parsing complete messages from serial port

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

S

Answer 1

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...