--- a/src/gui_xim.c
+++ b/src/gui_xim.c
@@ -73,16 +73,22 @@ xim_log(char *s, ...)
 static callback_T imaf_cb;	    // 'imactivatefunc' callback function
 static callback_T imsf_cb;	    // 'imstatusfunc' callback function
 
-    int
+    char *
 set_imactivatefunc_option(void)
 {
-    return option_set_callback_func(p_imaf, &imaf_cb);
+    if (option_set_callback_func(p_imaf, &imaf_cb) == FAIL)
+	return e_invalid_argument;
+
+    return NULL;
 }
 
-    int
+    char *
 set_imstatusfunc_option(void)
 {
-    return option_set_callback_func(p_imsf, &imsf_cb);
+    if (option_set_callback_func(p_imsf, &imsf_cb) == FAIL)
+	return e_invalid_argument;
+
+    return NULL;
 }
 
     static void

--- a/src/insexpand.c
+++ b/src/insexpand.c
@@ -2545,16 +2545,15 @@ copy_global_to_buflocal_cb(callback_T *g
  * name of a function (string), or function(<name>) or funcref(<name>) or a
  * lambda expression.
  */
-    int
+    char *
 set_completefunc_option(void)
 {
-    int	retval;
-
-    retval = option_set_callback_func(curbuf->b_p_cfu, &cfu_cb);
-    if (retval == OK)
-	set_buflocal_cfu_callback(curbuf);
-
-    return retval;
+    if (option_set_callback_func(curbuf->b_p_cfu, &cfu_cb) == FAIL)
+	return e_invalid_argument;
+
+    set_buflocal_cfu_callback(curbuf);
+
+    return NULL;
 }
 
 /*
@@ -2575,16 +2574,14 @@ set_buflocal_cfu_callback(buf_T *buf UNU
  * name of a function (string), or function(<name>) or funcref(<name>) or a
  * lambda expression.
  */
-    int
+    char *
 set_omnifunc_option(void)
 {
-    int	retval;
-
-    retval = option_set_callback_func(curbuf->b_p_ofu, &ofu_cb);
-    if (retval == OK)
-	set_buflocal_ofu_callback(curbuf);
-
-    return retval;
+    if (option_set_callback_func(curbuf->b_p_ofu, &ofu_cb) == FAIL)
+	return e_invalid_argument;
+
+    set_buflocal_ofu_callback(curbuf);
+    return NULL;
 }
 
 /*
@@ -2605,7 +2602,7 @@ set_buflocal_ofu_callback(buf_T *buf UNU
  * name of a function (string), or function(<name>) or funcref(<name>) or a
  * lambda expression.
  */
-    int
+    char *
 set_thesaurusfunc_option(void)
 {
     int	retval;
@@ -2622,7 +2619,7 @@ set_thesaurusfunc_option(void)
 	retval = option_set_callback_func(p_tsrfu, &tsrfu_cb);
     }
 
-    return retval;
+    return retval == FAIL ? e_invalid_argument : NULL;
 }
 
 /*

--- a/src/ops.c
+++ b/src/ops.c
@@ -3405,10 +3405,13 @@ static callback_T opfunc_cb;
  * Process the 'operatorfunc' option value.
  * Returns OK or FAIL.
  */
-    int
+    char *
 set_operatorfunc_option(void)
 {
-    return option_set_callback_func(p_opfunc, &opfunc_cb);
+    if (option_set_callback_func(p_opfunc, &opfunc_cb) == FAIL)
+	return e_invalid_argument;
+
+    return NULL;
 }
 
 #if defined(EXITFREE) || defined(PROTO)

--- a/src/optionstr.c
+++ b/src/optionstr.c
@@ -640,6 +640,1858 @@ check_stl_option(char_u *s)
 #endif
 
