Setting value equal to itself

Question

I don't know where to search for this (probably the standard but still don't know what to search for), so I will ask this here.

If in some execution array[i2] will be set to array[i] where i2 happens to be equal to i, then is this defined behavior?

I'm using C99 (with gcc 4.8.1), looking at the assembly with gcc -S, I don't see anything suspicious.

Answer 1

If variable i has valid value then assignment

i = i;

has perfectly defined behavior. This is equivalent to your question, as stated.

If there's something more tricky in your situation, you have to provide more details about it.

Answer 2

This is absolutely defined behaviour. The right-hand-side of an assignment is calculated first, then assigned to the left-hand-side. Note that the left-hand-side must resolve to an lvalue.

Answer 3

foo() {
  int i, j, a[5], b[5];
  i = i; // undefined because reading i is UB
  j = i; // undefined because reading i is UB
  a[i] = a[i]; // undefined because reading i is UB
  a[3] = a[3]; // undefined because reading a[3] is UB
  b[3] = a[3]; // undefined because reading a[3] is UB
}