defined _LIBC && defined USE_NONOPTION_FLAGS
+                    int argc, char *const *argv,
+#else
+                    int argc _U_, char *const *argv _U_,
+#endif
+                    const char *optstring,
                     struct _getopt_data *d, int posixly_correct)
 {
   /* Start processing options with ARGV-element 1 (since ARGV-element 0