Author: Jan Christoph Ebersbach <jceb@e-jc.de>
URL: http://dwm.suckless.org/patches/single_tagset
This patch addresses the multi-monitor setup. Instead of having separate tags
for every monitor there is just one list of tags for all monitors. Instead of
moving windows from one monitor to the other, the desired tag from the
other monitor can just be selected and all windows will be drawn on the
current monitor.

Several deep changes needed to be made:
1. Macro ISVISIBLE expects a second parameter, the monitor
2. Monitor->clients and Monitor->stack were moved to the global variable
   Clientlist cl. All monitors refer to this one list.
3. A new method attachclients was added. When changing between tags this
   function ensures that all clients are pointing to the right monitor.

Please be aware that this patch probably breaks any other patch!

Index: dwm/dwm.c
===================================================================
--- dwm/dwm.c.orig
+++ dwm/dwm.c
@@ -49,7 +49,7 @@
 #define CLEANMASK(mask)         (mask & ~(numlockmask|LockMask) & (ShiftMask|ControlMask|Mod1Mask|Mod2Mask|Mod3Mask|Mod4Mask|Mod5Mask))
 #define INTERSECT(x,y,w,h,m)    (MAX(0, MIN((x)+(w),(m)->wx+(m)->ww) - MAX((x),(m)->wx)) \
                                * MAX(0, MIN((y)+(h),(m)->wy+(m)->wh) - MAX((y),(m)->wy)))
-#define ISVISIBLE(C)            ((C->tags & C->mon->tagset[C->mon->seltags]))
+#define ISVISIBLE(C, M)         ((C->tags & M->tagset[M->seltags]))
 #define LENGTH(X)               (sizeof X / sizeof X[0])
 #define MOUSEMASK               (BUTTONMASK|PointerMotionMask)
 #define WIDTH(X)                ((X)->w + 2 * (X)->bw)
@@ -83,6 +83,7 @@ typedef struct {
 	const Arg arg;
 } Button;
 
+typedef struct Clientlist Clientlist;
 typedef struct Monitor Monitor;
 typedef struct Client Client;
 struct Client {
@@ -125,9 +126,8 @@ struct Monitor {
 	unsigned int tagset[2];
 	int showbar;
 	int topbar;
-	Client *clients;
+	Clientlist *cl;
 	Client *sel;
-	Client *stack;
 	Monitor *next;
 	Window barwin;
 	const Layout *lt[2];
@@ -142,12 +142,18 @@ typedef struct {
 	int monitor;
 } Rule;
 
+struct Clientlist {
+	Client *clients;
+	Client *stack;
+};
+
 /* function declarations */
 static void applyrules(Client *c);
 static int applysizehints(Client *c, int *x, int *y, int *w, int *h, int interact);
 static void arrange(Monitor *m);
 static void arrangemon(Monitor *m);
 static void attach(Client *c);
+static void attachclients(Monitor *m);
 static void attachstack(Client *c);
 static void buttonpress(XEvent *e);
 static void checkotherwm(void);
@@ -185,7 +191,7 @@ static void maprequest(XEvent *e);
 static void monocle(Monitor *m);
 static void motionnotify(XEvent *e);
 static void movemouse(const Arg *arg);
-static Client *nexttiled(Client *c);
+static Client *nexttiled(Client *c, Monitor *m);
 static void pop(Client *);
 static void propertynotify(XEvent *e);
 static void quit(const Arg *arg);
@@ -268,6 +274,7 @@ static Display *dpy;
 static Drw *drw;
 static Monitor *mons, *selmon;
 static Window root;
+static Clientlist *cl;
 