 /*
+ * The 'term' option is changed.
+ */
+    static char *
+did_set_term(int *opt_idx, long_u *free_oldval)
+{
+    char *errmsg = NULL;
+
+    if (T_NAME[0] == NUL)
+	errmsg = e_cannot_set_term_to_empty_string;
+#ifdef FEAT_GUI
+    else if (gui.in_use)
+	errmsg = e_cannot_change_term_in_GUI;
+    else if (term_is_gui(T_NAME))
+	errmsg = e_use_gui_to_start_GUI;
+#endif
+    else if (set_termname(T_NAME) == FAIL)
+	errmsg = e_not_found_in_termcap;
+    else
+    {
+	// Screen colors may have changed.
+	redraw_later_clear();
+
+	// Both 'term' and 'ttytype' point to T_NAME, only set the
+	// P_ALLOCED flag on 'term'.
+	*opt_idx = findoption((char_u *)"term");
+	*free_oldval = (get_option_flags(*opt_idx) & P_ALLOCED);
+    }
+
+    return errmsg;
+}
+
+/*
+ * The 'backupcopy' option is changed.
+ */
+    static char *
+did_set_backupcopy(
+    char_u	*oldval,
+    int		opt_flags)
+{
+    char_u		*bkc = p_bkc;
+    unsigned int	*flags = &bkc_flags;
+    char		*errmsg = NULL;
+
+    if (opt_flags & OPT_LOCAL)
+    {
+	bkc = curbuf->b_p_bkc;
+	flags = &curbuf->b_bkc_flags;
+    }
+
+    if ((opt_flags & OPT_LOCAL) && *bkc == NUL)
+	// make the local value empty: use the global value
+	*flags = 0;
+    else
+    {
+	if (opt_strings_flags(bkc, p_bkc_values, flags, TRUE) != OK)
+	    errmsg = e_invalid_argument;
+	if ((((int)*flags & BKC_AUTO) != 0)
+		+ (((int)*flags & BKC_YES) != 0)
+		+ (((int)*flags & BKC_NO) != 0) != 1)
+	{
+	    // Must have exactly one of "auto", "yes"  and "no".
+	    (void)opt_strings_flags(oldval, p_bkc_values, flags, TRUE);
+	    errmsg = e_invalid_argument;
+	}
+    }
+
+    return errmsg;
+}
+
+/*
+ * The 'backupext' or the 'patchmode' option is changed.
+ */
+    static char *
+did_set_backupext_or_patchmode(void)
+{
+    if (STRCMP(*p_bex == '.' ? p_bex + 1 : p_bex,
+		*p_pm == '.' ? p_pm + 1 : p_pm) == 0)
+	return e_backupext_and_patchmode_are_equal;
+
+    return NULL;
+}
+
+#ifdef FEAT_LINEBREAK
+/*
+ * The 'breakindentopt' option is changed.
+ */
+    static char *
+did_set_breakindentopt(void)
+{
+    char *errmsg = NULL;
+
+    if (briopt_check(curwin) == FAIL)
+	errmsg = e_invalid_argument;
+    // list setting requires a redraw
+    if (curwin->w_briopt_list)
+	redraw_all_later(UPD_NOT_VALID);
+
+    return errmsg;
+}
+#endif
+
+/*
+ * The 'isident' or the 'iskeyword' or the 'isprint' or the 'isfname' option is
+ * changed.
+ */
+    static char *
+did_set_isopt(int *did_chartab)
+{
+    if (init_chartab() == FAIL)
+    {
+	*did_chartab = TRUE;		// need to restore it below
+	return e_invalid_argument;	// error in value
+    }
+
+    return NULL;
+}
+
+/*
+ * The 'helpfile' option is changed.
+ */
+    static void
+did_set_helpfile(void)
+{
+    // May compute new values for $VIM and $VIMRUNTIME
+    if (didset_vim)
+	vim_unsetenv_ext((char_u *)"VIM");
+    if (didset_vimruntime)
+	vim_unsetenv_ext((char_u *)"VIMRUNTIME");
+}
+
+#ifdef FEAT_SYN_HL
+/*
+ * The 'cursorlineopt' option is changed.
+ */
+    static char *
+did_set_cursorlineopt(char_u **varp)
+{
+    if (**varp == NUL || fill_culopt_flags(*varp, curwin) != OK)
+	return e_invalid_argument;
+
+    return NULL;
+}
+#endif
+
+#ifdef FEAT_MULTI_LANG
+/*
+ * The 'helplang' option is changed.
+ */
+    static char *
+did_set_helplang(void)
+{
+    char *errmsg = NULL;
+
+    // Check for "", "ab", "ab,cd", etc.
+    for (char_u *s = p_hlg; *s != NUL; s += 3)
+    {
+	if (s[1] == NUL || ((s[2] != ',' || s[3] == NUL) && s[2] != NUL))
+	{
+	    errmsg = e_invalid_argument;
+	    break;
+	}
+	if (s[2] == NUL)
+	    break;
+    }
+
+    return errmsg;
+}
+#endif
+
+/*
+ * The 'highlight' option is changed.
+ */
+    static char *
+did_set_highlight(void)
+{
+    if (highlight_changed() == FAIL)
+	return e_invalid_argument;	// invalid flags
+
+    return NULL;
+}
+
+/*
+ * An option that accepts a list of flags is changed.
+ * e.g. 'viewoptions', 'switchbuf', 'casemap', etc.
+ */
+    static char *
+did_set_opt_flags(char_u *val, char **values, unsigned *flagp, int list)
+{
+    if (opt_strings_flags(val, values, flagp, list) == FAIL)
+	return e_invalid_argument;
+
+    return NULL;
+}
+
+/*
+ * An option that accepts a list of string values is changed.
+ * e.g. 'nrformats', 'scrollopt', 'wildoptions', etc.
+ */
+    static char *
+did_set_opt_strings(char_u *val, char **values, int list)
+{
+    return did_set_opt_flags(val, values, NULL, list);
+}
+
+#ifdef FEAT_SESSION
+/*
+ * The 'sessionoptions' option is changed.
+ */
+    static char *
+did_set_sessionoptions(char_u *oldval)
+{
+    if (opt_strings_flags(p_ssop, p_ssop_values, &ssop_flags, TRUE) != OK)
+	return e_invalid_argument;
+    if ((ssop_flags & SSOP_CURDIR) && (ssop_flags & SSOP_SESDIR))
+    {
+	// Don't allow both "sesdir" and "curdir".
+	(void)opt_strings_flags(oldval, p_ssop_values, &ssop_flags, TRUE);
+	return e_invalid_argument;
+    }
+
+    return NULL;
+}
+#endif
+
+/*
+ * The 'ambiwidth' option is changed.
+ */
+    static char *
+did_set_ambiwidth(void)
+{
+    if (check_opt_strings(p_ambw, p_ambw_values, FALSE) != OK)
+	return e_invalid_argument;
+
+    return check_chars_options();
+}
+
+/*
+ * The 'background' option is changed.
+ */
+    static char *
+did_set_background(void)
+{
+    if (check_opt_strings(p_bg, p_bg_values, FALSE) == FAIL)
+	return e_invalid_argument;
+
+#ifdef FEAT_EVAL
+    int dark = (*p_bg == 'd');
+#endif
+
+    init_highlight(FALSE, FALSE);
+
+#ifdef FEAT_EVAL
+    if (dark != (*p_bg == 'd')
+	    && get_var_value((char_u *)"g:colors_name") != NULL)
+    {
+	// The color scheme must have set 'background' back to another
+	// value, that's not what we want here.  Disable the color
+	// scheme and set the colors again.
+	do_unlet((char_u *)"g:colors_name", TRUE);
+	free_string_option(p_bg);
+	p_bg = vim_strsave((char_u *)(dark ? "dark" : "light"));
+	check_string_option(&p_bg);
+	init_highlight(FALSE, FALSE);
+    }
+#endif
+#ifdef FEAT_TERMINAL
+    term_update_colors_all();
+#endif
+
+    return NULL;
+}
+
+/*
+ * The 'wildmode' option is changed.
+ */
+    static char *
+did_set_wildmode(void)
+{
+    if (check_opt_wim() == FAIL)
+	return e_invalid_argument;
+    return NULL;
+}
+
+#ifdef FEAT_WAK
+/*
+ * The 'winaltkeys' option is changed.
+ */
+    static char *
+did_set_winaltkeys(void)
+{
+    char *errmsg = NULL;
+
+    if (*p_wak == NUL
+	    || check_opt_strings(p_wak, p_wak_values, FALSE) != OK)
+	errmsg = e_invalid_argument;
+# ifdef FEAT_MENU
+#  if defined(FEAT_GUI_MOTIF)
+    else if (gui.in_use)
+	gui_motif_set_mnemonics(p_wak[0] == 'y' || p_wak[0] == 'm');
+#  elif defined(FEAT_GUI_GTK)
+    else if (gui.in_use)
+	gui_gtk_set_mnemonics(p_wak[0] == 'y' || p_wak[0] == 'm');
+#  endif
+# endif
+    return errmsg;
+}
+#endif
+
+/*
+ * The 'eventignore' option is changed.
+ */
+    static char *
+did_set_eventignore(void)
+{
+    if (check_ei() == FAIL)
+	return e_invalid_argument;
+    return NULL;
+}
+
+/*
+ * One of the 'encoding', 'fileencoding', 'termencoding' or 'makeencoding'
+ * options is changed.
+ */
+    static char *
+did_set_encoding(char_u **varp, char_u **gvarp, int opt_flags)
+{
+    char	*errmsg = NULL;
+    char_u	*p;
+
+    if (gvarp == &p_fenc)
+    {
+	if (!curbuf->b_p_ma && opt_flags != OPT_GLOBAL)
+	    errmsg = e_cannot_make_changes_modifiable_is_off;
+	else if (vim_strchr(*varp, ',') != NULL)
+	    // No comma allowed in 'fileencoding'; catches confusing it
+	    // with 'fileencodings'.
+	    errmsg = e_invalid_argument;
+	else
+	{
+	    // May show a "+" in the title now.
+	    redraw_titles();
+	    // Add 'fileencoding' to the swap file.
+	    ml_setflags(curbuf);
+	}
+    }
+    if (errmsg == NULL)
+    {
+	// canonize the value, so that STRCMP() can be used on it
+	p = enc_canonize(*varp);
+	if (p != NULL)
+	{
+	    vim_free(*varp);
+	    *varp = p;
+	}
+	if (varp == &p_enc)
+	{
+	    errmsg = mb_init();
+	    redraw_titles();
+	}
+    }
+
+#if defined(FEAT_GUI_GTK)
+    if (errmsg == NULL && varp == &p_tenc && gui.in_use)
+    {
+	// GTK uses only a single encoding, and that is UTF-8.
+	if (STRCMP(p_tenc, "utf-8") != 0)
+	    errmsg = e_cannot_be_changed_in_gtk_GUI;
+    }
+#endif
+
+    if (errmsg == NULL)
+    {
+#ifdef FEAT_KEYMAP
+	// When 'keymap' is used and 'encoding' changes, reload the keymap
+	// (with another encoding).
+	if (varp == &p_enc && *curbuf->b_p_keymap != NUL)
+	    (void)keymap_init();
+#endif
+
+	// When 'termencoding' is not empty and 'encoding' changes or when
+	// 'termencoding' changes, need to setup for keyboard input and
+	// display output conversion.
+	if (((varp == &p_enc && *p_tenc != NUL) || varp == &p_tenc))
+	{
+	    if (convert_setup(&input_conv, p_tenc, p_enc) == FAIL
+		    || convert_setup(&output_conv, p_enc, p_tenc) == FAIL)
+	    {
+		semsg(_(e_cannot_convert_between_str_and_str),
+			p_tenc, p_enc);
+		errmsg = e_invalid_argument;
+	    }
+	}
+
+#if defined(MSWIN)
+	// $HOME may have characters in active code page.
+	if (varp == &p_enc)
+	    init_homedir();
+#endif
+    }
+
+    return errmsg;
+}
+
+#if defined(FEAT_POSTSCRIPT)
+/*
+ * The 'printencoding' option is changed.
+ */
+    static void
+did_set_printencoding(void)
+{
+    char_u	*s, *p;
+
+    // Canonize printencoding if VIM standard one
+    p = enc_canonize(p_penc);
+    if (p != NULL)
+    {
+	vim_free(p_penc);
+	p_penc = p;
+    }
+    else
+    {
+	// Ensure lower case and '-' for '_'
+	for (s = p_penc; *s != NUL; s++)
+	{
+	    if (*s == '_')
+		*s = '-';
+	    else
+		*s = TOLOWER_ASC(*s);
+	}
+    }
+}
+#endif
+
+#if defined(FEAT_XIM) && defined(FEAT_GUI_GTK)
+/*
+ * The 'imactivatekey' option is changed.
+ */
+    static char *
+did_set_imactivatekey(void)
+{
+    if (!im_xim_isvalid_imactivate())
+	return e_invalid_argument;
+    return NULL;
+}
+#endif
+
+#ifdef FEAT_KEYMAP
+/*
+ * The 'keymap' option is changed.
+ */
+    static char *
+did_set_keymap(char_u **varp, int opt_flags, int *value_checked)
+{
+    char *errmsg = NULL;
+
+    if (!valid_filetype(*varp))
+	errmsg = e_invalid_argument;
+    else
+    {
+	int	    secure_save = secure;
+
+	// Reset the secure flag, since the value of 'keymap' has
+	// been checked to be safe.
+	secure = 0;
+
+	// load or unload key mapping tables
+	errmsg = keymap_init();
+
+	secure = secure_save;
+
+	// Since we check the value, there is no need to set P_INSECURE,
+	// even when the value comes from a modeline.
+	*value_checked = TRUE;
+    }
+
+    if (errmsg == NULL)
+    {
+	if (*curbuf->b_p_keymap != NUL)
+	{
+	    // Installed a new keymap, switch on using it.
+	    curbuf->b_p_iminsert = B_IMODE_LMAP;
+	    if (curbuf->b_p_imsearch != B_IMODE_USE_INSERT)
+		curbuf->b_p_imsearch = B_IMODE_LMAP;
+	}
+	else
+	{
+	    // Cleared the keymap, may reset 'iminsert' and 'imsearch'.
+	    if (curbuf->b_p_iminsert == B_IMODE_LMAP)
+		curbuf->b_p_iminsert = B_IMODE_NONE;
+	    if (curbuf->b_p_imsearch == B_IMODE_LMAP)
+		curbuf->b_p_imsearch = B_IMODE_USE_INSERT;
+	}
+	if ((opt_flags & OPT_LOCAL) == 0)
+	{
+	    set_iminsert_global();
+	    set_imsearch_global();
+	}
+	status_redraw_curbuf();
+    }
+
+    return errmsg;
+}
+#endif
+
+/*
+ * The 'fileformat' option is changed.
+ */
+    static char *
+did_set_fileformat(char_u **varp, char_u *oldval, int opt_flags)
+{
+    if (!curbuf->b_p_ma && !(opt_flags & OPT_GLOBAL))
+	return e_cannot_make_changes_modifiable_is_off;
+    else if (check_opt_strings(*varp, p_ff_values, FALSE) != OK)
+	return e_invalid_argument;
+
+    // may also change 'textmode'
+    if (get_fileformat(curbuf) == EOL_DOS)
+	curbuf->b_p_tx = TRUE;
+    else
+	curbuf->b_p_tx = FALSE;
+    redraw_titles();
+    // update flag in swap file
+    ml_setflags(curbuf);
+    // Redraw needed when switching to/from "mac": a CR in the text
+    // will be displayed differently.
+    if (get_fileformat(curbuf) == EOL_MAC || *oldval == 'm')
+	redraw_curbuf_later(UPD_NOT_VALID);
+
+    return NULL;
+}
+
+/*
+ * The 'fileformats' option is changed.
+ */
+    static char *
+did_set_fileformats(void)
+{
+    if (check_opt_strings(p_ffs, p_ff_values, TRUE) != OK)
+	return e_invalid_argument;
+
+    // also change 'textauto'
+    if (*p_ffs == NUL)
+	p_ta = FALSE;
+    else
+	p_ta = TRUE;
+
+    return NULL;
+}
+
+#if defined(FEAT_CRYPT)
+/*
+ * The 'cryptkey' option is changed.
+ */
+    static void
+did_set_cryptkey(char_u *oldval)
+{
+    // Make sure the ":set" command doesn't show the new value in the
+    // history.
+    remove_key_from_history();
+
+    if (STRCMP(curbuf->b_p_key, oldval) != 0)
+	// Need to update the swapfile.
+    {
+	ml_set_crypt_key(curbuf, oldval,
+		*curbuf->b_p_cm == NUL ? p_cm : curbuf->b_p_cm);
+	changed_internal();
+    }
+}
+
+/*
+ * The 'cryptmethod' option is changed.
+ */
+    static char *
+did_set_cryptmethod(char_u *oldval, int opt_flags)
+{
+    char_u  *p;
+    char_u  *s;
+
+    if (opt_flags & OPT_LOCAL)
+	p = curbuf->b_p_cm;
+    else
+	p = p_cm;
+    if (check_opt_strings(p, p_cm_values, TRUE) != OK)
+	return e_invalid_argument;
+    else if (crypt_self_test() == FAIL)
+	return e_invalid_argument;
+
+    // When setting the global value to empty, make it "zip".
+    if (*p_cm == NUL)
+    {
+	free_string_option(p_cm);
+	p_cm = vim_strsave((char_u *)"zip");
+    }
+    // When using ":set cm=name" the local value is going to be empty.
+    // Do that here, otherwise the crypt functions will still use the
+    // local value.
+    if ((opt_flags & (OPT_LOCAL | OPT_GLOBAL)) == 0)
+    {
+	free_string_option(curbuf->b_p_cm);
+	curbuf->b_p_cm = empty_option;
+    }
+
+    // Need to update the swapfile when the effective method changed.
+    // Set "s" to the effective old value, "p" to the effective new
+    // method and compare.
+    if ((opt_flags & OPT_LOCAL) && *oldval == NUL)
+	s = p_cm;  // was previously using the global value
+    else
+	s = oldval;
+    if (*curbuf->b_p_cm == NUL)
+	p = p_cm;  // is now using the global value
+    else
+	p = curbuf->b_p_cm;
+    if (STRCMP(s, p) != 0)
+	ml_set_crypt_key(curbuf, curbuf->b_p_key, s);
+
+    // If the global value changes need to update the swapfile for all
+    // buffers using that value.
+    if ((opt_flags & OPT_GLOBAL) && STRCMP(p_cm, oldval) != 0)
+    {
+	buf_T	*buf;
+
+	FOR_ALL_BUFFERS(buf)
+	    if (buf != curbuf && *buf->b_p_cm == NUL)
+		ml_set_crypt_key(buf, buf->b_p_key, oldval);
+    }
+    return NULL;
+}
+#endif
+
+/*
+ * The 'matchpairs' option is changed.
+ */
+    static char *
+did_set_matchpairs(char_u **varp)
+{
+    char_u	*p;
+
+    if (has_mbyte)
+    {
+	for (p = *varp; *p != NUL; ++p)
+	{
+	    int x2 = -1;
+	    int x3 = -1;
+
+	    p += mb_ptr2len(p);
+	    if (*p != NUL)
+		x2 = *p++;
+	    if (*p != NUL)
+	    {
+		x3 = mb_ptr2char(p);
+		p += mb_ptr2len(p);
+	    }
+	    if (x2 != ':' || x3 == -1 || (*p != NUL && *p != ','))
+		return e_invalid_argument;
+	    if (*p == NUL)
+		break;
+	}
+    }
+    else
+    {
+	// Check for "x:y,x:y"
+	for (p = *varp; *p != NUL; p += 4)
+	{
+	    if (p[1] != ':' || p[2] == NUL || (p[3] != NUL && p[3] != ','))
+		return e_invalid_argument;
+	    if (p[3] == NUL)
+		break;
+	}
+    }
+
+    return NULL;
+}
+
+/*
+ * The 'comments' option is changed.
+ */
+    static char *
+did_set_comments(char_u **varp, char *errbuf)
+{
+    char_u	*s;
+    char	*errmsg = NULL;
+
+    for (s = *varp; *s; )
+    {
+	while (*s && *s != ':')
+	{
+	    if (vim_strchr((char_u *)COM_ALL, *s) == NULL
+		    && !VIM_ISDIGIT(*s) && *s != '-')
+	    {
+		errmsg = illegal_char(errbuf, *s);
+		break;
+	    }
+	    ++s;
+	}
+	if (*s++ == NUL)
+	    errmsg = e_missing_colon;
+	else if (*s == ',' || *s == NUL)
+	    errmsg = e_zero_length_string;
+	if (errmsg != NULL)
+	    break;
+	while (*s && *s != ',')
+	{
+	    if (*s == '\\' && s[1] != NUL)
+		++s;
+	    ++s;
+	}
+	s = skip_to_option_part(s);
+    }
+
+    return errmsg;
+}
+
+/*
+ * The global 'listchars' or 'fillchars' option is changed.
+ */
+    static char *
+did_set_global_listfillchars(char_u **varp, int opt_flags)
+{
+    char	*errmsg = NULL;
+    char_u	**local_ptr = varp == &p_lcs
+	? &curwin->w_p_lcs : &curwin->w_p_fcs;
+
+    // only apply the global value to "curwin" when it does not have a
+    // local value
+    errmsg = set_chars_option(curwin, varp,
+	    **local_ptr == NUL || !(opt_flags & OPT_GLOBAL));
+    if (errmsg != NULL)
+	return errmsg;
+
+    tabpage_T	*tp;
+    win_T	*wp;
+
+    // If the current window is set to use the global
+    // 'listchars'/'fillchars' value, clear the window-local value.
+    if (!(opt_flags & OPT_GLOBAL))
+	clear_string_option(local_ptr);
+    FOR_ALL_TAB_WINDOWS(tp, wp)
+    {
+	// If the current window has a local value need to apply it
+	// again, it was changed when setting the global value.
+	// If no error was returned above, we don't expect an error
+	// here, so ignore the return value.
+	local_ptr = varp == &p_lcs ? &wp->w_p_lcs : &wp->w_p_fcs;
+	if (**local_ptr == NUL)
+	    (void)set_chars_option(wp, local_ptr, TRUE);
+    }
+
+    redraw_all_later(UPD_NOT_VALID);
+
+    return NULL;
+}
+
+/*
+ * The 'verbosefile' option is changed.
+ */
+    static char *
+did_set_verbosefile(void)
+{
+    verbose_stop();
+    if (*p_vfile != NUL && verbose_open() == FAIL)
+	return e_invalid_argument;
+
+    return NULL;
+}
+
+#ifdef FEAT_VIMINFO
+/*
+ * The 'viminfo' option is changed.
+ */
+    static char *
+did_set_viminfo(char *errbuf)
+{
+    char_u	*s;
+    char	*errmsg = NULL;
+
+    for (s = p_viminfo; *s;)
+    {
+	// Check it's a valid character
+	if (vim_strchr((char_u *)"!\"%'/:<@cfhnrs", *s) == NULL)
+	{
+	    errmsg = illegal_char(errbuf, *s);
+	    break;
+	}
+	if (*s == 'n')	// name is always last one
+	    break;
+	else if (*s == 'r') // skip until next ','
+	{
+	    while (*++s && *s != ',')
+		;
+	}
+	else if (*s == '%')
+	{
+	    // optional number
+	    while (vim_isdigit(*++s))
+		;
+	}
+	else if (*s == '!' || *s == 'h' || *s == 'c')
+	    ++s;		// no extra chars
+	else		// must have a number
+	{
+	    while (vim_isdigit(*++s))
+		;
+
+	    if (!VIM_ISDIGIT(*(s - 1)))
+	    {
+		if (errbuf != NULL)
+		{
+		    sprintf(errbuf,
+			    _(e_missing_number_after_angle_str_angle),
+			    transchar_byte(*(s - 1)));
+		    errmsg = errbuf;
+		}
+		else
+		    errmsg = "";
+		break;
+	    }
+	}
+	if (*s == ',')
+	    ++s;
+	else if (*s)
+	{
+	    if (errbuf != NULL)
+		errmsg = e_missing_comma;
+	    else
+		errmsg = "";
+	    break;
+	}
+    }
+    if (*p_viminfo && errmsg == NULL && get_viminfo_parameter('\'') < 0)
+	errmsg = e_must_specify_a_value;
+
+    return errmsg;
+}
+#endif
+
+/*
+ * Some terminal option (t_xxx) is changed
+ */
+    static void
+did_set_term_option(char_u **varp, int *did_swaptcap UNUSED)
+{
+    // ":set t_Co=0" and ":set t_Co=1" do ":set t_Co="
+    if (varp == &T_CCO)
+    {
+	int colors = atoi((char *)T_CCO);
+
+	// Only reinitialize colors if t_Co value has really changed to
+	// avoid expensive reload of colorscheme if t_Co is set to the
+	// same value multiple times.
+	if (colors != t_colors)
+	{
+	    t_colors = colors;
+	    if (t_colors <= 1)
+	    {
+		vim_free(T_CCO);
+		T_CCO = empty_option;
+	    }
+#if defined(FEAT_VTP) && defined(FEAT_TERMGUICOLORS)
+	    if (is_term_win32())
+	    {
+		swap_tcap();
+		*did_swaptcap = TRUE;
+	    }
+#endif
+	    // We now have a different color setup, initialize it again.
+	    init_highlight(TRUE, FALSE);
+	}
+    }
+    ttest(FALSE);
+    if (varp == &T_ME)
+    {
+	out_str(T_ME);
+	redraw_later(UPD_CLEAR);
+#if defined(MSWIN) && (!defined(FEAT_GUI_MSWIN) || defined(VIMDLL))
+	// Since t_me has been set, this probably means that the user
+	// wants to use this as default colors.  Need to reset default
+	// background/foreground colors.
+# ifdef VIMDLL
+	if (!gui.in_use && !gui.starting)
+# endif
+	    mch_set_normal_colors();
+#endif
+    }
