In a structure, is it legal to use one array field to access another one?

Question

As an example, consider the following structure:

struct S {
  int a[4];
  int b[4];
} s;

Would it be legal to write s.a[6] and

Answer 1

As @MartinJames mentioned in a comment, if you need to guarantee that a and b are in contiguous memory (or at least able to be treated as such, (edit) unless your architecture/compiler uses an unusual memory block size/offset and forced alignment that would require padding to be added), you need to use a union.

union overlap {
    char all[8]; /* all the bytes in sequence */
    struct { /* (anonymous struct so its members can be accessed directly) */
        char a[4]; /* padding may be added after this if the alignment is not a sub-factor of 4 */
        char b[4];
    };
};

You can't directly access b from a (e.g. a[6], like you asked), but you can access the elements of both a and b by using all (e.g. all[6] refers to the same memory location as b[2]).

(Edit: You could replace 8 and 4 in the code above with 2*sizeof(int) and sizeof(int), respectively, to be more likely to match the architecture's alignment, especially if the code needs to be more portable, but then you have to be careful to avoid making any assumptions about how many bytes are in a, b, or all. However, this will work on what are probably the most common (1-, 2-, and 4-byte) memory alignments.)

Here is a simple example:

#include 

union overlap {
    char all[2*sizeof(int)]; /* all the bytes in sequence */
    struct { /* anonymous struct so its members can be accessed directly */
        char a[sizeof(int)]; /* low word */
        char b[sizeof(int)]; /* high word */
    };
};

int main()
{
    union overlap testing;
    testing.a[0] = 'a';
    testing.a[1] = 'b';
    testing.a[2] = 'c';
    testing.a[3] = '\0'; /* null terminator */
    testing.b[0] = 'e';
    testing.b[1] = 'f';
    testing.b[2] = 'g';
    testing.b[3] = '\0'; /* null terminator */
    printf("a=%s\n",testing.a); /* output: a=abc */
    printf("b=%s\n",testing.b); /* output: b=efg */
    printf("all=%s\n",testing.all); /* output: all=abc */

    testing.a[3] = 'd'; /* makes printf keep reading past the end of a */
    printf("a=%s\n",testing.a); /* output: a=abcdefg */
    printf("b=%s\n",testing.b); /* output: b=efg */
    printf("all=%s\n",testing.all); /* output: all=abcdefg */

    return 0;
}