Permit multiple prefix keys to be defined, separated by commas, for example:

set -g prefix ^a,^b

Any key in the list acts as the prefix. The send-prefix command always sends
the first key in the list.

cmd-send-prefix.c

@@ -43,13 +43,13 @@ cmd_send_prefix_exec(struct cmd *self, struct cmd_ctx *ctx)
 	struct cmd_target_data	*data = self->data;
 	struct session		*s;
 	struct window_pane	*wp;
-	int			 key;
+	struct keylist		*keylist;
 
 	if (cmd_find_pane(ctx, data->target, &s, &wp) == NULL)
 		return (-1);
 
-	key = options_get_number(&s->options, "prefix");
-	window_pane_key(wp, ctx->curclient, key);
+	keylist = options_get_data(&s->options, "prefix");
+	window_pane_key(wp, ctx->curclient, ARRAY_FIRST(keylist));
 
 	return (0);
 }

cmd-set-option.c

@@ -65,7 +65,7 @@ const struct set_option_entry set_option_table[] = {
 	{ "message-attr", SET_OPTION_ATTRIBUTES, 0, 0, NULL },
 	{ "message-bg", SET_OPTION_COLOUR, 0, 0, NULL },
 	{ "message-fg", SET_OPTION_COLOUR, 0, 0, NULL },
-	{ "prefix", SET_OPTION_KEY, 0, 0, NULL },
+	{ "prefix", SET_OPTION_KEYS, 0, 0, NULL },
 	{ "repeat-time", SET_OPTION_NUMBER, 0, SHRT_MAX, NULL },
 	{ "set-remain-on-exit", SET_OPTION_FLAG, 0, 0, NULL },
 	{ "set-titles", SET_OPTION_FLAG, 0, 0, NULL },
@@ -162,8 +162,8 @@ cmd_set_option_exec(struct cmd *self, struct cmd_ctx *ctx)
 		case SET_OPTION_NUMBER:
 			set_option_number(ctx, oo, entry, data->arg2);
 			break;
-		case SET_OPTION_KEY:
-			set_option_key(ctx, oo, entry, data->arg2);
+		case SET_OPTION_KEYS:
+			set_option_keys(ctx, oo, entry, data->arg2);
 			break;
 		case SET_OPTION_COLOUR:
 			set_option_colour(ctx, oo, entry, data->arg2);

cmd-set-window-option.c

@@ -140,8 +140,8 @@ cmd_set_window_option_exec(struct cmd *self, struct cmd_ctx *ctx)
 		case SET_OPTION_NUMBER:
 			set_option_number(ctx, oo, entry, data->arg2);
 			break;
-		case SET_OPTION_KEY:
-			set_option_key(ctx, oo, entry, data->arg2);
+		case SET_OPTION_KEYS:
+			set_option_keys(ctx, oo, entry, data->arg2);
 			break;
 		case SET_OPTION_COLOUR:
 			set_option_colour(ctx, oo, entry, data->arg2);

options-cmd.c

@@ -28,6 +28,8 @@ set_option_print(const struct set_option_entry *entry, struct options_entry *o)
 {
 	static char	out[BUFSIZ];
 	const char     *s;
+	struct keylist *keylist;
+	u_int		i;
 
 	*out = '\0';
 	switch (entry->type) {
@@ -37,9 +39,14 @@ set_option_print(const struct set_option_entry *entry, struct options_entry *o)
 		case SET_OPTION_NUMBER:
 			xsnprintf(out, sizeof out, "%lld", o->num);
 			break;
-		case SET_OPTION_KEY:
-			s = key_string_lookup_key(o->num);
-			xsnprintf(out, sizeof out, "%s", s);
+		case SET_OPTION_KEYS:
+			keylist = o->data;
+			for (i = 0; i < ARRAY_LENGTH(keylist); i++) {
+				strlcat(out, key_string_lookup_key(
+				    ARRAY_ITEM(keylist, i)), sizeof out);
+				if (i != ARRAY_LENGTH(keylist) - 1)
+					strlcat(out, ",", sizeof out);
+			}
 			break;
 		case SET_OPTION_COLOUR:
 			s = colour_tostring(o->num);
@@ -114,23 +121,35 @@ set_option_number(struct cmd_ctx *ctx, struct options *oo,
 }
 
 void
-set_option_key(struct cmd_ctx *ctx, struct options *oo,
+set_option_keys(struct cmd_ctx *ctx, struct options *oo,
     const struct set_option_entry *entry, char *value)
 {
 	struct options_entry	*o;
-	int			 key;
+	struct keylist		*keylist;
+	char			*copyvalue, *ptr, *str;
+	int		 	 key;
 
 	if (value == NULL) {
 		ctx->error(ctx, "empty value");
 		return;
 	}
 
-	if ((key = key_string_lookup_string(value)) == KEYC_NONE) {
-		ctx->error(ctx, "unknown key: %s", value);
-		return;
-	}
+	keylist = xmalloc(sizeof *keylist);
+	ARRAY_INIT(keylist);
 