+    if (varp == &T_BE && termcap_active)
+    {
+	MAY_WANT_TO_LOG_THIS;
+
+	if (*T_BE == NUL)
+	    // When clearing t_BE we assume the user no longer wants
+	    // bracketed paste, thus disable it by writing t_BD.
+	    out_str(T_BD);
+	else
+	    out_str(T_BE);
+    }
+}
+
+#ifdef FEAT_LINEBREAK
+/*
+ * The 'showbreak' option is changed.
+ */
+    static char *
+did_set_showbreak(char_u **varp)
+{
+    char_u	*s;
+
+    for (s = *varp; *s; )
+    {
+	if (ptr2cells(s) != 1)
+	    return e_showbreak_contains_unprintable_or_wide_character;
+	MB_PTR_ADV(s);
+    }
+
+    return NULL;
+}
+#endif
+
+#ifdef FEAT_GUI
+/*
+ * The 'guifont' option is changed.
+ */
+    static char *
+did_set_guifont(char_u *oldval, int *redraw_gui_only)
+{
+    char_u	*p;
+    char	*errmsg = NULL;
+
+    if (gui.in_use)
+    {
+	p = p_guifont;
+# if defined(FEAT_GUI_GTK)
+	// Put up a font dialog and let the user select a new value.
+	// If this is cancelled go back to the old value but don't
+	// give an error message.
+	if (STRCMP(p, "*") == 0)
+	{
+	    p = gui_mch_font_dialog(oldval);
+	    free_string_option(p_guifont);
+	    p_guifont = (p != NULL) ? p : vim_strsave(oldval);
+	}
+# endif
+	if (p != NULL && gui_init_font(p_guifont, FALSE) != OK)
+	{
+# if defined(FEAT_GUI_MSWIN) || defined(FEAT_GUI_PHOTON)
+	    if (STRCMP(p_guifont, "*") == 0)
+	    {
+		// Dialog was cancelled: Keep the old value without giving
+		// an error message.
+		free_string_option(p_guifont);
+		p_guifont = vim_strsave(oldval);
+	    }
+	    else
+# endif
+		errmsg = e_invalid_fonts;
+	}
+    }
+    *redraw_gui_only = TRUE;
+
+    return errmsg;
+}
+
+# ifdef FEAT_XFONTSET
+/*
+ * The 'guifontset' option is changed.
+ */
+    static char *
+did_set_guifontset(int *redraw_gui_only)
+{
+    char *errmsg = NULL;
+
+    if (STRCMP(p_guifontset, "*") == 0)
+	errmsg = e_cant_select_fontset;
+    else if (gui.in_use && gui_init_font(p_guifontset, TRUE) != OK)
+	errmsg = e_invalid_fontset;
+    *redraw_gui_only = TRUE;
+
+    return errmsg;
+}
+# endif
+
+/*
+ * The 'guifontwide' option is changed.
+ */
+    static char *
+did_set_guifontwide(int *redraw_gui_only)
+{
+    char *errmsg = NULL;
+
+    if (STRCMP(p_guifontwide, "*") == 0)
+	errmsg = e_cant_select_wide_font;
+    else if (gui_get_wide_font() == FAIL)
+	errmsg = e_invalid_wide_font;
+    *redraw_gui_only = TRUE;
+
+    return errmsg;
+}
+#endif
+
+#if defined(FEAT_GUI_GTK)
+    static void
+did_set_guiligatures(int *redraw_gui_only)
+{
+    gui_set_ligatures();
+    *redraw_gui_only = TRUE;
+}
+#endif
+
+#ifdef FEAT_MOUSESHAPE
+    static char *
+did_set_mouseshape(void)
+{
+    char *errmsg = NULL;
+
+    errmsg = parse_shape_opt(SHAPE_MOUSE);
+    update_mouseshape(-1);
+
+    return errmsg;
+}
+#endif
+
+/*
+ * The 'titlestring' or the 'iconstring' option is changed.
+ */
+    static void
+did_set_titleiconstring(char_u **varp UNUSED)
+{
+#ifdef FEAT_STL_OPT
+    int	flagval = (varp == &p_titlestring) ? STL_IN_TITLE : STL_IN_ICON;
+
+    // NULL => statusline syntax
+    if (vim_strchr(*varp, '%') && check_stl_option(*varp) == NULL)
+	stl_syntax |= flagval;
+    else
+	stl_syntax &= ~flagval;
+#endif
+    did_set_title();
+}
+
+#ifdef FEAT_GUI
+/*
+ * The 'guioptions' option is changed.
+ */
+    static void
+did_set_guioptions(char_u *oldval, int *redraw_gui_only)
+{
+    gui_init_which_components(oldval);
+    *redraw_gui_only = TRUE;
+}
+#endif
+
+#if defined(FEAT_GUI_TABLINE)
+    static void
+did_set_guitablabel(int *redraw_gui_only)
+{
+    redraw_tabline = TRUE;
+    *redraw_gui_only = TRUE;
+}
+#endif
+
+#if defined(UNIX) || defined(VMS)
+/*
+ * The 'ttymouse' option is changed.
+ */
+    static char *
+did_set_ttymouse(void)
+{
+    char *errmsg = NULL;
+
+    // Switch the mouse off before changing the escape sequences used for
+    // that.
+    mch_setmouse(FALSE);
+    if (opt_strings_flags(p_ttym, p_ttym_values, &ttym_flags, FALSE) != OK)
+	errmsg = e_invalid_argument;
+    else
+	check_mouse_termcode();
+    if (termcap_active)
+	setmouse();		// may switch it on again
+
+    return errmsg;
+}
+#endif
+
+/*
+ * The 'selection' option is changed.
+ */
+    static char *
+did_set_selection(void)
+{
+    if (*p_sel == NUL
+	    || check_opt_strings(p_sel, p_sel_values, FALSE) != OK)
+	return e_invalid_argument;
+
+    return NULL;
+}
+
+#ifdef FEAT_BROWSE
+/*
+ * The 'browsedir' option is changed.
+ */
+    static char *
+did_set_browsedir(void)
+{
+    if (check_opt_strings(p_bsdir, p_bsdir_values, FALSE) != OK
+	    && !mch_isdir(p_bsdir))
+	return e_invalid_argument;
+
+    return NULL;
+}
+#endif
+
+/*
+ * The 'keymodel' option is changed.
+ */
+    static char *
+did_set_keymodel(void)
+{
+    if (check_opt_strings(p_km, p_km_values, TRUE) != OK)
+	return e_invalid_argument;
+
+    km_stopsel = (vim_strchr(p_km, 'o') != NULL);
+    km_startsel = (vim_strchr(p_km, 'a') != NULL);
+    return NULL;
+}
+
+/*
+ * The 'keyprotocol' option is changed.
+ */
+    static char *
+did_set_keyprotocol(void)
+{
+    if (match_keyprotocol(NULL) == KEYPROTOCOL_FAIL)
+	return e_invalid_argument;
+
+    return NULL;
+}
+
+/*
+ * The 'mousemodel' option is changed.
+ */
+    static char *
+did_set_mousemodel(void)
+{
+    if (check_opt_strings(p_mousem, p_mousem_values, FALSE) != OK)
+	return e_invalid_argument;
+#if defined(FEAT_GUI_MOTIF) && defined(FEAT_MENU) && (XmVersion <= 1002)
+    else if (*p_mousem != *oldval)
+	// Changed from "extend" to "popup" or "popup_setpos" or vv: need
+	// to create or delete the popup menus.
+	gui_motif_update_mousemodel(root_menu);
+#endif
+
+    return NULL;
+}
+
+/*
+ * The 'display' option is changed.
+ */
+    static char *
+did_set_display(void)
+{
+    if (opt_strings_flags(p_dy, p_dy_values, &dy_flags, TRUE) != OK)
+	return e_invalid_argument;
+
+    (void)init_chartab();
+    return NULL;
+}
+
+#ifdef FEAT_SPELL
+/*
+ * The 'spellfile' option is changed.
+ */
+    static char *
+did_set_spellfile(char_u **varp)
+{
+    if (!valid_spellfile(*varp))
+	return e_invalid_argument;
+
+    // If there is a window for this buffer in which 'spell' is set load the
+    // wordlists.
+    return did_set_spell_option(TRUE);
+}
+
+/*
+ * The 'spell' option is changed.
+ */
+    static char *
+did_set_spell(char_u **varp)
+{
+    if (!valid_spelllang(*varp))
+	return e_invalid_argument;
+
+    // If there is a window for this buffer in which 'spell' is set load the
+    // wordlists.
+    return did_set_spell_option(FALSE);
+}
+
+/*
+ * The 'spellcapcheck' option is changed.
+ */
+    static char *
+did_set_spellcapcheck(void)
+{
+    // compile the regexp program.
+    return compile_cap_prog(curwin->w_s);
+}
+
+/*
+ * The 'spelloptions' option is changed.
+ */
+    static char *
+did_set_spelloptions(char_u **varp)
+{
+    if (**varp != NUL && STRCMP("camel", *varp) != 0)
+	return e_invalid_argument;
+
+    return NULL;
+}
+
+/*
+ * The 'spellsuggest' option is changed.
+ */
+    static char *
+did_set_spellsuggest(void)
+{
+    if (spell_check_sps() != OK)
+	return e_invalid_argument;
+
+    return NULL;
+}
+
+/*
+ * The 'mkspellmem' option is changed.
+ */
+    static char *
+did_set_mkspellmem(void)
+{
+    if (spell_check_msm() != OK)
+	return e_invalid_argument;
+
+    return NULL;
+}
+#endif
+
+/*
+ * The 'buftype' option is changed.
+ */
+    static char *
+did_set_buftype(void)
+{
+    if (check_opt_strings(curbuf->b_p_bt, p_buftype_values, FALSE) != OK)
+	return e_invalid_argument;
+
+    if (curwin->w_status_height)
+    {
+	curwin->w_redr_status = TRUE;
+	redraw_later(UPD_VALID);
+    }
+    curbuf->b_help = (curbuf->b_p_bt[0] == 'h');
+    redraw_titles();
+
+    return NULL;
+}
+
+#ifdef FEAT_STL_OPT
+/*
+ * The 'statusline' or the 'tabline' or the 'rulerformat' option is changed.
+ */
+    static char *
+did_set_statusline(char_u **varp)
+{
+    char_u	*s;
+    char	*errmsg = NULL;
+    int		wid;
+
+    if (varp == &p_ruf)	// reset ru_wid first
+	ru_wid = 0;
+    s = *varp;
+    if (varp == &p_ruf && *s == '%')
+    {
+	// set ru_wid if 'ruf' starts with "%99("
+	if (*++s == '-')	// ignore a '-'
+	    s++;
+	wid = getdigits(&s);
+	if (wid && *s == '(' && (errmsg = check_stl_option(p_ruf)) == NULL)
+	    ru_wid = wid;
+	else
+	    errmsg = check_stl_option(p_ruf);
+    }
+    // check 'statusline' or 'tabline' only if it doesn't start with "%!"
+    else if (varp == &p_ruf || s[0] != '%' || s[1] != '!')
+	errmsg = check_stl_option(s);
+    if (varp == &p_ruf && errmsg == NULL)
+	comp_col();
+
+    return errmsg;
+}
+#endif
+
+/*
+ * The 'complete' option is changed.
+ */
+    static char *
+did_set_complete(char_u **varp, char *errbuf)
+{
+    char_u	*s;
+
+    // check if it is a valid value for 'complete' -- Acevedo
+    for (s = *varp; *s;)
+    {
+	while (*s == ',' || *s == ' ')
+	    s++;
+	if (!*s)
+	    break;
+	if (vim_strchr((char_u *)".wbuksid]tU", *s) == NULL)
+	    return illegal_char(errbuf, *s);
+	if (*++s != NUL && *s != ',' && *s != ' ')
+	{
+	    if (s[-1] == 'k' || s[-1] == 's')
+	    {
+		// skip optional filename after 'k' and 's'
+		while (*s && *s != ',' && *s != ' ')
+		{
+		    if (*s == '\\' && s[1] != NUL)
+			++s;
+		    ++s;
+		}
+	    }
+	    else
+	    {
+		if (errbuf != NULL)
+		{
+		    sprintf((char *)errbuf,
+			    _(e_illegal_character_after_chr), *--s);
+		    return errbuf;
+		}
+		return "";
+	    }
+	}
+    }
+
+    return NULL;
+}
+
+/*
+ * The 'completeopt' option is changed.
+ */
+    static char *
+did_set_completeopt(void)
+{
+    if (check_opt_strings(p_cot, p_cot_values, TRUE) != OK)
+	return e_invalid_argument;
+
+    completeopt_was_set();
+    return NULL;
+}
+
+#ifdef FEAT_SIGNS
+/*
+ * The 'signcolumn' option is changed.
+ */
+    static char *
+did_set_signcolumn(char_u **varp, char_u *oldval)
+{
+    if (check_opt_strings(*varp, p_scl_values, FALSE) != OK)
+	return e_invalid_argument;
+    // When changing the 'signcolumn' to or from 'number', recompute the
+    // width of the number column if 'number' or 'relativenumber' is set.
+    if (((*oldval == 'n' && *(oldval + 1) == 'u')
+		|| (*curwin->w_p_scl == 'n' && *(curwin->w_p_scl + 1) =='u'))
+	    && (curwin->w_p_nu || curwin->w_p_rnu))
+	curwin->w_nrwidth_line_count = 0;
+
+    return NULL;
+}
+#endif
+
+#if defined(FEAT_TOOLBAR) && !defined(FEAT_GUI_MSWIN)
+/*
+ * The 'toolbar' option is changed.
+ */
+    static char *
+did_set_toolbar(void)
+{
+    if (opt_strings_flags(p_toolbar, p_toolbar_values,
+		&toolbar_flags, TRUE) != OK)
+	return e_invalid_argument;
+
+    out_flush();
+    gui_mch_show_toolbar((toolbar_flags &
+		(TOOLBAR_TEXT | TOOLBAR_ICONS)) != 0);
+    return NULL;
+}
+#endif
+
+#if defined(FEAT_TOOLBAR) && defined(FEAT_GUI_GTK)
+/*
+ * The 'toolbariconsize' option is changed.  GTK+ 2 only.
+ */
+    static char *
+did_set_toolbariconsize(void)
+{
+    if (opt_strings_flags(p_tbis, p_tbis_values, &tbis_flags, FALSE) != OK)
+	return e_invalid_argument;
+
+    out_flush();
+    gui_mch_show_toolbar((toolbar_flags &
+		(TOOLBAR_TEXT | TOOLBAR_ICONS)) != 0);
+    return NULL;
+}
+#endif
+
+/*
+ * The 'pastetoggle' option is changed.
+ */
+    static void
+did_set_pastetoggle(void)
+{
+    char_u	*p;
+
+    // translate key codes like in a mapping
+    if (*p_pt)
+    {
+	(void)replace_termcodes(p_pt, &p,
+		REPTERM_FROM_PART | REPTERM_DO_LT, NULL);
+	if (p != NULL)
+	{
+	    free_string_option(p_pt);
+	    p_pt = p;
+	}
+    }
+}
+
+/*
+ * The 'backspace' option is changed.
+ */
+    static char *
+did_set_backspace(void)
+{
+    if (VIM_ISDIGIT(*p_bs))
+    {
+	if (*p_bs > '3' || p_bs[1] != NUL)
+	    return e_invalid_argument;
+    }
+    else if (check_opt_strings(p_bs, p_bs_values, TRUE) != OK)
+	return e_invalid_argument;
+
+    return NULL;
+}
+
+/*
+ * The 'tagcase' option is changed.
+ */
+    static char *
+did_set_tagcase(int opt_flags)
+{
+    unsigned int	*flags;
+    char_u		*p;
+
+    if (opt_flags & OPT_LOCAL)
+    {
+	p = curbuf->b_p_tc;
+	flags = &curbuf->b_tc_flags;
+    }
+    else
+    {
+	p = p_tc;
+	flags = &tc_flags;
+    }
+
+    if ((opt_flags & OPT_LOCAL) && *p == NUL)
+	// make the local value empty: use the global value
+	*flags = 0;
+    else if (*p == NUL
+	    || opt_strings_flags(p, p_tc_values, flags, FALSE) != OK)
+	return e_invalid_argument;
+
+    return NULL;
+}
+
+#ifdef FEAT_DIFF
+/*
+ * The 'diffopt' option is changed.
+ */
+    static char *
+did_set_diffopt(void)
+{
+    if (diffopt_changed() == FAIL)
+	return e_invalid_argument;
+
+    return NULL;
+}
+#endif
+
+#ifdef FEAT_FOLDING
+/*
+ * The 'foldmethod' option is changed.
+ */
+    static char *
+did_set_foldmethod(char_u **varp)
+{
+    if (check_opt_strings(*varp, p_fdm_values, FALSE) != OK
+	    || *curwin->w_p_fdm == NUL)
+	return e_invalid_argument;
+
+    foldUpdateAll(curwin);
+    if (foldmethodIsDiff(curwin))
+	newFoldLevel();
+    return NULL;
+}
+
+/*
+ * The 'foldmarker' option is changed.
+ */
+    static char *
+did_set_foldmarker(char_u **varp)
+{
+    char_u	*p;
+
+    p = vim_strchr(*varp, ',');
+    if (p == NULL)
+	return e_comma_required;
+    else if (p == *varp || p[1] == NUL)
+	return e_invalid_argument;
+    else if (foldmethodIsMarker(curwin))
+	foldUpdateAll(curwin);
+
+    return NULL;
+}
+
+/*
+ * The 'commentstring' option is changed.
+ */
+    static char *
+did_set_commentstring(char_u **varp)
+{
+    if (**varp != NUL && strstr((char *)*varp, "%s") == NULL)
+	return e_commentstring_must_be_empty_or_contain_str;
+
+    return NULL;
+}
+
+/*
+ * The 'foldignore' option is changed.
+ */
+    static void
+did_set_foldignore(void)
+{
+    if (foldmethodIsIndent(curwin))
+	foldUpdateAll(curwin);
+}
+#endif
+
+/*
+ * The 'virtualedit' option is changed.
+ */
+    static char *
+did_set_virtualedit(char_u *oldval, int opt_flags)
+{
+    char_u		*ve = p_ve;
+    unsigned int	*flags = &ve_flags;
+
+    if (opt_flags & OPT_LOCAL)
+    {
+	ve = curwin->w_p_ve;
+	flags = &curwin->w_ve_flags;
+    }
+
+    if ((opt_flags & OPT_LOCAL) && *ve == NUL)
+	// make the local value empty: use the global value
+	*flags = 0;
+    else
+    {
+	if (opt_strings_flags(ve, p_ve_values, flags, TRUE) != OK)
+	    return e_invalid_argument;
+	else if (STRCMP(ve, oldval) != 0)
+	{
+	    // Recompute cursor position in case the new 've' setting
+	    // changes something.
+	    validate_virtcol();
+	    coladvance(curwin->w_virtcol);
+	}
+    }
+
+    return NULL;
+}
+
+#if defined(FEAT_CSCOPE) && defined(FEAT_QUICKFIX)
+/*
+ * The 'cscopequickfix' option is changed.
+ */
+    static char *
+did_set_cscopequickfix(void)
+{
+    char_u	*p;
+
+    if (p_csqf == NULL)
+	return NULL;
+
+    p = p_csqf;
+    while (*p != NUL)
+    {
+	if (vim_strchr((char_u *)CSQF_CMDS, *p) == NULL
+		|| p[1] == NUL
+		|| vim_strchr((char_u *)CSQF_FLAGS, p[1]) == NULL
+		|| (p[2] != NUL && p[2] != ','))
+	    return e_invalid_argument;
+	else if (p[2] == NUL)
+	    break;
+	else
+	    p += 3;
+    }
+
+    return NULL;
+}
+#endif
+
+/*
+ * The 'cinoptions' option is changed.
+ */
+    static void
+did_set_cinoptions(void)
+{
+    // TODO: recognize errors
+    parse_cino(curbuf);
+}
+
+/*
+ * The 'lispoptions' option is changed.
+ */
+    static char *
+did_set_lispoptions(char_u **varp)
+{
+    if (**varp != NUL && STRCMP(*varp, "expr:0") != 0
+	    && STRCMP(*varp, "expr:1") != 0)
+	return e_invalid_argument;
+
+    return NULL;
+}
+
+#if defined(FEAT_RENDER_OPTIONS)
+/*
+ * The 'renderoptions' option is changed.
+ */
+    static char *
+did_set_renderoptions(void)
+{
+    if (!gui_mch_set_rendering_options(p_rop))
+	return e_invalid_argument;
+
+    return NULL;
+}
+#endif
+
+/*
+ * The 'filetype' or the 'syntax' option is changed.
+ */
+    static char *
+did_set_filetype_or_syntax(
+    char_u	**varp,
+    char_u	*oldval,
+    int		*value_checked,
+    int		*value_changed)
+{
+    if (!valid_filetype(*varp))
+	return e_invalid_argument;
+
+    *value_changed = STRCMP(oldval, *varp) != 0;
+
+    // Since we check the value, there is no need to set P_INSECURE,
+    // even when the value comes from a modeline.
+    *value_checked = TRUE;
+
+    return NULL;
+}
+
+#ifdef FEAT_TERMINAL
+/*
+ * The 'termwinkey' option is changed.
+ */
+    static char *
+did_set_termwinkey(void)
+{
+    if (*curwin->w_p_twk != NUL
+	    && string_to_key(curwin->w_p_twk, TRUE) == 0)
+	return e_invalid_argument;
+
+    return NULL;
+}
+
+/*
+ * The 'termwinsize' option is changed.
+ */
+    static char *
+did_set_termwinsize(void)
+{
+    char_u	*p;
+
+    if (*curwin->w_p_tws == NUL)
+	return NULL;
+
+    p = skipdigits(curwin->w_p_tws);
+    if (p == curwin->w_p_tws
+	    || (*p != 'x' && *p != '*')
+	    || *skipdigits(p + 1) != NUL)
+	return e_invalid_argument;
+
+    return NULL;
+}
+#endif
+
+#ifdef FEAT_VARTABS
+/*
+ * The 'varsofttabstop' option is changed.
+ */
+    static char *
+did_set_varsofttabstop(char_u **varp)
+{
+    char_u *cp;
+
+    if (!(*varp)[0] || ((*varp)[0] == '0' && !(*varp)[1]))
+    {
+	if (curbuf->b_p_vsts_array)
+	{
+	    vim_free(curbuf->b_p_vsts_array);
+	    curbuf->b_p_vsts_array = 0;
+	}
+    }
+    else
+    {
+	for (cp = *varp; *cp; ++cp)
+	{
+	    if (vim_isdigit(*cp))
+		continue;
+	    if (*cp == ',' && cp > *varp && *(cp-1) != ',')
+		continue;
+	    return e_invalid_argument;
+	}
+
+	int *oldarray = curbuf->b_p_vsts_array;
+	if (tabstop_set(*varp, &(curbuf->b_p_vsts_array)) == OK)
+	{
+	    if (oldarray)
+		vim_free(oldarray);
+	}
+	else
+	    return e_invalid_argument;
+    }
+
+    return NULL;
+}
+
+/*
+ * The 'vartabstop' option is changed.
+ */
+    static char *
+did_set_vartabstop(char_u **varp)
+{
+    char_u *cp;
+
+    if (!(*varp)[0] || ((*varp)[0] == '0' && !(*varp)[1]))
+    {
+	if (curbuf->b_p_vts_array)
+	{
+	    vim_free(curbuf->b_p_vts_array);
+	    curbuf->b_p_vts_array = NULL;
+	}
+    }
+    else
+    {
+	for (cp = *varp; *cp; ++cp)
+	{
+	    if (vim_isdigit(*cp))
+		continue;
+	    if (*cp == ',' && cp > *varp && *(cp-1) != ',')
+		continue;
+	    return e_invalid_argument;
+	}
+
+	int *oldarray = curbuf->b_p_vts_array;
+
+	if (tabstop_set(*varp, &(curbuf->b_p_vts_array)) == OK)
+	{
+	    vim_free(oldarray);
+# ifdef FEAT_FOLDING
+	    if (foldmethodIsIndent(curwin))
+		foldUpdateAll(curwin);
+# endif
+	}
+	else
+	    return e_invalid_argument;
+    }
+
+    return NULL;
+}
+#endif
+
+#ifdef FEAT_PROP_POPUP
+/*
+ * The 'previewpopup' option is changed.
+ */
+    static char *
+did_set_previewpopup(void)
+{
+    if (parse_previewpopup(NULL) == FAIL)
+	return e_invalid_argument;
+
+    return NULL;
+}
+
+# ifdef FEAT_QUICKFIX
+/*
+ * The 'completepopup' option is changed.
+ */
+    static char *
+did_set_completepopup(void)
+{
+    if (parse_completepopup(NULL) == FAIL)
+	return e_invalid_argument;
+
+    popup_close_info();
+    return NULL;
+}
+# endif
+#endif
+
+#ifdef FEAT_EVAL
+/*
+ * One of the '*expr' options is changed: 'balloonexpr', 'diffexpr',
+ * 'foldexpr', 'foldtext', 'formatexpr', 'includeexpr', 'indentexpr',
+ * 'patchexpr', 'printexpr' and 'charconvert'.
+ *
+ */
+    static void
+did_set_optexpr(char_u **varp)
+{
+    // If the option value starts with <SID> or s:, then replace that with
+    // the script identifier.
+    char_u *name = get_scriptlocal_funcname(*varp);
+    if (name != NULL)
+    {
+	free_string_option(*varp);
+	*varp = name;
+    }
+
+# ifdef FEAT_FOLDING
+    if (varp == &curwin->w_p_fde && foldmethodIsExpr(curwin))
+	foldUpdateAll(curwin);
+# endif
+}
+#endif
+
+/*
  * Handle string options that need some action to perform when changed.
  * The new value must be allocated.
  * Returns NULL for success, or an unstranslated error message for an error.
@@ -664,9 +2516,7 @@ did_set_string_option(
     int		redraw_gui_only = FALSE;
 #endif
     int		value_changed = FALSE;
-#if defined(FEAT_VTP) && defined(FEAT_TERMGUICOLORS)
     int		did_swaptcap = FALSE;
-#endif
 
