parsing and sending dataframes in C - c

Question

I have some trouble to implement an existing protocol in C, escpecially with creating the data packets.
The Packet looks like this:
HeaderData 1(1Byte) | HeaderData 2(1 Byte) | Length_Data (2 Byte) | Data (Length variable)
I get some data via serial port and it is saved in returnMessage:
int getMessage(char * returnMessage){
int length = 0;
//static char replay[REPLAY_MAX_SIZE];
memset(replay, 0x00, REPLAY_MAX_SIZE);
// goes into the while-loop, if some data comes in
// after a timeout of 20 seconds the program will be terminated
if(select(fd +1 , &set, NULL, NULL, &timeout )){
length = read(fd, returnMessage, REPLAY_MAX_SIZE);
//reinitialize fd_set values
FD_ZERO(&set);
FD_SET(fd, &set);
return length;
}
//printf("[%s] ERROR CANNOT GET MESSAGE\n", __FUNCTION__);
return -1;
}
This data in returnMessage I will enhance to my packet to send it further. But I dont know how.
Maybe there is a way with structs?
struct Packet{
char header1[1];
char header2[2];
char lengthData[20];
char data[MAX_DATA_LENGTH];
};
struct Packet packet;
But how can I copy the data in returnMessage to packet.data[]? And how can I concatenate the data in the struct in order to send the packet.
Have someone an introduction of creating/parsing packets for simple protocols?
Thanks a lot,
Florian

Answer 1

Assuming you get the full packet in your char buffer (*), it is then simple :
struct Packet{
char header1[1];
char header2[2];
short lengthData;
char data[MAX_DATA_LENGTH];
};
void buftopacket(char *buffer, Packet *pkt) {
pkt->header1 = buffer[0];
pkt->header2 = buffer[1];
pkt->lengthData = buffer[2] << 8 + buffer[3]; /* assuming network order */
memcpy(pkt->data, buffer + 4, (pkt->lengthData <= MAX_DATA_LENGTH)
? pkt->lengthData : MAX_DATA_LENGTH); /* do not write pass buffer end*/
}
(*) but nothing ensures that you get full packet in one single read ...