Are multiple variable-length arrays in a structure in C possible?

Question

Is something like this possible in C? It would be really nice to have both of these.

    typedef struct {
        int r;
        char a0[0];
        char a1[0];
    }

Answer 1

No. Rather use dynamically allocated memory. Something like:

typedef struct {
    int *data1;
    size_t len1

    int *data;
    size_t len2;
} sometype;

sometype *alloc_sometype(size_t len1, size_t len2) {
    sometype *n = malloc(sizeof(sometype));

    if (!n) return NULL;

    n->len1 = len1;
    n->len2 = len2;

    n->data1 = malloc(sizeof(int) * len1);
    n->data2 = malloc(sizeof(int) * len2);

    // Error handling if those malloc calls fail

    return n;
}

Answer 2

The flexible array must be the last member in the struct so you can't have two. However, if both arrays are supposed to have the same size, you could combine them:

#include <stdio.h>
#include <stdlib.h>

typedef struct { // a struct to keep one a0 a1 pair
    char a0;
    char a1;
} A;

typedef struct {
    size_t len;
    A a[];        // flexible array of a0 a1 pairs
} data;

data *data_create(size_t len) {
    data *d = malloc(sizeof *d + len * sizeof *d->a);
    if(d) d->len = len;
    return d;
}

int main() {
    data *d = data_create(10);

    for(size_t i = 0; i < d->len; ++i) {
        d->a[i].a0 = i;
        d->a[i].a1 = i+1;
    }
    
    free(d);
}

Answer 3

C99 compilation will allow this, but some strict compiler throw error. Now a1 never be used, because a0 is itself variable length. If you want to divide data into two part, first read into a single array then use pointer forwarding to read data into two part.