Appending CRC (Checksum) to data stream serial port communication C++ -

i appending crc data stream. using serial port libary rs-232 linux , windows.

i want same value 1 displayed on comport tool picture:

i using const char[4086]="123456789"; buffer.

crc returned bb3d convert unsigned short char *.

#define serialport_bufsize       4088  char *im_buf = (char *)malloc(serialport_bufsize  + 8);  char *tx_buf = (char *)malloc(serialport_rcv_bufsize  + 8);  const char buf[serialport_bufsize]="123456789";  strcpy(im_buf, buf); writetoserialport(); // call function  void serialportcom::writetoserialport(void) {     if(!port_open) return;      int i,n, tmp, tmp2;      if(newline == 1)     {         strcat(im_buf, "0a");     }     else if(newline == 2)     {         strcat(im_buf, "0d");     }     else if(newline == 3)     {         strcat(im_buf, "0d0a");     }      n = strlen(im_buf) / 2;      if(n < 1) return;      for(i=0; i< n; i++)     {         tmp = im_buf[i*2];          if((tmp >= '0') && (tmp <= '9'))         {             tmp -= '0';         }         else if((tmp >= 'a') && (tmp <= 'f'))         {             tmp -= ('a' - 10);         }         else if((tmp >= 'a') && (tmp <= 'f'))         {             tmp -= ('a' - 10);         }          tmp *= 16;          tmp2 = im_buf[(i*2)+1];          if((tmp2 >= '0') && (tmp2 <= '9'))         {             tmp2 -= '0';         }         else if((tmp2 >= 'a') && (tmp2 <= 'f'))         {             tmp2 -= ('a' - 10);         }         else if((tmp2 >= 'a') && (tmp2 <= 'f'))         {             tmp2 -= ('a' - 10);         }          tmp += tmp2;          tx_buf[i] = tmp;      }     //generate crc     unsigned short crc  = getcrc16(im_buf, strlen(im_buf));     //append crc     memcpy( tx_buf + n, &crc, 2 );     //send serial port     rs232_sendbuf(comport_nr, (unsigned char *)tx_buf, n+2);  } 

what get:   (12 34 56 78 70 11 4a 3d)
want:(12 34 56 78 7b 34 fd fd fd fd)

how achieve expected result?

unsigned short serialportcom::getcrc16(const char* instr,unsigned int len) {     //crc16     unsigned short crc16table[256];     unsigned int i,j;     unsigned short crc;           unsigned char crchigh = 0xff, crclow = 0xff;      (i = 0; < 256; i++)     {         crc = i;         (j = 0; j < 8; j++)         {             if(crc & 0x1)                 crc = (crc >> 1) ^ 0xa001;             else                 crc >>= 1;         }         crc16table[i] = crc;     }     //crc16     crc=0x0000;     for(i=0; i<len; i++)     {         crc = (crc >> 8) ^ crc16table[(crc & 0xff) ^(uint8_t) instr[i]];      }     //crc ^= 0x0000;      return crc; } 

first, notes :

• your inputs ascii hex. each character half-byte, input string has odd number of characters : discard last character.
• what expect 12 34 56 78 7b 34 : 0x347b crc 0x12 0x34 0x56 0x78. additional fds looks irrelevant end of buffer (or bug ?).

the problems program :

you need data buffer send rs232_sendbuf. convert ascii data , put in tx_buf , take crc. that's fine, use strcat append crc buffer ; can't that, strcat text. strcat expect 2 null terminated strings, have 2 data buffers.

you crc function has problem : have superfluous mask & 0x7f removes last bit of each crc byte. 0xff mask sufficient (and not needed actually).

you allocate buffer new char[2], never delete it. work it's ugly.

note crc 2 bytes ; return crc directly unsigned short.

after that, append crc buffer memcpy.
in case, memcpy( tx_buf + n, &my_crc_short, 2 );