     // Get the global option to compare with, otherwise we would have to check
     // two values for all local options.
@@ -692,76 +2542,20 @@ did_set_string_option(
 
     // 'term'
     else if (varp == &T_NAME)
-    {
-	if (T_NAME[0] == NUL)
-	    errmsg = e_cannot_set_term_to_empty_string;
-#ifdef FEAT_GUI
-	else if (gui.in_use)
-	    errmsg = e_cannot_change_term_in_GUI;
-	else if (term_is_gui(T_NAME))
-	    errmsg = e_use_gui_to_start_GUI;
-#endif
-	else if (set_termname(T_NAME) == FAIL)
-	    errmsg = e_not_found_in_termcap;
-	else
-	{
-	    // Screen colors may have changed.
-	    redraw_later_clear();
-
-	    // Both 'term' and 'ttytype' point to T_NAME, only set the
-	    // P_ALLOCED flag on 'term'.
-	    opt_idx = findoption((char_u *)"term");
-	    free_oldval = (get_option_flags(opt_idx) & P_ALLOCED);
-	}
-    }
+	errmsg = did_set_term(&opt_idx, &free_oldval);
 
     // 'backupcopy'
     else if (gvarp == &p_bkc)
-    {
-	char_u		*bkc = p_bkc;
-	unsigned int	*flags = &bkc_flags;
-
-	if (opt_flags & OPT_LOCAL)
-	{
-	    bkc = curbuf->b_p_bkc;
-	    flags = &curbuf->b_bkc_flags;
-	}
-
-	if ((opt_flags & OPT_LOCAL) && *bkc == NUL)
-	    // make the local value empty: use the global value
-	    *flags = 0;
-	else
-	{
-	    if (opt_strings_flags(bkc, p_bkc_values, flags, TRUE) != OK)
-		errmsg = e_invalid_argument;
-	    if ((((int)*flags & BKC_AUTO) != 0)
-		    + (((int)*flags & BKC_YES) != 0)
-		    + (((int)*flags & BKC_NO) != 0) != 1)
-	    {
-		// Must have exactly one of "auto", "yes"  and "no".
-		(void)opt_strings_flags(oldval, p_bkc_values, flags, TRUE);
-		errmsg = e_invalid_argument;
-	    }
-	}
-    }
+	errmsg = did_set_backupcopy(oldval, opt_flags);
 
     // 'backupext' and 'patchmode'
     else if (varp == &p_bex || varp == &p_pm)
-    {
-	if (STRCMP(*p_bex == '.' ? p_bex + 1 : p_bex,
-		     *p_pm == '.' ? p_pm + 1 : p_pm) == 0)
-	    errmsg = e_backupext_and_patchmode_are_equal;
-    }
+	errmsg = did_set_backupext_or_patchmode();
+
 #ifdef FEAT_LINEBREAK
     // 'breakindentopt'
     else if (varp == &curwin->w_p_briopt)
-    {
-	if (briopt_check(curwin) == FAIL)
-	    errmsg = e_invalid_argument;
-	// list setting requires a redraw
-	if (curwin->w_briopt_list)
-	    redraw_all_later(UPD_NOT_VALID);
-    }
+	errmsg = did_set_breakindentopt();
 #endif
 
