# packet.c

Functions:- | The packet-sniffing engine for reading data packets from GPS devices.
Notes based on code as of Tue Apr 6 10:17:55 2010 -0400.
`static void nextstate(struct gps_packet_t *lexer, unsigned char c)` | This is the main packet-sniffer loop. It scans the character against the definitions of all the packet structures known to gpsd and, if possible, sets a new packet state. If the state is xxx_RECOGNIZED, the packet_parse routine will despatch the packet to the appropriate driver.
`static void packet_accept(struct gps_packet_t *lexer, int packet_type)` | This shifts a packet that has been recognized into the output buffer, provided it is not bigger than the buffer.
`static void packet_discard(struct gps_packet_t *lexer)` | This clears a handled packet out of the input buffer.
`static void character_discard(struct gps_packet_t *lexer)` | This is called if the `nextstate()` function returns GROUND_STATE. In this case the character does not match any pattern, so to discard it, the input buffer is shifted back by one character to overwrite the bad character.
`ssize_t packet_parse(struct gps_packet_t *lexer, size_t fix)` | Call the `nextstate()` function to process the available data and set the recognition state correctly. When a packet is matched to a driver, call `packet_accept()` and `packet_discard()` to handle the packet. If it is not matched, call `packet_discard()` and set the state to GROUND_STATE Return the number of characters handled.
`ssize_t packet_get(int fd, struct gps_packet_t *lexer)` | Reads raw data from the input port. Returns the number of characters read (0 or more) or BAD_PACKET if there was an error in reading. Errors EAGAIN and EINTR are not classed as failures and cause a return of 0. In case of a good read of more than 0 characters, the return value is the output from a call to `packet_parse()`.
`void packet_reset(struct gps_packet_t *lexer)` | This simply resets the entire packet state machine to the ground state.
`void packet_nit(struct gps_packet_t *lexer)` | Zeros some counters, then resets the entire packet state machine to the ground state.
`void packet_pushback(struct gps_packet_t *lexer)` | This pushes back the last packet from the output buffer to the input buffer, provided doing so would not overflow the input buffer.