15.5 Structure Layout

The rest of this chapter covers advanced topics about structures. If you are just learning C, you can skip it.

The precise layout of a struct type is crucial when using it to overlay hardware registers, to access data structures in shared memory, or to assemble and disassemble packets for network communication. It is also important for avoiding memory waste when the program makes many objects of that type. However, the layout depends on the target platform. Each platform has conventions for structure layout, which compilers need to follow.

Here are the conventions used on most platforms.

The structure’s fields appear in the structure layout in the order they are declared. When possible, consecutive fields occupy consecutive bytes within the structure. However, if a field’s type demands more alignment than it would get that way, C gives it the alignment it requires by leaving a gap after the previous field.

Once all the fields have been laid out, it is possible to determine the structure’s alignment and size. The structure’s alignment is the maximum alignment of any of the fields in it. Then the structure’s size is rounded up to a multiple of its alignment. That may require leaving a gap at the end of the structure.

Here are some examples, where we assume that char has size and alignment 1 (always true), and int has size and alignment 4 (true on most kinds of computers):

struct foo
{
  char a, b;
  int c;
};

This structure occupies 8 bytes, with an alignment of 4. a is at offset 0, b is at offset 1, and c is at offset 4. There is a gap of 2 bytes before c.

Contrast that with this structure:

struct foo
{
  char a;
  int c;
  char b;
};

This structure has size 12 and alignment 4. a is at offset 0, c is at offset 4, and b is at offset 8. There are two gaps: three bytes before c, and three bytes at the end.

These two structures have the same contents at the C level, but one takes 8 bytes and the other takes 12 bytes due to the ordering of the fields. A reliable way to avoid this sort of wastage is to order the fields by size, biggest fields first.