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Code Block

bgColor	#ffcccc
lang	c

#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);

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

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index may have changed.
value may have changed (that is, the value of the value variable).
value may no longer be the maximum value in the array.

Compliant Solution (Mutex)

This compliant solution uses a mutex to prevent the data from being modified during the operation. Although this code is thread-safe, it is no longer lock-free.

Code Block

bgColor	#ccccff
lang	c

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

void func(void) {
  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 */
  }
}

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Old Version 38

New Version 39

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Compliant Solution (Mutex)