diff --git a/source/common/cutscenes/movieplayer.cpp b/source/common/cutscenes/movieplayer.cpp
index a13d0c91e..d808fd60b 100644
--- a/source/common/cutscenes/movieplayer.cpp
+++ b/source/common/cutscenes/movieplayer.cpp
@@ -687,7 +687,7 @@ MoviePlayer* OpenMovie(const char* filename, TArray<int>& ans, const int* framet
 	{
 		size_t nLen = strlen(filename);
 		// Strip the drive letter and retry.
-		if (nLen >= 3 && isalpha(filename[0]) && filename[1] == ':' && filename[2] == '/')
+		if (nLen >= 3 && isalpha((uint8_t)filename[0]) && filename[1] == ':' && filename[2] == '/')
 		{
 			filename += 3;
 			fr = fileSystem.OpenFileReader(filename);
diff --git a/source/common/utility/cmdlib.cpp b/source/common/utility/cmdlib.cpp
index 0dd6f7750..02ab18959 100644
--- a/source/common/utility/cmdlib.cpp
+++ b/source/common/utility/cmdlib.cpp
@@ -986,7 +986,7 @@ bool IsAbsPath(const char *name)
     if (IsSeperator(name[0])) return true;
 #ifdef _WIN32
     /* [A-Za-z]: (for Windows) */
-    if (isalpha(name[0]) && name[1] == ':')    return true;
+    if (isalpha((uint8_t)name[0]) && name[1] == ':')    return true;
 #endif /* _WIN32 */
     return 0;
 }
diff --git a/source/core/ct_chat.cpp b/source/core/ct_chat.cpp
index bbdeb8909..714022fa5 100644
--- a/source/core/ct_chat.cpp
+++ b/source/core/ct_chat.cpp
@@ -379,7 +379,7 @@ static bool DoSubstitution (FString &out, const char *in)
 		out.AppendCStrPart(a, b - a);
 
 		a = ++b;
-		while (*b && isalpha(*b))
+		while (*b && isalpha((uint8_t) * b))
 		{
 			++b;
 		}
diff --git a/source/core/searchpaths.cpp b/source/core/searchpaths.cpp
index 169c3c2d9..c09da94bf 100644
--- a/source/core/searchpaths.cpp
+++ b/source/core/searchpaths.cpp
@@ -233,7 +233,7 @@ TArray<FString> CollectSearchPaths()
 				if (nice.Len() > 0)
 				{
 #ifdef _WIN32
-					if (isalpha(nice[0] && nice[1] == ':' && nice[2] != '/')) continue;	// ignore drive relative paths because they are meaningless.
+					if (isalpha((uint8_t)nice[0] && nice[1] == ':' && nice[2] != '/')) continue;	// ignore drive relative paths because they are meaningless.
 #endif
 					// A path ending with "/*" means to add all subdirectories.
 					if (nice[nice.Len()-2] == '/' && nice[nice.Len()-1] == '*')
diff --git a/source/games/blood/src/barf.cpp b/source/games/blood/src/barf.cpp
index 634cf80e3..1a34f244c 100644
--- a/source/games/blood/src/barf.cpp
+++ b/source/games/blood/src/barf.cpp
@@ -59,7 +59,7 @@ struct define_t
 
 define_t gCmdDefines[kMaxCmdLineDefines];
 
-void addMemoryResource(char* fileName, char flags, int ID);
+void addMemoryResource(const char* fileName, int flags, int ID);
 
 struct tag_t {
 	const char* _value;
@@ -320,7 +320,7 @@ void RFS::ScriptError(const char* message)
 	char* p = _pStartLine;
 	while (*p != '\n')
 	{
-		if (isprint(*p))
+		if (isprint((uint8_t) *p))
 			msg.Push(*p);
 		else
 			msg.Push(' ');
@@ -354,7 +354,7 @@ uint8_t RFS::GetNextTag()
 	// skip any space characters
 	do {
 		Increment();
-	} while (isspace(_curChar));
+	} while (isspace((uint8_t)_curChar));
 
 	if (_curChar == '\0') {
 		return kTagEnd;
@@ -409,12 +409,12 @@ uint8_t RFS::GetNextTag()
 
 			isNegative = true;
 
-			if (!isdigit(_curChar)) {
+			if (!isdigit((uint8_t)_curChar)) {
 				UnsetMark();
 			}
 		}
 
