Question

Is there any convention over the algorithm used to make the layouts of structs on C?

I want to be able to have a code running in a vm to be able to have structures compatible with their C counterparts, just like C# interop works. For this I will need to know how the alignment algorithm works. I gather there must be a convetion for that, as it works nicely on C#. I have in mind the probable algorithm they have used to work this out, but I haven’t found any proof it is the right one.

Here’s how I think it works:

for each declared field (by order of declaration)

• See if the field fits in the remaining bytes (until next alignment)
• If it doesn’t fit, align this field; otherwise add it to current offset

for example, on a 32-bit system for a struct like:

{
byte b1;
byte b2;
int32 i1;
byte b3;
}

would be like this with this algorithm:

{
byte b1;
byte b2;
byte[2] align1;
int32 i1;
byte b3;
byte[3] align2;
}

Answer 1

In general, structure alignment in C depends on the compiler used, and especially the compiler options in effect at the time the structure declaration is processed. You can’t make any general assumptions except to say that for a particular structure in a particular program compiled with particular settings, the structure layout can be determined.

That said, your guess closely matches what most compilers are likely to do with default alignment settings.