[1000100_1000101]

More than implementation defined, for some you need context that simply isn't given. On the ones with mixed-type structs, even if you know what system it's compiled for you don't know if someone has used pragma pack 1 to byte pack the data instead of standard packing. Just seeing the struct, you still don't know.

[ryao]

Good point, although that is not part of standard C.

[ksherlock]

'#pragma pack' isn't part of the C standard, but #pragma is and "causes the implementation to behave in an implementation-defined manner."