-	o = options_set_number(oo, entry->name, key);
+	ptr = copyvalue = xstrdup(value);
+	while ((str = strsep(&ptr, ",")) != NULL) {
+		if ((key = key_string_lookup_string(str)) == KEYC_NONE) {
+			xfree(keylist);
+			ctx->error(ctx, "unknown key: %s", str);
+			xfree(copyvalue);
+			return;
+		}
+		ARRAY_ADD(keylist, key);
+	}
+	xfree(copyvalue);
+
+	o = options_set_data(oo, entry->name, keylist, xfree);
 	ctx->info(
 	    ctx, "set option: %s -> %s", o->name, set_option_print(entry, o));
 }

options.c

@@ -54,6 +54,8 @@ options_free(struct options *oo)
 		xfree(o->name);
 		if (o->type == OPTIONS_STRING)
 			xfree(o->str);
+		else if (o->type == OPTIONS_DATA)
+			o->freefn(o->data);
 		xfree(o);
 	}
 }
@@ -95,6 +97,8 @@ options_remove(struct options *oo, const char *name)
 	xfree(o->name);
 	if (o->type == OPTIONS_STRING)
 		xfree(o->str);
+	else if (o->type == OPTIONS_DATA)
+		o->freefn(o->data);
 	xfree(o);
 }
 
@@ -110,6 +114,8 @@ options_set_string(struct options *oo, const char *name, const char *fmt, ...)
 		SPLAY_INSERT(options_tree, &oo->tree, o);
 	} else if (o->type == OPTIONS_STRING)
 		xfree(o->str);
+	else if (o->type == OPTIONS_DATA)
+		o->freefn(o->data);
 
 	va_start(ap, fmt);
 	o->type = OPTIONS_STRING;
@@ -141,6 +147,8 @@ options_set_number(struct options *oo, const char *name, long long value)
 		SPLAY_INSERT(options_tree, &oo->tree, o);
 	} else if (o->type == OPTIONS_STRING)
 		xfree(o->str);
+	else if (o->type == OPTIONS_DATA)
+		o->freefn(o->data);
 
 	o->type = OPTIONS_NUMBER;
 	o->num = value;
@@ -158,3 +166,36 @@ options_get_number(struct options *oo, const char *name)
 		fatalx("option not a number");
 	return (o->num);
 }
+
+struct options_entry *
+options_set_data(
+    struct options *oo, const char *name, void *value, void (*freefn)(void *))
+{
+	struct options_entry	*o;
+
+	if ((o = options_find1(oo, name)) == NULL) {
+		o = xmalloc(sizeof *o);
+		o->name = xstrdup(name);
+		SPLAY_INSERT(options_tree, &oo->tree, o);
+	} else if (o->type == OPTIONS_STRING)
+		xfree(o->str);
+	else if (o->type == OPTIONS_DATA)
+		o->freefn(o->data);
+
+	o->type = OPTIONS_DATA;
+	o->data = value;
+	o->freefn = freefn;
+	return (o);
+}
+
+void *
+options_get_data(struct options *oo, const char *name)
+{
+	struct options_entry	*o;
+
+	if ((o = options_find(oo, name)) == NULL)
+		fatalx("missing option");
+	if (o->type != OPTIONS_DATA)
+		fatalx("option not data");
+	return (o->data);
+}

server.c

@@ -798,8 +798,9 @@ server_handle_client(struct client *c)
 	struct screen		*s;
 	struct timeval	 	 tv;
 	struct key_binding	*bd;
-	int		 	 key, prefix, status, xtimeout;
-	int			 mode;
+	struct keylist		*keylist;
+	int		 	 key, status, xtimeout, mode, isprefix;
+	u_int			 i;
 	u_char			 mouse[3];
 
 	xtimeout = options_get_number(&c->session->options, "repeat-time");
@@ -811,7 +812,7 @@ server_handle_client(struct client *c)
 	}
 
 	/* Process keys. */
-	prefix = options_get_number(&c->session->options, "prefix");
+	keylist = options_get_data(&c->session->options, "prefix");
 	while (tty_keys_next(&c->tty, &key, mouse) == 0) {
 		server_activity = time(NULL);
 
@@ -844,9 +845,18 @@ server_handle_client(struct client *c)
 			continue;
 		}
 
+		/* Is this a prefix key? */
+		isprefix = 0;
+		for (i = 0; i < ARRAY_LENGTH(keylist); i++) {
+			if (key == ARRAY_ITEM(keylist, i)) {
+				isprefix = 1;
+				break;
+			}
+		}
+
 		/* No previous prefix key. */
 		if (!(c->flags & CLIENT_PREFIX)) {
-			if (key == prefix)
+			if (isprefix)
 				c->flags |= CLIENT_PREFIX;
 			else {
 				/* Try as a non-prefix key binding. */
@@ -864,7 +874,7 @@ server_handle_client(struct client *c)
 			/* If repeating, treat this as a key, else ignore. */
 			if (c->flags & CLIENT_REPEAT) {
 				c->flags &= ~CLIENT_REPEAT;
-				if (key == prefix)
+				if (isprefix)
 					c->flags |= CLIENT_PREFIX;
 				else
 					window_pane_key(wp, c, key);
@@ -875,7 +885,7 @@ server_handle_client(struct client *c)
 		/* If already repeating, but this key can't repeat, skip it. */
 		if (c->flags & CLIENT_REPEAT && !bd->can_repeat) {
 			c->flags &= ~CLIENT_REPEAT;
-			if (key == prefix)
+			if (isprefix)
 				c->flags |= CLIENT_PREFIX;
 			else
 				window_pane_key(wp, c, key);