-		if (isdigit(_curChar))
+		if (isdigit((uint8_t)_curChar))
 		{
 			// left path
 			if (_curChar == '0')
@@ -428,14 +428,14 @@ uint8_t RFS::GetNextTag()
 					while (1)
 					{
 						Increment();
-						if (!isxdigit(_curChar)) { // isxdigit() checks for a hex value
+						if (!isxdigit((uint8_t)_curChar)) { // isxdigit() checks for a hex value
 							break;
 						}
 
 						// hex version of atoi?
 						scriptValue *= 16;
-						if (!isdigit(_curChar)) {
-							scriptValue += toupper(_curChar) - 55;
+						if (!isdigit((uint8_t)_curChar)) {
+							scriptValue += toupper((uint8_t)_curChar) - 55;
 						}
 						else {
 							scriptValue += _curChar - '0';
@@ -518,7 +518,7 @@ uint8_t RFS::GetNextTag()
 
 			i = 0;
 
-			while (isalnum(_curChar))
+			while (isalnum((uint8_t)_curChar))
 			{
 				scriptBuffer[i] = _curChar;
 				SetMark();
@@ -987,7 +987,7 @@ void ParseScript(int lumpnum)
 //
 //---------------------------------------------------------------------------
 
-void addMemoryResource(char* filePath, char flags, int ID)
+void addMemoryResource(const char* filePath, int flags, int ID)
 {
 	char zDirectory[BMAX_PATH];
 	char zFilename[BMAX_PATH];
diff --git a/source/games/blood/src/inifile.cpp b/source/games/blood/src/inifile.cpp
index e2ee8d533..1cc2f6d7c 100644
--- a/source/games/blood/src/inifile.cpp
+++ b/source/games/blood/src/inifile.cpp
@@ -116,7 +116,7 @@ void IniFile::LoadRes(void *res)
         char *pBuffer = buffer;
 
         // remove whitespace from buffer
-        while (isspace(*pBuffer)) {
+        while (isspace((uint8_t)*pBuffer)) {
             pBuffer++;
         }
 
@@ -239,7 +239,7 @@ bool IniFile::FindKey(const char *key)
         assert(pEqual != NULL);
 
         // remove whitespace
-        while (isspace(*(pEqual - 1))) {
+        while (isspace((uint8_t) *(pEqual - 1))) {
             pEqual--;
         }
 
@@ -251,7 +251,7 @@ bool IniFile::FindKey(const char *key)
             // strings match
             *pEqual = c;
             _13 = ++pEqualStart;
-            while (isspace(*_13)) {
+            while (isspace((uint8_t)*_13)) {
                 _13++;
             }
 
diff --git a/source/games/blood/src/levels.cpp b/source/games/blood/src/levels.cpp
index 0c11d1517..6a4957b7e 100644
--- a/source/games/blood/src/levels.cpp
+++ b/source/games/blood/src/levels.cpp
@@ -156,7 +156,7 @@ static FString cleanPath(const char* pth)
 	FString path = pth;
 	FixPathSeperator(path);
 	if (fileSystem.FileExists(path)) return path;
-	if (path.Len() > 3 && path[1] == ':' && isalpha(path[0]) && path[2] == '/')
+	if (path.Len() > 3 && path[1] == ':' && isalpha((uint8_t)path[0]) && path[2] == '/')
 	{
 		path = path.Mid(3);
 		if (fileSystem.FileExists(path)) return path;
diff --git a/source/games/blood/src/nnexts.cpp b/source/games/blood/src/nnexts.cpp
index ea8af77c6..a8db79160 100644
--- a/source/games/blood/src/nnexts.cpp
+++ b/source/games/blood/src/nnexts.cpp
@@ -6682,7 +6682,7 @@ void useIncDecGen(DBloodActor* sourceactor, int objType, sectortype* destSect, w
 //
 //---------------------------------------------------------------------------
 
-void sprite2sectorSlope(DBloodActor* actor, sectortype* pSector, char rel, bool forcez)
+void sprite2sectorSlope(DBloodActor* actor, sectortype* pSector, int rel, bool forcez)
 {
 	int slope = 0, z = 0;
 	switch (rel) {
diff --git a/source/games/blood/src/triggers.cpp b/source/games/blood/src/triggers.cpp
index 7acc20ea4..f30cc1017 100644
--- a/source/games/blood/src/triggers.cpp
+++ b/source/games/blood/src/triggers.cpp
@@ -839,7 +839,7 @@ void PathSound(sectortype* pSector, int nSound)
 //
 //---------------------------------------------------------------------------
 
