diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4e83018c401b3..2c1443daf5d5d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -212,7 +212,9 @@ IF(MSVC)
 		add_definitions(-D_X86_)
 	endif()
 ELSE()
-	SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror -Wall -Wextra -Woverloaded-virtual -Wpedantic -std=c++14")
+	SET(CATA_WARNINGS
+		"-Werror -Wall -Wextra -Woverloaded-virtual -Wpedantic -Wmissing-declarations")
+	SET(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${CATA_WARNINGS} -std=c++14")
 	SET(CMAKE_CXX_FLAGS_DEBUG "-Og -g")
 ENDIF()
 
diff --git a/Makefile b/Makefile
index daf3e07e178c2..533710fc720ea 100644
--- a/Makefile
+++ b/Makefile
@@ -83,7 +83,7 @@
 # PROFILE is for use with gprof or a similar program -- don't bother generally.
 # RELEASE_FLAGS is flags for release builds.
 RELEASE_FLAGS =
-WARNINGS = -Werror -Wall -Wextra -Woverloaded-virtual -Wpedantic
+WARNINGS = -Werror -Wall -Wextra -Woverloaded-virtual -Wpedantic -Wmissing-declarations
 # Uncomment below to disable warnings
 #WARNINGS = -w
 DEBUGSYMS = -g
diff --git a/src/debug.cpp b/src/debug.cpp
index 6bf29dccde8ee..4b0356966d19b 100644
--- a/src/debug.cpp
+++ b/src/debug.cpp
@@ -822,17 +822,41 @@ std::string game_info::operating_system()
 #endif
 }
 
+#if defined (__linux__) || defined(unix) || defined(__unix__) || defined(__unix) || ( defined(__APPLE__) && defined(__MACH__) ) || defined(BSD) // linux; unix; MacOs; BSD
+/** Execute a command with the shell by using `popen()`.
+ * @param command The full command to execute.
+ * @note The output buffer is limited to 512 characters.
+ * @returns The result of the command (only stdout) or an empty string if there was a problem.
+ */
+static std::string shell_exec( const std::string &command )
+{
+    std::vector<char> buffer( 512 );
+    std::string output;
+    try {
+        std::unique_ptr<FILE, decltype( &pclose )> pipe( popen( command.c_str(), "r" ), pclose );
+        if( pipe ) {
+            while( fgets( buffer.data(), buffer.size(), pipe.get() ) != nullptr ) {
+                output += buffer.data();
+            }
+        }
+    } catch( ... ) {
+        output = "";
+    }
+    return output;
+}
+#endif
+
+#if defined (__ANDROID__)
 /** Get a precise version number for Android systems.
  * @note see:
  *    - https://stackoverflow.com/a/19777977/507028
  *    - https://github.com/pytorch/cpuinfo/blob/master/test/build.prop/galaxy-s7-us.log
  * @returns If successful, a string containing the Android system version, otherwise an empty string.
  */
-std::string android_version()
+static std::string android_version()
 {
     std::string output;
 
-#if defined (__ANDROID__)
     // buffer used for the __system_property_get() function.
     // note: according to android sources, it can't be greater than 92 chars (see 'PROP_VALUE_MAX' define in system_properties.h)
     std::vector<char> buffer( 255 );
@@ -859,60 +883,36 @@ std::string android_version()
         }
         output.append( string_format( "%s: %s; ", entry.second, value ) );
     }
-#endif
     return output;
 }
 
