C array padding

Question

I have many data array defined in my code with the length only specified in the array initializer.

int a[] = {1, 2, 3, 4};
int b[] = {1, 2, 3, 4, 5};

For cache line coherence consideration, I want to padding them to cache line size (32). The following code can be used without scarifying the exact size information

struct {
   int payload[4];
} __attribute__((aligned(32))) a = {
   .payload = {1, 2, 3, 4}
};

struct {
   int payload[5];
} __attribute__((aligned(32))) b = {
   .payload = {1, 2, 3, 4, 5}
};

// sizeof(a.payload) == 4 * sizeof(int), sizeof(b.payload) == 5 * sizeof(int)

However, there are quite a bundle of such data, it is hard to manually rewrite them one by one. Is there a way to define a macro to handle this, like

#define ARRAY_INITIALIZER (array, initializer) ...
#define ARRAY_PAYLOAD(array) array.payload
ARRAY_INITIALIZER(a, {1, 2, 3, 4});
ARRAY_INITIALIZER(b, {1, 2, 3, 4, 5});

The most difficulty thing here is how to calculate the length of initializer here, don't get any direction for it, can anyone help here?

Answer 1

This initialization produces the aligned addresses either in C or C++:

#include <stdalign.h>

alignas(32) int a[] = {1, 2, 3, 4} ;
alignas(32) int b[] = {1, 2, 3, 4, 5};