+/* Launch a process to filter data written to a file descriptor. fd_out is the
+ * file descriptor where the filtered data should be written. program is the
+ * filter program to execute (a single string which will be interpreted by
+ * /bin/sh). The return value is a file descriptor to which the data to be
+ * filtered should be written. The process ID of the filter process is stored
+ * at address filter_pid if non-NULL. */
+int spawn_filter(int fd_out, const char *program, pid_t *filter_pid);
+