C CLP: Pointers and Arrays

This domain tests the array–pointer relationship precisely: when an array name decays, which pointer arithmetic is defined, and how multidimensional arrays and parameters change type.

Decay and indexing

• In most expressions an array converts to a pointer to its first element. The exceptions are the operand of sizeof, the operand of unary &, and a string literal used to initialize an array.
• a[i] is defined as *(a + i), so i[a] is equally valid and &a[i] == a + i.
• Pointer arithmetic is scaled by the pointed-to type: for int *p, p + 1 advances sizeof(int) bytes. Pointers may be subtracted only within the same array object (one past the end is allowed as an address, not as a dereference); the result has type ptrdiff_t.
• int *p and int (*q)[5] are different: p + 1 moves one int, q + 1 moves a whole five-element array.

Arrays as parameters

• A parameter written int a[10] or int a[] is adjusted to int *a, so sizeof a inside the function is the pointer size. Pass the length as a separate argument.
• For a two-dimensional array only the first dimension may be omitted: void f(int m[][4]) means int (*m)[4].

Worked example

int a[5] = {10, 20, 30, 40, 50};
int *p = a;
int (*q)[5] = &a;
printf("%d %zu %zu\n", *(p + 2), sizeof a, sizeof *q);

p + 2 points at a[2], so the first value printed is 30. sizeof a is 5 * sizeof(int), typically 20. q points at the whole array, so *q is the array and sizeof *q is also 20. In bytes, p + 1 advances 4 while q + 1 advances 20.

Traps

• Assuming sizeof still reports the array size inside a function; after decay it reports the pointer size.
• Forming a pointer more than one past the end of an object is undefined even if it is never dereferenced.
• char *s = "literal"; s[0] = 'X'; is undefined; string literals are not modifiable.
• Confusing *p++ (dereference the current target, then advance p) with (*p)++ (increment the pointed-to value).
• int *a, b; declares one pointer and one int.