     // 'isident', 'iskeyword', 'isprint or 'isfname' option: refill g_chartab[]
@@ -771,32 +2565,17 @@ did_set_string_option(
 	    || varp == &(curbuf->b_p_isk)
 	    || varp == &p_isp
 	    || varp == &p_isf)
-    {
-	if (init_chartab() == FAIL)
-	{
-	    did_chartab = TRUE;	    // need to restore it below
-	    errmsg = e_invalid_argument;	    // error in value
-	}
-    }
+	errmsg = did_set_isopt(&did_chartab);
 
     // 'helpfile'
     else if (varp == &p_hf)
-    {
-	// May compute new values for $VIM and $VIMRUNTIME
-	if (didset_vim)
-	    vim_unsetenv_ext((char_u *)"VIM");
-	if (didset_vimruntime)
-	    vim_unsetenv_ext((char_u *)"VIMRUNTIME");
-    }
+	did_set_helpfile();
 
 #ifdef FEAT_SYN_HL
     // 'cursorlineopt'
     else if (varp == &curwin->w_p_culopt
 				  || gvarp == &curwin->w_allbuf_opt.wo_culopt)
-    {
-	if (**varp == NUL || fill_culopt_flags(*varp, curwin) != OK)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_cursorlineopt(varp);
 
     // 'colorcolumn'
     else if (varp == &curwin->w_p_cc)
@@ -806,739 +2585,158 @@ did_set_string_option(
 #ifdef FEAT_MULTI_LANG
     // 'helplang'
     else if (varp == &p_hlg)
-    {
-	// Check for "", "ab", "ab,cd", etc.
-	for (s = p_hlg; *s != NUL; s += 3)
-	{
-	    if (s[1] == NUL || ((s[2] != ',' || s[3] == NUL) && s[2] != NUL))
-	    {
-		errmsg = e_invalid_argument;
-		break;
-	    }
-	    if (s[2] == NUL)
-		break;
-	}
-    }
+	errmsg = did_set_helplang();
 #endif
 
     // 'highlight'
     else if (varp == &p_hl)
-    {
-	if (highlight_changed() == FAIL)
-	    errmsg = e_invalid_argument;	// invalid flags
-    }
+	errmsg = did_set_highlight();
 
     // 'nrformats'
     else if (gvarp == &p_nf)
-    {
-	if (check_opt_strings(*varp, p_nf_values, TRUE) != OK)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_opt_strings(*varp, p_nf_values, TRUE);
 
 #ifdef FEAT_SESSION
     // 'sessionoptions'
     else if (varp == &p_ssop)
-    {
-	if (opt_strings_flags(p_ssop, p_ssop_values, &ssop_flags, TRUE) != OK)
-	    errmsg = e_invalid_argument;
-	if ((ssop_flags & SSOP_CURDIR) && (ssop_flags & SSOP_SESDIR))
-	{
-	    // Don't allow both "sesdir" and "curdir".
-	    (void)opt_strings_flags(oldval, p_ssop_values, &ssop_flags, TRUE);
-	    errmsg = e_invalid_argument;
-	}
-    }
+	errmsg = did_set_sessionoptions(oldval);
+
     // 'viewoptions'
     else if (varp == &p_vop)
-    {
-	if (opt_strings_flags(p_vop, p_ssop_values, &vop_flags, TRUE) != OK)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_opt_flags(p_vop, p_ssop_values, &vop_flags, TRUE);
 #endif
 
     // 'scrollopt'
     else if (varp == &p_sbo)
-    {
-	if (check_opt_strings(p_sbo, p_scbopt_values, TRUE) != OK)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_opt_strings(p_sbo, p_scbopt_values, TRUE);
 
     // 'ambiwidth'
     else if (varp == &p_ambw || varp == &p_emoji)
-    {
-	if (check_opt_strings(p_ambw, p_ambw_values, FALSE) != OK)
-	    errmsg = e_invalid_argument;
-	else
-	    errmsg = check_chars_options();
-    }
+	errmsg = did_set_ambiwidth();
 
     // 'background'
     else if (varp == &p_bg)
-    {
-	if (check_opt_strings(p_bg, p_bg_values, FALSE) == OK)
-	{
-#ifdef FEAT_EVAL
-	    int dark = (*p_bg == 'd');
-#endif
-
-	    init_highlight(FALSE, FALSE);
-
-#ifdef FEAT_EVAL
-	    if (dark != (*p_bg == 'd')
-			  && get_var_value((char_u *)"g:colors_name") != NULL)
-	    {
-		// The color scheme must have set 'background' back to another
-		// value, that's not what we want here.  Disable the color
-		// scheme and set the colors again.
-		do_unlet((char_u *)"g:colors_name", TRUE);
-		free_string_option(p_bg);
-		p_bg = vim_strsave((char_u *)(dark ? "dark" : "light"));
-		check_string_option(&p_bg);
-		init_highlight(FALSE, FALSE);
-	    }
-#endif
-#ifdef FEAT_TERMINAL
-	    term_update_colors_all();
-#endif
-	}
-	else
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_background();
 
     // 'wildmode'
     else if (varp == &p_wim)
-    {
-	if (check_opt_wim() == FAIL)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_wildmode();
 
     // 'wildoptions'
     else if (varp == &p_wop)
-    {
-	if (check_opt_strings(p_wop, p_wop_values, TRUE) != OK)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_opt_strings(p_wop, p_wop_values, TRUE);
 
 #ifdef FEAT_WAK
     // 'winaltkeys'
     else if (varp == &p_wak)
-    {
-	if (*p_wak == NUL
-		|| check_opt_strings(p_wak, p_wak_values, FALSE) != OK)
-	    errmsg = e_invalid_argument;
-# ifdef FEAT_MENU
-#  if defined(FEAT_GUI_MOTIF)
-	else if (gui.in_use)
-	    gui_motif_set_mnemonics(p_wak[0] == 'y' || p_wak[0] == 'm');
-#  elif defined(FEAT_GUI_GTK)
-	else if (gui.in_use)
-	    gui_gtk_set_mnemonics(p_wak[0] == 'y' || p_wak[0] == 'm');
-#  endif
-# endif
-    }
+	errmsg = did_set_winaltkeys();
 #endif
 
     // 'eventignore'
     else if (varp == &p_ei)
-    {
-	if (check_ei() == FAIL)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_eventignore();
 
     // 'encoding', 'fileencoding', 'termencoding' and 'makeencoding'
     else if (varp == &p_enc || gvarp == &p_fenc || varp == &p_tenc
 							   || gvarp == &p_menc)
-    {
-	if (gvarp == &p_fenc)
-	{
-	    if (!curbuf->b_p_ma && opt_flags != OPT_GLOBAL)
-		errmsg = e_cannot_make_changes_modifiable_is_off;
-	    else if (vim_strchr(*varp, ',') != NULL)
-		// No comma allowed in 'fileencoding'; catches confusing it
-		// with 'fileencodings'.
-		errmsg = e_invalid_argument;
-	    else
-	    {
-		// May show a "+" in the title now.
-		redraw_titles();
-		// Add 'fileencoding' to the swap file.
-		ml_setflags(curbuf);
-	    }
-	}
-	if (errmsg == NULL)
-	{
-	    // canonize the value, so that STRCMP() can be used on it
-	    p = enc_canonize(*varp);
-	    if (p != NULL)
-	    {
-		vim_free(*varp);
-		*varp = p;
-	    }
-	    if (varp == &p_enc)
-	    {
-		errmsg = mb_init();
-		redraw_titles();
-	    }
-	}
-
-#if defined(FEAT_GUI_GTK)
-	if (errmsg == NULL && varp == &p_tenc && gui.in_use)
-	{
-	    // GTK uses only a single encoding, and that is UTF-8.
-	    if (STRCMP(p_tenc, "utf-8") != 0)
-		errmsg = e_cannot_be_changed_in_gtk_GUI;
-	}
-#endif
-
-	if (errmsg == NULL)
-	{
-#ifdef FEAT_KEYMAP
-	    // When 'keymap' is used and 'encoding' changes, reload the keymap
-	    // (with another encoding).
-	    if (varp == &p_enc && *curbuf->b_p_keymap != NUL)
-		(void)keymap_init();
-#endif
-
-	    // When 'termencoding' is not empty and 'encoding' changes or when
-	    // 'termencoding' changes, need to setup for keyboard input and
-	    // display output conversion.
-	    if (((varp == &p_enc && *p_tenc != NUL) || varp == &p_tenc))
-	    {
-		if (convert_setup(&input_conv, p_tenc, p_enc) == FAIL
-			|| convert_setup(&output_conv, p_enc, p_tenc) == FAIL)
-		{
-		    semsg(_(e_cannot_convert_between_str_and_str),
-								p_tenc, p_enc);
-		    errmsg = e_invalid_argument;
-		}
-	    }
-
-#if defined(MSWIN)
-	    // $HOME may have characters in active code page.
-	    if (varp == &p_enc)
-		init_homedir();
-#endif
-	}
-    }
+	errmsg = did_set_encoding(varp, gvarp, opt_flags);
 
 #if defined(FEAT_POSTSCRIPT)
     else if (varp == &p_penc)
-    {
-	// Canonize printencoding if VIM standard one
-	p = enc_canonize(p_penc);
-	if (p != NULL)
-	{
-	    vim_free(p_penc);
-	    p_penc = p;
-	}
-	else
-	{
-	    // Ensure lower case and '-' for '_'
-	    for (s = p_penc; *s != NUL; s++)
-	    {
-		if (*s == '_')
-		    *s = '-';
-		else
-		    *s = TOLOWER_ASC(*s);
-	    }
-	}
-    }
+	did_set_printencoding();
 #endif
 
 #if defined(FEAT_XIM) && defined(FEAT_GUI_GTK)
     else if (varp == &p_imak)
-    {
-	if (!im_xim_isvalid_imactivate())
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_imactivatekey();
 #endif
 
 #ifdef FEAT_KEYMAP
     else if (varp == &curbuf->b_p_keymap)
-    {
-	if (!valid_filetype(*varp))
-	    errmsg = e_invalid_argument;
-	else
-	{
-	    int	    secure_save = secure;
-
-	    // Reset the secure flag, since the value of 'keymap' has
-	    // been checked to be safe.
-	    secure = 0;
-
-	    // load or unload key mapping tables
-	    errmsg = keymap_init();
-
-	    secure = secure_save;
-
-	    // Since we check the value, there is no need to set P_INSECURE,
-	    // even when the value comes from a modeline.
-	    *value_checked = TRUE;
-	}
-
-	if (errmsg == NULL)
-	{
-	    if (*curbuf->b_p_keymap != NUL)
-	    {
-		// Installed a new keymap, switch on using it.
-		curbuf->b_p_iminsert = B_IMODE_LMAP;
-		if (curbuf->b_p_imsearch != B_IMODE_USE_INSERT)
-		    curbuf->b_p_imsearch = B_IMODE_LMAP;
-	    }
-	    else
-	    {
-		// Cleared the keymap, may reset 'iminsert' and 'imsearch'.
-		if (curbuf->b_p_iminsert == B_IMODE_LMAP)
-		    curbuf->b_p_iminsert = B_IMODE_NONE;
-		if (curbuf->b_p_imsearch == B_IMODE_LMAP)
-		    curbuf->b_p_imsearch = B_IMODE_USE_INSERT;
-	    }
-	    if ((opt_flags & OPT_LOCAL) == 0)
-	    {
-		set_iminsert_global();
-		set_imsearch_global();
-	    }
-	    status_redraw_curbuf();
-	}
-    }
+	errmsg = did_set_keymap(varp, opt_flags, value_checked);
 #endif
 
     // 'fileformat'
     else if (gvarp == &p_ff)
-    {
-	if (!curbuf->b_p_ma && !(opt_flags & OPT_GLOBAL))
-	    errmsg = e_cannot_make_changes_modifiable_is_off;
-	else if (check_opt_strings(*varp, p_ff_values, FALSE) != OK)
-	    errmsg = e_invalid_argument;
-	else
-	{
-	    // may also change 'textmode'
-	    if (get_fileformat(curbuf) == EOL_DOS)
-		curbuf->b_p_tx = TRUE;
-	    else
-		curbuf->b_p_tx = FALSE;
-	    redraw_titles();
-	    // update flag in swap file
-	    ml_setflags(curbuf);
-	    // Redraw needed when switching to/from "mac": a CR in the text
-	    // will be displayed differently.
-	    if (get_fileformat(curbuf) == EOL_MAC || *oldval == 'm')
-		redraw_curbuf_later(UPD_NOT_VALID);
-	}
-    }
+	errmsg = did_set_fileformat(varp, oldval, opt_flags);
 
     // 'fileformats'
     else if (varp == &p_ffs)
-    {
-	if (check_opt_strings(p_ffs, p_ff_values, TRUE) != OK)
-	    errmsg = e_invalid_argument;
-	else
-	{
-	    // also change 'textauto'
-	    if (*p_ffs == NUL)
-		p_ta = FALSE;
-	    else
-		p_ta = TRUE;
-	}
-    }
+	errmsg = did_set_fileformats();
 
 #if defined(FEAT_CRYPT)
     // 'cryptkey'
     else if (gvarp == &p_key)
-    {
-	// Make sure the ":set" command doesn't show the new value in the
-	// history.
-	remove_key_from_history();
-
-	if (STRCMP(curbuf->b_p_key, oldval) != 0)
-	    // Need to update the swapfile.
-	{
-	    ml_set_crypt_key(curbuf, oldval,
-			      *curbuf->b_p_cm == NUL ? p_cm : curbuf->b_p_cm);
-	    changed_internal();
-	}
-    }
-
+	did_set_cryptkey(oldval);
+
+    // 'cryptmethod'
     else if (gvarp == &p_cm)
-    {
-	if (opt_flags & OPT_LOCAL)
-	    p = curbuf->b_p_cm;
-	else
-	    p = p_cm;
-	if (check_opt_strings(p, p_cm_values, TRUE) != OK)
-	    errmsg = e_invalid_argument;
-	else if (crypt_self_test() == FAIL)
-	    errmsg = e_invalid_argument;
-	else
-	{
-	    // When setting the global value to empty, make it "zip".
-	    if (*p_cm == NUL)
-	    {
-		free_string_option(p_cm);
-		p_cm = vim_strsave((char_u *)"zip");
-	    }
-	    // When using ":set cm=name" the local value is going to be empty.
-	    // Do that here, otherwise the crypt functions will still use the
-	    // local value.
-	    if ((opt_flags & (OPT_LOCAL | OPT_GLOBAL)) == 0)
-	    {
-		free_string_option(curbuf->b_p_cm);
-		curbuf->b_p_cm = empty_option;
-	    }
-
-	    // Need to update the swapfile when the effective method changed.
-	    // Set "s" to the effective old value, "p" to the effective new
-	    // method and compare.
-	    if ((opt_flags & OPT_LOCAL) && *oldval == NUL)
-		s = p_cm;  // was previously using the global value
-	    else
-		s = oldval;
-	    if (*curbuf->b_p_cm == NUL)
-		p = p_cm;  // is now using the global value
-	    else
-		p = curbuf->b_p_cm;
-	    if (STRCMP(s, p) != 0)
-		ml_set_crypt_key(curbuf, curbuf->b_p_key, s);
-
-	    // If the global value changes need to update the swapfile for all
-	    // buffers using that value.
-	    if ((opt_flags & OPT_GLOBAL) && STRCMP(p_cm, oldval) != 0)
-	    {
-		buf_T	*buf;
-
-		FOR_ALL_BUFFERS(buf)
-		    if (buf != curbuf && *buf->b_p_cm == NUL)
-			ml_set_crypt_key(buf, buf->b_p_key, oldval);
-	    }
-	}
-    }
+	errmsg = did_set_cryptmethod(oldval, opt_flags);
 #endif
 
