Basierend auf dieser Antwort:https://stackoverflow.com/a/19765782/1606345

#include <stdlib.h>

typedef struct {
    int *arr1;
    int *arr2;
} myStruct;

myStruct *allocMyStruct(int num)
{
    myStruct *p;

    if ((p = malloc(sizeof *p +
                 10 * sizeof *p->arr1 +
                 10 * num * sizeof *p->arr2)) != NULL)
    {
        p->arr1 = (int *)(p + 1);
        p->arr2 = p->arr1 + 10;
    }
    return p;
}

void initMyStruct(myStruct * a, int num)
{
    int i;

    for (i = 0; i < 10; i++) a->arr1[i] = 0;
    for (i = 0; i < 10 * num; i++) a->arr2[i] = -1;
}

int main (void)
{
    int num = 3;

    myStruct *a = allocMyStruct(num);
    initMyStruct(a, num);
    free(a);
    return 1;
}

Es ist sicher zuzuweisenp->arr1 an die Adresse von(p + 1)?

p->arr1 = (int *)(p + 1);