 /* configuration, allows nested code to access above variables */
 #include "config.h"
@@ -300,7 +307,7 @@ applyrules(Client *c)
 		{
 			c->isfloating = r->isfloating;
 			c->tags |= r->tags;
-			for (m = mons; m && m->num != r->monitor; m = m->next);
+			for (m = mons; m && (m->tagset[m->seltags] & c->tags) == 0; m = m->next) ;
 			if (m)
 				c->mon = m;
 		}
@@ -382,9 +389,9 @@ void
 arrange(Monitor *m)
 {
 	if (m)
-		showhide(m->stack);
+		showhide(m->cl->stack);
 	else for (m = mons; m; m = m->next)
-		showhide(m->stack);
+		showhide(m->cl->stack);
 	if (m) {
 		arrangemon(m);
 		restack(m);
@@ -403,15 +410,48 @@ arrangemon(Monitor *m)
 void
 attach(Client *c)
 {
-	c->next = c->mon->clients;
-	c->mon->clients = c;
+	c->next = c->mon->cl->clients;
+	c->mon->cl->clients = c;
+}
+
+void
+attachclients(Monitor *m) {
+	/* attach clients to the specified monitor */
+	Monitor *tm;
+	Client *c;
+	unsigned int utags = 0;
+	Bool rmons = False;
+	if(!m)
+		return;
+
+	/* collect information about the tags in use */
+	for (tm = mons; tm; tm = tm->next)
+		if(tm != m)
+			utags |= tm->tagset[tm->seltags];
+
+	for (c = m->cl->clients; c; c = c->next)
+		if(ISVISIBLE(c, m)) {
+			/* if client is also visible on other tags that are displayed on
+			 * other monitors, remove these tags */
+			if(c->tags & utags) {
+				c->tags = c->tags & m->tagset[m->seltags];
+				rmons = True;
+			}
+			unfocus(c, True);
+			c->mon = m;
+		}
+
+	if (rmons)
+		for (tm = mons; tm; tm = tm->next)
+			if(tm != m)
+				arrange(tm);
 }
 
 void
 attachstack(Client *c)
 {
-	c->snext = c->mon->stack;
-	c->mon->stack = c;
+	c->snext = c->mon->cl->stack;
+	c->mon->cl->stack = c;
 }
 
 void
@@ -476,8 +516,8 @@ cleanup(void)
 	view(&a);
 	selmon->lt[selmon->sellt] = &foo;
 	for (m = mons; m; m = m->next)
-		while (m->stack)
-			unmanage(m->stack, 0);
+		while (m->cl->stack)
+			unmanage(m->cl->stack, 0);
 	XUngrabKey(dpy, AnyKey, AnyModifier, root);
 	while (mons)
 		cleanupmon(mons);
@@ -533,7 +573,7 @@ clientmessage(XEvent *e)
 			setfullscreen(c, (cme->data.l[0] == 1 /* _NET_WM_STATE_ADD    */
 			              || (cme->data.l[0] == 2 /* _NET_WM_STATE_TOGGLE */ && !c->isfullscreen)));
 	} else if (cme->message_type == netatom[NetActiveWindow]) {
-		if (!ISVISIBLE(c)) {
+		if (!ISVISIBLE(c, c->mon)) {
 			c->mon->seltags ^= 1;
 			c->mon->tagset[c->mon->seltags] = c->tags;
 		}
@@ -577,7 +617,7 @@ configurenotify(XEvent *e)
 			drw_resize(drw, sw, bh);
 			updatebars();
 			for (m = mons; m; m = m->next) {
-				for (c = m->clients; c; c = c->next)
+				for (c = m->cl->clients; c; c = c->next)
 					if (c->isfullscreen)
 						resizeclient(c, m->mx, m->my, m->mw, m->mh);
 				XMoveResizeWindow(dpy, m->barwin, m->wx, m->by, m->ww, bh);
@@ -623,7 +663,7 @@ configurerequest(XEvent *e)
 				c->y = m->my + (m->mh / 2 - HEIGHT(c) / 2); /* center in y direction */
 			if ((ev->value_mask & (CWX|CWY)) && !(ev->value_mask & (CWWidth|CWHeight)))
 				configure(c);
-			if (ISVISIBLE(c))
+			if (ISVISIBLE(c, m))
 				XMoveResizeWindow(dpy, c->win, c->x, c->y, c->w, c->h);
 		} else
 			configure(c);
@@ -643,10 +683,31 @@ configurerequest(XEvent *e)
 Monitor *
 createmon(void)
 {
-	Monitor *m;
+	Monitor *m, *tm;
+	int i;
 
+	/* bail out if the number of monitors exceeds the number of tags */
+	for (i=1, tm=mons; tm; i++, tm=tm->next);
+	if (i > LENGTH(tags)) {
+		fprintf(stderr, "dwm: failed to add monitor, number of tags exceeded\n");
+		return NULL;
+	}
+	/* find the first tag that isn't in use */
+	for (i=0; i < LENGTH(tags); i++) {
+		for (tm=mons; tm && !(tm->tagset[tm->seltags] & (1<<i)); tm=tm->next);
+		if (!tm)
+			break;
+	}
+	/* reassign all tags to monitors since there's currently no free tag for the
+	 * new monitor */
+	if (i >= LENGTH(tags))
+		for (i=0, tm=mons; tm; tm=tm->next, i++) {
+			tm->seltags ^= 1;
+			tm->tagset[tm->seltags] = (1<<i) & TAGMASK;
+		}
 	m = ecalloc(1, sizeof(Monitor));
-	m->tagset[0] = m->tagset[1] = 1;
+	m->cl = cl;
+	m->tagset[0] = m->tagset[1] = (1<<i) & TAGMASK;
 	m->mfact = mfact;
 	m->nmaster = nmaster;
 	m->showbar = showbar;
@@ -672,7 +733,7 @@ detach(Client *c)
 {
 	Client **tc;
 