     // 'matchpairs'
     else if (gvarp == &p_mps)
-    {
-	if (has_mbyte)
-	{
-	    for (p = *varp; *p != NUL; ++p)
-	    {
-		int x2 = -1;
-		int x3 = -1;
-
-		p += mb_ptr2len(p);
-		if (*p != NUL)
-		    x2 = *p++;
-		if (*p != NUL)
-		{
-		    x3 = mb_ptr2char(p);
-		    p += mb_ptr2len(p);
-		}
-		if (x2 != ':' || x3 == -1 || (*p != NUL && *p != ','))
-		{
-		    errmsg = e_invalid_argument;
-		    break;
-		}
-		if (*p == NUL)
-		    break;
-	    }
-	}
-	else
-	{
-	    // Check for "x:y,x:y"
-	    for (p = *varp; *p != NUL; p += 4)
-	    {
-		if (p[1] != ':' || p[2] == NUL || (p[3] != NUL && p[3] != ','))
-		{
-		    errmsg = e_invalid_argument;
-		    break;
-		}
-		if (p[3] == NUL)
-		    break;
-	    }
-	}
-    }
+	errmsg = did_set_matchpairs(varp);
 
     // 'comments'
     else if (gvarp == &p_com)
-    {
-	for (s = *varp; *s; )
-	{
-	    while (*s && *s != ':')
-	    {
-		if (vim_strchr((char_u *)COM_ALL, *s) == NULL
-					     && !VIM_ISDIGIT(*s) && *s != '-')
-		{
-		    errmsg = illegal_char(errbuf, *s);
-		    break;
-		}
-		++s;
-	    }
-	    if (*s++ == NUL)
-		errmsg = e_missing_colon;
-	    else if (*s == ',' || *s == NUL)
-		errmsg = e_zero_length_string;
-	    if (errmsg != NULL)
-		break;
-	    while (*s && *s != ',')
-	    {
-		if (*s == '\\' && s[1] != NUL)
-		    ++s;
-		++s;
-	    }
-	    s = skip_to_option_part(s);
-	}
-    }
+	errmsg = did_set_comments(varp, errbuf);
 
     // global 'listchars' or 'fillchars'
     else if (varp == &p_lcs || varp == &p_fcs)
-    {
-	char_u **local_ptr = varp == &p_lcs
-					 ? &curwin->w_p_lcs : &curwin->w_p_fcs;
-
-	// only apply the global value to "curwin" when it does not have a
-	// local value
-	errmsg = set_chars_option(curwin, varp,
-			      **local_ptr == NUL || !(opt_flags & OPT_GLOBAL));
-	if (errmsg == NULL)
-	{
-	    tabpage_T	*tp;
-	    win_T	*wp;
-
-	    // If the current window is set to use the global
-	    // 'listchars'/'fillchars' value, clear the window-local value.
-	    if (!(opt_flags & OPT_GLOBAL))
-		clear_string_option(local_ptr);
-	    FOR_ALL_TAB_WINDOWS(tp, wp)
-	    {
-		// If the current window has a local value need to apply it
-		// again, it was changed when setting the global value.
-		// If no error was returned above, we don't expect an error
-		// here, so ignore the return value.
-		local_ptr = varp == &p_lcs ? &wp->w_p_lcs : &wp->w_p_fcs;
-		if (**local_ptr == NUL)
-		    (void)set_chars_option(wp, local_ptr, TRUE);
-	    }
-
-	    redraw_all_later(UPD_NOT_VALID);
-	}
-    }
+	errmsg = did_set_global_listfillchars(varp, opt_flags);
+
     // local 'listchars'
     else if (varp == &curwin->w_p_lcs)
 	errmsg = set_chars_option(curwin, varp, TRUE);
 
     // local 'fillchars'
     else if (varp == &curwin->w_p_fcs)
-    {
 	errmsg = set_chars_option(curwin, varp, TRUE);
-    }
 
     // 'cedit'
     else if (varp == &p_cedit)
-    {
 	errmsg = check_cedit();
-    }
 
     // 'verbosefile'
     else if (varp == &p_vfile)
-    {
-	verbose_stop();
-	if (*p_vfile != NUL && verbose_open() == FAIL)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_verbosefile();
 
 #ifdef FEAT_VIMINFO
     // 'viminfo'
     else if (varp == &p_viminfo)
-    {
-	for (s = p_viminfo; *s;)
-	{
-	    // Check it's a valid character
-	    if (vim_strchr((char_u *)"!\"%'/:<@cfhnrs", *s) == NULL)
-	    {
-		errmsg = illegal_char(errbuf, *s);
-		break;
-	    }
-	    if (*s == 'n')	// name is always last one
-		break;
-	    else if (*s == 'r') // skip until next ','
-	    {
-		while (*++s && *s != ',')
-		    ;
-	    }
-	    else if (*s == '%')
-	    {
-		// optional number
-		while (vim_isdigit(*++s))
-		    ;
-	    }
-	    else if (*s == '!' || *s == 'h' || *s == 'c')
-		++s;		// no extra chars
-	    else		// must have a number
-	    {
-		while (vim_isdigit(*++s))
-		    ;
-
-		if (!VIM_ISDIGIT(*(s - 1)))
-		{
-		    if (errbuf != NULL)
-		    {
-			sprintf(errbuf,
-				_(e_missing_number_after_angle_str_angle),
-						    transchar_byte(*(s - 1)));
-			errmsg = errbuf;
-		    }
-		    else
-			errmsg = "";
-		    break;
-		}
-	    }
-	    if (*s == ',')
-		++s;
-	    else if (*s)
-	    {
-		if (errbuf != NULL)
-		    errmsg = e_missing_comma;
-		else
-		    errmsg = "";
-		break;
-	    }
-	}
-	if (*p_viminfo && errmsg == NULL && get_viminfo_parameter('\'') < 0)
-	    errmsg = e_must_specify_a_value;
-    }
+	errmsg = did_set_viminfo(errbuf);
 #endif // FEAT_VIMINFO
 
     // terminal options
     else if (istermoption_idx(opt_idx) && full_screen)
-    {
-	// ":set t_Co=0" and ":set t_Co=1" do ":set t_Co="
-	if (varp == &T_CCO)
-	{
-	    int colors = atoi((char *)T_CCO);
-
-	    // Only reinitialize colors if t_Co value has really changed to
-	    // avoid expensive reload of colorscheme if t_Co is set to the
-	    // same value multiple times.
-	    if (colors != t_colors)
-	    {
-		t_colors = colors;
-		if (t_colors <= 1)
-		{
-		    vim_free(T_CCO);
-		    T_CCO = empty_option;
-		}
-#if defined(FEAT_VTP) && defined(FEAT_TERMGUICOLORS)
-		if (is_term_win32())
-		{
-		    swap_tcap();
-		    did_swaptcap = TRUE;
-		}
-#endif
-		// We now have a different color setup, initialize it again.
-		init_highlight(TRUE, FALSE);
-	    }
-	}
-	ttest(FALSE);
-	if (varp == &T_ME)
-	{
-	    out_str(T_ME);
-	    redraw_later(UPD_CLEAR);
-#if defined(MSWIN) && (!defined(FEAT_GUI_MSWIN) || defined(VIMDLL))
-	    // Since t_me has been set, this probably means that the user
-	    // wants to use this as default colors.  Need to reset default
-	    // background/foreground colors.
-# ifdef VIMDLL
-	    if (!gui.in_use && !gui.starting)
-# endif
-		mch_set_normal_colors();
-#endif
-	}
-	if (varp == &T_BE && termcap_active)
-	{
-	    MAY_WANT_TO_LOG_THIS;
-
-	    if (*T_BE == NUL)
-		// When clearing t_BE we assume the user no longer wants
-		// bracketed paste, thus disable it by writing t_BD.
-		out_str(T_BD);
-	    else
-		out_str(T_BE);
-	}
-    }
+	did_set_term_option(varp, &did_swaptcap);
 
 #ifdef FEAT_LINEBREAK
     // 'showbreak'
     else if (gvarp == &p_sbr)
-    {
-	for (s = *varp; *s; )
-	{
-	    if (ptr2cells(s) != 1)
-		errmsg = e_showbreak_contains_unprintable_or_wide_character;
-	    MB_PTR_ADV(s);
-	}
-    }
+	errmsg = did_set_showbreak(varp);
 #endif
 
 #ifdef FEAT_GUI
     // 'guifont'
     else if (varp == &p_guifont)
-    {
-	if (gui.in_use)
-	{
-	    p = p_guifont;
-# if defined(FEAT_GUI_GTK)
-	    // Put up a font dialog and let the user select a new value.
-	    // If this is cancelled go back to the old value but don't
-	    // give an error message.
-	    if (STRCMP(p, "*") == 0)
-	    {
-		p = gui_mch_font_dialog(oldval);
-		free_string_option(p_guifont);
-		p_guifont = (p != NULL) ? p : vim_strsave(oldval);
-	    }
-# endif
-	    if (p != NULL && gui_init_font(p_guifont, FALSE) != OK)
-	    {
-# if defined(FEAT_GUI_MSWIN) || defined(FEAT_GUI_PHOTON)
-		if (STRCMP(p_guifont, "*") == 0)
-		{
-		    // Dialog was cancelled: Keep the old value without giving
-		    // an error message.
-		    free_string_option(p_guifont);
-		    p_guifont = vim_strsave(oldval);
-		}
-		else
+	errmsg = did_set_guifont(oldval, &redraw_gui_only);
+
+# ifdef FEAT_XFONTSET
+    // 'guifontset'
+    else if (varp == &p_guifontset)
+	errmsg = did_set_guifontset(&redraw_gui_only);
 # endif
-		    errmsg = e_invalid_fonts;
-	    }
-	}
-	redraw_gui_only = TRUE;
-    }
-# ifdef FEAT_XFONTSET
-    else if (varp == &p_guifontset)
-    {
-	if (STRCMP(p_guifontset, "*") == 0)
-	    errmsg = e_cant_select_fontset;
-	else if (gui.in_use && gui_init_font(p_guifontset, TRUE) != OK)
-	    errmsg = e_invalid_fontset;
-	redraw_gui_only = TRUE;
-    }
-# endif
+
+    // 'guifontwide'
     else if (varp == &p_guifontwide)
-    {
-	if (STRCMP(p_guifontwide, "*") == 0)
-	    errmsg = e_cant_select_wide_font;
-	else if (gui_get_wide_font() == FAIL)
-	    errmsg = e_invalid_wide_font;
-	redraw_gui_only = TRUE;
-    }
+	errmsg = did_set_guifontwide(&redraw_gui_only);
 #endif
-# if defined(FEAT_GUI_GTK)
+#if defined(FEAT_GUI_GTK)
     else if (varp == &p_guiligatures)
-    {
-	gui_set_ligatures();
-	redraw_gui_only = TRUE;
-    }
-# endif
+	did_set_guiligatures(&redraw_gui_only);
+#endif
 
 #ifdef CURSOR_SHAPE
     // 'guicursor'
@@ -1549,10 +2747,7 @@ did_set_string_option(
 #ifdef FEAT_MOUSESHAPE
     // 'mouseshape'
     else if (varp == &p_mouseshape)
-    {
-	errmsg = parse_shape_opt(SHAPE_MOUSE);
-	update_mouseshape(-1);
-    }
+	errmsg = did_set_mouseshape();
 #endif
 
 #ifdef FEAT_PRINTER
@@ -1578,152 +2773,74 @@ did_set_string_option(
 
     // 'titlestring' and 'iconstring'
     else if (varp == &p_titlestring || varp == &p_iconstring)
-    {
-#ifdef FEAT_STL_OPT
-	int	flagval = (varp == &p_titlestring) ? STL_IN_TITLE : STL_IN_ICON;
-
-	// NULL => statusline syntax
-	if (vim_strchr(*varp, '%') && check_stl_option(*varp) == NULL)
-	    stl_syntax |= flagval;
-	else
-	    stl_syntax &= ~flagval;
-#endif
-	did_set_title();
-    }
+	did_set_titleiconstring(varp);
 
 #ifdef FEAT_GUI
     // 'guioptions'
     else if (varp == &p_go)
-    {
-	gui_init_which_components(oldval);
-	redraw_gui_only = TRUE;
-    }
+	did_set_guioptions(oldval, &redraw_gui_only);
 #endif
 
 #if defined(FEAT_GUI_TABLINE)
     // 'guitablabel'
     else if (varp == &p_gtl)
-    {
-	redraw_tabline = TRUE;
-	redraw_gui_only = TRUE;
-    }
+	did_set_guitablabel(&redraw_gui_only);
     // 'guitabtooltip'
     else if (varp == &p_gtt)
-    {
 	redraw_gui_only = TRUE;
-    }
 #endif
 
 #if defined(UNIX) || defined(VMS)
     // 'ttymouse'
     else if (varp == &p_ttym)
-    {
-	// Switch the mouse off before changing the escape sequences used for
-	// that.
-	mch_setmouse(FALSE);
-	if (opt_strings_flags(p_ttym, p_ttym_values, &ttym_flags, FALSE) != OK)
-	    errmsg = e_invalid_argument;
-	else
-	    check_mouse_termcode();
-	if (termcap_active)
-	    setmouse();		// may switch it on again
-    }
+	errmsg = did_set_ttymouse();
 #endif
 
     // 'selection'
     else if (varp == &p_sel)
-    {
-	if (*p_sel == NUL
-		|| check_opt_strings(p_sel, p_sel_values, FALSE) != OK)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_selection();
 
     // 'selectmode'
     else if (varp == &p_slm)
-    {
-	if (check_opt_strings(p_slm, p_slm_values, TRUE) != OK)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_opt_strings(p_slm, p_slm_values, TRUE);
 