-#if defined (__linux__) || defined(unix) || defined(__unix__) || defined(__unix) || ( defined(__APPLE__) && defined(__MACH__) ) || defined(BSD) // linux; unix; MacOs; BSD
-/** Execute a command with the shell by using `popen()`.
- * @param command The full command to execute.
- * @note The output buffer is limited to 512 characters.
- * @returns The result of the command (only stdout) or an empty string if there was a problem.
- */
-std::string shell_exec( const std::string &command )
-{
-    std::vector<char> buffer( 512 );
-    std::string output;
-    try {
-        std::unique_ptr<FILE, decltype( &pclose )> pipe( popen( command.c_str(), "r" ), pclose );
-        if( pipe ) {
-            while( fgets( buffer.data(), buffer.size(), pipe.get() ) != nullptr ) {
-                output += buffer.data();
-            }
-        }
-    } catch( ... ) {
-        output = "";
-    }
-    return output;
-}
-#endif
+#elif defined(BSD)
 
 /** Get a precise version number for BSD systems.
  * @note The code shells-out to call `uname -a`.
  * @returns If successful, a string containing the Linux system version, otherwise an empty string.
  */
-std::string bsd_version()
+static std::string bsd_version()
 {
     std::string output;
-#if defined(BSD)
     output = shell_exec( "uname -a" );
     if( !output.empty() ) {
         // remove trailing '\n', if any.
         output.erase( std::remove( output.begin(), output.end(), '\n' ),
                       output.end() );
     }
-#endif
     return output;
 }
 
+#elif defined(__linux__)
+
 /** Get a precise version number for Linux systems.
  * @note The code shells-out to call `lsb_release -a`.
  * @returns If successful, a string containing the Linux system version, otherwise an empty string.
  */
-std::string linux_version()
+static std::string linux_version()
 {
     std::string output;
-#if defined(__linux__)
     output = shell_exec( "lsb_release -a" );
     if( !output.empty() ) {
         // replace '\n' and '\t' in output.
@@ -927,18 +927,18 @@ std::string linux_version()
             }
         }
     }
-#endif
     return output;
 }
 
+#elif defined(__APPLE__) && defined(__MACH__) && !defined(BSD)
+
 /** Get a precise version number for MacOs systems.
  * @note The code shells-out to call `sw_vers` with various options.
  * @returns If successful, a string containing the MacOS system version, otherwise an empty string.
  */
-std::string mac_os_version()
+static std::string mac_os_version()
 {
     std::string output;
-#if defined(__APPLE__) && defined(__MACH__) && !defined(BSD)
     std::vector<std::pair<std::string,  std::string>> commands = {
         { "sw_vers -productName", "Name" },
         { "sw_vers -productVersion", "Version" },
@@ -956,20 +956,20 @@ std::string mac_os_version()
         }
         output.append( string_format( "%s: %s; ", entry.second, command_result ) );
     }
-#endif
     return output;
 }
 
+#elif defined (_WIN32)
+
 /** Get a precise version number for Windows systems.
  * @note Since Windows 10 all version-related APIs lie about the underlying system if the application is not Manifested (see VerifyVersionInfoA
  *     or GetVersionEx documentation for further explanation). In this function we use the registry or the native RtlGetVersion which both
  *     report correct versions and are compatible down to XP.
  * @returns If successful, a string containing the Windows system version number, otherwise an empty string.
  */
-std::string windows_version()
+static std::string windows_version()
 {
     std::string output;
-#if defined (_WIN32)
     HKEY handle_key;
     bool success = RegOpenKeyExA( HKEY_LOCAL_MACHINE, R"(SOFTWARE\Microsoft\Windows NT\CurrentVersion)",
                                   0,
@@ -1026,9 +1026,9 @@ std::string windows_version()
             }
         }
     }
-#endif
     return output;
 }
+#endif // Various OS define tests
 
 std::string game_info::operating_system_version()
 {
diff --git a/src/wincurse.cpp b/src/wincurse.cpp
index 02f39bd313209..fd8fbfa19e53b 100644
--- a/src/wincurse.cpp
+++ b/src/wincurse.cpp
@@ -62,7 +62,7 @@ static int TERMINAL_HEIGHT;
 // Declare this locally, because it's not generally cross-compatible in cursesport.h
 LRESULT CALLBACK ProcessMessages( HWND__ *hWnd, std::uint32_t Msg, WPARAM wParam, LPARAM lParam );
 
