On #pragma pack()

#pragma  pack(n),  where  n  is  1,  2,  4,  8,  or  16:
each  structure  member  after  the  first  is  stored  on  the  smaller  member  type  or  n-byte  boundaries.

A example will give a more direct impression:

#pragma pack(8)
struct A {
    char c;
    int i;
};

so sizeof(A) is 8;
while

#pragma pack(1)
struct A {
    char c;
    int i;
};

then sizeof(A) is 5.

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PS. There is a little funny when I first encounter this preprocessor. I guess pack(8) is to force to align at 8b, but in fact the true meaning is 8B.