 #ifdef FEAT_BROWSE
     // 'browsedir'
     else if (varp == &p_bsdir)
-    {
-	if (check_opt_strings(p_bsdir, p_bsdir_values, FALSE) != OK
-		&& !mch_isdir(p_bsdir))
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_browsedir();
 #endif
 
     // 'keymodel'
     else if (varp == &p_km)
-    {
-	if (check_opt_strings(p_km, p_km_values, TRUE) != OK)
-	    errmsg = e_invalid_argument;
-	else
-	{
-	    km_stopsel = (vim_strchr(p_km, 'o') != NULL);
-	    km_startsel = (vim_strchr(p_km, 'a') != NULL);
-	}
-    }
+	errmsg = did_set_keymodel();
 
     // 'keyprotocol'
     else if (varp == &p_kpc)
-    {
-	if (match_keyprotocol(NULL) == KEYPROTOCOL_FAIL)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_keyprotocol();
 
     // 'mousemodel'
     else if (varp == &p_mousem)
-    {
-	if (check_opt_strings(p_mousem, p_mousem_values, FALSE) != OK)
-	    errmsg = e_invalid_argument;
-#if defined(FEAT_GUI_MOTIF) && defined(FEAT_MENU) && (XmVersion <= 1002)
-	else if (*p_mousem != *oldval)
-	    // Changed from "extend" to "popup" or "popup_setpos" or vv: need
-	    // to create or delete the popup menus.
-	    gui_motif_update_mousemodel(root_menu);
-#endif
-    }
+	errmsg = did_set_mousemodel();
 
     // 'switchbuf'
     else if (varp == &p_swb)
-    {
-	if (opt_strings_flags(p_swb, p_swb_values, &swb_flags, TRUE) != OK)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_opt_flags(p_swb, p_swb_values, &swb_flags, TRUE);
 
     // 'splitkeep'
     else if (varp == &p_spk)
-    {
-	if (check_opt_strings(p_spk, p_spk_values, FALSE) != OK)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_opt_strings(p_spk, p_spk_values, FALSE);
 
     // 'debug'
     else if (varp == &p_debug)
-    {
-	if (check_opt_strings(p_debug, p_debug_values, TRUE) != OK)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_opt_strings(p_debug, p_debug_values, TRUE);
 
     // 'display'
     else if (varp == &p_dy)
-    {
-	if (opt_strings_flags(p_dy, p_dy_values, &dy_flags, TRUE) != OK)
-	    errmsg = e_invalid_argument;
-	else
-	    (void)init_chartab();
-
-    }
+	errmsg = did_set_display();
 
     // 'eadirection'
     else if (varp == &p_ead)
-    {
-	if (check_opt_strings(p_ead, p_ead_values, FALSE) != OK)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_opt_strings(p_ead, p_ead_values, FALSE);
 
 #ifdef FEAT_CLIPBOARD
     // 'clipboard'
@@ -1732,146 +2849,53 @@ did_set_string_option(
 #endif
 
 #ifdef FEAT_SPELL
-    // When 'spelllang' or 'spellfile' is set and there is a window for this
-    // buffer in which 'spell' is set load the wordlists.
-    else if (varp == &(curwin->w_s->b_p_spl)
-	    || varp == &(curwin->w_s->b_p_spf))
-    {
-	int	is_spellfile = varp == &(curwin->w_s->b_p_spf);
-
-	if ((is_spellfile && !valid_spellfile(*varp))
-		|| (!is_spellfile && !valid_spelllang(*varp)))
-	    errmsg = e_invalid_argument;
-	else
-	    errmsg = did_set_spell_option(is_spellfile);
-    }
-    // When 'spellcapcheck' is set compile the regexp program.
+    // 'spellfile'
+    else if (varp == &(curwin->w_s->b_p_spf))
+	errmsg = did_set_spellfile(varp);
+
+    // 'spell'
+    else if (varp == &(curwin->w_s->b_p_spl))
+	errmsg = did_set_spell(varp);
+
+    // 'spellcapcheck'
     else if (varp == &(curwin->w_s->b_p_spc))
-    {
-	errmsg = compile_cap_prog(curwin->w_s);
-    }
+	errmsg = did_set_spellcapcheck();
+
     // 'spelloptions'
     else if (varp == &(curwin->w_s->b_p_spo))
-    {
-	if (**varp != NUL && STRCMP("camel", *varp) != 0)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_spelloptions(varp);
+
     // 'spellsuggest'
     else if (varp == &p_sps)
-    {
-	if (spell_check_sps() != OK)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_spellsuggest();
+
     // 'mkspellmem'
     else if (varp == &p_msm)
-    {
-	if (spell_check_msm() != OK)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_mkspellmem();
 #endif
 
     // When 'bufhidden' is set, check for valid value.
     else if (gvarp == &p_bh)
-    {
-	if (check_opt_strings(curbuf->b_p_bh, p_bufhidden_values, FALSE) != OK)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_opt_strings(curbuf->b_p_bh, p_bufhidden_values,
+								FALSE);
 
     // When 'buftype' is set, check for valid value.
     else if (gvarp == &p_bt)
-    {
-	if (check_opt_strings(curbuf->b_p_bt, p_buftype_values, FALSE) != OK)
-	    errmsg = e_invalid_argument;
-	else
-	{
-	    if (curwin->w_status_height)
-	    {
-		curwin->w_redr_status = TRUE;
-		redraw_later(UPD_VALID);
-	    }
-	    curbuf->b_help = (curbuf->b_p_bt[0] == 'h');
-	    redraw_titles();
-	}
-    }
+	errmsg = did_set_buftype();
 
 #ifdef FEAT_STL_OPT
     // 'statusline', 'tabline' or 'rulerformat'
     else if (gvarp == &p_stl || varp == &p_tal || varp == &p_ruf)
-    {
-	int wid;
-
-	if (varp == &p_ruf)	// reset ru_wid first
-	    ru_wid = 0;
-	s = *varp;
-	if (varp == &p_ruf && *s == '%')
-	{
-	    // set ru_wid if 'ruf' starts with "%99("
-	    if (*++s == '-')	// ignore a '-'
-		s++;
-	    wid = getdigits(&s);
-	    if (wid && *s == '(' && (errmsg = check_stl_option(p_ruf)) == NULL)
-		ru_wid = wid;
-	    else
-		errmsg = check_stl_option(p_ruf);
-	}
-	// check 'statusline' or 'tabline' only if it doesn't start with "%!"
-	else if (varp == &p_ruf || s[0] != '%' || s[1] != '!')
-	    errmsg = check_stl_option(s);
-	if (varp == &p_ruf && errmsg == NULL)
-	    comp_col();
-    }
+	errmsg = did_set_statusline(varp);
 #endif
 
-    // check if it is a valid value for 'complete' -- Acevedo
+    // 'complete'
     else if (gvarp == &p_cpt)
-    {
-	for (s = *varp; *s;)
-	{
-	    while (*s == ',' || *s == ' ')
-		s++;
-	    if (!*s)
-		break;
-	    if (vim_strchr((char_u *)".wbuksid]tU", *s) == NULL)
-	    {
-		errmsg = illegal_char(errbuf, *s);
-		break;
-	    }
-	    if (*++s != NUL && *s != ',' && *s != ' ')
-	    {
-		if (s[-1] == 'k' || s[-1] == 's')
-		{
-		    // skip optional filename after 'k' and 's'
-		    while (*s && *s != ',' && *s != ' ')
-		    {
-			if (*s == '\\' && s[1] != NUL)
-			    ++s;
-			++s;
-		    }
-		}
-		else
-		{
-		    if (errbuf != NULL)
-		    {
-			sprintf((char *)errbuf,
-				       _(e_illegal_character_after_chr), *--s);
-			errmsg = errbuf;
-		    }
-		    else
-			errmsg = "";
-		    break;
-		}
-	    }
-	}
-    }
+	errmsg = did_set_complete(varp, errbuf);
 
     // 'completeopt'
     else if (varp == &p_cot)
-    {
-	if (check_opt_strings(p_cot, p_cot_values, TRUE) != OK)
-	    errmsg = e_invalid_argument;
-	else
-	    completeopt_was_set();
-    }
+	errmsg = did_set_completeopt();
 
 #ifdef BACKSLASH_IN_FILENAME
     // 'completeslash'
@@ -1886,418 +2910,150 @@ did_set_string_option(
 #ifdef FEAT_SIGNS
     // 'signcolumn'
     else if (varp == &curwin->w_p_scl)
-    {
-	if (check_opt_strings(*varp, p_scl_values, FALSE) != OK)
-	    errmsg = e_invalid_argument;
-	// When changing the 'signcolumn' to or from 'number', recompute the
-	// width of the number column if 'number' or 'relativenumber' is set.
-	if (((*oldval == 'n' && *(oldval + 1) == 'u')
-		|| (*curwin->w_p_scl == 'n' && *(curwin->w_p_scl + 1) =='u'))
-		&& (curwin->w_p_nu || curwin->w_p_rnu))
-	    curwin->w_nrwidth_line_count = 0;
-    }
+	errmsg = did_set_signcolumn(varp, oldval);
 #endif
 
     // 'showcmdloc'
     else if (varp == &p_sloc)
-    {
-	if (check_opt_strings(p_sloc, p_sloc_values, FALSE) != OK)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_opt_strings(p_sloc, p_sloc_values, FALSE);
 
 #if defined(FEAT_TOOLBAR) && !defined(FEAT_GUI_MSWIN)
     // 'toolbar'
     else if (varp == &p_toolbar)
-    {
-	if (opt_strings_flags(p_toolbar, p_toolbar_values,
-			      &toolbar_flags, TRUE) != OK)
-	    errmsg = e_invalid_argument;
-	else
-	{
-	    out_flush();
-	    gui_mch_show_toolbar((toolbar_flags &
-				  (TOOLBAR_TEXT | TOOLBAR_ICONS)) != 0);
-	}
-    }
+	errmsg = did_set_toolbar();
 #endif
 
 #if defined(FEAT_TOOLBAR) && defined(FEAT_GUI_GTK)
     // 'toolbariconsize': GTK+ 2 only
     else if (varp == &p_tbis)
-    {
-	if (opt_strings_flags(p_tbis, p_tbis_values, &tbis_flags, FALSE) != OK)
-	    errmsg = e_invalid_argument;
-	else
-	{
-	    out_flush();
-	    gui_mch_show_toolbar((toolbar_flags &
-				  (TOOLBAR_TEXT | TOOLBAR_ICONS)) != 0);
-	}
-    }
+	errmsg = did_set_toolbariconsize();
 #endif
 
-    // 'pastetoggle': translate key codes like in a mapping
+    // 'pastetoggle'
     else if (varp == &p_pt)
-    {
-	if (*p_pt)
-	{
-	    (void)replace_termcodes(p_pt, &p,
-				      REPTERM_FROM_PART | REPTERM_DO_LT, NULL);
-	    if (p != NULL)
-	    {
-		free_string_option(p_pt);
-		p_pt = p;
-	    }
-	}
-    }
+	did_set_pastetoggle();
 
     // 'backspace'
     else if (varp == &p_bs)
-    {
-	if (VIM_ISDIGIT(*p_bs))
-	{
-	    if (*p_bs > '3' || p_bs[1] != NUL)
-		errmsg = e_invalid_argument;
-	}
-	else if (check_opt_strings(p_bs, p_bs_values, TRUE) != OK)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_backspace();
+
+    // 'belloff'
     else if (varp == &p_bo)
-    {
-	if (opt_strings_flags(p_bo, p_bo_values, &bo_flags, TRUE) != OK)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_opt_flags(p_bo, p_bo_values, &bo_flags, TRUE);
 
     // 'tagcase'
     else if (gvarp == &p_tc)
-    {
-	unsigned int	*flags;
-
-	if (opt_flags & OPT_LOCAL)
-	{
-	    p = curbuf->b_p_tc;
-	    flags = &curbuf->b_tc_flags;
-	}
-	else
-	{
-	    p = p_tc;
-	    flags = &tc_flags;
-	}
-
-	if ((opt_flags & OPT_LOCAL) && *p == NUL)
-	    // make the local value empty: use the global value
-	    *flags = 0;
-	else if (*p == NUL
-		|| opt_strings_flags(p, p_tc_values, flags, FALSE) != OK)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_tagcase(opt_flags);
 
     // 'casemap'
     else if (varp == &p_cmp)
-    {
-	if (opt_strings_flags(p_cmp, p_cmp_values, &cmp_flags, TRUE) != OK)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_opt_flags(p_cmp, p_cmp_values, &cmp_flags, TRUE);
 
 #ifdef FEAT_DIFF
     // 'diffopt'
     else if (varp == &p_dip)
-    {
-	if (diffopt_changed() == FAIL)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_diffopt();
 #endif
 
 #ifdef FEAT_FOLDING
     // 'foldmethod'
     else if (gvarp == &curwin->w_allbuf_opt.wo_fdm)
-    {
-	if (check_opt_strings(*varp, p_fdm_values, FALSE) != OK
-		|| *curwin->w_p_fdm == NUL)
-	    errmsg = e_invalid_argument;
-	else
-	{
-	    foldUpdateAll(curwin);
-	    if (foldmethodIsDiff(curwin))
-		newFoldLevel();
-	}
-    }
+	errmsg = did_set_foldmethod(varp);
+
     // 'foldmarker'
     else if (gvarp == &curwin->w_allbuf_opt.wo_fmr)
-    {
-	p = vim_strchr(*varp, ',');
-	if (p == NULL)
-	    errmsg = e_comma_required;
-	else if (p == *varp || p[1] == NUL)
-	    errmsg = e_invalid_argument;
-	else if (foldmethodIsMarker(curwin))
-	    foldUpdateAll(curwin);
-    }
+	errmsg = did_set_foldmarker(varp);
+
     // 'commentstring'
     else if (gvarp == &p_cms)
-    {
-	if (**varp != NUL && strstr((char *)*varp, "%s") == NULL)
-	    errmsg = e_commentstring_must_be_empty_or_contain_str;
-    }
+	errmsg = did_set_commentstring(varp);
+
     // 'foldopen'
     else if (varp == &p_fdo)
-    {
-	if (opt_strings_flags(p_fdo, p_fdo_values, &fdo_flags, TRUE) != OK)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_opt_flags(p_fdo, p_fdo_values, &fdo_flags, TRUE);
+
     // 'foldclose'
     else if (varp == &p_fcl)
-    {
-	if (check_opt_strings(p_fcl, p_fcl_values, TRUE) != OK)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_opt_strings(p_fcl, p_fcl_values, TRUE);
+
     // 'foldignore'
     else if (gvarp == &curwin->w_allbuf_opt.wo_fdi)
-    {
-	if (foldmethodIsIndent(curwin))
-	    foldUpdateAll(curwin);
-    }
+	did_set_foldignore();
 #endif
 