-void TranslateSector(sectortype* pSector, int a2, int a3, int a4, int a5, int a6, int a7, int a8, int a9, int a10, int a11, char bAllWalls)
+void TranslateSector(sectortype* pSector, int a2, int a3, int a4, int a5, int a6, int a7, int a8, int a9, int a10, int a11, bool bAllWalls)
 {
 	int x, y;
 	XSECTOR* pXSector = &pSector->xs();
@@ -1450,13 +1450,13 @@ int StepRotateBusy(sectortype* pSector, unsigned int a2, DBloodActor* initiator)
 	{
 		vbp = pXSector->data + marker0->int_ang();
 		int nWave = pXSector->busyWaveA;
-		TranslateSector(pSector, GetWaveValue(pXSector->busy, nWave), GetWaveValue(a2, nWave), marker0->int_pos().X, marker0->int_pos().Y, marker0->int_pos().X, marker0->int_pos().Y, pXSector->data, marker0->int_pos().X, marker0->int_pos().Y, vbp, 1);
+		TranslateSector(pSector, GetWaveValue(pXSector->busy, nWave), GetWaveValue(a2, nWave), marker0->int_pos().X, marker0->int_pos().Y, marker0->int_pos().X, marker0->int_pos().Y, pXSector->data, marker0->int_pos().X, marker0->int_pos().Y, vbp, true);
 	}
 	else
 	{
 		vbp = pXSector->data - marker0->int_ang();
 		int nWave = pXSector->busyWaveB;
-		TranslateSector(pSector, GetWaveValue(pXSector->busy, nWave), GetWaveValue(a2, nWave), marker0->int_pos().X, marker0->int_pos().Y, marker0->int_pos().X, marker0->int_pos().Y, vbp, marker0->int_pos().X, marker0->int_pos().Y, pXSector->data, 1);
+		TranslateSector(pSector, GetWaveValue(pXSector->busy, nWave), GetWaveValue(a2, nWave), marker0->int_pos().X, marker0->int_pos().Y, marker0->int_pos().X, marker0->int_pos().Y, vbp, marker0->int_pos().X, marker0->int_pos().Y, pXSector->data, true);
 	}
 	pXSector->busy = a2;
 	if (pXSector->command == kCmdLink && pXSector->txID)
@@ -1510,7 +1510,7 @@ int PathBusy(sectortype* pSector, unsigned int a2, DBloodActor* initiator)
 	if (!basepath || !marker0 || !marker1) return 0;
 
 	int nWave = marker0->xspr.wave;
-	TranslateSector(pSector, GetWaveValue(pXSector->busy, nWave), GetWaveValue(a2, nWave), basepath->int_pos().X, basepath->int_pos().Y, marker0->int_pos().X, marker0->int_pos().Y, marker0->int_ang(), marker1->int_pos().X, marker1->int_pos().Y, marker1->int_ang(), 1);
+	TranslateSector(pSector, GetWaveValue(pXSector->busy, nWave), GetWaveValue(a2, nWave), basepath->int_pos().X, basepath->int_pos().Y, marker0->int_pos().X, marker0->int_pos().Y, marker0->int_ang(), marker1->int_pos().X, marker1->int_pos().Y, marker1->int_ang(), true);
 	ZTranslateSector(pSector, pXSector, a2, nWave);
 	pXSector->busy = a2;
 	if ((a2 & 0xffff) == 0)
@@ -1555,9 +1555,9 @@ void OperateDoor(sectortype* pSector, EVENT event, BUSYID busyWave)
 			}
 		}
 		else {
-			char t = !pXSector->state; int nDelta;
+			int nDelta;
 
-			if (t) nDelta = 65536 / ClipLow((pXSector->busyTimeA * 120) / 10, 1);
+			if (!pXSector->state) nDelta = 65536 / ClipLow((pXSector->busyTimeA * 120) / 10, 1);
 			else nDelta = -65536 / ClipLow((pXSector->busyTimeB * 120) / 10, 1);
 
