Question

I want to understand this syntax to fix it for crc16 function :

unsigned short crc16(void *memptr, int len) {
  int j;
  unsigned short crc = CRC16_INIT ;
  while(len--){
    crc ^= *((unsigned char *)memptr)++;
    for(j=0; j<8; j++){
      if(crc & 1)
        crc =(USHORT)( (crc>>1) ^ CRC16_POLY );
      else
        crc =(USHORT)( crc>>1);
    }//for
  }//while
  return crc ;
}

it was working code on older compiler and now I ‘ve got

error : IntelliSense: expression must be a modifiable lvalue

on this line:

crc ^= *((unsigned char *)memptr)++;

Compiler error : error C2105: ‘++’ needs l-value

Recoded to it (hope it’s correct):

unsigned char oldValue = *((unsigned char *)memptr);
++*((unsigned char *)memptr);
crc ^= oldValue; // <--- WRONG



crc ^= (*((unsigned char *)memptr))++;  // <--- WRONG

Answer 1

ASM-version of this function (not that optimized, but still 0x59 bytes vs my compiler’s 0x69 of C++ version).

If you are familiar with ASM, it will be enough to understand mentioned C++ code segment and the whole function overall.

unsigned short crc16(void *memptr, int len)
{
 unsigned short crc = 0;//CRC16_INIT;

 __asm
 {
  MOV       ESI, len
while_s:
  DEC       ESI
  CMP       ESI, 0
  JL        while_e
  XOR       EAX, EAX
  MOV       AX, WORD PTR DS:[crc]
  MOV       EBX, len
  SUB       EBX, ESI
  DEC       EBX
  MOV       ECX, memptr
  MOV       BL, BYTE PTR DS:[ECX+EBX]
  XOR       AL, BL
  XOR       BL, BL
 for_s:
  CMP       BL, 8
  JGE       for_e
  INC       BL
  MOV       DX, AX
  SHR       AX, 1
  AND       DX, 1
  JE        for_j
  XOR       AX, 0xA001//CRC16_POLY
 for_j:
  JMP       for_s
 for_e:
  MOV       WORD PTR DS:[crc], AX
  JMP       while_s
while_e:
  XOR       EAX, EAX
  MOV       AX, WORD PTR DS:[crc]
 }
}