diff --git a/runtime/tools/ccfilter.c b/runtime/tools/ccfilter.c
index 43489f16c..ae1443e20 100644
--- a/runtime/tools/ccfilter.c
+++ b/runtime/tools/ccfilter.c
@@ -249,7 +249,7 @@ int main( int argc, char *argv[] )
 
 	    stay = (echogets(Line2, echo) != NULL);
 	    while ( stay && (Line2[0] == '|') )
-	      { for (p=&Line2[2]; (*p) && (isspace(*p)); p++);
+	      { for (p=&Line2[2]; (*p) && (isspace((unsigned char)*p)); p++);
 		strcat( Reason, ": " );
 		strcat( Reason, p );
 		Line2[0] = 0;
@@ -265,7 +265,7 @@ int main( int argc, char *argv[] )
 	    ok	      = 0;
 	    if ( !strncmp(Line, "cfe: ", 5) )
 	      { p = &Line[5];
-		Severity = tolower(*p);
+		Severity = tolower((unsigned char)*p);
 		p = strchr( &Line[5], ':' );
 		if (p == NULL)
 		  { ok = 0;
@@ -313,7 +313,7 @@ int main( int argc, char *argv[] )
 	}
        else
 	{
-	  for (p=Reason; (*p) && (isspace(*p)); p++);
+	  for (p=Reason; (*p) && (isspace((unsigned char)*p)); p++);
 	  if ( BasePath[CWDlen] == 0 )
 	      printf( "%s:%lu:%lu:%c:%s\n", FileName, Row, Col, Severity, p );
 	  else
diff --git a/runtime/tools/xcmdsrv_client.c b/runtime/tools/xcmdsrv_client.c
index e1aea1066..81ca66ceb 100644
--- a/runtime/tools/xcmdsrv_client.c
+++ b/runtime/tools/xcmdsrv_client.c
@@ -336,7 +336,7 @@ LookupName(
     for (p = regProp; (p - regProp) < numItems; )
     {
 	entry = p;
-	while ((*p != 0) && (!isspace(*p)))
+	while ((*p != 0) && (!isspace((unsigned char)*p)))
 	    p++;
 	if ((*p != 0) && (strcasecmp(name, p + 1) == 0))
 	{
@@ -353,7 +353,7 @@ LookupName(
 	for (p = regProp; (p - regProp) < numItems; )
 	{
 	    entry = p;
-	    while ((*p != 0) && (!isspace(*p)))
+	    while ((*p != 0) && (!isspace((unsigned char)*p)))
 		p++;
 	    if ((*p != 0) && IsSerialName(p + 1)
 		    && (strncmp(name, p + 1, strlen(name)) == 0))
@@ -574,5 +574,5 @@ IsSerialName(char *str)
 {
     int len = strlen(str);
 
-    return (len > 1 && isdigit(str[len - 1]));
+    return (len > 1 && isdigit((unsigned char)str[len - 1]));
 }
diff --git a/src/buffer.c b/src/buffer.c
index 9ee74f54d..64e492647 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -4022,7 +4022,7 @@ maketitle(void)
 					buf + off, SPACE_FOR_DIR - off, TRUE);
 #ifdef BACKSLASH_IN_FILENAME
 		// avoid "c:/name" to be reduced to "c"
-		if (isalpha(buf[off]) && buf[off + 1] == ':')
+		if (SAFE_isalpha(buf[off]) && buf[off + 1] == ':')
 		    off += 2;
 #endif
 		// remove the file name
@@ -5671,7 +5671,7 @@ chk_modeline(
 			&& (s[0] != 'V'
 				  || STRNCMP(skipwhite(e + 1), "set", 3) == 0)
 			&& (s[3] == ':'
-			    || (VIM_VERSION_100 >= vers && isdigit(s[3]))
+			    || (VIM_VERSION_100 >= vers && SAFE_isdigit(s[3]))
 			    || (VIM_VERSION_100 < vers && s[3] == '<')
 			    || (VIM_VERSION_100 > vers && s[3] == '>')
 			    || (VIM_VERSION_100 == vers && s[3] == '=')))
diff --git a/src/charset.c b/src/charset.c
index bda3f911b..0e4dbbe1d 100644
--- a/src/charset.c
+++ b/src/charset.c
@@ -1958,7 +1958,7 @@ vim_islower(int c)
 	if (enc_latin1like)
 	    return (latin1flags[c] & LATIN1LOWER) == LATIN1LOWER;
     }
-    return islower(c);
+    return SAFE_islower(c);
 }
 
     int
@@ -1982,7 +1982,7 @@ vim_isupper(int c)
 	if (enc_latin1like)
 	    return (latin1flags[c] & LATIN1UPPER) == LATIN1UPPER;
     }
-    return isupper(c);
+    return SAFE_isupper(c);
 }
 
     int
diff --git a/src/cmdhist.c b/src/cmdhist.c
index 96a9b3e95..6342f02bd 100644
--- a/src/cmdhist.c
+++ b/src/cmdhist.c
@@ -674,7 +674,7 @@ remove_key_from_history(void)
 	return;
 
     for ( ; *p; ++p)
-	if (STRNCMP(p, "key", 3) == 0 && !isalpha(p[3]))
+	if (STRNCMP(p, "key", 3) == 0 && !SAFE_isalpha(p[3]))
 	{
 	    p = vim_strchr(p + 3, '=');
 	    if (p == NULL)
diff --git a/src/diff.c b/src/diff.c
index 158870402..9b8c816b5 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -1712,7 +1712,7 @@ diff_read(
 		// --- file1       2018-03-20 13:23:35.783153140 +0100
 		// +++ file2       2018-03-20 13:23:41.183156066 +0100
 		// @@ -1,3 +1,5 @@
-		if (isdigit(*line))
+		if (SAFE_isdigit(*line))
 		    diffstyle = DIFF_ED;
 		else if ((STRNCMP(line, "@@ ", 3) == 0))
 		    diffstyle = DIFF_UNIFIED;
@@ -1730,7 +1730,7 @@ diff_read(
 
