Organization of Structs

[pradeep]

Hello friends,

If we have a struct definition like this

typedef struct s {
char a;
int b;
char c;
} _s;

where because of alignment effects the layout given results in memory
inefficiency, do we need to manually arrange things (char then char
then int) or will a good optimizing compiler automatically reorder the
fields in the struct?

Also if we do arrange it as char then char then int, the first dword
in the struct will have two char bytes and two padding bytes. How will
the compiler organize the space and why? Will it be
padding - padding - char - char
or padding - char - padding - char
or char - char - padding - padding
etc.?

Thanks.

 

[Ian Collins]

Hello friends,

If we have a struct definition like this

typedef struct s {
char a;
int b;
char c;
} _s;

where because of alignment effects the layout given results in memory
inefficiency, do we need to manually arrange things (char then char
then int) or will a good optimizing compiler automatically reorder the
fields in the struct?

No, the order is fixed.

Also if we do arrange it as char then char then int, the first dword
in the struct will have two char bytes and two padding bytes. How will
the compiler organize the space and why? Will it be

That is entirely up to the compiler.

 

[Ben Bacarisse]

If we have a struct definition like this

typedef struct s {
char a;
int b;
char c;
} _s;

where because of alignment effects the layout given results in memory
inefficiency, do we need to manually arrange things (char then char
then int) or will a good optimizing compiler automatically reorder the
fields in the struct?

The compiler can't re-order the struct.

Also if we do arrange it as char then char then int, the first dword
in the struct will have two char bytes and two padding bytes. How will
the compiler organize the space and why? Will it be
padding - padding - char - char
or padding - char - padding - char
or char - char - padding - padding

The only options are ones that have no padding at the start. That
leaves three possibilities. I'd lay odds on it using char, char,
padding, but you can't assume that it will do so.

As to why, the usually strategy is that the compiler puts a field at
the first offset that will incur no access penalty.