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#include <stdatomic.h>
 
/*
 * Sets index to point to index of maximum element in array
 * and value to contain maximum array value.
 */
void find_max_element(atomic_int array[], size_t *index, int *value);

static size_t index;
static int value;
static atomic_int array[];

void func(void) {
  size_t index;
  int value;
  find_max_element(array, &index, &value);
  /* ... */
  if (!atomic_compare_exchange_strong(array[index], value, 0)) {
    /* Handle error */
  }
}

#include <stdatomic.h>
#include <threads.h>
 
static atomic_int array[];
static size_t index;
static int value;
static mtx_t array_mutex;

void func(void) {
  size_t index;
  int value;
  if (thrd_success != mtx_lock(&array_mutex)) {
    /* Handle error */
  }
  find_max_element(array, &index, &value);
  /* ... */
  if (!atomic_compare_exchange_strong(array[index], value, 0)) {
    /* Handle error */
  }
  if (thrd_success != mtx_unlock(&array_mutex)) {
    /* Handle error */
  }
}