What other compilers do I need to worry about struct packing?

Question

In GCC, I need to use __attribute__((packed)) to make structs take the least amount of space, for example, if I have a large array of structs I should pack them. What other common compilers do struct padding, and how do I pack structs in these other compilers?

Answer 1

The premise of your question is mistaken. Packed structs are not something you're "supposed to use" to save space. They're a dubious nonstandard feature with lots of problems that should be avoided whenever possible. (Ultimately it's always possible to avoid them, but some people balk at the tradeoffs involved.) For example, whenever you use packed structures, any use of pointers to members is potentially unsafe because the pointer value is not necessarily a valid (properly aligned) pointer to the type it points to. The only time there is a "need" for packed structures is when you're using them to access memory-mapped hardware registers that are misaligned, or to access in-file/on-disk data structures that are misaligned (but the latter won't be portable anyway since representation/endianness may not match, and both problems are solved together much better with a proper serialization/deserialization function).

If your goal is to save space, then as long as you control the definition of the structure, simply order it so as not to leave unnecessary padding space. This can be achieved simply by ordering the members in order of decreasing size; if you do that, then on any reasonable implementation, the wasted space will be at most the difference between the size of the largest member and the size of the smallest.