#pragma pack - Data Alignment İçindir

Giriş

#pragma pack(N) data alignment için en portable yöntem

1 Byte Padding

Örnek
Şöyle yaparız:

#pragma pack(push, 1)
typedef struct {
  unsigned char __reserved : 1;
  unsigned char dont_fragment : 1;
  unsigned char more_fragment : 1;
  unsigned short fragment_offset : 13; 
} ipv4_fragmenting;
#pragma pack(pop)

Örnek

Şöyle yaparız.

#pragma pack(push, 1)
struct Sensor1Telemetry {
  int16_t temperature;
  uint32_t timestamp;
  uint16_t voltageMv;
  // etc...
};
#pragma pack(pop)

Örnek

Şöyle yaparız

int main() {
#pragma pack(push, 1)
  struct align1 {
    struct {
      double d;   // 8 bytes
      bool b1;    //+1 byte (+ 0 bytes padding) = 9 bytes
    } subStruct;
    bool b2;      //+1 byte (+ 0 bytes padding) = 10 bytes
    char pad_[6]; //+6 bytes (+ 0 bytes padding) = 16 bytes 
  };
#pragma pack(pop)
  struct align2 {
    double d;    // 8 bytes
    bool b1, b2; //+2 byte (+ 6 bytes padding) = 16 bytes
  };

  std::cout << "align1: " << sizeof(align1) << " bytes\n";    // 16 bytes
  std::cout << "align2: " << sizeof(align2) << " bytes\n";    // 16 bytes

  return 0;
}

Çıktı olarak şunu alırız

align1: 16 bytes
align2: 16 bytes

4 Byte Padding

Örnek

Şöyle yaparız

#pragma pack(push,4)
struct  MyStruct
{  
  uint32_t i1;        /* offset=0  size=4 */
  uint32_t i2;        /* offset=4  size=4 */
  uint16_t s1;        /* offset=8  size=2 */
  unsigned char c[8]; /* offset=10 size=8 */
  uint16_t s2;        /* offset=18 size=2 */
  uint16_t s3;        /* offset=20 size=2 */
                        /* offset=22 padding=2 (needed to align MyStruct) */
} ; // total size is 24