diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 6d7ecff08d8f8eedc2bc948fe6ba026bdfd12224..486f4e913057af02c22a6acd6c554bd7b76801b3 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -245,6 +245,7 @@ target_compile_options(SRB2SDL2 PRIVATE
 	# This is a direct translation from versions.mk
 	$<$<AND:$<COMPILE_LANGUAGE:C>,$<C_COMPILER_ID:GNU>>:
 		-Wall
+		-Wextra
 		-Wno-trigraphs
 		-W # Was controlled by RELAXWARNINGS
 		-Wfloat-equal
@@ -325,7 +326,9 @@ target_compile_options(SRB2SDL2 PRIVATE
 	$<$<AND:$<COMPILE_LANGUAGE:C>,$<OR:$<C_COMPILER_ID:AppleClang>,$<C_COMPILER_ID:Clang>>>:
 		-Wall
 		-Wno-absolute-value
+		-Wextra
 		-Wno-trigraphs
+		-Wconditional-uninitialized
 		-Wno-error=non-literal-null-conversion
 		-Wno-error=constant-conversion
 		-Wno-error=unused-but-set-variable
@@ -342,6 +345,9 @@ target_compile_options(SRB2SDL2 PRIVATE
 	# C++, GNU, Clang and Apple Clang
 	$<$<AND:$<COMPILE_LANGUAGE:CXX>,$<OR:$<C_COMPILER_ID:GNU>,$<C_COMPILER_ID:AppleClang>,$<C_COMPILER_ID:Clang>>>:
 		-Wall
+		-Wextra
+		-Wno-trigraphs
+		-Wconditional-uninitialized
 	>
 
 	# C++, MSVC
diff --git a/src/blua/lvm.c b/src/blua/lvm.c
index b74fef4ee3d72d3081fdb11776ab759c337020f3..627685b389dcdea401dffc72ed02ef061394a1b6 100644
--- a/src/blua/lvm.c
+++ b/src/blua/lvm.c
@@ -134,7 +134,7 @@ void luaV_gettable (lua_State *L, TValue *t, TValue *key, StkId val) {
 void luaV_settable (lua_State *L, TValue *t, TValue *key, StkId val) {
   int loop;
   for (loop = 0; loop < MAXTAGLOOP; loop++) {
-    TValue *tm;
+    TValue *tm = NULL;
     if (ttistable(t)) {  /* `t' is a table? */
       Table *h = hvalue(t);
       TValue *oldval = luaH_set(L, h, key); /* do a primitive set */
diff --git a/src/g_demo.c b/src/g_demo.c
index f64f34168efd8286ba21199841be95bc60fceb3d..859ac5cba25b2215875e19bdea586160a6989e28 100644
--- a/src/g_demo.c
+++ b/src/g_demo.c
@@ -1650,7 +1650,7 @@ static void G_LoadDemoExtraFiles(UINT8 **pp, UINT16 this_demo_version)
 	UINT16 totalfiles;
 	char filename[MAX_WADPATH];
 	UINT8 md5sum[16];
-	filestatus_t ncs;
+	filestatus_t ncs = FS_NOTFOUND;
 	boolean toomany = false;
 	boolean alreadyloaded;
 	UINT16 i, j;
diff --git a/src/g_game.c b/src/g_game.c
index 6a99381e741544030ad9208f93860ffbc0f249d6..0e7e2b344e9d08b78df1246d5af45cc8e26696f5 100644
--- a/src/g_game.c
+++ b/src/g_game.c
@@ -5403,7 +5403,7 @@ void G_FreeMapSearch(mapsearchfreq_t *freq, INT32 freqc)
 INT32 G_FindMapByNameOrCode(const char *mapname, char **realmapnamep)
 {
 	boolean usemapcode = false;
-	INT32 newmapnum;
+	INT32 newmapnum = -1;
 	size_t mapnamelen = strlen(mapname);
 	char *p;
 
diff --git a/src/hardware/r_opengl/r_opengl.c b/src/hardware/r_opengl/r_opengl.c
index acd09f614318f433567a5764f2cf453d26315e9d..75a92c2fb33ad542b0931786a9b4688a9f9dccbc 100644
--- a/src/hardware/r_opengl/r_opengl.c
+++ b/src/hardware/r_opengl/r_opengl.c
@@ -180,7 +180,7 @@ FUNCPRINTF void GL_DBG_Printf(const char *format, ...)
 // GL_MSG_Warning   : Raises a warning.
 // -----------------+
 
-static void GL_MSG_Warning(const char *format, ...)
+FUNCPRINTF static void GL_MSG_Warning(const char *format, ...)
 {
 	char str[4096] = "";
 	va_list arglist;
@@ -203,7 +203,7 @@ static void GL_MSG_Warning(const char *format, ...)
 // GL_MSG_Error     : Raises an error.
 // -----------------+
 
