Question

I got a structure like this:

struct bar {
    char x;
    char *y;
};

I can assume that on a 32 bit system, that padding for char will make it 4 bytes total, and a pointer in 32 bit is 4, so the total size will be 8 right?

I know it’s all implementation specific, but I think if it’s within 1-4, it should be padded to 4, within 5-8 to 8 and 9-16 within 16, is this right? it seems to work.

Would I be right to say that the struct will be 12 bytes in a x64 arch, because pointers are 8 bytes? Or what do you think it should be?

Answer 1

I can assume that on a 32 bit system,
that padding for char will make it 4
bytes total, and a pointer in 32 bit
is 4, so the total size will be 8
right?

It’s not safe to assume that, but that will often be the case, yes. For x86, fields are usually 32-bit aligned. The reason for this is to increase the system’s performance at the cost of memory usage (see here).

Would I be right to say that the
struct will be 12 bytes in a x64 arch,
because pointers are 8 bytes? Or what
do you think it should be?

Similarly, for x64, fields are usually 64-bit/8-byte aligned, so sizeof(bar) would be 16.

As Anders points out, however, all this goes flying out the window once you start playing with alignment via /Zp, the pack directive, or whatever else your compiler supports.