PACK Compiler Directive

• By default, the compiler assumes that structure members are aligned on byte boundaries. PACK(2) may increase the size of a structure since alignment gaps are inserted for padding.

#pragma pack(1)     /* byte alignment */

struct s1  {
  int  i1;     // i1 has offset 0
  char c1;     // c1 has offset 2
  int  i2;     // i2 has offset 3
  char c2;     // c2 has offset 5
  int  i3;     // i3 has offset 6
  char z1;     // z1 has offset 8
};

#pragma pack()    /* reset to default alignment */

struct s2  {
  int  i1;     // i1 has offset 0
  char c1;     // c1 has offset 2
  int  i2;     // i2 has offset 4
  char c2;     // c2 has offset 6
  int  i3;     // i3 has offset 8
  char z1;     // z1 has offset 10
};