C: How to determine sizeof(array) / sizeof(struct) for external array?

Question

Defines the type x and an array X of that type.

x.h:

typedef struct _x {int p, q, r;} x;
extern x X[];

Separate file to keep the huge honking array X.

x.c:

#include "x.h"
x X[] = {/* lotsa stuff */};

Now I want to use X:

main.c:

#include "x.h"

int main()
{
    int i;

    for (i = 0; i < sizeof(X)/sizeof(x); i++) /* error here */
        invert(X[i]);

    return 0;
}

main.c won't compile; the error is:

error: invalid application of ‘sizeof’ to incomplete type ‘struct x[]’

How do I get the size of X without hardcoding it?

Answer 1

In x.h add:

extern size_t x_count;

In x.c add:

size_t x_count = sizeof(X)/sizeof(x);

Then use the variable x_count in your loop.

The division has to be done in the compilation unit that contains the array initializer, so it knows the size of the whole array.

Answer 2

If it is possible to place a termination indicator at the end of the array, such as:

x X[] = {/* lotsa stuff */, NULL};

It might be that the number of elements in the array would be irrelevant:

#include "x.h"

int main()
   {
   x *ptr = X;

   while(ptr)
      invert(ptr++);

   return 0;
   }

If the number of elements in the array is needed, the above method can be also be used to count the elements.