 			AddBusy(pSector, busyWave, nDelta);
@@ -1623,7 +1623,7 @@ void OperateTeleport(sectortype* pSector)
 		if (actor->spr.statnum == kStatDude)
 		{
 			PLAYER* pPlayer;
-			char bPlayer = actor->IsPlayerActor();
+			bool bPlayer = actor->IsPlayerActor();
 			if (bPlayer)
 				pPlayer = &gPlayer[actor->spr.type - kDudePlayer1];
 			else
diff --git a/source/games/duke/src/ccmds.cpp b/source/games/duke/src/ccmds.cpp
index 08068e09d..7ab62daa6 100644
--- a/source/games/duke/src/ccmds.cpp
+++ b/source/games/duke/src/ccmds.cpp
@@ -73,7 +73,7 @@ static int ccmd_spawn(CCmdFuncPtr parm)
 		pal = (uint8_t)atol(parm->parms[1]); set |= 1;
 		[[fallthrough]];
 	case 1: // tile number
-		if (isdigit(parm->parms[0][0])) {
+		if (isdigit((uint8_t)parm->parms[0][0])) {
 			picnum = (unsigned short)atol(parm->parms[0]);
 		}
 		else {
diff --git a/source/games/duke/src/gamedef.cpp b/source/games/duke/src/gamedef.cpp
index 307b38831..1c41fe24e 100644
--- a/source/games/duke/src/gamedef.cpp
+++ b/source/games/duke/src/gamedef.cpp
@@ -407,7 +407,7 @@ bool ConCompiler::ispecial(char c)
 static bool isaltok(char c)
 {
 	// isalnum pukes on negative input.
-	return c > 0 && (isalnum(c) || c == '{' || c == '}' || c == '/' || c == '*' || c == '-' || c == '_' || c == '.');
+	return c > 0 && (isalnum((uint8_t)c) || c == '{' || c == '}' || c == '/' || c == '*' || c == '-' || c == '_' || c == '.');
 }
 
 //---------------------------------------------------------------------------
@@ -532,7 +532,7 @@ void ConCompiler::getlabel(void)
 
 	skipcomments();
 
-	while (isalnum(*textptr & 0xff) == 0)
+	while (isalnum((uint8_t) * textptr & 0xff) == 0)
 	{
 		if (*textptr == 0x0a) line_number++;
 		textptr++;
@@ -719,7 +719,7 @@ int ConCompiler::transnum(int type)
 		}
 	}
 
-	if (isdigit(*textptr) == 0 && *textptr != '-')
+	if (isdigit((uint8_t) *textptr) == 0 && *textptr != '-')
 	{
 		ReportError(ERROR_PARMUNDEFINED);
 		errorcount++;
@@ -2935,7 +2935,7 @@ int ConCompiler::parsecommand()
 #if 0
 		voxel_map[j].name[i] = 0;
 		voxel_map[j].voxnum = -2;	// flag to load later
-		while ((i < 12) && (isalnum(*textptr) || *textptr == '.'))
+		while ((i < 12) && (isalnum((uint8_t) * textptr) || *textptr == '.'))
 		{
 			voxel_map[j].name[i++] = *textptr++;
 		}
diff --git a/source/games/sw/src/sprite.cpp b/source/games/sw/src/sprite.cpp
index d08586ce5..fcefab2c7 100644
--- a/source/games/sw/src/sprite.cpp
+++ b/source/games/sw/src/sprite.cpp
@@ -4323,7 +4323,7 @@ void getzrangepoint(int x, int y, int z, sectortype* sect,
     int j, k, l, dax, day, daz, xspan, yspan, xoff, yoff;
     int x1, y1, x2, y2, x3, y3, x4, y4, cosang, sinang, tilenum;
     short cstat;
-    char clipyou;
+    uint8_t clipyou;
 
     if (sect == nullptr)
     {
diff --git a/source/games/sw/src/weapon.cpp b/source/games/sw/src/weapon.cpp
index 2013b4147..ebd4c873b 100644
--- a/source/games/sw/src/weapon.cpp
+++ b/source/games/sw/src/weapon.cpp
@@ -15001,7 +15001,6 @@ int InitUzi(PLAYER* pp)
     int xvect,yvect,zvect;
     ESpriteFlags cstat = 0;
     uint8_t pal = 0;
-    //static char alternate=0;
     static int uziclock=0;
     int clockdiff=0;
     bool FireSnd = false;