-change to uid 0 before executing the process. Standard output from the process
-is redirected to the debug log. Note that the process is invoked directly, not
-via a shell, so shell metacharcters (e.g., C<< 2>&1 >>) will not function as
-expected. If C<$log_stderr> is set, then the filter's standard error is sent
-to the debug log; otherwise, it is sent to the parent process's stderr.
+change to uid 0 before executing the process. Note that the process is
+invoked directly, not via a shell, so shell metacharcters (e.g., C<< 2>&1 >>)
+will not function as expected. This method create a pipe for the process
+stderr and the caller must read it or a hang may occur.
+
+ $xfp->get_stderr_fd()
+
+Return the file descriptor of the stderr pipe to read from.