[SOLVED] C array padding

C array padding

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?

Solution

Here's one idea using variadic macros:

#include <stdalign.h> // for "alignas" - not needed since C23

#define VA_COUNT(...) (sizeof((int[]){0, __VA_ARGS__}) / sizeof(int) - 1)

#define ARRAY_INITIALIZER(V, ...)                       \
    struct {                                            \
        alignas(32) int payload[VA_COUNT(__VA_ARGS__)]; \
    } V = {.payload = {__VA_ARGS__}}

Note that the standard _Alignas (alignas) is used to align the array. The payload size is calculated by using VA_COUNT.

Example use:

#include <stdio.h>

// the macro definitions from above goes here

#define SIZE(x) (sizeof(x) / sizeof *(x))

int main(void) {
    ARRAY_INITIALIZER(a, 1, 2, 3, 4);

    for (unsigned i = 0; i < SIZE(a.payload); ++i) {
        printf("%d\n", a.payload[i]);
    }
}

Output: