parsing complete messages from serial port - c++

Question

I am trying to read complete messages from my GPS via serial port.
The message I am looking for starts with:
0xB5 0x62 0x02 0x13
So I read from the serial port like so
while (running !=0)
{
int n = read (fd, input_buffer, sizeof input_buffer);
for (int i=0; i<BUFFER_SIZE; i++)
{
if (input_buffer[i]==0xB5 && input_buffer[i+1]== 0x62 && input_buffer[i+2]== 0x02 && input_buffer[i+3]== 0x13 && i<(BUFFER_SIZE-1) )
{
// process the message.
}
}
The problem I am having is that I need to get a complete message. Half of a message could be in the buffer one iteration. And the other half could come into the message the next iteration.
Somebody suggested that free the buffer up from the complete message. And then I move the rest of data in the buffer to the beginning of the buffer.
How do I do that or any other way that make sure I get every complete selected message that comes in?
edit//
I want a particular class and ID. But I can also read in the length

Answer 1

To minimize the overhead of making many read() syscalls of small byte counts, use an intermediate buffer in your code.
The read()s should be in blocking mode to avoid a return code of zero bytes.
#define BLEN 1024
unsigned char rbuf[BLEN];
unsigned char *rp = &rbuf[BLEN];
int bufcnt = 0;
static unsigned char getbyte(void)
{
if ((rp - rbuf) >= bufcnt) {
/* buffer needs refill */
bufcnt = read(fd, rbuf, BLEN);
if (bufcnt <= 0) {
/* report error, then abort */
}
rp = rbuf;
}
return *rp++;
}
For proper termios initialization code for the serial terminal, see this answer. You should increase the VMIN parameter to something closer to the BLEN value.
Now you can conveniently access the received data a byte at a time with minimal performance penalty.
#define MLEN 1024 /* choose appropriate value for message protocol */
unsigned char mesg[MLEN];
while (1) {
while (getbyte() != 0xB5)
/* hunt for 1st sync */ ;
retry_sync:
if ((sync = getbyte()) != 0x62) {
if (sync == 0xB5)
goto retry_sync;
else
continue; /* restart sync hunt */
}
class = getbyte();
id = getbyte();
length = getbyte();
length += getbyte() << 8;
if (length > MLEN) {
/* report error, then restart sync hunt */
continue;
}
for (i = 0; i < length; i++) {
mesg[i] = getbyte();
/* accumulate checksum */
}
chka = getbyte();
chkb = getbyte();
if ( /* valid checksum */ )
break; /* verified message */
/* report error, and restart sync hunt */
}
/* process the message */
switch (class) {
case 0x02:
if (id == 0x13) {
...
...

Answer 2

You can break the read into three parts. Find the start of a message. Then get the LENGTH. Then read the rest of the message.
// Should probably clear these in case data left over from a previous read
input_buffer[0] = input_buffer[1] = 0;
// First make sure first char is 0xB5
do {
n = read(fd, input_buffer, 1);
} while (0xB5 != input_buffer[0]);
// Check for 2nd sync char
n = read(fd, &input_buffer[1], 1);
if (input_buffer[1] != 0x62) {
// Error
return;
}
// Read up to LENGTH
n = read(fd, &input_buffer[2], 4);
// Parse length
//int length = *((int *)&input_buffer[4]);
// Since I don't know what size an int is on your system, this way is better
int length = input_buffer[4] | (input_buffer[5] << 8);
// Read rest of message
n = read(fd, &input_buffer[6], length);
// input_buffer should now have a complete message
You should add error checking...