C macro to ensure element is at start of struct

Question

Is there a way to design a macro that could ensure an element is at the start of a struct during it's definition? For example:

typedef struct {
  START(int a);
} b;
// Becomes
typedef struct {
  int a;
} b;

But generate a compiletime error when it isn't the first element?

typedef struct {
  int c;
  START(int a);
} b;
// Generate an error

I was thinking you could use a combo of the OFFSETOF and BUILD_BUG_ON_ZERO macros but this would require knowing the struct layout while initializing it, and produces an error because the variable is undeclared.

Is this possible in C?

Answer 1

Use a compile time assertion at the locations you actually assume that layout, instead of at the definition site. Of course you will need to actually define it at the start in order to pass the assertion.

Answer 2

Perhaps something like this would work for you:

#define typedef_header(x) typedef struct { x

typedef_header(int a);
  int c;
} b;

int main()
{
  b x;
}