tmux.1

@@ -1048,6 +1048,7 @@ characters.
 All arguments are sent sequentially from first to last.
 .It Ic send-prefix Op Fl t Ar target-pane
 Send the prefix key to a window as if it was pressed.
+If multiple prefix keys are configured, only the first is sent.
 .It Xo Ic unbind-key
 .Op Fl cn
 .Op Fl t Ar key-table
@@ -1258,8 +1259,11 @@ from the 256-colour palette, or
 .Ic default .
 .It Ic message-fg Ar colour
 Set status line message foreground colour.
-.It Ic prefix Ar key
-Set the current prefix key.
+.It Ic prefix Ar keys
+Set the keys accepted as a prefix key.
+.Ar keys
+is a comma-separated list of key names, each of which individually behave as
+the prefix key.
 .It Ic repeat-time Ar time
 Allow multiple commands to be entered without pressing the prefix-key again
 in the specified

tmux.c

@@ -309,6 +309,7 @@ main(int argc, char **argv)
 	enum msgtype		 msg;
 	struct passwd		*pw;
 	struct options		*so, *wo;
+	struct keylist		*keylist;
 	char			*s, *path, *label, *home, *cause, **var;
 	char			 cwd[MAXPATHLEN];
 	void			*buf;
@@ -409,7 +410,6 @@ main(int argc, char **argv)
 	options_set_number(so, "message-attr", 0);
 	options_set_number(so, "message-bg", 3);
 	options_set_number(so, "message-fg", 0);
-	options_set_number(so, "prefix", '\002');
 	options_set_number(so, "repeat-time", 500);
 	options_set_number(so, "set-remain-on-exit", 0);
 	options_set_number(so, "set-titles", 0);
@@ -439,6 +439,11 @@ main(int argc, char **argv)
 	options_set_number(so, "visual-bell", 0);
 	options_set_number(so, "visual-content", 0);
 
+	keylist = xmalloc(sizeof *keylist);
+	ARRAY_INIT(keylist);
+	ARRAY_ADD(keylist, '\002');
+	options_set_data(so, "prefix", keylist, xfree);
+
 	options_init(&global_w_options, NULL);
 	wo = &global_w_options;
 	options_set_number(wo, "aggressive-resize", 0);

tmux.h

@@ -544,10 +544,14 @@ struct options_entry {
 	enum {
 		OPTIONS_STRING,
 		OPTIONS_NUMBER,
+		OPTIONS_DATA,
 	} type;
 
 	char		*str;
 	long long	 num;
+	void		*data;
+
+	void		 (*freefn)(void *);
 
 	SPLAY_ENTRY(options_entry) entry;
 };
@@ -557,6 +561,9 @@ struct options {
 	struct options	*parent;
 };
 
+/* Key list for prefix option. */
+ARRAY_DECL(keylist, int);
+
 /* Screen selection. */
 struct screen_sel {
 	int		 flag;
@@ -1085,7 +1092,7 @@ struct set_option_entry {
 	enum {
 		SET_OPTION_STRING,
 		SET_OPTION_NUMBER,
-		SET_OPTION_KEY,
+		SET_OPTION_KEYS,
 		SET_OPTION_COLOUR,
 		SET_OPTION_ATTRIBUTES,
 		SET_OPTION_FLAG,
@@ -1165,6 +1172,9 @@ char   *options_get_string(struct options *, const char *);
 struct options_entry *options_set_number(
     	    struct options *, const char *, long long);
 long long options_get_number(struct options *, const char *);
+struct options_entry *options_set_data(
+    	    struct options *, const char *, void *, void (*)(void *));
+void   *options_get_data(struct options *, const char *);
 
 /* environ.c */
 int	environ_cmp(struct environ_entry *, struct environ_entry *);
@@ -1245,7 +1255,7 @@ void	set_option_string(struct cmd_ctx *,
 	    struct options *, const struct set_option_entry *, char *, int);
 void	set_option_number(struct cmd_ctx *,
     	    struct options *, const struct set_option_entry *, char *);
-void	set_option_key(struct cmd_ctx *,
+void	set_option_keys(struct cmd_ctx *,
     	    struct options *, const struct set_option_entry *, char *);
 void	set_option_colour(struct cmd_ctx *,
     	    struct options *, const struct set_option_entry *, char *);