c++ - 为什么 pthread_create() 返回 0 但线程永远不会启动

Question

我有 Java 背景，很乐意使用 Java 的同步进行多线程编程。最近，我开始涉足 C++，不得不处理一些多线程代码。以下是我面临的问题的最小工作示例。

class TxStatus {
    private:
        int numOperations;
        bool commitStatus;
        pthread_mutex_t statusLock;

    public:
        TxStatus() {
            this->commitStatus = false;
            this->numOperations = 0;
        }
        void addNewOperation() {
            pthread_mutex_lock(&statusLock);
            numOperations++;
            pthread_mutex_unlock(&statusLock);
        }
        void operationCompleted() {
            pthread_mutex_lock(&statusLock);
            numOperations--;
            pthread_mutex_unlock(&statusLock);
        }
        void commit() {
            this->commitStatus = true;
        }

};

class TxManager {
    private:
        unsigned long long globalFrontier;
        unsigned long long txId;

        pthread_mutex_t gfLock;
        pthread_mutex_t txIdLock;

        std::map<unsigned long long, TxStatus> txStatusMap;

    public:
        TxManager() {
            pthread_mutex_lock(&gfLock);
            globalFrontier = 1;
            pthread_mutex_unlock(&gfLock);

            pthread_mutex_lock(&txIdLock);
            txId = 1;
            pthread_mutex_unlock(&txIdLock);
        }

        unsigned long long beginNewTx() {
            pthread_mutex_lock(&txIdLock);
            unsigned long long newId = txId;
            txId++;
            pthread_mutex_unlock(&txIdLock);
            TxStatus statusObj;
            txStatusMap.insert(std::make_pair(newId,statusObj));
            return newId;
        }

        void addUnflushedOperation(unsigned long long txId) {
            txStatusMap[txId].addNewOperation();
        }
        void markOperationAsFlushed(unsigned long long txId) {
            txStatusMap[txId].operationCompleted();
        }
        void markCommitted(unsigned long long txId) {
            txStatusMap[txId].commit();
        }
};


void * thread( void *args){

    TxManager txManager;
    fprintf(stderr,"Inside thread");
    unsigned long long newTxId = txManager.beginNewTx();

    fprintf(stderr,"Tx Started: %d", newTxId );
    txManager.addUnflushedOperation(newTxId);
    pthread_exit(NULL);   

}

int main(){
    pthread_t tx_thread;
    fprintf(stderr,"Inside main");
    int ret = pthread_create(&tx_thread, NULL, thread, NULL);
    if (ret != 0)
    {
        fprintf(stderr,"Error launching thread");
    }else{
        fprintf(stderr,"Thread launched successfully");
    }


    if (pthread_join(tx_thread, NULL) != 0)
    {
        fprintf(stderr,"Join pthread failed");
    }

    return 0;

}

线程已成功启动，但我从未看到线程本身执行的函数(即 thread())的输出之一。如果我删除 join 调用，那么程序将在 main 方法中打印语句后终止。

Answer 1

Pthreads 是一个 C 库。它的数据类型是哑 C 类型。如果您只是将它们放在一个类或结构中，它们将不会自动初始化。你需要初始化它们。最简单的方法是使用 PTHREAD_MUTEX_INITIALIZER(或者使用 pthread_mutex_init)，在 C++ 中，您可以在成员声明旁边执行此操作。

这不会挂起:

#include <pthread.h>
#include <map>
#include <stdio.h>
using namespace std;
class TxStatus {
    private:
        int numOperations;
        bool commitStatus;
        pthread_mutex_t statusLock = PTHREAD_MUTEX_INITIALIZER;

    public:
        TxStatus() {
            this->commitStatus = false;
            this->numOperations = 0;
        }
        void addNewOperation() {
            pthread_mutex_lock(&statusLock);
            numOperations++;
            pthread_mutex_unlock(&statusLock);
        }
        void operationCompleted() {
            pthread_mutex_lock(&statusLock);
            numOperations--;
            pthread_mutex_unlock(&statusLock);
        }
        void commit() {
            this->commitStatus = true;
        }

};

class TxManager {
    private:
        unsigned long long globalFrontier;
        unsigned long long txId;

        pthread_mutex_t gfLock = PTHREAD_MUTEX_INITIALIZER;
        pthread_mutex_t txIdLock = PTHREAD_MUTEX_INITIALIZER;

        std::map<unsigned long long, TxStatus> txStatusMap;

    public:
        TxManager() {
            pthread_mutex_lock(&gfLock);
            globalFrontier = 1;
            pthread_mutex_unlock(&gfLock);

            pthread_mutex_lock(&txIdLock);
            txId = 1;
            pthread_mutex_unlock(&txIdLock);
        }

        unsigned long long beginNewTx() {
            pthread_mutex_lock(&txIdLock);
            unsigned long long newId = txId;
            txId++;
            pthread_mutex_unlock(&txIdLock);
            TxStatus statusObj;
            txStatusMap.insert(std::make_pair(newId,statusObj));
            return newId;
        }

        void addUnflushedOperation(unsigned long long txId) {
            txStatusMap[txId].addNewOperation();
        }
        void markOperationAsFlushed(unsigned long long txId) {
            txStatusMap[txId].operationCompleted();
        }
        void markCommitted(unsigned long long txId) {
            txStatusMap[txId].commit();
        }
};


void * thread( void *args){

    TxManager txManager;
    fprintf(stderr,"Inside thread\n");
    unsigned long long newTxId = txManager.beginNewTx();

    fprintf(stderr,"Tx Started: %llu", newTxId );
    txManager.addUnflushedOperation(newTxId);
    pthread_exit(NULL);   

}

int main(){
    pthread_t tx_thread;
    fprintf(stderr,"Inside main\n");
    int ret = pthread_create(&tx_thread, NULL, thread, NULL);
    if (ret != 0)
    {
        fprintf(stderr,"Error launching thread");
    }else{
        fprintf(stderr,"Thread launched successfully");
    }


    if (pthread_join(tx_thread, NULL) != 0)
    {
        fprintf(stderr,"Join pthread failed");
    }

    return 0;

}