Convert Big to Little Endian, vice versa

 걸리버 여행기에 나오는 Big & Little Endians. 메모리에 data 를 저장하고 전송하는 두 가지 방법. Big endian 은 most significant bit 을 left-most memory address 에, 반대로 Little endian 은 right-most memory address 에 저장하는 방법. Big endian 에서는 high byte transferred fisrt, Little endian 에서는 low byte transferred first. 

 

 예를 들면, integer value인 1234 값이 있다고 해보자. 이는 hexadecimal value로 변환하면 4D2 이다. 1 byte는 8 bits, hexadecimal one digit 은 half byte (4 bits), hexadecimal two digits 는 one byte, int 는 4 bytes size, unsigned char 는 1 byte size. 따라서 1234 (int) = 0x000004D2 이고, Big or Little Endian 에 따라서 메모리에 0x00 0x00 0x04 0xD2 (Big Endian), 0xD2 0x04 0x00 0x00 (Little Endian)으로 기록된다. 

#include 

int isBigEndian();
void change_endian(int);

int main() {
  /********************************************************
   * check endianness of System
   *******************************************************/
  if (isBigEndian()) {
    printf("This system is Big Endian.\n");
  } else {
    printf("This system is Little Endian.\n");
  }

  /********************************************************
   * Intel x86-64 is little endian
   *******************************************************/
  unsigned char b[sizeof(int)];
  int n = 1234;  // hex 0x04D2
  memcpy(b, &n, sizeof(int));
  printf("%d\n", sizeof(b));
  for (int i = 0; i < 4; i++) {
    printf("%x ", b[i]);
  }  // d2 4 0 0
  printf("\n");

  /********************************************************
   * another example
   *******************************************************/
  n = 1234;
  const int arrayLength = sizeof(int);
  unsigned char *bytePtr = (unsigned char *)&n;

  for (int i = 0; i < arrayLength; i++) {
    printf("[%X]", bytePtr[i]);
  }
  printf("\n");

  /********************************************************
   * change endian
   *******************************************************/
  n = 1234;
  change_endian(n);

  return 0;
}

/**********************************************************
 * Andrew Ippoliti's Blog: Handling Endianness in C
 * http://blog.acipo.com/handling-endianness-in-c/
 * Test will be stored as [0x00,0x01] on a big endian machine
 * and as [0x01, 0x00] on a little endian machine.
 * (if int is 2 bytes)
 *********************************************************/
int isBigEndian() {
  int test = 1;             // int is 4 bytes
  char *p = (char *)&test;  // char is 1 byte

  return p[0] == 0;
}

/**********************************************************
 * Convert Endian
 *********************************************************/
void change_endian(int num) {
  unsigned char *p = (unsigned char *)#
  if (isBigEndian()) {
    printf("Convert Big to Little Endian: ");
    for (int i = 0; i < sizeof(int); i++) {
      printf("%x ", *(p + i));
    }
  } else {
    printf("Convert Little to Big Endian:  ");
    for (int i = sizeof(int) - 1; i >= 0; i--) {
      printf("%x ", *(p + i));
    }
  }
  printf("\n");
}