 	    if (diffstyle == DIFF_ED)
 	    {
-		if (!isdigit(*line))
+		if (!SAFE_isdigit(*line))
 		    continue;	// not the start of a diff block
 		if (parse_diff_ed(line, hunk) == FAIL)
 		    continue;
diff --git a/src/dosinst.c b/src/dosinst.c
index 4eae5aadc..35625a794 100644
--- a/src/dosinst.c
+++ b/src/dosinst.c
@@ -2760,7 +2760,7 @@ main(int argc, char **argv)
 	    rewind(stdin);
 	    if (scanf("%99s", buf) == 1)
 	    {
-		if (isdigit(buf[0]))
+		if (isdigit((unsigned char)buf[0]))
 		{
 		    // Change a choice.
 		    i = atoi(buf);
diff --git a/src/edit.c b/src/edit.c
index 9435fd6fc..f89d43eec 100644
--- a/src/edit.c
+++ b/src/edit.c
@@ -5318,7 +5318,7 @@ ins_ctrl_ey(int tc)
 	    // was typed after a CTRL-V, and pretend 'textwidth'
 	    // wasn't set.  Digits, 'o' and 'x' are special after a
 	    // CTRL-V, don't use it for these.
-	    if (c < 256 && !isalnum(c))
+	    if (c < 256 && !SAFE_isalnum(c))
 		AppendToRedobuff((char_u *)CTRL_V_STR);	// CTRL-V
 	    tw_save = curbuf->b_p_tw;
 	    curbuf->b_p_tw = -1;
diff --git a/src/eval.c b/src/eval.c
index 8563aa6d7..815d13d42 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -4276,7 +4276,7 @@ eval9(
 	return FAIL;
     end_leader = *arg;
 
-    if (**arg == '.' && (!isdigit(*(*arg + 1)) || in_old_script(2)))
+    if (**arg == '.' && (!SAFE_isdigit(*(*arg + 1)) || in_old_script(2)))
     {
 	semsg(_(e_invalid_expression_str), *arg);
 	++*arg;
diff --git a/src/evalfunc.c b/src/evalfunc.c
index f87c08ffc..513ddccdb 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -6744,7 +6744,7 @@ f_has(typval_T *argvars, typval_T *rettv)
 				     && has_patch(atoi(end + 3))))));
 		}
 	    }
-	    else if (isdigit(name[5]))
+	    else if (SAFE_isdigit(name[5]))
 		n = has_patch(atoi((char *)name + 5));
 	}
 	else if (STRICMP(name, "vim_starting") == 0)
diff --git a/src/evalvars.c b/src/evalvars.c
index ea039cbed..8e42c5a30 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -3332,7 +3332,7 @@ find_var(char_u *name, hashtab_T **htp, int no_autoload)
     dictitem_T *
 find_var_also_in_script(char_u *name, hashtab_T **htp, int no_autoload)
 {
-    if (STRNCMP(name, "<SNR>", 5) == 0 && isdigit(name[5]))
+    if (STRNCMP(name, "<SNR>", 5) == 0 && SAFE_isdigit(name[5]))
     {
 	char_u	    *p = name + 5;
 	int	    sid = getdigits(&p);
@@ -4975,7 +4975,7 @@ get_callback(typval_T *arg)
     else
     {
 	if (arg->v_type == VAR_STRING && arg->vval.v_string != NULL
-					       && isdigit(*arg->vval.v_string))
+					       && SAFE_isdigit(*arg->vval.v_string))
 	    r = FAIL;
 	else if (arg->v_type == VAR_FUNC || arg->v_type == VAR_STRING)
 	{
diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index d21493362..d8e891c2b 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -3707,7 +3707,7 @@ skip_substitute(char_u *start, int delimiter)
     static int
 check_regexp_delim(int c)
 {
-    if (isalpha(c))
+    if (SAFE_isalpha(c))
     {
 	emsg(_(e_regular_expressions_cant_be_delimited_by_letters));
 	return FAIL;
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 534cd7e03..c18a9107e 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -7996,7 +7996,7 @@ ex_winsize(exarg_T *eap)
     char_u	*arg = eap->arg;
     char_u	*p;
 
-    if (!isdigit(*arg))
+    if (!SAFE_isdigit(*arg))
     {
 	semsg(_(e_invalid_argument_str), arg);
 	return;
@@ -8387,7 +8387,7 @@ ex_later(exarg_T *eap)
 
     if (*p == NUL)
 	count = 1;
-    else if (isdigit(*p))
+    else if (SAFE_isdigit(*p))
     {
 	count = getdigits(&p);
 	switch (*p)
@@ -8490,7 +8490,7 @@ ex_redir(exarg_T *eap)
 			arg++;
 		    // Make register empty when not using @A-@Z and the
 		    // command is valid.
-		    if (*arg == NUL && !isupper(redir_reg))
+		    if (*arg == NUL && !SAFE_isupper(redir_reg))
 			write_reg_contents(redir_reg, (char_u *)"", -1, FALSE);
 		}
 	    }
diff --git a/src/filepath.c b/src/filepath.c
index 1ea062386..cbf2da413 100644
--- a/src/filepath.c
+++ b/src/filepath.c
@@ -2953,7 +2953,7 @@ get_past_head(char_u *path)
 
