請(qǐng)教一個(gè)關(guān)于線程取消的問題

qiumupo

<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);
}

這段程序中, 在s = pthread_cond_wait(&cond, &mtx);    /* A cancellation point */時(shí)發(fā)生線程取消的話, 為何需要在處理函數(shù)cleanupHandler中對(duì)互斥量mtx進(jìn)行解鎖,
pthread_cond_wait(&cond, &mtx)在等待的時(shí)候不是已經(jīng)解鎖了mtk了么

羽劍天涯

回復(fù) 1# qiumupo


    pthread_cond_wait這個(gè)函數(shù)調(diào)用后，會(huì)解鎖傳遞的mutex，同時(shí)進(jìn)入阻塞，等待傳遞的cond，（此時(shí)mutex可以被別的線程加鎖，并通知cond）,當(dāng)cond通知收到后，會(huì)嘗試重新去加鎖，加鎖成功后，該函數(shù)會(huì)返回，（此時(shí)mutex重新被當(dāng)前線程加鎖），也就是在該函數(shù)調(diào)用前和調(diào)用后，mutex都是本線程加鎖狀態(tài)，在該函數(shù)調(diào)用中，mutex才會(huì)處于解鎖狀態(tài)。
    另外，調(diào)用pthread_cancel時(shí)，如未特別指定，取消狀態(tài)為PTHREAD_CANCEL_ENABLE，取消類型為PTHREAD_CANCEL_DEFERED，也就是執(zhí)行了下述函數(shù)之一后采取行動(dòng)：pthread_join,pthread_cond_wait,pthread_cond_timedwait,pthread_testcancel,sem_wait或sigwait。從你的線程代碼中可以看到，是在pthread_con_wait結(jié)束后行動(dòng)，此時(shí)已經(jīng)加鎖了。
    另外該線程在自己退出時(shí)也是調(diào)用cleanupHandler，這樣也需要進(jìn)行free和解鎖。

guojinshuai

回復(fù) 2# 羽劍天涯


    是不是設(shè)置下線程分離比較好。pthread_detach().  這樣線程結(jié)束時(shí)自動(dòng)回收資源。。

羽劍天涯

回復(fù) 3# guojinshuai


    pthread_detach只是將線程設(shè)置為不需要join，回收的僅僅是線程的退出狀態(tài)等一些資源，鎖、條件變量等其他資源并不會(huì)自動(dòng)回收（這些資源也沒有直接屬于該線程）……