线程清理函数：pthread_cleanup机制详解
pthread_cleanup函数族概述
pthread_cleanup_push()和pthread_cleanup_pop()是POSIX线程库提供的线程清理机制，用于注册和执行线程退出时的清理函数。这些函数通常用于释放资源、解锁互斥量等操作，确保线程异常终止时不会泄漏资源。
清理函数注册与执行机制
pthread_cleanup_push()接受一个清理函数指针和参数，将二者压入线程的清理栈。清理函数需满足void (cleanup_routine)(void)签名。当线程通过pthread_exit()显式终止、被其他线程取消或执行pthread_cleanup_pop(1)时，栈顶的清理函数会被执行。
pthread_cleanup_pop()参数控制是否执行栈顶清理函数：非零值执行，零值仅弹出。该函数必须与pthread_cleanup_push()成对出现且在相同词法作用域内，因宏实现可能包含{和}。
典型使用场景
资源释放场景：动态分配的内存、打开的文件描述符需在线程终止时释放。清理函数中调用free()或close()可避免泄漏。
互斥量解锁场景：线程持有锁时若被取消，将导致死锁。通过清理函数解锁可预防：
pthread_mutex_t mutex;
void cleanup_unlock(void* arg) {
pthread_mutex_unlock((pthread_mutex_t*)arg);
}
void* thread_func(void* arg) {
pthread_mutex_lock(&mutex);
pthread_cleanup_push(cleanup_unlock, &mutex);
// 临界区操作
pthread_cleanup_pop(1); // 执行清理并弹出
return NULL;
}
取消点与清理时序
线程取消仅发生在取消点（如sleep()、read()等系统调用）。若线程无限循环且无取消点，需用pthread_testcancel()手动添加。清理函数执行时机包括：
- 线程调用pthread_exit()
- 线程响应取消请求
- pthread_cleanup_pop(非零)
错误处理实践
忽略pthread_cleanup_pop()会导致未定义行为。建议通过RAII模式封装：
#define SCOPED_CLEANUP(func, arg) \
pthread_cleanup_push(func, arg); \
for(int _done=0; !_done; pthread_cleanup_pop(1), _done=1)
与C++析构对比
C++线程中更推荐使用RAII对象而非清理函数。例如std::lock_guard在析构时自动解锁，但清理机制在纯C或需要跨语言兼容时仍有价值。
性能考量
清理栈通常实现为线程局部存储的链表结构，push/pop操作时间复杂度为O(1)。频繁注册/执行可能影响性能，建议在必要场景使用。
线程取消类型
清理行为受pthread_setcanceltype()影响：
- 延迟取消（PTHREAD_CANCEL_DEFERRED）：默认，在取消点处理
- 异步取消（PTHREAD_CANCEL_ASYNCHRONOUS）：立即终止，可能跳过清理
绝大多数场景应使用延迟取消以确保清理执行。
跨平台兼容性
某些系统（如FreeBSD）要求push/pop严格配对。Linux允许嵌套但必须平衡。Windows的pthread实现可能行为不同，需验证目标平台文档。
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