#pragma pack(n)

Use #pragma pack(n), to affect member packing of a structure. By default, members of a structure are aligned on their natural boundaries; one byte for a char, two bytes for a short, four bytes for an integer etc. If n is present, it must be zero or a power of 2 specifying the strictest natural alignment for any structure member.

You can use #pragma pack(n) to specify a different aligned of a structure 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 Intel, eight on SPARC v8, and 16 on SPARC v9), the directive has the effect of natural alignment. Also, if n is omitted, member alignment reverts to the natural alignment boundaries.

The #pragma pack(n) directive applies to all structure definitions which follow it until the next pack directive. If the same structure 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 the 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 to be packed. Follow the packed structure immediately with #pragma pack( ).