applied improved version of Martti Kühne's dmenu/multi monitor approach from dwm, no dmenuspawn required

config.def.h

@@ -50,7 +50,8 @@ static const Layout layouts[] = {
 #define SHCMD(cmd) { .v = (const char*[]){ "/bin/sh", "-c", cmd, NULL } }
 
 /* commands */
-static const char *dmenucmd[] = { "dmenu_run", "-fn", font, "-nb", normbgcolor, "-nf", normfgcolor, "-sb", selbgcolor, "-sf", selfgcolor, NULL };
+static char dmenumon[2] = "0"; /* component of dmenucmd, manipulated in spawn() */
+static const char *dmenucmd[] = { "dmenu_run", "-m", dmenumon, "-fn", font, "-nb", normbgcolor, "-nf", normfgcolor, "-sb", selbgcolor, "-sf", selfgcolor, NULL };
 static const char *termcmd[]  = { "st", NULL };
 
 static Key keys[] = {

dwm.c

@@ -1569,6 +1569,8 @@ sigchld(int unused) {
 
 void
 spawn(const Arg *arg) {
+	if(arg->v == dmenucmd)
+		dmenumon[0] = '0' + selmon->num;
 	if(fork() == 0) {
 		if(dpy)
 			close(ConnectionNumber(dpy));