#pragma pack(n)
Use #pragma pack(n)to affect member packing of a structure or a union. By default, members of a structure or union are aligned on their natural boundaries; one byte for a char, two bytes for a short, four bytes for an integer, and so on. If n is present, it must be a power of 2 specifying the strictest natural alignment for any structure or union member. Zero is not accepted.
The #pragma pack(n) directive applies to all structure or union definitions that follow it until the next pack directive. If the same structure or union is defined in different translation units with different packing, your program may fail in unpredictable ways. In particular, you should not use #pragma pack(n) prior to including a header that defines the interface of a precompiled library. The recommended usage of #pragma pack(n) is to place it in your program code immediately before any structure or union to be packed. Follow the packed structure immediately with #pragma pack( ).
You can use #pragma pack(n) to specify an alignment boundary for a structure or union member. For example, #pragma pack(2) aligns int, long, long long, float, double, long double, and pointers on two byte boundaries instead of their natural alignment boundaries.
If n is the same or greater than the strictest alignment on your platform, (four on x86 with -m32, eight on SPARC with —m32, and 16 with -m64), the directive has the effect of natural alignment. Also, if n is omitted, member alignment reverts to the natural alignment boundaries.
Note that when you use #pragma pack, the alignment of the packed structure or union itself is the same as its more strictly aligned member. Therefore any declaration of that struct or union will be at the pack alignment. For example, a struct with only chars has no alignment restrictions, whereas a struct containing a double would be aligned on an 8-byte boundary.