 #if defined(MSWIN)
     // may skip "c:"
-    if (isalpha(path[0]) && path[1] == ':')
+    if (SAFE_isalpha(path[0]) && path[1] == ':')
 	retval = path + 2;
     else
 	retval = path;
diff --git a/src/findfile.c b/src/findfile.c
index 246a81898..263660925 100644
--- a/src/findfile.c
+++ b/src/findfile.c
@@ -1847,7 +1847,7 @@ grab_file_name(long count, linenr_T *file_lnum)
 	if (get_visual_text(NULL, &ptr, &len) == FAIL)
 	    return NULL;
 	// Only recognize ":123" here
-	if (file_lnum != NULL && ptr[len] == ':' && isdigit(ptr[len + 1]))
+	if (file_lnum != NULL && ptr[len] == ':' && SAFE_isdigit(ptr[len + 1]))
 	{
 	    char_u *p = ptr + len + 1;
 
@@ -1981,10 +1981,10 @@ file_name_in_line(
 	    p = skipwhite(p);
 	if (*p != NUL)
 	{
-	    if (!isdigit(*p))
+	    if (!SAFE_isdigit(*p))
 		++p;		    // skip the separator
 	    p = skipwhite(p);
-	    if (isdigit(*p))
+	    if (SAFE_isdigit(*p))
 		*file_lnum = (int)getdigits(&p);
 	}
     }
diff --git a/src/getchar.c b/src/getchar.c
index da0503370..3427a9f8d 100644
--- a/src/getchar.c
+++ b/src/getchar.c
@@ -870,7 +870,7 @@ start_redo(long count, int old_redo)
 	{
 	    c = read_redo(FALSE, old_redo);
 	    add_char_buff(&readbuf2, c);
-	    if (!isdigit(c))
+	    if (!SAFE_isdigit(c))
 		break;
 	}
 	c = read_redo(FALSE, old_redo);
@@ -1873,7 +1873,7 @@ vgetc(void)
 
 		    // Handle <SID>{sid};  Do up to 20 digits for safety.
 		    last_used_sid = 0;
-		    for (j = 0; j < 20 && isdigit(c = vgetorpeek(TRUE)); ++j)
+		    for (j = 0; j < 20 && SAFE_isdigit(c = vgetorpeek(TRUE)); ++j)
 			last_used_sid = last_used_sid * 10 + (c - '0');
 		    last_used_map = NULL;
 		    continue;
diff --git a/src/gui.c b/src/gui.c
index 9c9aa3cbe..16c1b54c6 100644
--- a/src/gui.c
+++ b/src/gui.c
@@ -5030,7 +5030,7 @@ display_errors(void)
 
     // avoid putting up a message box with blanks only
     for (p = (char_u *)error_ga.ga_data; *p != NUL; ++p)
-	if (!isspace(*p))
+	if (!SAFE_isspace(*p))
 	{
 	    // Truncate a very long message, it will go off-screen.
 	    if (STRLEN(p) > 2000)
diff --git a/src/gui_haiku.cc b/src/gui_haiku.cc
index c8b05e1be..d08322654 100644
--- a/src/gui_haiku.cc
+++ b/src/gui_haiku.cc
@@ -4031,17 +4031,6 @@ gui_mch_mousehide(int hide)
     //	TODO
 }
 
-    static int
-hex_digit(int c)
-{
-    if (isdigit(c))
-	return c - '0';
-    c = TOLOWER_ASC(c);
-    if (c >= 'a' && c <= 'f')
-	return c - 'a' + 10;
-    return -1000;
-}
-
 /*
  * This function has been lifted from gui_w32.c and extended a bit.
  *
diff --git a/src/gui_photon.c b/src/gui_photon.c
index b987e9b8a..9bce94ddf 100644
--- a/src/gui_photon.c
+++ b/src/gui_photon.c
@@ -977,7 +977,7 @@ gui_ph_is_buffer_item(vimmenu_T *menu, vimmenu_T *parent)
 	if (mark != NULL)
 	{
 	    mark++;
-	    while (isdigit(*mark))
+	    while (SAFE_isdigit(*mark))
 		mark++;
 
 	    if (*mark == ')')
@@ -2545,7 +2545,7 @@ gui_mch_add_menu(vimmenu_T *menu, int index)
 
 	    if (menu->mnemonic != 0)
 	    {
-		PtAddHotkeyHandler(gui.vimWindow, tolower(menu->mnemonic),
+		PtAddHotkeyHandler(gui.vimWindow, SAFE_tolower(menu->mnemonic),
 			Pk_KM_Alt, 0, menu, gui_ph_handle_pulldown_menu);
 	    }
 	}
@@ -2829,7 +2829,7 @@ gui_ph_parse_font_name(
 	{
 	    while (*mark != NUL && *mark++ == ':')
 	    {
-		switch (tolower(*mark++))
+		switch (SAFE_tolower(*mark++))
 		{
 		    case 'a': *font_flags |= PF_STYLE_ANTIALIAS; break;
 		    case 'b': *font_flags |= PF_STYLE_BOLD; break;
diff --git a/src/gui_xim.c b/src/gui_xim.c
index 88de7a4e9..c9b1c6cb4 100644
--- a/src/gui_xim.c
+++ b/src/gui_xim.c
@@ -1471,7 +1471,7 @@ xim_real_init(Window x11_window, Display *x11_display)
 		break;
 	    if ((ns = end = strchr(s, ',')) == NULL)
 		end = s + strlen(s);
-	    while (isspace(((char_u *)end)[-1]))
+	    while (SAFE_isspace(end[-1]))
 		end--;
 	    *end = NUL;
 
