add -o/--output option

src/include/options.h

@@ -54,11 +54,12 @@ struct opts_s {           /* structure describing run-time options */
 	bool stop_at_size;             /* set if we stop at "size" bytes */
 	bool sync_after_write;         /* set if we sync after every write */
 	bool direct_io;                /* set if O_DIRECT is to be used */
-	bool discard_input;            /* set to write nothing to stdout */
+	bool discard_input;            /* set to write nothing to output */
 	double interval;               /* interval between updates */
 	double delay_start;            /* delay before first display */
 	pid_t watch_pid;	       /* process to watch fds of */
 	int watch_fd;		       /* fd to watch */
+	/*@keep@*/ /*@null@*/ char *output; /* fd to write output to */
 	unsigned int average_rate_window; /* time window in seconds for average rate calculations */
 	unsigned int width;            /* screen width */
 	unsigned int height;           /* screen height */

src/include/pv-internal.h

@@ -123,6 +123,8 @@ struct pvstate_s {
 		double delay_start;              /* delay before first display */
 		pid_t watch_pid;		 /* process to watch fds of */
 		int watch_fd;			 /* fd to watch */
+		int output_fd;                   /* fd to write output to */
+		/*@null@*/ char *output_name;    /* name of the output, for diagnostics */
 		unsigned int average_rate_window; /* time window in seconds for average rate calculations */
 		unsigned int width;              /* screen width */
 		unsigned int height;             /* screen height */

src/include/pv.h

@@ -186,6 +186,7 @@ extern void pv_state_name_set(pvstate_t, /*@null@*/ const char *);
 extern void pv_state_format_string_set(pvstate_t, /*@null@*/ const char *);
 extern void pv_state_watch_pid_set(pvstate_t, pid_t);
 extern void pv_state_watch_fd_set(pvstate_t, int);
+extern void pv_state_output_set(pvstate_t, int, const char *);
 extern void pv_state_average_rate_window_set(pvstate_t, unsigned int);
 
 extern void pv_state_inputfiles(pvstate_t, unsigned int, const char **);