Is struct packing deterministic?

Question

For example, say I have two equivalent structs a and b in different projects:

typedef struct _a
{
    int a;
    double b;
    char         


        

Answer 1

Any particular compiler ought to be deterministic, but between any two compilers, or even the same compiler with different compilation options, or even between different versions of the same compiler, all bets are off.

You're much better off if you don't depend on the details of the structure, or if you do, you should embed code to check at runtime that the structure is actually as you depend.

A good example of this is the recent change from 32 to 64 bit architectures, where even if you didn't change the size of integers used in a structure, the default packing of partial integers changed; where previously 3 32bit integers in a row would pack perfectly, now they pack into two 64 bit slots.

You can't possibly anticipate what changes may occur in the future; if you depend on details that are not guaranteed by the language, such as structure packing, you ought to verify your assumptions at runtime.