@@ -1533,7 +1533,7 @@ xim_real_init(Window x11_window, Display *x11_display)
     strcpy(tmp, gui.rsrc_preedit_type_name);
     for (s = tmp; s && !found; )
     {
-	while (*s && isspace((unsigned char)*s))
+	while (*s && SAFE_isspace(*s))
 	    s++;
 	if (!*s)
 	    break;
@@ -1541,7 +1541,7 @@ xim_real_init(Window x11_window, Display *x11_display)
 	    ns++;
 	else
 	    end = s + strlen(s);
-	while (isspace((unsigned char)*end))
+	while (SAFE_isspace(*end))
 	    end--;
 	*end = '\0';
 
diff --git a/src/hardcopy.c b/src/hardcopy.c
index e91a7dc2d..8abfff210 100644
--- a/src/hardcopy.c
+++ b/src/hardcopy.c
@@ -1745,7 +1745,7 @@ prt_resfile_skip_nonws(int offset)
     idx = prt_resfile.line_start + offset;
     while (idx < prt_resfile.line_end)
     {
-	if (isspace(prt_resfile.buffer[idx]))
+	if (SAFE_isspace(prt_resfile.buffer[idx]))
 	    return idx - prt_resfile.line_start;
 	idx++;
     }
@@ -1760,7 +1760,7 @@ prt_resfile_skip_ws(int offset)
     idx = prt_resfile.line_start + offset;
     while (idx < prt_resfile.line_end)
     {
-	if (!isspace(prt_resfile.buffer[idx]))
+	if (!SAFE_isspace(prt_resfile.buffer[idx]))
 	    return idx - prt_resfile.line_start;
 	idx++;
     }
diff --git a/src/highlight.c b/src/highlight.c
index 31c3280e8..318564c74 100644
--- a/src/highlight.c
+++ b/src/highlight.c
@@ -2307,7 +2307,7 @@ gui_adjust_rgb(guicolor_T c)
     static int
 hex_digit(int c)
 {
-    if (isdigit(c))
+    if (SAFE_isdigit(c))
 	return c - '0';
     c = TOLOWER_ASC(c);
     if (c >= 'a' && c <= 'f')
diff --git a/src/if_py_both.h b/src/if_py_both.h
index f8438639e..42db5101d 100644
--- a/src/if_py_both.h
+++ b/src/if_py_both.h
@@ -3240,7 +3240,7 @@ FunctionNew(PyTypeObject *subtype, char_u *name, int argc, typval_T *argv,
     if (self == NULL)
 	return NULL;
 
-    if (isdigit(*name))
+    if (isdigit((unsigned char)*name))
     {
 	if (!translated_function_exists(name, FALSE))
 	{
diff --git a/src/if_xcmdsrv.c b/src/if_xcmdsrv.c
index d9408398d..716665a7b 100644
--- a/src/if_xcmdsrv.c
+++ b/src/if_xcmdsrv.c
@@ -657,7 +657,7 @@ serverGetVimNames(Display *dpy)
     for (p = regProp; (long_u)(p - regProp) < numItems; p++)
     {
 	entry = p;
-	while (*p != 0 && !isspace(*p))
+	while (*p != 0 && !SAFE_isspace(*p))
 	    p++;
 	if (*p != 0)
 	{
@@ -928,7 +928,7 @@ LookupName(
     for (p = regProp; (long_u)(p - regProp) < numItems; )
     {
 	entry = p;
-	while (*p != 0 && !isspace(*p))
+	while (*p != 0 && !SAFE_isspace(*p))
 	    p++;
 	if (*p != 0 && STRICMP(name, p + 1) == 0)
 	{
@@ -945,7 +945,7 @@ LookupName(
 	for (p = regProp; (long_u)(p - regProp) < numItems; )
 	{
 	    entry = p;
-	    while (*p != 0 && !isspace(*p))
+	    while (*p != 0 && !SAFE_isspace(*p))
 		p++;
 	    if (*p != 0 && IsSerialName(p + 1)
 		    && STRNICMP(name, p + 1, STRLEN(name)) == 0)
diff --git a/src/macros.h b/src/macros.h
index cc2d11fdd..190778eca 100644
--- a/src/macros.h
+++ b/src/macros.h
@@ -50,6 +50,28 @@
  */
 #define BUFEMPTY() (curbuf->b_ml.ml_line_count == 1 && *ml_get((linenr_T)1) == NUL)
 
+// The is*() and to*() functions declared in <ctype.h> have
+// undefined behavior for values other than EOF outside the range of
+// unsigned char.  If plain char is signed, a call with a negative
+// value has undefined behavior.  These macros cast the argument to
+// unsigned char.  (Most implementations behave more or less sanely
+// with negative values, and most character values in practice are
+// positive, but we want to avoid undefined behavior anyway.)
+#define SAFE_isalnum(c)  (isalnum ((unsigned char)(c)))
+#define SAFE_isalpha(c)  (isalpha ((unsigned char)(c)))
+#define SAFE_isblank(c)  (isblank ((unsigned char)(c)))
+#define SAFE_iscntrl(c)  (iscntrl ((unsigned char)(c)))
+#define SAFE_isdigit(c)  (isdigit ((unsigned char)(c)))
+#define SAFE_isgraph(c)  (isgraph ((unsigned char)(c)))
+#define SAFE_islower(c)  (islower ((unsigned char)(c)))
+#define SAFE_isprint(c)  (isprint ((unsigned char)(c)))
+#define SAFE_ispunct(c)  (ispunct ((unsigned char)(c)))
+#define SAFE_isspace(c)  (isspace ((unsigned char)(c)))
+#define SAFE_isupper(c)  (isupper ((unsigned char)(c)))
+#define SAFE_isxdigit(c) (isxdigit((unsigned char)(c)))
+#define SAFE_tolower(c)  (tolower ((unsigned char)(c)))
+#define SAFE_toupper(c)  (toupper ((unsigned char)(c)))
+
 /*
  * toupper() and tolower() that use the current locale.
  * On some systems toupper()/tolower() only work on lower/uppercase
@@ -64,11 +86,11 @@
 #  define TOLOWER_LOC(c)	tolower_tab[(c) & 255]
 #else
 # ifdef BROKEN_TOUPPER
-#  define TOUPPER_LOC(c)	(islower(c) ? toupper(c) : (c))
-#  define TOLOWER_LOC(c)	(isupper(c) ? tolower(c) : (c))
+#  define TOUPPER_LOC(c)	(SAFE_islower(c) ? SAFE_toupper(c) : (c))
+#  define TOLOWER_LOC(c)	(SAFE_isupper(c) ? SAFE_tolower(c) : (c))
 # else
-#  define TOUPPER_LOC		toupper
-#  define TOLOWER_LOC		tolower
+#  define TOUPPER_LOC		SAFE_toupper
+#  define TOLOWER_LOC		SAFE_tolower
 # endif
 #endif
 
