Add p format modifier for padding to width.

format.c

@@ -1299,7 +1299,7 @@ format_build_modifiers(struct format_tree *ft, const char **s, u_int *count)
 		}
 
 		/* Now try single character with arguments. */
-		if (strchr("mCs=", cp[0]) == NULL)
+		if (strchr("mCs=p", cp[0]) == NULL)
 			break;
 		c = cp[0];
 
@@ -1565,7 +1565,7 @@ format_replace(struct format_tree *ft, const char *key, size_t keylen,
 	char			 *copy0, *condition, *found, *new;
 	char			 *value, *left, *right;
 	size_t			  valuelen;
-	int			  modifiers = 0, limit = 0, j;
+	int			  modifiers = 0, limit = 0, width = 0, j;
 	struct format_modifier   *list, *fm, *cmp = NULL, *search = NULL;
 	struct format_modifier	**sub = NULL;
 	u_int			  i, count, nsub = 0;
@@ -1611,6 +1611,14 @@ format_replace(struct format_tree *ft, const char *key, size_t keylen,
 				if (fm->argc >= 2 && fm->argv[1] != NULL)
 					marker = fm->argv[1];
 				break;
+			case 'p':
+				if (fm->argc < 1)
+					break;
+				width = strtonum(fm->argv[0], INT_MIN, INT_MAX,
+				    &errptr);
+				if (errptr != NULL)
+					width = 0;
+				break;
 			case 'l':
 				modifiers |= FORMAT_LITERAL;
 				break;
@@ -1845,6 +1853,19 @@ done:
 		format_log(ft, "applied length limit %d: %s", limit, value);
 	}
 
+	/* Pad the value if needed. */
+	if (width > 0) {
+		new = utf8_padcstr(value, width);
+		free(value);
+		value = new;
+		format_log(ft, "applied padding width %d: %s", width, value);
+	} else if (width < 0) {
+		new = utf8_rpadcstr(value, -width);
+		free(value);
+		value = new;
+		format_log(ft, "applied padding width %d: %s", width, value);
+	}
+
 	/* Expand the buffer and copy in the value. */
 	valuelen = strlen(value);
 	while (*len - *off < valuelen + 1) {

tmux.1

@@ -4101,6 +4101,12 @@ appended or prepended to the string if the length has been trimmed, for example
 will append
 .Ql ...
 if the pane title is more than five characters.
+Similarly,
+.Ql p
+pads the string to a given width, for example
+.Ql #{p10:pane_title}
+will result in a width of at least 10 characters.
+A positive width pads on the left, a negative on the right.
 .Pp
 Prefixing a time variable with
 .Ql t:\&

tmux.h

@@ -2638,6 +2638,7 @@ struct utf8_data *utf8_fromcstr(const char *);
 char		*utf8_tocstr(struct utf8_data *);
 u_int		 utf8_cstrwidth(const char *);
 char		*utf8_padcstr(const char *, u_int);
+char		*utf8_rpadcstr(const char *, u_int);
 int		 utf8_cstrhas(const char *, const struct utf8_data *);
 
 /* procname.c */

utf8.c

@@ -390,7 +390,7 @@ utf8_cstrwidth(const char *s)
 	return (width);
 }
 
-/* Pad UTF-8 string to width. Caller frees. */
+/* Pad UTF-8 string to width on the left. Caller frees. */
 char *
 utf8_padcstr(const char *s, u_int width)
 {
@@ -411,6 +411,27 @@ utf8_padcstr(const char *s, u_int width)
 	return (out);
 }
 
+/* Pad UTF-8 string to width on the right. Caller frees. */
+char *
+utf8_rpadcstr(const char *s, u_int width)
+{
+	size_t	 slen;
+	char	*out;
+	u_int	  n, i;
+
+	n = utf8_cstrwidth(s);
+	if (n >= width)
+		return (xstrdup(s));
+
+	slen = strlen(s);
+	out = xmalloc(slen + 1 + (width - n));
+	for (i = 0; i < width - n; i++)
+		out[i] = ' ';
+	memcpy(out + i, s, slen);
+	out[i + slen] = '\0';
+	return (out);
+}
+
 int
 utf8_cstrhas(const char *s, const struct utf8_data *ud)
 {