diff --git a/osx.c b/osx.c
index 40035f6..9b1e16d 100644
--- a/osx.c
+++ b/osx.c
@@ -93,6 +93,10 @@ static int flocking(FILE *fh, int mode, __int64 ofs, __int64 len)
         len = _filelengthi64(_fileno(fh));
         if (len < 0)
             goto fail;
+
+        len -= ofs;
+        if (len < 0)
+            len = 0;
         if (len > LONG_MAX) {
             errno = ERANGE;
             goto fail;