diff --git a/src/mbyte.c b/src/mbyte.c
index 34592bd56..ee2834cf4 100644
--- a/src/mbyte.c
+++ b/src/mbyte.c
@@ -4628,7 +4628,7 @@ enc_canonize(char_u *enc)
     }
 
     // "iso-8859n" -> "iso-8859-n"
-    if (STRNCMP(p, "iso-8859", 8) == 0 && isdigit(p[8]))
+    if (STRNCMP(p, "iso-8859", 8) == 0 && SAFE_isdigit(p[8]))
     {
 	STRMOVE(p + 9, p + 8);
 	p[8] = '-';
@@ -4705,7 +4705,7 @@ enc_locale_env(char *locale)
     if ((p = (char *)vim_strchr((char_u *)s, '.')) != NULL)
     {
 	if (p > s + 2 && STRNICMP(p + 1, "EUC", 3) == 0
-			&& !isalnum((int)p[4]) && p[4] != '-' && p[-3] == '_')
+			&& !SAFE_isalnum((int)p[4]) && p[4] != '-' && p[-3] == '_')
 	{
 	    // copy "XY.EUC" to "euc-XY" to buf[10]
 	    STRCPY(buf + 10, "euc-");
@@ -4721,7 +4721,7 @@ enc_locale_env(char *locale)
     {
 	if (s[i] == '_' || s[i] == '-')
 	    buf[i] = '-';
-	else if (isalnum((int)s[i]))
+	else if (SAFE_isalnum(s[i]))
 	    buf[i] = TOLOWER_ASC(s[i]);
 	else
 	    break;
diff --git a/src/normal.c b/src/normal.c
index 61a19c13a..fd89b00fd 100644
--- a/src/normal.c
+++ b/src/normal.c
@@ -4470,11 +4470,11 @@ nv_brackets(cmdarg_T *cap)
 		return;
 
 	    find_pattern_in_path(ptr, 0, len, TRUE,
-		cap->count0 == 0 ? !isupper(cap->nchar) : FALSE,
+		cap->count0 == 0 ? !SAFE_isupper(cap->nchar) : FALSE,
 		((cap->nchar & 0xf) == ('d' & 0xf)) ?  FIND_DEFINE : FIND_ANY,
 		cap->count1,
-		isupper(cap->nchar) ? ACTION_SHOW_ALL :
-			    islower(cap->nchar) ? ACTION_SHOW : ACTION_GOTO,
+		SAFE_isupper(cap->nchar) ? ACTION_SHOW_ALL :
+			    SAFE_islower(cap->nchar) ? ACTION_SHOW : ACTION_GOTO,
 		cap->cmdchar == ']' ? curwin->w_cursor.lnum + 1 : (linenr_T)1,
 		(linenr_T)MAXLNUM);
 	    vim_free(ptr);
@@ -5217,7 +5217,7 @@ v_visop(cmdarg_T *cap)
 
     // Uppercase means linewise, except in block mode, then "D" deletes till
     // the end of the line, and "C" replaces till EOL
-    if (isupper(cap->cmdchar))
+    if (SAFE_isupper(cap->cmdchar))
     {
 	if (VIsual_mode != Ctrl_V)
 	{
diff --git a/src/ops.c b/src/ops.c
index 46101a435..f6d765bd7 100644
--- a/src/ops.c
+++ b/src/ops.c
@@ -2734,7 +2734,7 @@ do_addsub(
 	{
 	    if (CharOrd(firstdigit) < Prenum1)
 	    {
-		if (isupper(firstdigit))
+		if (SAFE_isupper(firstdigit))
 		    firstdigit = 'A';
 		else
 		    firstdigit = 'a';
@@ -2746,7 +2746,7 @@ do_addsub(
 	{
 	    if (26 - CharOrd(firstdigit) - 1 < Prenum1)
 	    {
-		if (isupper(firstdigit))
+		if (SAFE_isupper(firstdigit))
 		    firstdigit = 'Z';
 		else
 		    firstdigit = 'z';
@@ -2875,9 +2875,9 @@ do_addsub(
 	save_pos = curwin->w_cursor;
 	for (i = 0; i < todel; ++i)
 	{
-	    if (c < 0x100 && isalpha(c))
+	    if (c < 0x100 && SAFE_isalpha(c))
 	    {
-		if (isupper(c))
+		if (SAFE_isupper(c))
 		    hexupper = TRUE;
 		else
 		    hexupper = FALSE;
diff --git a/src/option.c b/src/option.c
index 4d12834f2..7cac89e5c 100644
--- a/src/option.c
+++ b/src/option.c
@@ -7697,7 +7697,7 @@ ExpandSettings(
 	{
 	    for (opt_idx = 0; (str = get_termcode(opt_idx)) != NULL; opt_idx++)
 	    {
-		if (!isprint(str[0]) || !isprint(str[1]))
+		if (!SAFE_isprint(str[0]) || !SAFE_isprint(str[1]))
 		    continue;
 
