diff --git a/usr.bin/diff/diff.c b/usr.bin/diff/diff.c
--- a/usr.bin/diff/diff.c
+++ b/usr.bin/diff/diff.c
@@ -137,10 +137,9 @@
 int
 main(int argc, char **argv)
 {
-	const char *errstr = NULL;
-	char *ep, **oargv;
-	long  l;
-	int   ch, dflags, lastch, gotstdin, prevoptind, newarg;
+	const char *errstr;
+	char **oargv;
+	int ch, dflags, lastch, gotstdin, prevoptind, newarg;
 
 	oargv = argv;
 	gotstdin = 0;
@@ -177,7 +176,7 @@
 			}
 
 			if (diff_algorithm == D_DIFFNONE) {
-				printf("unknown algorithm: %s\n", optarg);
+				warnx("unknown algorithm: %s", optarg);
 				usage();
 			}
 			break;
@@ -194,10 +193,13 @@
 			cflag = true;
 			diff_format = D_CONTEXT;
 			if (optarg != NULL) {
-				l = strtol(optarg, &ep, 10);
-				if (*ep != '\0' || l < 0 || l >= INT_MAX)
+				diff_context = (int) strtonum(optarg,
+				    1, INT_MAX, &errstr);
+				if (errstr != NULL) {
+					warnx("context size is %s: %s",
+					    errstr, optarg);
 					usage();
-				diff_context = (int)l;
+				}
 			}
 			break;
 		case 'd':
@@ -294,10 +296,13 @@
 				conflicting_format();
 			diff_format = D_UNIFIED;
 			if (optarg != NULL) {
-				l = strtol(optarg, &ep, 10);
-				if (*ep != '\0' || l < 0 || l >= INT_MAX)
+				diff_context = (int) strtonum(optarg,
+				    0, INT_MAX, &errstr);
+				if (errstr != NULL) {
+					warnx("context size is %s: %s",
+					    errstr, optarg);
 					usage();
-				diff_context = (int)l;
+				}
 			}
 			break;
 		case 'w':
@@ -305,8 +310,8 @@
 			break;
 		case 'W':
 			width = (int) strtonum(optarg, 1, INT_MAX, &errstr);
-			if (errstr) {
-				warnx("Invalid argument for width");
+			if (errstr != NULL) {
+				warnx("width is %s: %s", errstr, optarg);
 				usage();
 			}
 			break;
@@ -346,8 +351,8 @@
 			break;
 		case OPT_TSIZE:
 			tabsize = (int) strtonum(optarg, 1, INT_MAX, &errstr);
-			if (errstr) {
-				warnx("Invalid argument for tabsize");
+			if (errstr != NULL) {
+				warnx("tabsize is %s: %s", errstr, optarg);
 				usage();
 			}
 			break;
@@ -364,9 +369,12 @@
 				colorflag = COLORFLAG_ALWAYS;
 			else if (strncmp(optarg, "never", 5) == 0)
 				colorflag = COLORFLAG_NEVER;
-			else
-				errx(2, "unsupported --color value '%s' (must be always, auto, or never)",
-					optarg);
+			else {
+				warnx("unsupported --color value "
+				    "(must be always, auto, or never): "
+				    "%s", optarg);
+				usage();
+			}
 			break;
 		case OPT_NO_DEREFERENCE:
 			noderef = true;
diff --git a/usr.bin/diff/tests/diff_test.sh b/usr.bin/diff/tests/diff_test.sh
--- a/usr.bin/diff/tests/diff_test.sh
+++ b/usr.bin/diff/tests/diff_test.sh
@@ -397,11 +397,11 @@
 {
 	echo $'x\na\ny' >a
 	echo $'x\nb\ny' >b
-	atf_check -s exit:2 -e ignore diff -C$(((1<<31)-1)) a b
+	atf_check -s exit:2 -e ignore diff -C$((1<<31)) a b
 	atf_check -s exit:1 -o match:'--- 1,3 ---' \
-	    diff -C$(((1<<31)-2)) a b
+	    diff -C$(((1<<31)-1)) a b
 	atf_check -s exit:1 -o match:'--- 1,3 ---' \
-	    diff -Astone -C$(((1<<31)-2)) a b
+	    diff -Astone -C$(((1<<31)-1)) a b
 }
 
 bigu_head()
@@ -412,11 +412,11 @@
 {
 	echo $'x\na\ny' >a
 	echo $'x\nb\ny' >b
-	atf_check -s exit:2 -e ignore diff -U$(((1<<31)-1)) a b
+	atf_check -s exit:2 -e ignore diff -U$((1<<31)) a b
 	atf_check -s exit:1 -o match:'^@@ -1,3 \+1,3 @@$' \
-	    diff -U$(((1<<31)-2)) a b
+	    diff -U$(((1<<31)-1)) a b
 	atf_check -s exit:1 -o match:'^@@ -1,3 \+1,3 @@$' \
-	    diff -Astone -U$(((1<<31)-2)) a b
+	    diff -Astone -U$(((1<<31)-1)) a b
 }
 
 prleak_head()