请教一个关于线程取消的问题
<The linux programming interface>Listing 32-2: Using cleanup handlers
––––––––––––––––––––––––––– ––––––––––––––––––––––– threads/thread_cleanup.c
#include <pthread.h>
#include "tlpi_hdr.h"
static pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
static pthread_mutex_t mtx = PTHREAD_MUTEX_INITIALIZER;
static int glob = 0; /* Predicate variable */
static void /* Free memory pointed to by 'arg' and unlock mutex */
cleanupHandler(void *arg)
{
int s;
printf("cleanup: freeing block at %p\n", arg);
free(arg);
printf("cleanup: unlocking mutex\n");
s = pthread_mutex_unlock(&mtx);
if (s != 0)
errExitEN(s, "pthread_mutex_unlock");
}
static void *
threadFunc(void *arg)
{
int s;
void *buf = NULL; /* Buffer allocated by thread */
buf = malloc(0x10000); /* Not a cancellation point */
printf("thread:allocated memory at %p\n", buf);
s = pthread_mutex_lock(&mtx); /* Not a cancellation point */
if (s != 0)
errExitEN(s, "pthread_mutex_lock");
pthread_cleanup_push(cleanupHandler, buf);
while (glob == 0) {
s = pthread_cond_wait(&cond, &mtx); /* A cancellation point */
if (s != 0)
errExitEN(s, "pthread_cond_wait");
}
printf("thread:condition wait loop completed\n");
pthread_cleanup_pop(1); /* Executes cleanup handler */
return NULL;
}
int
main(int argc, char *argv[])
{
pthread_t thr;
void *res;
int s;
s = pthread_create(&thr, NULL, threadFunc, NULL);
if (s != 0)
errExitEN(s, "pthread_create");
sleep(2); /* Give thread a chance to get started */
if (argc == 1) { /* Cancel thread */
printf("main: about to cancel thread\n");
s = pthread_cancel(thr);
if (s != 0)
errExitEN(s, "pthread_cancel");
} else { /* Signal condition variable */
printf("main: about to signal condition variable\n");
glob = 1;
s = pthread_cond_signal(&cond);
if (s != 0)
errExitEN(s, "pthread_cond_signal");
}
s = pthread_join(thr, &res);
if (s != 0)
errExitEN(s, "pthread_join");
if (res == PTHREAD_CANCELED)
printf("main: thread was canceled\n");
else
printf("main: thread terminated normally\n");
exit(EXIT_SUCCESS);
}
这段程序中, 在s = pthread_cond_wait(&cond, &mtx); /* A cancellation point */时发生线程取消的话, 为何需要在处理函数cleanupHandler中对互斥量mtx进行解锁,
pthread_cond_wait(&cond, &mtx)在等待的时候不是已经解锁了mtk了么 本帖最后由 羽剑天涯 于 2015-03-30 15:46 编辑
回复 1# qiumupo
pthread_cond_wait这个函数调用后，会解锁传递的mutex，同时进入阻塞，等待传递的cond，（此时mutex可以被别的线程加锁，并通知cond）,当cond通知收到后，会尝试重新去加锁，加锁成功后，该函数会返回，（此时mutex重新被当前线程加锁），也就是在该函数调用前和调用后，mutex都是本线程加锁状态，在该函数调用中，mutex才会处于解锁状态。
另外，调用pthread_cancel时，如未特别指定，取消状态为PTHREAD_CANCEL_ENABLE，取消类型为PTHREAD_CANCEL_DEFERED，也就是执行了下述函数之一后采取行动：pthread_join,pthread_cond_wait,pthread_cond_timedwait,pthread_testcancel,sem_wait或sigwait。从你的线程代码中可以看到，是在pthread_con_wait结束后行动，此时已经加锁了。
另外该线程在自己退出时也是调用cleanupHandler，这样也需要进行free和解锁。 回复 2# 羽剑天涯
是不是设置下线程分离比较好。pthread_detach().这样线程结束时自动回收资源。。 回复 3# guojinshuai
pthread_detach只是将线程设置为不需要join，回收的仅仅是线程的退出状态等一些资源，锁、条件变量等其他资源并不会自动回收（这些资源也没有直接属于该线程）……
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