 		name_buf[0] = 't';
diff --git a/src/os_mswin.c b/src/os_mswin.c
index 46f73752f..21b7db31f 100644
--- a/src/os_mswin.c
+++ b/src/os_mswin.c
@@ -675,7 +675,7 @@ display_errors(void)
 	{
 	    // avoid putting up a message box with blanks only
 	    for (p = (char_u *)error_ga.ga_data; *p; ++p)
-		if (!isspace(*p))
+		if (!SAFE_isspace(*p))
 		{
 		    // Only use a dialog when not using --gui-dialog-file:
 		    // write text to a file.
@@ -759,7 +759,7 @@ mch_chdir(char *path)
 	smsg("chdir(%s)", path);
 	verbose_leave();
     }
-    if (isalpha(path[0]) && path[1] == ':')	// has a drive name
+    if (SAFE_isalpha(path[0]) && path[1] == ':')	// has a drive name
     {
 	// If we can change to the drive, skip that part of the path.  If we
 	// can't then the current directory may be invalid, try using chdir()
@@ -1751,7 +1751,7 @@ is_reparse_point_included(LPCWSTR fname)
     WCHAR	buf[MAX_PATH];
     DWORD	attr;
 
-    if (isalpha(p[0]) && p[1] == L':' && is_path_sep(p[2]))
+    if (SAFE_isalpha(p[0]) && p[1] == L':' && is_path_sep(p[2]))
 	p += 3;
     else if (is_path_sep(p[0]) && is_path_sep(p[1]))
 	p += 2;
diff --git a/src/os_unix.c b/src/os_unix.c
index 11448c5c6..af8f00604 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -6171,7 +6171,7 @@ mch_signal_job(job_T *job, char_u *how)
     else if (STRCMP(how, "winch") == 0)
 	sig = SIGWINCH;
 #endif
-    else if (isdigit(*how))
+    else if (SAFE_isdigit(*how))
 	sig = atoi((char *)how);
     else
 	return FAIL;
diff --git a/src/os_win32.c b/src/os_win32.c
index e5cfac729..dbfc4eeca 100644
--- a/src/os_win32.c
+++ b/src/os_win32.c
@@ -7443,7 +7443,7 @@ notsgr:
 	{
 	    int l = 2;
 
-	    if (isdigit(s[l]))
+	    if (SAFE_isdigit(s[l]))
 		l++;
 	    if (s[l] == ' ' && s[l + 1] == 'q')
 	    {
diff --git a/src/spellfile.c b/src/spellfile.c
index d3351ef23..24df042b7 100644
--- a/src/spellfile.c
+++ b/src/spellfile.c
@@ -2890,7 +2890,7 @@ spell_read_aff(spellinfo_T *spin, char_u *fname)
 		     || is_aff_rule(items, itemcnt, "REPSAL", 2))
 	    {
 		// Ignore REP/REPSAL count
-		if (!isdigit(*items[1]))
+		if (!SAFE_isdigit(*items[1]))
 		    smsg(_("Expected REP(SAL) count in %s line %d"),
 								 fname, lnum);
 	    }
@@ -2925,7 +2925,7 @@ spell_read_aff(spellinfo_T *spin, char_u *fname)
 		{
 		    // First line contains the count.
 		    found_map = TRUE;
-		    if (!isdigit(*items[1]))
+		    if (!SAFE_isdigit(*items[1]))
 			smsg(_("Expected MAP count in %s line %d"),
 								 fname, lnum);
 		}
diff --git a/src/tag.c b/src/tag.c
index 8003156f0..893415f69 100644
--- a/src/tag.c
+++ b/src/tag.c
@@ -1194,7 +1194,7 @@ add_llist_tags(
 	// Get the line number or the search pattern used to locate
 	// the tag.
 	lnum = 0;
-	if (isdigit(*tagp.command))
+	if (SAFE_isdigit(*tagp.command))
 	    // Line number is used to locate the tag
 	    lnum = atol((char *)tagp.command);
 	else
diff --git a/src/term.c b/src/term.c
index 0fdb33e6d..dd59fdf65 100644
--- a/src/term.c
+++ b/src/term.c
@@ -3434,7 +3434,7 @@ ttest(int pairs)
 #endif
     {
 	env_colors = mch_getenv((char_u *)"COLORS");
-	if (env_colors != NULL && isdigit(*env_colors))
+	if (env_colors != NULL && SAFE_isdigit(*env_colors))
 	{
 	    int colors = atoi((char *)env_colors);
 