-static void GL_MSG_Error(const char *format, ...)
+FUNCPRINTF static void GL_MSG_Error(const char *format, ...)
 {
 	char str[4096] = "";
 	va_list arglist;
diff --git a/src/netcode/d_net.c b/src/netcode/d_net.c
index 5a2e229d3f47c9eadf66729a1250a1d2b0719343..b24409db158909970a73fb5bd7630500a4f3d63f 100644
--- a/src/netcode/d_net.c
+++ b/src/netcode/d_net.c
@@ -940,7 +940,7 @@ void Command_Droprate(void)
 static boolean ShouldDropPacket(void)
 {
 	return (packetdropquantity[netbuffer->packettype])
-		|| (packetdroprate != 0 && rand() < (RAND_MAX * (packetdroprate / 100.f))) || packetdroprate == 100;
+		|| (packetdroprate != 0 && rand() < (((double)RAND_MAX) * (packetdroprate / 100.f))) || packetdroprate == 100;
 }
 #endif
 
diff --git a/src/netcode/http-mserv.c b/src/netcode/http-mserv.c
index 2b52380cf506f00f119dc01b5f0ab3b47ff4e6f6..b8c662a51e215965470310d758331236359378b7 100644
--- a/src/netcode/http-mserv.c
+++ b/src/netcode/http-mserv.c
@@ -134,7 +134,7 @@ HMS_on_read (char *s, size_t _1, size_t n, void *userdata)
 	return n;
 }
 
-static struct HMS_buffer *
+FUNCDEBUG static struct HMS_buffer *
 HMS_connect (int proto, const char *format, ...)
 {
 	va_list ap;
diff --git a/src/p_user.c b/src/p_user.c
index 49b4b2c94468c332016dd9125298dff1b41ebc7c..9fc1f8c9b4dcbf9dcb7f589b5283f474ff5ac843 100644
--- a/src/p_user.c
+++ b/src/p_user.c
@@ -11307,7 +11307,7 @@ void P_DoTailsOverlay(player_t *player, mobj_t *tails)
 	fixed_t backwards = -1*FRACUNIT;
 	boolean doswim = (player->panim == PA_ABILITY && (player->mo->eflags & MFE_UNDERWATER));
 	boolean doroll = (player->panim == PA_ROLL || (player->panim == PA_JUMP && !(player->charflags & SF_NOJUMPSPIN)) || doswim);
-	angle_t rollangle;
+	angle_t rollangle = 0;
 	boolean panimchange;
 	INT32 ticnum = 0;
 	statenum_t chosenstate;
@@ -12089,7 +12089,7 @@ void P_PlayerThink(player_t *player)
 	// deez New User eXperiences.
 	{
 		angle_t oldang = player->drawangle, diff = 0;
-		UINT8 factor;
+		UINT8 factor = 0;
 		// Directionchar!
 		// Camera angle stuff.
 		if (player->exiting // no control, no modification
diff --git a/src/r_translation.c b/src/r_translation.c
index 3905458026df20945e566861fc6c7a087bcab244..a53b30e9f6fc6196b72df18b3f6063534bda3219 100644
--- a/src/r_translation.c
+++ b/src/r_translation.c
@@ -556,7 +556,7 @@ static boolean ParseDecimal(tokenizer_t *sc, double *out)
 	return M_StringToDecimal(tkn, out);
 }
 
-static struct PaletteRemapParseResult *ThrowError(const char *format, ...)
+FUNCPRINTF static struct PaletteRemapParseResult *ThrowError(const char *format, ...)
 {
 	const size_t err_size = 512 * sizeof(char);
 
@@ -792,7 +792,7 @@ static struct PaletteRemapParseResult *PaletteRemap_ParseTranslation(const char
 	return result;
 }
 
-static void PrintError(const char *name, const char *format, ...)
+FUNCDEBUG static void PrintError(const char *name, const char *format, ...)
 {
 	char error[256];
 
diff --git a/src/sdl/i_system.c b/src/sdl/i_system.c
index 8db6a487c14fce730aaddeebd760c4814bddefef..9f0fa0250a664dfe020d649ac65ccb347a63a3c8 100644
--- a/src/sdl/i_system.c
+++ b/src/sdl/i_system.c
@@ -2857,7 +2857,7 @@ size_t I_GetRandomBytes(char *destination, size_t count)
 {
 #if defined (__unix__) || defined (UNIXCOMMON) || defined(__APPLE__)
 	FILE *rndsource;
-	size_t actual_bytes;
+	size_t actual_bytes = 0;
 
 	if (!(rndsource = fopen("/dev/urandom", "r")))
 		if (!(rndsource = fopen("/dev/random", "r")))
diff --git a/src/st_stuff.c b/src/st_stuff.c
index 342c7dcd824b71a9e044fef37114abb7e322e70d..b3e392499e7db22dd027ead283b82de5b5a1ed25 100644
--- a/src/st_stuff.c
+++ b/src/st_stuff.c
@@ -815,7 +815,7 @@ static inline void ST_drawRings(void)
 
 static void ST_drawLivesArea(void)
 {
-	INT32 v_colmap = V_YELLOWMAP, livescount;
+	INT32 v_colmap = V_YELLOWMAP, livescount = -1;
 	boolean notgreyedout = false;
 
 	if (!stplyr->skincolor)