-	for (tc = &c->mon->clients; *tc && *tc != c; tc = &(*tc)->next);
+	for (tc = &c->mon->cl->clients; *tc && *tc != c; tc = &(*tc)->next);
 	*tc = c->next;
 }
 
@@ -681,11 +742,11 @@ detachstack(Client *c)
 {
 	Client **tc, *t;
 
-	for (tc = &c->mon->stack; *tc && *tc != c; tc = &(*tc)->snext);
+	for (tc = &c->mon->cl->stack; *tc && *tc != c; tc = &(*tc)->snext);
 	*tc = c->snext;
 
 	if (c == c->mon->sel) {
-		for (t = c->mon->stack; t && !ISVISIBLE(t); t = t->snext);
+		for (t = c->mon->cl->stack; t && !ISVISIBLE(t, c->mon); t = t->snext);
 		c->mon->sel = t;
 	}
 }
@@ -721,7 +782,7 @@ drawbar(Monitor *m)
 		drw_text(drw, m->ww - sw, 0, sw, bh, lrpad / 2 - 2, stext, 0);
 	}
 
-	for (c = m->clients; c; c = c->next) {
+	for(c = m->cl->clients; c; c = c->next) {
 		occ |= c->tags;
 		if (c->isurgent)
 			urg |= c->tags;
@@ -796,8 +857,8 @@ expose(XEvent *e)
 void
 focus(Client *c)
 {
-	if (!c || !ISVISIBLE(c))
-		for (c = selmon->stack; c && !ISVISIBLE(c); c = c->snext);
+	if (!c || !ISVISIBLE(c, selmon))
+		for (c = selmon->cl->stack; c && !ISVISIBLE(c, selmon); c = c->snext);
 	/* was if (selmon->sel) */
 	if (selmon->sel && selmon->sel != c)
 		unfocus(selmon->sel, 0);
@@ -852,16 +913,16 @@ focusstack(const Arg *arg)
 	if (!selmon->sel)
 		return;
 	if (arg->i > 0) {
-		for (c = selmon->sel->next; c && !ISVISIBLE(c); c = c->next);
+		for (c = selmon->sel->next; c && !ISVISIBLE(c, selmon); c = c->next);
 		if (!c)
-			for (c = selmon->clients; c && !ISVISIBLE(c); c = c->next);
+			for (c = selmon->cl->clients; c && !ISVISIBLE(c, selmon); c = c->next);
 	} else {
-		for (i = selmon->clients; i != selmon->sel; i = i->next)
-			if (ISVISIBLE(i))
+		for (i = selmon->cl->clients; i != selmon->sel; i = i->next)
+			if (ISVISIBLE(i, selmon))
 				c = i;
 		if (!c)
 			for (; i; i = i->next)
-				if (ISVISIBLE(i))
+				if (ISVISIBLE(i, selmon))
 					c = i;
 	}
 	if (c) {
@@ -1123,12 +1184,12 @@ monocle(Monitor *m)
 	unsigned int n = 0;
 	Client *c;
 
-	for (c = m->clients; c; c = c->next)
-		if (ISVISIBLE(c))
+	for(c = m->cl->clients; c; c = c->next)
+		if(ISVISIBLE(c, m))
 			n++;
 	if (n > 0) /* override layout symbol */
 		snprintf(m->ltsymbol, sizeof m->ltsymbol, "[%d]", n);
-	for (c = nexttiled(m->clients); c; c = nexttiled(c->next))
+	for (c = nexttiled(m->cl->clients, m); c; c = nexttiled(c->next, m))
 		resize(c, m->wx, m->wy, m->ww - 2 * c->bw, m->wh - 2 * c->bw, 0);
 }
 
@@ -1213,9 +1274,9 @@ movemouse(const Arg *arg)
 }
 
 Client *
-nexttiled(Client *c)
+nexttiled(Client *c, Monitor *m)
 {
-	for (; c && (c->isfloating || !ISVISIBLE(c)); c = c->next);
+	for (; c && (c->isfloating || !ISVISIBLE(c, m)); c = c->next);
 	return c;
 }
 
