diff --git a/usr.bin/diff/diffreg_new.c b/usr.bin/diff/diffreg_new.c
--- a/usr.bin/diff/diffreg_new.c
+++ b/usr.bin/diff/diffreg_new.c
@@ -15,10 +15,12 @@
  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  */
 
+#include <sys/param.h>
 #include <sys/capsicum.h>
+#ifndef DIFF_NO_MMAP
 #include <sys/mman.h>
+#endif
 #include <sys/stat.h>
-#include <sys/types.h>
 
 #include <capsicum_helpers.h>
 #include <err.h>
@@ -266,10 +268,12 @@
 	diff_result_free(result);
 	diff_data_free(&left);
 	diff_data_free(&right);
+#ifndef DIFF_NO_MMAP
 	if (str1)
 		munmap(str1, st1.st_size);
 	if (str2)
 		munmap(str2, st2.st_size);
+#endif
 	fclose(f1);
 	fclose(f2);
 
@@ -297,6 +301,8 @@
 	if (*p == MAP_FAILED)
 #endif
 		*p = NULL; /* fall back on file I/O */
+	if (*p == NULL)
+		setbuffer(f, NULL, MAXBSIZE);
 
 	return f;
 }