This addresses all comments received.

deraadt suggested that snprintf is overkill and to use strlcpy. I don't
see either as a clear winner, we just don't have a string joining
function in libc...

I'm with tb that using the canonical overflow check causes the least
amount of cognitive load. You see the idiom is used correctly and move
on, so I went with that.

OK?

diff --git watch.c watch.c
index 50dcab12a42..0599b4140bf 100644
--- watch.c
+++ watch.c
@@ -77,6 +77,7 @@ int xflag = 0;
 #define	WCWIDTH(_x)	((wcwidth((_x)) > 0)? wcwidth((_x)) : 1)
 
 static char	 *cmdstr;
+static size_t	  cmdlen;
 static char	**cmdv;
 
 typedef wchar_t BUFFER[MAXLINE][MAXCOLUMN + 1];
@@ -114,8 +115,9 @@ set_interval(const char *str)
 int
 main(int argc, char *argv[])
 {
-	int i, ch, cmdsiz = 0;
-	char *s;
+	size_t len, rem;
+	int i, ch;
+	char *p;
 
 	while ((ch = getopt(argc, argv, "cls:wx")) != -1)
 		switch (ch) {
@@ -153,24 +155,33 @@ main(int argc, char *argv[])
 	if ((cmdv = calloc(argc + 1, sizeof(char *))) == NULL)
 		err(1, "calloc");
 
-	cmdstr = "";
+	cmdlen = 0;
 	for (i = 0; i < argc; i++) {
 		cmdv[i] = argv[i];
-		while (strlen(cmdstr) + strlen(argv[i]) + 3 > cmdsiz) {
-			if (cmdsiz == 0) {
-				cmdsiz = 128;
-				s = calloc(cmdsiz, 1);
-			} else {
-				cmdsiz *= 2;
-				s = realloc(cmdstr, cmdsiz);
-			}
-			if (s == NULL)
-				err(1, "malloc");
-			cmdstr = s;
-		}
+		cmdlen += strlen(argv[i]);
 		if (i != 0)
-			strlcat(cmdstr, " ", cmdsiz);
-		strlcat(cmdstr, argv[i], cmdsiz);
+			cmdlen++;
+	}
+
+	if ((cmdstr = malloc(cmdlen + 1)) == NULL)
+		err(1, "malloc");
+
+	p = cmdstr;
+	rem = cmdlen + 1;
+
+	if ((len = strlcpy(p, argv[0], rem)) >= rem)
+		errx(1, "overflow");
+	rem -= len;
+	p += len;
+	for (i = 1; i < argc; i++) {
+		if ((len = strlcpy(p, " ", rem)) >= rem)
+			errx(1, "overflow");
+		rem -= len;
+		p += len;
+		if ((len = strlcpy(p, argv[i], rem)) >= rem)
+			errx(1, "overflow");
+		rem -= len;
+		p += len;
 	}
 	cmdv[i++] = NULL;
 
@@ -278,7 +289,7 @@ display(BUFFER * cur, BUFFER * prev, highlight_mode_t hm)
 	else
 		printw("Every %.4gs: ", opt_interval.d);
 
-	if ((int)strlen(cmdstr) > COLS - 47)
+	if (cmdlen > COLS - 47)
 		printw("%-.*s...", COLS - 49, cmdstr);
 	else
 		printw("%s", cmdstr);

-- 
In my defence, I have been left unsupervised.