__sync_val_compare_and_swap

__sync_val_compare_and_swap是GCC内置函数之一，用于实现原子操作。它的作用是比较内存地址中的值和给定值，如果相等，则将内存地址中的值替换为新值，并返回原来的值。这个操作是原子的，即在执行期间不会被中断。这个函数通常用于多线程编程中，用于保证共享变量的同步和一致性。

相关问题

__sync_val_compare_and_swap()

__sync_val_compare_and_swap() is a built-in function in GCC that performs an atomic compare-and-swap operation on a given memory location. This function compares the value of a memory location with a given old value and, if they match, replaces the old value with a new value. The function returns the original value of the memory location, allowing the caller to determine if the compare-and-swap operation was successful.

The function takes the following parameters:

- A pointer to the memory location to perform the operation on
- The old value to compare against
- The new value to replace the old value with

The function is useful in multi-threaded programming, where multiple threads may attempt to update the same memory location simultaneously. By using this function, only one thread will successfully update the memory location, while the others will receive an indication that the update failed. This can help prevent race conditions and other synchronization issues.

__sync_val_compare_and_swap()示例

以下是一个使用__sync_val_compare_and_swap()函数的示例：

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

#define THREADS 4
#define LOOP 1000000

int counter = 0;

void *increment(void *arg)
{
    int i;
    int local_counter = 0;

    for (i = 0; i < LOOP; i++) {
        int old_value = counter;
        int new_value = old_value + 1;

        while (__sync_val_compare_and_swap(&counter, old_value, new_value) != old_value) {
            old_value = counter;
            new_value = old_value + 1;
        }

        local_counter++;
    }

    printf("Thread %ld incremented counter %d times\n", (long)arg, local_counter);

    pthread_exit(NULL);
}

int main(void)
{
    pthread_t threads[THREADS];
    int i;

    for (i = 0; i < THREADS; i++) {
        if (pthread_create(&threads[i], NULL, increment, (void *)(long)i)) {
            fprintf(stderr, "Error creating thread\n");
            exit(1);
        }
    }

    for (i = 0; i < THREADS; i++) {
        if (pthread_join(threads[i], NULL)) {
            fprintf(stderr, "Error joining thread\n");
            exit(1);
        }
    }

    printf("Counter value: %d\n", counter);

    return 0;
}

这个程序创建了4个线程，每个线程执行1000000次循环。在每次循环中，线程将计数器的旧值存储在一个变量中，然后将计数器的值增加1。如果__sync_val_compare_and_swap()函数返回了旧值，说明另一个线程已经更新了计数器，这个线程需要重试。如果函数返回了新值，说明这个线程成功更新了计数器的值。

这个程序使用了__sync_val_compare_and_swap()函数来保证计数器的并发更新是正确的。这个函数可以原子地读取、比较和更新一个变量的值，它可以确保线程之间的同步，并避免竞态条件的出现。