- * Run \var{cmd} with a read pipe connected to its stdout.
- * When data is received through the pipe, \var{handler} is called
- * with that data.
+ * Run \var{cmd} in directory \var{wd} with a read pipe connected to its
+ * stdout and stderr.
+ * When data is received through one of these pipes, \var{h} or \var{errh}
+ * is called with that data. When the pipe is closed, the handler is called
+ * with \code{nil} argument. The PID of the new process is returned, or
+ * -1 on error.