-std::wstring widen( const std::string &s )
+static std::wstring widen( const std::string &s )
 {
     if( s.empty() ) {
         // MultiByteToWideChar can not handle this case
@@ -76,7 +76,7 @@ std::wstring widen( const std::string &s )
 }
 
 // Registers, creates, and shows the Window!!
-bool WinCreate()
+static bool WinCreate()
 {
     // Get current process handle
     WindowINST = GetModuleHandle( 0 );
@@ -128,7 +128,7 @@ bool WinCreate()
 }
 
 // Unregisters, releases the DC if needed, and destroys the window.
-void WinDestroy()
+static void WinDestroy()
 {
     if( ( WindowDC != NULL ) && ( ReleaseDC( WindowHandle, WindowDC ) == 0 ) ) {
         WindowDC = 0;
@@ -142,7 +142,7 @@ void WinDestroy()
 }
 
 // Creates a backbuffer to prevent flickering
-void create_backbuffer()
+static void create_backbuffer()
 {
     if( WindowDC != NULL ) {
         ReleaseDC( WindowHandle, WindowDC );
@@ -207,7 +207,7 @@ static void begin_alt_code()
     alt_buffer_len = 0;
 }
 
-void add_alt_code( char c )
+static void add_alt_code( char c )
 {
     // Not exactly how it works, but acceptable
     if( c >= '0' && c <= '9' ) {
@@ -534,7 +534,7 @@ void cata_cursesport::curses_drawwindow( const catacurses::window &w )
 }
 
 // Check for any window messages (keypress, paint, mousemove, etc)
-void CheckMessages()
+static void CheckMessages()
 {
     MSG msg;
     while( PeekMessage( &msg, 0, 0, 0, PM_REMOVE ) ) {
@@ -636,7 +636,7 @@ void catacurses::init_interface()
 }
 
 // A very accurate and responsive timer (NEVER use GetTickCount)
-uint64_t GetPerfCount()
+static uint64_t GetPerfCount()
 {
     uint64_t Count;
     QueryPerformanceCounter( ( PLARGE_INTEGER )&Count );
diff --git a/tests/test_main.cpp b/tests/test_main.cpp
index e830dbca7dba6..39ad2c8354228 100644
--- a/tests/test_main.cpp
+++ b/tests/test_main.cpp
@@ -212,7 +212,7 @@ static option_overrides_t extract_option_overrides( std::vector<const char *> &a
     return ret;
 }
 
-std::string extract_user_dir( std::vector<const char *> &arg_vec )
+static std::string extract_user_dir( std::vector<const char *> &arg_vec )
 {
     std::string option_user_dir = extract_argument( arg_vec, "--user-dir=" );
     if( option_user_dir.empty() ) {
diff --git a/tools/format/getpost.h b/tools/format/getpost.h
index 87365fb4708e4..3e584cb837db8 100644
--- a/tools/format/getpost.h
+++ b/tools/format/getpost.h
@@ -31,7 +31,7 @@ THE SOFTWARE.
 #include <map>
 #include <new>
 
-std::string urlDecode( std::string str )
+inline std::string urlDecode( std::string str )
 {
     std::string temp;
     int i;
@@ -60,7 +60,7 @@ std::string urlDecode( std::string str )
     return temp;
 }
 
-void initializeGet( std::map <std::string, std::string> &Get )
+inline void initializeGet( std::map <std::string, std::string> &Get )
 {
     std::string tmpkey, tmpvalue;
     std::string *tmpstr = &tmpkey;
@@ -92,7 +92,7 @@ void initializeGet( std::map <std::string, std::string> &Get )
     }
 }
 
-void initializePost( std::map <std::string, std::string> &Post )
+inline void initializePost( std::map <std::string, std::string> &Post )
 {
     std::string tmpkey, tmpvalue;
     std::string *tmpstr = &tmpkey;