PR70975 Pass valid offset argument to sendfile

	PR libstdc++/70975
	* src/filesystem/ops.cc (do_copy_file) [_GLIBCXX_USE_SENDFILE]:
	Pass non-null pointer to sendfile for offset argument.

From-SVN: r241629

libstdc++-v3/ChangeLog

+2016-10-27  Uros Bizjak  <ubizjak@gmail.com>
+
+	PR libstdc++/70975
+	* src/filesystem/ops.cc (do_copy_file) [_GLIBCXX_USE_SENDFILE]:
+	Pass non-null pointer to sendfile for offset argument.
+
 2016-10-27  Jonathan Wakely  <jwakely@redhat.com>
 
 	* testsuite/ext/random/uniform_inside_sphere_distribution/cons/

libstdc++-v3/src/filesystem/ops.cc

@@ -444,7 +444,8 @@ namespace
       }
 
 #ifdef _GLIBCXX_USE_SENDFILE
-    const auto n = ::sendfile(out.fd, in.fd, nullptr, from_st->st_size);
+    off_t offset = 0;
+    const auto n = ::sendfile(out.fd, in.fd, &offset, from_st->st_size);
     if (n < 0 && (errno == ENOSYS || errno == EINVAL))
       {
 #endif