     // 'virtualedit'
     else if (gvarp == &p_ve)
-    {
-	char_u		*ve = p_ve;
-	unsigned int	*flags = &ve_flags;
-
-	if (opt_flags & OPT_LOCAL)
-	{
-	    ve = curwin->w_p_ve;
-	    flags = &curwin->w_ve_flags;
-	}
-
-	if ((opt_flags & OPT_LOCAL) && *ve == NUL)
-	    // make the local value empty: use the global value
-	    *flags = 0;
-	else
-	{
-	    if (opt_strings_flags(ve, p_ve_values, flags, TRUE) != OK)
-		errmsg = e_invalid_argument;
-	    else if (STRCMP(ve, oldval) != 0)
-	    {
-		// Recompute cursor position in case the new 've' setting
-		// changes something.
-		validate_virtcol();
-		coladvance(curwin->w_virtcol);
-	    }
-	}
-    }
+	errmsg = did_set_virtualedit(oldval, opt_flags);
 
 #if defined(FEAT_CSCOPE) && defined(FEAT_QUICKFIX)
+    // 'cscopequickfix'
     else if (varp == &p_csqf)
-    {
-	if (p_csqf != NULL)
-	{
-	    p = p_csqf;
-	    while (*p != NUL)
-	    {
-		if (vim_strchr((char_u *)CSQF_CMDS, *p) == NULL
-			|| p[1] == NUL
-			|| vim_strchr((char_u *)CSQF_FLAGS, p[1]) == NULL
-			|| (p[2] != NUL && p[2] != ','))
-		{
-		    errmsg = e_invalid_argument;
-		    break;
-		}
-		else if (p[2] == NUL)
-		    break;
-		else
-		    p += 3;
-	    }
-	}
-    }
+	errmsg = did_set_cscopequickfix();
 #endif
 
     // 'cinoptions'
     else if (gvarp == &p_cino)
-    {
-	// TODO: recognize errors
-	parse_cino(curbuf);
-    }
+	did_set_cinoptions();
 
     // 'lispoptions'
     else if (gvarp == &p_lop)
-    {
-	if (**varp != NUL && STRCMP(*varp, "expr:0") != 0
-					       && STRCMP(*varp, "expr:1") != 0)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_lispoptions(varp);
 
 #if defined(FEAT_RENDER_OPTIONS)
     // 'renderoptions'
     else if (varp == &p_rop)
-    {
-	if (!gui_mch_set_rendering_options(p_rop))
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_renderoptions();
 #endif
 
+    // 'filetype'
     else if (gvarp == &p_ft)
-    {
-	if (!valid_filetype(*varp))
-	    errmsg = e_invalid_argument;
-	else
-	{
-	    value_changed = STRCMP(oldval, *varp) != 0;
-
-	    // Since we check the value, there is no need to set P_INSECURE,
-	    // even when the value comes from a modeline.
-	    *value_checked = TRUE;
-	}
-    }
+	errmsg = did_set_filetype_or_syntax(varp, oldval, value_checked,
+							&value_changed);
 
 #ifdef FEAT_SYN_HL
+    // 'syntax'
     else if (gvarp == &p_syn)
-    {
-	if (!valid_filetype(*varp))
-	    errmsg = e_invalid_argument;
-	else
-	{
-	    value_changed = STRCMP(oldval, *varp) != 0;
-
-	    // Since we check the value, there is no need to set P_INSECURE,
-	    // even when the value comes from a modeline.
-	    *value_checked = TRUE;
-	}
-    }
+	errmsg = did_set_filetype_or_syntax(varp, oldval, value_checked,
+							&value_changed);
 #endif
 
 #ifdef FEAT_TERMINAL
     // 'termwinkey'
     else if (varp == &curwin->w_p_twk)
-    {
-	if (*curwin->w_p_twk != NUL
-				  && string_to_key(curwin->w_p_twk, TRUE) == 0)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_termwinkey();
+
     // 'termwinsize'
     else if (varp == &curwin->w_p_tws)
-    {
-	if (*curwin->w_p_tws != NUL)
-	{
-	    p = skipdigits(curwin->w_p_tws);
-	    if (p == curwin->w_p_tws
-		    || (*p != 'x' && *p != '*')
-		    || *skipdigits(p + 1) != NUL)
-		errmsg = e_invalid_argument;
-	}
-    }
+	errmsg = did_set_termwinsize();
+
     // 'wincolor'
     else if (varp == &curwin->w_p_wcr)
 	term_update_wincolor(curwin);
 # if defined(MSWIN)
     // 'termwintype'
     else if (varp == &p_twt)
-    {
-	if (check_opt_strings(*varp, p_twt_values, FALSE) != OK)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_opt_strings(p_twt, p_twt_values, FALSE);
 # endif
 #endif
 
 #ifdef FEAT_VARTABS
     // 'varsofttabstop'
     else if (varp == &(curbuf->b_p_vsts))
-    {
-	char_u *cp;
-
-	if (!(*varp)[0] || ((*varp)[0] == '0' && !(*varp)[1]))
-	{
-	    if (curbuf->b_p_vsts_array)
-	    {
-		vim_free(curbuf->b_p_vsts_array);
-		curbuf->b_p_vsts_array = 0;
-	    }
-	}
-	else
-	{
-	    for (cp = *varp; *cp; ++cp)
-	    {
-		if (vim_isdigit(*cp))
-		    continue;
-		if (*cp == ',' && cp > *varp && *(cp-1) != ',')
-		    continue;
-		errmsg = e_invalid_argument;
-		break;
-	    }
-	    if (errmsg == NULL)
-	    {
-		int *oldarray = curbuf->b_p_vsts_array;
-		if (tabstop_set(*varp, &(curbuf->b_p_vsts_array)) == OK)
-		{
-		    if (oldarray)
-			vim_free(oldarray);
-		}
-		else
-		    errmsg = e_invalid_argument;
-	    }
-	}
-    }
+	errmsg = did_set_varsofttabstop(varp);
 
     // 'vartabstop'
     else if (varp == &(curbuf->b_p_vts))
-    {
-	char_u *cp;
-
-	if (!(*varp)[0] || ((*varp)[0] == '0' && !(*varp)[1]))
-	{
-	    if (curbuf->b_p_vts_array)
-	    {
-		vim_free(curbuf->b_p_vts_array);
-		curbuf->b_p_vts_array = NULL;
-	    }
-	}
-	else
-	{
-	    for (cp = *varp; *cp; ++cp)
-	    {
-		if (vim_isdigit(*cp))
-		    continue;
-		if (*cp == ',' && cp > *varp && *(cp-1) != ',')
-		    continue;
-		errmsg = e_invalid_argument;
-		break;
-	    }
-	    if (errmsg == NULL)
-	    {
-		int *oldarray = curbuf->b_p_vts_array;
-
-		if (tabstop_set(*varp, &(curbuf->b_p_vts_array)) == OK)
-		{
-		    vim_free(oldarray);
-#ifdef FEAT_FOLDING
-		    if (foldmethodIsIndent(curwin))
-			foldUpdateAll(curwin);
-#endif
-		}
-		else
-		    errmsg = e_invalid_argument;
-	    }
-	}
-    }
+	errmsg = did_set_vartabstop(varp);
 #endif
 
 #ifdef FEAT_PROP_POPUP
     // 'previewpopup'
     else if (varp == &p_pvp)
-    {
-	if (parse_previewpopup(NULL) == FAIL)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = did_set_previewpopup();
 # ifdef FEAT_QUICKFIX
     // 'completepopup'
     else if (varp == &p_cpp)
-    {
-	if (parse_completepopup(NULL) == FAIL)
-	    errmsg = e_invalid_argument;
-	else
-	    popup_close_info();
-    }
+	errmsg = did_set_completepopup();
 # endif
 #endif
 
@@ -2326,86 +3082,48 @@ did_set_string_option(
 	    varp == &p_pexpr ||
 # endif
 	    varp == &p_ccv)
-    {
-	// If the option value starts with <SID> or s:, then replace that with
-	// the script identifier.
-	char_u *name = get_scriptlocal_funcname(*varp);
-	if (name != NULL)
-	{
-	    free_string_option(*varp);
-	    *varp = name;
-	}
-
-# ifdef FEAT_FOLDING
-	if (varp == &curwin->w_p_fde && foldmethodIsExpr(curwin))
-	    foldUpdateAll(curwin);
-# endif
-    }
+	did_set_optexpr(varp);
 #endif
 
 #ifdef FEAT_COMPL_FUNC
     // 'completefunc'
     else if (gvarp == &p_cfu)
-    {
-	if (set_completefunc_option() == FAIL)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = set_completefunc_option();
 
     // 'omnifunc'
     else if (gvarp == &p_ofu)
-    {
-	if (set_omnifunc_option() == FAIL)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = set_omnifunc_option();
 
     // 'thesaurusfunc'
     else if (gvarp == &p_tsrfu)
-    {
-	if (set_thesaurusfunc_option() == FAIL)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = set_thesaurusfunc_option();
 #endif
 
 #if defined(FEAT_EVAL) && \
      (defined(FEAT_XIM) || defined(IME_WITHOUT_XIM) || defined(VIMDLL))
     // 'imactivatefunc'
     else if (gvarp == &p_imaf)
-    {
-	if (set_imactivatefunc_option() == FAIL)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = set_imactivatefunc_option();
 
     // 'imstatusfunc'
     else if (gvarp == &p_imsf)
-    {
-	if (set_imstatusfunc_option() == FAIL)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = set_imstatusfunc_option();
 #endif
 
     // 'operatorfunc'
     else if (varp == &p_opfunc)
-    {
-	if (set_operatorfunc_option() == FAIL)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = set_operatorfunc_option();
 
 #ifdef FEAT_QUICKFIX
     // 'quickfixtextfunc'
     else if (varp == &p_qftf)
-    {
-	if (qf_process_qftf_option() == FAIL)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = qf_process_qftf_option();
 #endif
 
 #ifdef FEAT_EVAL
     // 'tagfunc'
     else if (gvarp == &p_tfu)
-    {
-	if (set_tagfunc_option() == FAIL)
-	    errmsg = e_invalid_argument;
-    }
+	errmsg = set_tagfunc_option();
 #endif
 
     // Options that are a list of flags.

--- a/src/proto/gui_xim.pro
+++ b/src/proto/gui_xim.pro
@@ -1,6 +1,6 @@
 /* gui_xim.c */
-int set_imactivatefunc_option(void);
-int set_imstatusfunc_option(void);
+char *set_imactivatefunc_option(void);
+char *set_imstatusfunc_option(void);
 void free_xim_stuff(void);
 int set_ref_in_im_funcs(int copyID);
 void im_set_active(int active);

--- a/src/proto/insexpand.pro
+++ b/src/proto/insexpand.pro
@@ -44,11 +44,11 @@ int ins_compl_bs(void);
 void ins_compl_addleader(int c);
 void ins_compl_addfrommatch(void);
 int ins_compl_prep(int c);
-int set_completefunc_option(void);
+char *set_completefunc_option(void);
 void set_buflocal_cfu_callback(buf_T *buf);
-int set_omnifunc_option(void);
+char *set_omnifunc_option(void);
 void set_buflocal_ofu_callback(buf_T *buf);
-int set_thesaurusfunc_option(void);
+char *set_thesaurusfunc_option(void);
 int set_ref_in_insexpand_funcs(int copyID);
 void f_complete(typval_T *argvars, typval_T *rettv);
 void f_complete_add(typval_T *argvars, typval_T *rettv);

--- a/src/proto/ops.pro
+++ b/src/proto/ops.pro
@@ -17,7 +17,7 @@ void block_prep(oparg_T *oap, struct blo
 void op_addsub(oparg_T *oap, linenr_T Prenum1, int g_cmd);
 void clear_oparg(oparg_T *oap);
 void cursor_pos_info(dict_T *dict);
-int set_operatorfunc_option(void);
+char *set_operatorfunc_option(void);
 void free_operatorfunc_option(void);
 int set_ref_in_opfunc(int copyID);
 void do_pending_operator(cmdarg_T *cap, int old_col, int gui_yank);

--- a/src/proto/quickfix.pro
+++ b/src/proto/quickfix.pro
@@ -15,7 +15,7 @@ void ex_cclose(exarg_T *eap);
 void ex_copen(exarg_T *eap);
 void ex_cbottom(exarg_T *eap);
 linenr_T qf_current_entry(win_T *wp);
-int qf_process_qftf_option(void);
+char *qf_process_qftf_option(void);
 int grep_internal(cmdidx_T cmdidx);
 void ex_make(exarg_T *eap);
 int qf_get_size(exarg_T *eap);

--- a/src/proto/tag.pro
+++ b/src/proto/tag.pro
@@ -1,5 +1,5 @@
 /* tag.c */
-int set_tagfunc_option(void);
+char *set_tagfunc_option(void);
 void free_tagfunc_option(void);
 int set_ref_in_tagfunc(int copyID);
 void set_buflocal_tfu_callback(buf_T *buf);

--- a/src/quickfix.c
+++ b/src/quickfix.c
@@ -4533,10 +4533,13 @@ qf_find_buf(qf_info_T *qi)
  * Process the 'quickfixtextfunc' option value.
  * Returns OK or FAIL.
  */
-    int
+    char *
 qf_process_qftf_option(void)
 {
-    return option_set_callback_func(p_qftf, &qftf_cb);
+    if (option_set_callback_func(p_qftf, &qftf_cb) == FAIL)
+	return e_invalid_argument;
+
+    return NULL;
 }
 
 /*

--- a/src/spell.c
+++ b/src/spell.c
@@ -4386,7 +4386,7 @@ valid_spellfile(char_u *val)
 }
 
 /*
- * Handle side effects of setting 'spell'.
+ * Handle side effects of setting 'spell' or 'spellfile'
  * Return an error message or NULL for success.
  */
     char *

--- a/src/tag.c
+++ b/src/tag.c
@@ -170,7 +170,7 @@ static callback_T tfu_cb;	    // 'tagfun
  * Invoked when the 'tagfunc' option is set. The option value can be a name of
  * a function (string), or function(<name>) or funcref(<name>) or a lambda.
  */
-    int
+    char *
 set_tagfunc_option(void)
 {
 #ifdef FEAT_EVAL
@@ -178,15 +178,15 @@ set_tagfunc_option(void)
     free_callback(&curbuf->b_tfu_cb);
 
     if (*curbuf->b_p_tfu == NUL)
-	return OK;
+	return NULL;
 
     if (option_set_callback_func(curbuf->b_p_tfu, &tfu_cb) == FAIL)
-	return FAIL;
+	return e_invalid_argument;
 
     copy_callback(&curbuf->b_tfu_cb, &tfu_cb);
 #endif
 
-    return OK;
+    return NULL;
 }
 #endif