ax.25 UI framer, non working crc. Ugly code...

aframe.c

// Jimmy Carter - KG4SGP 
// ax.25 UI framer
// compile with gcc aframe.c -o aframe

#include <stdio.h>
#include <string.h>

#define poly (0x1201)

unsigned char flag = 0x7e;          //ax.25 start/stop flag
unsigned short crc = 0xffff;
unsigned char head[74];             //headder buffer
unsigned char scall[7];             //Source callsign
unsigned char dcall[7];             //Destination callsign
unsigned char sssid = 0xE0;          //
unsigned char dssid = 0x60;          //
unsigned char rxseq = 0;            //
unsigned char txseq = 0;            //
unsigned char poll = 0;             //poll bit
unsigned char sfcf = 0;             //Supervisory Frame Control Field
unsigned char ufcf = 0;             //Unnumbered Frame Control Field
unsigned char control = 0;
unsigned char pid = 0;                  //protocol indentifier
int txbits = 0;                     //number of tx data bits
unsigned char txdata[307];          //txdata (info portion)
unsigned char packet[512];          //entire packet
int packetLen;
char type = 2;
int starting = 75;                  //number of starting flags
int ending = 20;                    //number of ending flags
int bytepos = 0;
int bitpos = 0;
int packbitpos = 0;
int bitstuff = 0;
int bitStuffCount = 0;
int oncrc = 0;
int writebit = 0;

int addToPacket(char* , int);
//int setTxCall(char* , int);
//int setRxCall(char* , int );
void leftShift(char* , long unsigned int);
void docrc(unsigned short);
unsigned char revChar(char);

void charLeftShift(char *array, long unsigned int size){
  int i;
  for(i = 0; i < size; i++){
    array[i] = array[i] << 1;
  }
}

unsigned char revChar(char in){

  unsigned char temp = 0;
  int i = 0;
  for(i = 0; i < 8; i++){
    if ((in>>i & 0x01) == 0){
      temp &= (((0x01)<<(7-i))^(0xff));
    } else {
      temp |= ((0x01)<<(7-i));
    }
  }

  return temp;
}

// this function is fed byte per byte of MSB first data
void updateCrc(unsigned short ch)
{


}

int addToPacket(char* data, int size){

  int i , j = 0;
  for(i = 0; i < size; i++){

    if(oncrc == 1){
      updateCrc(data[i]);
    }

    for(j = 0; j < 8; j++){

      int move = j;

      // NRZI, if readbuff is 0 at i change state of write bit
      if(((data[i]>>move) & 0x01) == 0){
        writebit = !writebit;
      }

      //printf(" %d", writebit);

      if(writebit == 0){

        //printf("0");
        packet[bytepos] &= ((0x01)<<(7-packbitpos))^(0xff);
        packbitpos++;
        bitStuffCount = 0;

        if(packbitpos > 7){
          packbitpos = 0;
          bytepos++;
          // printf("\n");
        }

      } else {
        if(bitstuff == 1 && bitStuffCount > 4){
          //printf("0");
          packet[bytepos] &= ((0x01)<<(7-packbitpos))^(0xff);
          bitStuffCount++;
          packbitpos++;
          bitStuffCount = 0;

          if(packbitpos > 7){
            packbitpos = 0;
            bytepos++;
            //printf("\n");
          }

        }
        //printf("1");
        packet[bytepos] |= (0x01)<<(7-packbitpos);
        bitStuffCount++;
        packbitpos++;

        if(packbitpos > 7){
          bytepos++;
          packbitpos = 0;
          //printf("\n");
        }

      }

    }
  }

}  


int main(int argc[], char* argv[]){

  int i;

  // open input file
  FILE* fin;
  if (strcmp(argv[1], "-") == 0){
    fin = stdin;
  } else if( (fin = fopen(argv[1], "rb")) == NULL){
    printf("Error opening input file...\n");
    return;
  }

  // open output file
  FILE* fout;
  if (strcmp(argv[2], "-") == 0){
    fout = stdout;
  } else if( (fout = fopen(argv[2], "wb")) == NULL){
    printf("Error opening input file...\n");
    return;
  }

  // flush buffers if piped
  if (fin == stdin) fflush(stdin);
  if (fout == stdin) fflush(stdout);

  // set callsigns
  char txc[6] = { 'A', 'P', 'R', 'S', ' ', ' '};
  char rxc[6] = { 'K', 'G', '4', 'S', 'G', 'P'};
  
  // shift callsign to conform to standard
  charLeftShift(txc, sizeof(txc));
  charLeftShift(rxc, sizeof(rxc));
  
  // make lsb of ssid (last bit of address frame) 1 to conform to standard
  dssid |= 0x01;

  // turn bit stuffing off
  bitstuff = 0;
  bitStuffCount = 0;  

  // prepair crc
  crc = 0xffff;
  oncrc = 0;

  //add starting flags
  for(i = 0; i < starting; i++){
    addToPacket(&flag, sizeof(flag));
  }

  // turn bit stuffing on
  bitstuff = 1;
  bitStuffCount = 0;

  // turn on crc
  oncrc = 1;

  //add callsigns and ssids
  addToPacket(&txc[0], sizeof(txc));
  addToPacket(&sssid, sizeof(dssid));
  addToPacket(&rxc[0], sizeof(rxc));
  addToPacket(&dssid, sizeof(sssid));

  // add un-numbered control field as per standard
  control = 0x03;
  addToPacket(&control, sizeof(control));

  // add protocol identifier, no layer 3 implemented
  pid = 0xf0;
  addToPacket(&pid, sizeof(pid));

  // file read buffer
  // read one character at a time from a file and add it to the packet
  char info = 0;
  while(!feof(fin)){
    fread(&info, sizeof(char), 1, fin);
    addToPacket(&info, (sizeof(info)));
  }

  //turn off crc
  oncrc = 0;


  // split crc into individual characters

  // if we change the place of the high and low crc then flip them
  // it flips the entire order of the 16bit sequence... its a hack and needs to
  // be fixed at some point

  // if this is fixed with our crc implementation it could be as simple as
  // passing the crc high and low byte to addToPacket()

  char highcrc = crc & 0x00ff;
  char lowcrc = (crc & 0xff00)>>8;
  highcrc = revChar(highcrc);
  lowcrc = revChar(lowcrc);  
  addToPacket(&lowcrc, sizeof(lowcrc));
  addToPacket(&highcrc, sizeof(highcrc));

  // turn bit stuffing off
  bitstuff = 0;
  bitStuffCount = 0;

  // add ending flags
  for(i = 0; i < ending; i++){
    addToPacket(&flag, sizeof(flag));
  }

  // write created packet to file (in binary form)
  for(i = 0; i<bytepos; i++){
    fwrite(&packet[i], sizeof(char), 1, fout);
    //printf("%c", packet[i]);
  }
}