Index: usr.bin/xargs/strnsubst.c
===================================================================
--- usr.bin/xargs/strnsubst.c
+++ usr.bin/xargs/strnsubst.c
@@ -16,7 +16,13 @@
 #include <string.h>
 #include <unistd.h>
 
-void	strnsubst(char **, const char *, const char *, size_t);
+enum error {
+	STRNSUBST_NOERROR = 0,
+	STRNSUBST_ERROR,
+	STRNSUBST_TRUNCATED
+};
+
+int	strnsubst(char **, const char *, const char *, size_t);
 
 /*
  * Replaces str with a string consisting of str with match replaced with
@@ -28,14 +34,15 @@
  * that we can still pretend to do somewhat meaningful substitution.
  * No value is returned.
  */
-void
+int
 strnsubst(char **str, const char *match, const char *replstr, size_t maxsize)
 {
 	char *s1, *s2, *this;
+	int error = STRNSUBST_NOERROR;
 
 	s1 = *str;
 	if (s1 == NULL)
-		return;
+		return STRNSUBST_ERROR;
 	/*
 	 * If maxsize is 0 then set it to the length of s1, because we have
 	 * to duplicate s1.  XXX we maybe should double-check whether the match
@@ -67,6 +74,7 @@
 		if ((strlen(s2) + strlen(s1) + strlen(replstr) -
 		    strlen(match) + 1) > maxsize) {
 			strlcat(s2, s1, maxsize);
+			error = STRNSUBST_TRUNCATED;
 			goto done;
 		}
 		strncat(s2, s1, (uintptr_t)this - (uintptr_t)s1);
@@ -76,7 +84,7 @@
 	strcat(s2, s1);
 done:
 	*str = s2;
-	return;
+	return error;
 }
 
 #ifdef TEST
Index: usr.bin/xargs/xargs.c
===================================================================
--- usr.bin/xargs/xargs.c
+++ usr.bin/xargs/xargs.c
@@ -73,7 +73,7 @@
 static int	prompt(void);
 static void	run(char **);
 static void	usage(void);
-void		strnsubst(char **, const char *, const char *, size_t);
+int		strnsubst(char **, const char *, const char *, size_t);
 static pid_t	xwait(int block, int *status);
 static void	xexit(const char *, const int);
 static void	waitchildren(const char *, int);
@@ -517,7 +517,10 @@
 	while (--argc) {
 		*tmp = *avj++;
 		if (repls && strstr(*tmp, replstr) != NULL) {
-			strnsubst(tmp++, replstr, inpline, (size_t)Sflag);
+			if (strnsubst(tmp++, replstr, inpline, (size_t)Sflag) > 0) {
+				warnx("comamnd line cannot be assembled, too long");
+				xexit(*argv, 1);
+			}
 			if (repls > 0)
 				repls--;
 		} else {