Infinite loop when using size_t in a count down for loop

Question

So I\'m using size_t instead of int in any indexing for loop to prevent negative indices. But when counting down, this leads to an overflow:

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Answer 1

size_t i = 10; i >= 0; is never false as size_t is some unsigned type and all values are greater than or equal to zero.

... size_t which is the unsigned integer type of the result of the sizeof operator; ...
C11 §7.19 2

A good compiler with warnings enabled would have warned about this.
Hopefully, that infinite loop would never had occurred as an investigation to the warning would have first rectified the problem.

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Best alternative depends on coding goals

Good code avoids magic numbers like this naked 10. Better if code derived that. In this simple case, it should have been 11.

#define A_SIZE 11
int array[A_SIZE];
...
for (size_t i = A_SIZE; i-- > 0; ) {
    // Do something, f.ex. array[i] = i
}

OTOH, code may have had break conditions in the loop and needs i in later code to indicate array[] usages

size_t i = A_SIZE;
while (i > 0) {
  if (...) break; 
  i--;
  // Do something, f.ex. array[i] = i
  if (...) break; 
}
// Do something with i

Code may have a contract requirement to use a 10 in various places.

// Contract says loop must handle indexes 0 to N, inclusive
#define N 10
int array[N + 1];

for (size_t i = N; i + 1 > 0; i--) {
  // Do something, f.ex. array[i] = i
}

Good optimizing compilers will not perform a +1 on each i + 1 > 0, but create equivalent efficient code.

Code is a fashion that best conveys the overall meaning of the code.