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/*
* gpscheck.c -- test to see if specified device is a GPS
*
* This will detect NMEA devices at 4800 baud or up, 8N1, including
* all SiRF-II-based GPS mice. It won't find newer Garmin or old
* Zodiac GPSes, which speak binary protocols; nor will it find
* oddballs like the San Jose Navigation FV18 that run at 7N2.
* However, it should catch over 80% of the consumer-grade GPSes
* available in 2005.
*
* Return value: -1 if any of the TTY mode sets fails, 0 if they succeeed
* but it's not a GPS, one of the values B4800, B9600, B19200, or B38400
* if it's a GPS.
*
* Compile with -DTESTMAIN to produce a test executable that takes the name
* of the device as a command-line argument.
*
* By Eric S. Raymond, February 2005.
*/
#include <unistd.h>
#include <stdlib.h>
#include <termios.h>
#include <errno.h>
#include <ctype.h>
#include <string.h>
#include <fcntl.h>
#include <stdio.h>
#define MAX_NMEA 82 /* max chars per NMEA sentence */
int gpscheck(int ttyfd)
{
struct termios ttyset, ttyset_old;
/* every rate we're likely to see on a GPS */
static unsigned int rates[] = {B4800, B9600, B19200, B38400, B57600};
unsigned int *ip;
/* Save original terminal parameters */
if (tcgetattr(ttyfd,&ttyset_old) != 0)
return -1;
memcpy(&ttyset, &ttyset_old,sizeof(ttyset));
ttyset.c_cflag &= ~(PARENB | CRTSCTS);
ttyset.c_cflag |= (CSIZE & CS8) | CREAD | CLOCAL;
ttyset.c_iflag = ttyset.c_oflag = ttyset.c_lflag = (tcflag_t) 0;
ttyset.c_oflag = (ONLCR);
for (ip = rates; ip < rates + sizeof(rates)/sizeof(rates[0]); ip++)
{
char buf[MAX_NMEA * 3 + 1], *sp, csum[3];
int n;
unsigned char sum;
/* might be able to avoid delay if device started at 4800 */
if (cfgetispeed(&ttyset) != *ip)
{
tcflush(ttyfd, TCIOFLUSH);
cfsetispeed(&ttyset, (speed_t)*ip);
if (tcsetattr(ttyfd, TCSANOW, &ttyset) != 0)
return -1;
tcflush(ttyfd, TCIOFLUSH);
#ifdef TESTMAIN
fprintf(stderr, "gpscheck: checking at rate %d\n", *ip);
#endif /* TESTMAIN */
}
/* assumes the fd is opened for ordinary (blocking) read */
if ((n = read(ttyfd, buf, sizeof(buf)-1)) == -1)
return -1;
/*
* Stuff that is specific to NMEA devices starts here.
*/
/* If no valid NMEA prefix in the read buffer, crap out */
if (!(sp = strstr(buf, "$GP"))) {
#ifdef TESTMAIN
fprintf(stderr, "gpscheck: no NMEA prefix found\n");
#endif /* TESTMAIN */
tcsetattr(ttyfd, TCSAFLUSH, &ttyset_old);
continue;
}
/* Check to see if we actually have a valid NMEA packet here. */
sum = 0;
for (++sp; *sp != '*' && *sp != '\0'; sp++)
sum ^= *sp;
sprintf(csum, "%02X", sum);
if (*sp == '\0'
|| toupper(csum[0])!=toupper(sp[1])
|| toupper(csum[1])!=toupper(sp[2])) {
#ifdef TESTMAIN
fprintf(stderr, "gpscheck: checksum incorrect\n");
#endif /* TESTMAIN */
tcsetattr(ttyfd, TCSAFLUSH, &ttyset_old);
continue;
}
/*
* Trailing garbage means that the data accidentally looked
* like NMEA or that old data that really was NMEA happened to
* be sitting in the TTY buffer unread, but the new data we
* read is not sentences. Second case shouldn't happen,
* because we flush the buffer after each speed change, but
* welcome to serial-programming hell.
*/
while (sp < buf + n)
if (!isascii(*sp++)) {
#ifdef TESTMAIN
fprintf(stderr, "gpscheck: trailing garbage in buffer\n");
#endif /* TESTMAIN */
tcsetattr(ttyfd, TCSAFLUSH, &ttyset_old);
return 0;
}
/* NMEA-device-specic stuff ends here */
/* passed all tests, looks like GPS */
switch (*ip) {
case B4800: return(4800);
case B9600: return(9600);
case B19200: return(19200);
default: return(38400);
}
}
return 0;
}
#ifdef TESTMAIN
int main(int argc, char **argv)
{
int fd, st;
fd = open(argv[1], O_RDONLY);
if (fd < 0) {
perror("gpscheck");
exit(0);
}
if ((st = gpscheck(fd)) > 0)
printf("%s appears to be a GPS.\n", argv[1]);
else
printf("%s does not appear to be a GPS.\n", argv[1]);
close(fd);
exit(st <= 0);
}
#endif /* TESTMAIN */
/*
Local Variables:
compile-command: "cc -DTESTMAIN gpscheck.c -o gpscheck"
End:
*/
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