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cstructstructurepaddingpacking

Structure padding and packing

Consider:

struct mystruct_A
{
   char a;
   int b;
   char c;
} x;

struct mystruct_B
{
   int b;
   char a;
} y;

The sizes of the structures are 12 and 8 respectively.

Are these structures padded or packed?

When does padding or packing take place?

Solution

  • Padding aligns structure members to "natural" address boundaries - say, int members would have offsets, which are mod(4) == 0 on 32-bit platform. Padding is on by default. It inserts the following "gaps" into your first structure:

    struct mystruct_A {
    char a;
    char gap_0[3]; /* inserted by compiler: for alignment of b */
    int b;
    char c;
    char gap_1[3]; /* -"-: for alignment of the whole struct in an array */
} x;

    Packing, on the other hand prevents compiler from doing padding - this has to be explicitly requested - under GCC it's __attribute__((__packed__)), so the following:

    struct __attribute__((__packed__)) mystruct_A {
    char a;
    int b;
    char c;
};

    would produce structure of size 6 on a 32-bit architecture.

    A note though - unaligned memory access is slower on architectures that allow it (like x86 and amd64), and is explicitly prohibited on strict alignment architectures like SPARC.