Index: usr.bin/cmp/cmp.1
===================================================================
--- usr.bin/cmp/cmp.1
+++ usr.bin/cmp/cmp.1
@@ -41,6 +41,7 @@
 .Nm
 .Op Fl l | s | x
 .Op Fl hz
+.Op Fl -bytes Ns Cm = Ns Ar num
 .Ar file1 file2
 .Op Ar skip1 Op Ar skip2
 .Sh DESCRIPTION
@@ -62,6 +63,10 @@
 .It Fl l , Fl -verbose
 Print the byte number (decimal) and the differing
 byte values (octal) for each difference.
+.It Fl n Ar num , Fl -bytes= Ns num
+Only compare up to
+.Ar num
+bytes.
 .It Fl s , Fl -silent , Fl -quiet
 Print nothing for differing files; return exit
 status only.
@@ -165,6 +170,7 @@
 compatible.
 The
 .Fl h ,
+.Fl n ,
 .Fl x ,
 and
 .Fl z
Index: usr.bin/cmp/cmp.c
===================================================================
--- usr.bin/cmp/cmp.c
+++ usr.bin/cmp/cmp.c
@@ -62,11 +62,13 @@
 
 #include "extern.h"
 
+off_t	limit;
 bool	lflag, sflag, xflag, zflag;
 
 static const struct option long_opts[] =
 {
 	{"verbose",	no_argument,		NULL, 'l'},
+	{"bytes",	required_argument,	NULL, 'n'},
 	{"silent",	no_argument,		NULL, 's'},
 	{"quiet",	no_argument,		NULL, 's'},
 	{NULL,		no_argument,		NULL, 0}
@@ -85,8 +87,15 @@
 
 	skip1 = skip2 = 0;
 	oflag = O_RDONLY;
-	while ((ch = getopt_long(argc, argv, "+hlsxz", long_opts, NULL)) != -1)
+	while ((ch = getopt_long(argc, argv, "+hln:sxz", long_opts, NULL)) != -1)
 		switch (ch) {
+		case 'n':		/* Limit */
+			if (expand_number(optarg, &limit) < 0 || limit < 0) {
+				fprintf(stderr, "Invalid --bytes: '%s'\n",
+				    optarg);
+				usage();
+			}
+			break;
 		case 'h':		/* Don't follow symlinks */
 			oflag |= O_NOFOLLOW;
 			break;
Index: usr.bin/cmp/extern.h
===================================================================
--- usr.bin/cmp/extern.h
+++ usr.bin/cmp/extern.h
@@ -44,4 +44,5 @@
 void	diffmsg(const char *, const char *, off_t, off_t);
 void	eofmsg(const char *);
 
+extern off_t limit;
 extern bool lflag, sflag, xflag, zflag;
Index: usr.bin/cmp/link.c
===================================================================
--- usr.bin/cmp/link.c
+++ usr.bin/cmp/link.c
@@ -72,7 +72,8 @@
 
 	dfound = 0;
 	byte = 1;
-	for (p1 = buf1 + skip1, p2 = buf2 + skip2; *p1 && *p2; p1++, p2++) {
+	for (p1 = buf1 + skip1, p2 = buf2 + skip2;
+	    *p1 && *p2 && (limit == 0 || byte <= limit); p1++, p2++) {
 		if ((ch = *p1) != *p2) {
 			if (xflag) {
 				dfound = 1;
Index: usr.bin/cmp/regular.c
===================================================================
--- usr.bin/cmp/regular.c
+++ usr.bin/cmp/regular.c
@@ -86,6 +86,8 @@
 	off2 = ROUNDPAGE(skip2);
 
 	length = MIN(len1, len2);
+	if (limit > 0)
+		length = MIN(length, limit);
 
 	if ((m1 = remmap(NULL, fd1, off1)) == NULL) {
 		c_special(fd1, file1, skip1, fd2, file2, skip2);
Index: usr.bin/cmp/special.c
===================================================================
--- usr.bin/cmp/special.c
+++ usr.bin/cmp/special.c
@@ -76,7 +76,7 @@
 		if (getc(fp2) == EOF)
 			goto eof;
 
-	for (byte = line = 1;; ++byte) {
+	for (byte = line = 1; limit == 0 || byte <= limit; ++byte) {
 		ch1 = getc(fp1);
 		ch2 = getc(fp2);
 		if (ch1 == EOF || ch2 == EOF)
Index: usr.bin/cmp/tests/cmp_test2.sh
===================================================================
--- usr.bin/cmp/tests/cmp_test2.sh
+++ usr.bin/cmp/tests/cmp_test2.sh
@@ -91,10 +91,33 @@
 	atf_check -s exit:0 cmp -s a b 1k 1k
 }
 
+atf_test_case limit
+limit_head()
+{
+	atf_set "descr" "Test cmp(1) -n (limit)"
+}
+limit_body()
+{
+	echo -n "aaaabbbb" > a
+	echo -n "aaaaxxxx" > b
+
+	atf_check -s exit:1 -o ignore cmp -s a b
+	atf_check -s exit:0 cmp -sn 4 a b
+	atf_check -s exit:0 cmp -sn 3 a b
+	atf_check -s exit:1 -o ignore cmp -sn 5 a b
+
+	# Test special, too.  The implementation for link is effectively
+	# identical.
+	atf_check -s exit:0 -e empty -x "cat a | cmp -sn 4 b -"
+	atf_check -s exit:0 -e empty -x "cat a | cmp -sn 3 b -"
+	atf_check -s exit:1 -o ignore -x "cat a | cmp -sn 5 b -"
+}
+
 atf_init_test_cases()
 {
 	atf_add_test_case special
 	atf_add_test_case symlink
 	atf_add_test_case pr252542
 	atf_add_test_case skipsuff
+	atf_add_test_case limit
 }