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
@@ -24,8 +24,8 @@
 atf_test_case noderef
 atf_test_case ignorecase
 atf_test_case dirloop
-atf_test_case bigc
-atf_test_case bigu
+atf_test_case crange
+atf_test_case urange
 atf_test_case prleak
 atf_test_case same
 
@@ -389,34 +389,48 @@
 	atf_check diff -r a b
 }
 
-bigc_head()
+crange_head()
 {
-	atf_set "descr" "Context diff with very large context"
+	atf_set "descr" "Context diff context length range"
 }
-bigc_body()
+crange_body()
 {
-	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:1 -o match:'--- 1,3 ---' \
-	    diff -C$(((1<<31)-2)) a b
-	atf_check -s exit:1 -o match:'--- 1,3 ---' \
-	    diff -Astone -C$(((1<<31)-2)) a b
+	echo $'x\nx\na\ny\ny' >a
+	echo $'x\nx\nb\ny\ny' >b
+	atf_check -s exit:2 -e match:'too small' \
+	    diff -C-1 a b
+	atf_check -s exit:2 -e match:'too small' \
+	    diff -C0 a b
+	atf_check -s exit:1 -o match:'--- 2,4 ---' \
+	    diff -C1 a b
+	atf_check -s exit:1 -o match:'--- 2,4 ---' \
+	    diff -Astone -C1 a b
+	atf_check -s exit:2 -e match:'too large' \
+	    diff -C$((1<<31)) a b
+	atf_check -s exit:1 -o match:'--- 1,5 ---' \
+	    diff -C$(((1<<31)-1)) a b
+	atf_check -s exit:1 -o match:'--- 1,5 ---' \
+	    diff -Astone -C$(((1<<31)-1)) a b
 }
 
-bigu_head()
+urange_head()
 {
-	atf_set "descr" "Unified diff with very large context"
+	atf_set "descr" "Unified diff context length range"
 }
-bigu_body()
+urange_body()
 {
-	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:1 -o match:'^@@ -1,3 \+1,3 @@$' \
-	    diff -U$(((1<<31)-2)) a b
-	atf_check -s exit:1 -o match:'^@@ -1,3 \+1,3 @@$' \
-	    diff -Astone -U$(((1<<31)-2)) a b
+	echo $'x\nx\na\ny\ny' >a
+	echo $'x\nx\nb\ny\ny' >b
+	atf_check -s exit:2 -e match:'too small' \
+	    diff -U-1 a b
+	atf_check -s exit:1 -o match:'^@@ -3 \+3 @@$' \
+	    diff -U0 a b
+	atf_check -s exit:2 -e match:'too large' \
+	    diff -U$((1<<31)) a b
+	atf_check -s exit:1 -o match:'^@@ -1,5 \+1,5 @@$' \
+	    diff -U$(((1<<31)-1)) a b
+	atf_check -s exit:1 -o match:'^@@ -1,5 \+1,5 @@$' \
+	    diff -Astone -U$(((1<<31)-1)) a b
 }
 
 prleak_head()
@@ -485,8 +499,8 @@
 	atf_add_test_case noderef
 	atf_add_test_case ignorecase
 	atf_add_test_case dirloop
-	atf_add_test_case bigc
-	atf_add_test_case bigu
+	atf_add_test_case crange
+	atf_add_test_case urange
 	atf_add_test_case prleak
 	atf_add_test_case same
 }