@@ -5849,7 +5849,7 @@ handle_dcs(char_u *tp, char_u *argp, int len, char_u *key_name, int *slen)
 	// characters.
 	for (i = j + 3; i < len; ++i)
 	{
-	    if (i - j == 3 && !isdigit(tp[i]))
+	    if (i - j == 3 && !SAFE_isdigit(tp[i]))
 		break;
 	    if (i - j == 4 && tp[i] != ' ')
 		break;
@@ -6083,7 +6083,7 @@ check_termcode(
 			// The mouse termcode "ESC [" is also the prefix of
 			// "ESC [ I" (focus gained) and other keys.  Check some
 			// more bytes to find out.
-			if (!isdigit(tp[2]))
+			if (!SAFE_isdigit(tp[2]))
 			{
 			    // ESC [ without number following: Only use it when
 			    // there is no other match.
@@ -6166,7 +6166,7 @@ check_termcode(
 			    // Skip over the digits, the final char must
 			    // follow. URXVT can use a negative value, thus
 			    // also accept '-'.
-			    for (j = slen - 2; j < len && (isdigit(tp[j])
+			    for (j = slen - 2; j < len && (SAFE_isdigit(tp[j])
 				       || tp[j] == '-' || tp[j] == ';'); ++j)
 				;
 			    ++j;
diff --git a/src/terminal.c b/src/terminal.c
index f79d102e8..a641a850b 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -853,13 +853,13 @@ ex_terminal(exarg_T *eap)
 	    else
 		opt.jo_term_api = NULL;
 	}
-	else if (OPTARG_HAS("rows") && ep != NULL && isdigit(ep[1]))
+	else if (OPTARG_HAS("rows") && ep != NULL && SAFE_isdigit(ep[1]))
 	{
 	    opt.jo_set2 |= JO2_TERM_ROWS;
 	    opt.jo_term_rows = atoi((char *)ep + 1);
 	    p = skiptowhite(cmd);
 	}
-	else if (OPTARG_HAS("cols") && ep != NULL && isdigit(ep[1]))
+	else if (OPTARG_HAS("cols") && ep != NULL && SAFE_isdigit(ep[1]))
 	{
 	    opt.jo_set2 |= JO2_TERM_COLS;
 	    opt.jo_term_cols = atoi((char *)ep + 1);
@@ -5463,11 +5463,11 @@ read_dump_file(FILE *fd, VTermPos *cursor_pos)
 		    // use same attr as previous cell
 		    c = fgetc(fd);
 		}
-		else if (isdigit(c))
+		else if (SAFE_isdigit(c))
 		{
 		    // get the decimal attribute
 		    attr = 0;
-		    while (isdigit(c))
+		    while (SAFE_isdigit(c))
 		    {
 			attr = attr * 10 + (c - '0');
 			c = fgetc(fd);
@@ -5499,9 +5499,9 @@ read_dump_file(FILE *fd, VTermPos *cursor_pos)
 			    c = fgetc(fd);
 			    blue = (blue << 4) + hex2nr(c);
 			    c = fgetc(fd);
-			    if (!isdigit(c))
+			    if (!SAFE_isdigit(c))
 				dump_is_corrupt(&ga_text);
-			    while (isdigit(c))
+			    while (SAFE_isdigit(c))
 			    {
 				index = index * 10 + (c - '0');
 				c = fgetc(fd);
@@ -5565,7 +5565,7 @@ read_dump_file(FILE *fd, VTermPos *cursor_pos)
 		for (;;)
 		{
 		    c = fgetc(fd);
-		    if (!isdigit(c))
+		    if (!SAFE_isdigit(c))
 			break;
 		    count = count * 10 + (c - '0');
 		}
diff --git a/src/typval.c b/src/typval.c
index 35c9e24eb..af96b3170 100644
--- a/src/typval.c
+++ b/src/typval.c
@@ -2425,7 +2425,7 @@ eval_string(char_u **arg, typval_T *rettv, int evaluate, int interpolate)
 			  if (vim_isxdigit(p[1]))
 			  {
 			      int	n, nr;
-			      int	c = toupper(*p);
+			      int	c = SAFE_toupper(*p);
 
 			      if (c == 'X')
 				  n = 2;
diff --git a/src/uninstall.c b/src/uninstall.c
index 78b2b7d3c..907ed3797 100644
--- a/src/uninstall.c
+++ b/src/uninstall.c
@@ -29,7 +29,7 @@ confirm(void)
     char	answer[10];
 
     fflush(stdout);
-    return (scanf(" %c", answer) == 1 && toupper(answer[0]) == 'Y');
+    return (scanf(" %c", answer) == 1 && toupper((unsigned char)answer[0]) == 'Y');
 }
 
     static int
diff --git a/src/userfunc.c b/src/userfunc.c
index c0a2487b4..64761ecdb 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -77,7 +77,7 @@ one_function_arg(
 
     while (ASCII_ISALNUM(*p) || *p == '_')
 	++p;
-    if (arg == p || isdigit(*arg)
+    if (arg == p || SAFE_isdigit(*arg)
 	    || (argtypes == NULL
 		&& ((p - arg == 9 && STRNCMP(arg, "firstline", 9) == 0)
 		    || (p - arg == 8 && STRNCMP(arg, "lastline", 8) == 0))))
@@ -2272,7 +2272,7 @@ func_requires_g_prefix(ufunc_T *ufunc)
     return ufunc->uf_name[0] != K_SPECIAL
 	    && (ufunc->uf_flags & FC_LAMBDA) == 0
 	    && vim_strchr(ufunc->uf_name, AUTOLOAD_CHAR) == NULL
-	    && !isdigit(ufunc->uf_name[0]);
+	    && !SAFE_isdigit(ufunc->uf_name[0]);
 }
 
 /*
@@ -2448,8 +2448,8 @@ cleanup_function_call(funccall_T *fc)
     static int
 numbered_function(char_u *name)
 {
-    return isdigit(*name)
-	    || (name[0] == 'g' && name[1] == ':' && isdigit(name[2]));
+    return SAFE_isdigit(*name)
+	    || (name[0] == 'g' && name[1] == ':' && SAFE_isdigit(name[2]));
 }
 