@@ -1379,8 +1440,8 @@ restack(Monitor *m)
 	if (m->lt[m->sellt]->arrange) {
 		wc.stack_mode = Below;
 		wc.sibling = m->barwin;
-		for (c = m->stack; c; c = c->snext)
-			if (!c->isfloating && ISVISIBLE(c)) {
+		for (c = m->cl->stack; c; c = c->snext)
+			if (!c->isfloating && ISVISIBLE(c, m)) {
 				XConfigureWindow(dpy, c->win, CWSibling|CWStackMode, &wc);
 				wc.sibling = c->win;
 			}
@@ -1433,11 +1494,9 @@ sendmon(Client *c, Monitor *m)
 	if (c->mon == m)
 		return;
 	unfocus(c, 1);
-	detach(c);
 	detachstack(c);
 	c->mon = m;
 	c->tags = m->tagset[m->seltags]; /* assign tags of target monitor */
-	attach(c);
 	attachstack(c);
 	focus(NULL);
 	arrange(NULL);
@@ -1558,6 +1617,8 @@ setup(void)
 	screen = DefaultScreen(dpy);
 	sw = DisplayWidth(dpy, screen);
 	sh = DisplayHeight(dpy, screen);
+	if(!(cl = (Clientlist *)calloc(1, sizeof(Clientlist))))
+		die("fatal: could not malloc() %u bytes\n", sizeof(Clientlist));
 	root = RootWindow(dpy, screen);
 	drw = drw_create(dpy, screen, root, sw, sh);
 	if (!drw_fontset_create(drw, fonts, LENGTH(fonts)))
@@ -1607,7 +1668,7 @@ showhide(Client *c)
 {
 	if (!c)
 		return;
-	if (ISVISIBLE(c)) {
+	if (ISVISIBLE(c, c->mon)) {
 		/* show clients top down */
 		XMoveWindow(dpy, c->win, c->x, c->y);
 		if ((!c->mon->lt[c->mon->sellt]->arrange || c->isfloating) && !c->isfullscreen)
@@ -1647,7 +1708,22 @@ spawn(const Arg *arg)
 void
 tag(const Arg *arg)
 {
+	Monitor *m;
+	unsigned int newtags;
 	if (selmon->sel && arg->ui & TAGMASK) {
+		newtags = arg->ui & TAGMASK;
+		for (m = mons; m; m = m->next)
+			/* if tag is visible on another monitor, move client to the new monitor */
+			if (m != selmon && m->tagset[m->seltags] & newtags) {
+				/* prevent moving client to all tags (MODKEY-Shift-0) when multiple monitors are connected */
+				if(newtags & selmon->tagset[selmon->seltags])
+					return;
+				selmon->sel->tags = newtags;
+				selmon->sel->mon = m;
+				arrange(m);
+				break;
+			}
+		/* workaround in case just one monitor is connected */
 		selmon->sel->tags = arg->ui & TAGMASK;
 		focus(NULL);
 		arrange(selmon);
@@ -1668,7 +1744,7 @@ tile(Monitor *m)
 	unsigned int i, n, h, mw, my, ty;
 	Client *c;
 
-	for (n = 0, c = nexttiled(m->clients); c; c = nexttiled(c->next), n++);
+	for (n = 0, c = nexttiled(m->cl->clients, m); c; c = nexttiled(c->next, m), n++);
 	if (n == 0)
 		return;
 
@@ -1676,7 +1752,7 @@ tile(Monitor *m)
 		mw = m->nmaster ? m->ww * m->mfact : 0;
 	else
 		mw = m->ww;
-	for (i = my = ty = 0, c = nexttiled(m->clients); c; c = nexttiled(c->next), i++)
+	for (i = my = ty = 0, c = nexttiled(m->cl->clients, m); c; c = nexttiled(c->next, m), i++)
 		if (i < m->nmaster) {
 			h = (m->wh - my) / (MIN(n, m->nmaster) - i);
 			resize(c, m->wx, m->wy + my, mw - (2*c->bw), h - (2*c->bw), 0);
@@ -1714,12 +1790,17 @@ togglefloating(const Arg *arg)
 void
 toggletag(const Arg *arg)
 {
+	Monitor *m;
 	unsigned int newtags;
 
