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 etc. 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.
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, 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 or union definitions which 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( ).
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.
If you use #pragma pack to align struct or union members on boundaries other than their natural boundaries, accessing these fields usually leads to a bus error on SPARC. In order to avoid such an error, be sure to also specify the -xmemalign option. See B.2.111 -xmemalign=ab , for the optimal way to compile such programs.