 /*
@@ -4653,7 +4653,7 @@ list_functions(regmatch_T *regmatch)
 		    && (regmatch == NULL
 			? !message_filtered(fp->uf_name)
 			    && !func_name_refcount(fp->uf_name)
-			: !isdigit(*fp->uf_name)
+			: !SAFE_isdigit(*fp->uf_name)
 			    && vim_regexec(regmatch, fp->uf_name, 0)))
 	    {
 		if (list_func_head(fp, FALSE) == FAIL)
diff --git a/src/version.c b/src/version.c
index 42d58897e..3dc537d18 100644
--- a/src/version.c
+++ b/src/version.c
@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    6,
 /**/
     5,
 /**/
@@ -1328,9 +1330,9 @@ do_intro_line(
 	if (highest_patch())
 	{
 	    // Check for 9.9x or 9.9xx, alpha/beta version
-	    if (isalpha((int)vers[3]))
+	    if (SAFE_isalpha((int)vers[3]))
 	    {
-		int len = (isalpha((int)vers[4])) ? 5 : 4;
+		int len = (SAFE_isalpha((int)vers[4])) ? 5 : 4;
 		sprintf((char *)vers + len, ".%d%s", highest_patch(),
 							 mediumVersion + len);
 	    }
diff --git a/src/vim9compile.c b/src/vim9compile.c
index ad1f6b4aa..d6faa7bb9 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -3159,7 +3159,7 @@ get_cmd_count(char_u *line, exarg_T *eap)
     // skip over colons and white space
     for (p = line; *p == ':' || VIM_ISWHITE(*p); ++p)
 	;
-    if (!isdigit(*p))
+    if (!SAFE_isdigit(*p))
     {
 	// The command or modifiers must be following.  Assume a lower case
 	// character means there is a modifier.
diff --git a/src/vim9expr.c b/src/vim9expr.c
index 31fa824a7..38a65d442 100644
--- a/src/vim9expr.c
+++ b/src/vim9expr.c
@@ -1950,7 +1950,7 @@ get_compare_type(char_u *p, int *len, int *type_is)
 			if (p[2] == 'n' && p[3] == 'o' && p[4] == 't')
 			    *len = 5;
 			i = p[*len];
-			if (!isalnum(i) && i != '_')
+			if (!SAFE_isalnum(i) && i != '_')
 			{
 			    type = *len == 2 ? EXPR_IS : EXPR_ISNOT;
 			    *type_is = TRUE;
diff --git a/src/viminfo.c b/src/viminfo.c
index 58bf41919..540422c8c 100644
--- a/src/viminfo.c
+++ b/src/viminfo.c
@@ -1068,7 +1068,7 @@ barline_parse(vir_T *virp, char_u *text, garray_T *values)
 	    }
 	}
 
-	if (isdigit(*p))
+	if (SAFE_isdigit(*p))
 	{
 	    value->bv_type = BVAL_NR;
 	    value->bv_nr = getdigits(&p);
@@ -2485,7 +2485,7 @@ read_viminfo_filemark(vir_T *virp, int force)
     str = virp->vir_line + 1;
     if (*str <= 127
 	    && ((*virp->vir_line == '\''
-				       && (VIM_ISDIGIT(*str) || isupper(*str)))
+				       && (VIM_ISDIGIT(*str) || SAFE_isupper(*str)))
 	     || (*virp->vir_line == '-' && *str == '\'')))
     {
 	if (*str == '\'')
diff --git a/src/xxd/xxd.c b/src/xxd/xxd.c
index c90bc027e..26f8d8365 100644
--- a/src/xxd/xxd.c
+++ b/src/xxd/xxd.c
@@ -96,8 +96,8 @@
 # include <unistd.h>
 #endif
 #include <stdlib.h>
-#include <string.h>	/* for strncmp() */
-#include <ctype.h>	/* for isalnum() */
+#include <string.h>
+#include <ctype.h>
 #include <limits.h>
 #if __MWERKS__ && !defined(BEBOX)
 # include <unix.h>	/* for fdopen() on MAC */
@@ -208,7 +208,7 @@ char hexxa[] = "0123456789abcdef0123456789ABCDEF", *hexx = hexxa;
 #define HEX_BITS 3		/* not hex a dump, but bits: 01111001 */
 #define HEX_LITTLEENDIAN 4
 
-#define CONDITIONAL_CAPITALIZE(c) (capitalize ? toupper((int)c) : c)
+#define CONDITIONAL_CAPITALIZE(c) (capitalize ? toupper((unsigned char)(c)) : (c))
 
 #define COLOR_PROLOGUE \
 l[c++] = '\033'; \
@@ -952,9 +952,9 @@ main(int argc, char *argv[])
 
       if (varname != NULL)
 	{
-	  FPRINTF_OR_DIE((fpo, "unsigned char %s", isdigit((int)varname[0]) ? "__" : ""));
+	  FPRINTF_OR_DIE((fpo, "unsigned char %s", isdigit((unsigned char)varname[0]) ? "__" : ""));
 	  for (e = 0; (c = varname[e]) != 0; e++)
-	    putc_or_die(isalnum(c) ? CONDITIONAL_CAPITALIZE(c) : '_', fpo);
+	    putc_or_die(isalnum((unsigned char)c) ? CONDITIONAL_CAPITALIZE(c) : '_', fpo);
 	  fputs_or_die("[] = {\n", fpo);
 	}
 
@@ -972,9 +972,9 @@ main(int argc, char *argv[])
       if (varname != NULL)
 	{
 	  fputs_or_die("};\n", fpo);
-	  FPRINTF_OR_DIE((fpo, "unsigned int %s", isdigit((int)varname[0]) ? "__" : ""));
+	  FPRINTF_OR_DIE((fpo, "unsigned int %s", isdigit((unsigned char)varname[0]) ? "__" : ""));
 	  for (e = 0; (c = varname[e]) != 0; e++)
-	    putc_or_die(isalnum(c) ? CONDITIONAL_CAPITALIZE(c) : '_', fpo);
+	    putc_or_die(isalnum((unsigned char)c) ? CONDITIONAL_CAPITALIZE(c) : '_', fpo);
 	  FPRINTF_OR_DIE((fpo, "_%s = %d;\n", capitalize ? "LEN" : "len", p));
 	}