 	if (!selmon->sel)
 		return;
 	newtags = selmon->sel->tags ^ (arg->ui & TAGMASK);
 	if (newtags) {
+		/* prevent adding tags that are in use on other monitors */
+		for (m = mons; m; m = m->next)
+			if (m != selmon && newtags & m->tagset[m->seltags])
+				return;
 		selmon->sel->tags = newtags;
 		focus(NULL);
 		arrange(selmon);
@@ -1729,12 +1810,17 @@ toggletag(const Arg *arg)
 void
 toggleview(const Arg *arg)
 {
+	Monitor *m;
 	unsigned int newtagset = selmon->tagset[selmon->seltags] ^ (arg->ui & TAGMASK);
 
 	if (newtagset) {
+		/* prevent displaying the same tags on multiple monitors */
+		for(m = mons; m; m = m->next)
+			if(m != selmon && newtagset & m->tagset[m->seltags])
+				return;
 		selmon->tagset[selmon->seltags] = newtagset;
 		focus(NULL);
-		arrange(selmon);
+		attachclients(selmon);
 	}
 }
 
@@ -1832,7 +1918,7 @@ updateclientlist()
 
 	XDeleteProperty(dpy, root, netatom[NetClientList]);
 	for (m = mons; m; m = m->next)
-		for (c = m->clients; c; c = c->next)
+		for (c = m->cl->clients; c; c = c->next)
 			XChangeProperty(dpy, root, netatom[NetClientList],
 			                XA_WINDOW, 32, PropModeAppend,
 			                (unsigned char *) &(c->win), 1);
@@ -1862,8 +1948,10 @@ updategeom(void)
 		if (n <= nn) {
 			for (i = 0; i < (nn - n); i++) { /* new monitors available */
 				for (m = mons; m && m->next; m = m->next);
-				if (m)
+				if (m) {
 					m->next = createmon();
+					attachclients(m->next);
+				}
 				else
 					mons = createmon();
 			}
@@ -1884,17 +1972,13 @@ updategeom(void)
 			/* less monitors available nn < n */
 			for (i = nn; i < n; i++) {
 				for (m = mons; m && m->next; m = m->next);
-				while (m->clients) {
-					dirty = 1;
-					c = m->clients;
-					m->clients = c->next;
-					detachstack(c);
-					c->mon = mons;
-					attach(c);
-					attachstack(c);
-				}
 				if (m == selmon)
 					selmon = mons;
+				for (c = m->cl->clients; c; c = c->next) {
+					dirty = True;
+					if (c->mon == m)
+						c->mon = selmon;
+				}
 				cleanupmon(m);
 			}
 		}
@@ -2030,13 +2114,31 @@ updatewmhints(Client *c)
 void
 view(const Arg *arg)
 {
+	Monitor *m;
+	unsigned int newtagset = selmon->tagset[selmon->seltags ^ 1];
 	if ((arg->ui & TAGMASK) == selmon->tagset[selmon->seltags])
 		return;
+	/* swap tags when trying to display a tag from another monitor */
+	if (arg->ui & TAGMASK)
+		newtagset = arg->ui & TAGMASK;
+	for (m = mons; m; m = m->next)
+		if (m != selmon && newtagset & m->tagset[m->seltags]) {
+			/* prevent displaying all tags (MODKEY-0) when multiple monitors
+			 * are connected */
+			if (newtagset & selmon->tagset[selmon->seltags])
+				return;
+			m->sel = selmon->sel;
+			m->seltags ^= 1;
+			m->tagset[m->seltags] = selmon->tagset[selmon->seltags];
+			attachclients(m);
+			arrange(m);
+			break;
+		}
 	selmon->seltags ^= 1; /* toggle sel tagset */
 	if (arg->ui & TAGMASK)
 		selmon->tagset[selmon->seltags] = arg->ui & TAGMASK;
 	focus(NULL);
-	arrange(selmon);
+	attachclients(selmon);
 }
 
 Client *
@@ -2046,7 +2148,7 @@ wintoclient(Window w)
 	Monitor *m;
 
 	for (m = mons; m; m = m->next)
-		for (c = m->clients; c; c = c->next)
+		for (c = m->cl->clients; c; c = c->next)
 			if (c->win == w)
 				return c;
 	return NULL;
@@ -2113,8 +2215,8 @@ zoom(const Arg *arg)
 	if (!selmon->lt[selmon->sellt]->arrange
 	|| (selmon->sel && selmon->sel->isfloating))
 		return;
-	if (c == nexttiled(selmon->clients))
-		if (!c || !(c = nexttiled(c->next)))
+	if (c == nexttiled(selmon->cl->clients, selmon))
+		if (!c || !(c = nexttiled(c->next, selmon)))
 			return;
 	pop(c);
 }