c++多線程編程

原文鏈接：https://blog.csdn.net/hitwengqi/article/details/8015646

一直對多線程編程這一塊很陌生，決定花一點時間整理一下。

os:ubuntu 10.04 c++

1.最基礎，進程同時創建5個線程，各自調用同一個函數


#include <iostream>
#include <pthread.h> //多線程相關操作頭文件，可移植衆多平臺
 
using namespace std;
 
#define NUM_THREADS 5 //線程數
 
void* say_hello( void* args )
{
    cout << "hello..." << endl;
} //函數返回的是函數指針，便於後面作爲參數
 
int main()
{
    pthread_t tids[NUM_THREADS]; //線程id
    for( int i = 0; i < NUM_THREADS; ++i )
    {
        int ret = pthread_create( &tids[i], NULL, say_hello, NULL ); //參數：創建的線程id，線程參數，線程運行函數的起始地址，運行函數的參數
        if( ret != 0 ) //創建線程成功返回0
        {
            cout << "pthread_create error:error_code=" << ret << endl;
        }
    }
    pthread_exit( NULL ); //等待各個線程退出後，進程才結束，否則進程強制結束，線程處於未終止的狀態
}

輸入命令：g++ -o muti_thread_test_1 muti_thread_test_1.cpp -lpthread

注意：

1）此爲c++程序，故用g++來編譯生成可執行文件，並且要調用處理多線程操作相關的靜態鏈接庫文件pthread。

2）-lpthread 編譯選項到位置可任意，如g++ -lpthread -o muti_thread_test_1 muti_thread_test_1.cpp

3）注意gcc和g++的區別，轉到此文：點擊打開鏈接

測試結果：


wq@wq-desktop:~/coding/muti_thread$ ./muti_thread_test_1
hello...hello...
hello...
hello...
 
hello...


wq@wq-desktop:~/coding/muti_thread$ ./muti_thread_test_1
hello...hello...hello...
 
hello...
hello...

可知，兩次運行的結果會有差別，這不是多線程的特點吧？這顯然沒有同步？還有待進一步探索...

多線程的運行是混亂的，混亂就是正常？

2.線程調用到函數在一個類中，那必須將該函數聲明爲靜態函數函數

因爲靜態成員函數屬於靜態全局區，線程可以共享這個區域，故可以各自調用。


#include <iostream>
#include <pthread.h>
 
using namespace std;
 
#define NUM_THREADS 5
 
class Hello
{
public:
    static void* say_hello( void* args )
    {
        cout << "hello..." << endl;
    }
};
 
int main()
{
    pthread_t tids[NUM_THREADS];
    for( int i = 0; i < NUM_THREADS; ++i )
    {
        int ret = pthread_create( &tids[i], NULL, Hello::say_hello, NULL );
        if( ret != 0 )
        {
            cout << "pthread_create error:error_code" << ret << endl;
        }
    }
    pthread_exit( NULL );
}

測試結果：


wq@wq-desktop:~/coding/muti_thread$ ./muti_thread_test_2
hello...
hello...
hello...
hello...
hello...


wq@wq-desktop:~/coding/muti_thread$ ./muti_thread_test_2
hello...hello...hello...
 
 
hello...
hello...

3.如何在線程調用函數時傳入參數呢？

先看下面修改的代碼，傳入線程編號作爲參數：


#include <iostream>
#include <pthread.h> //多線程相關操作頭文件，可移植衆多平臺
 
using namespace std;
 
#define NUM_THREADS 5 //線程數
 
void* say_hello( void* args )
{
    int i = *( (int*)args ); //對傳入的參數進行強制類型轉換，由無類型指針轉變爲整形指針，再用*讀取其指向到內容
    cout << "hello in " << i <<  endl;
} //函數返回的是函數指針，便於後面作爲參數
 
int main()
{
    pthread_t tids[NUM_THREADS]; //線程id
    cout << "hello in main.." << endl;
    for( int i = 0; i < NUM_THREADS; ++i )
    {
        int ret = pthread_create( &tids[i], NULL, say_hello, (void*)&i ); //傳入到參數必須強轉爲void*類型，即無類型指針，&i表示取i的地址，即指向i的指針
        cout << "Current pthread id = " << tids[i] << endl; //用tids數組打印創建的進程id信息
        if( ret != 0 ) //創建線程成功返回0
        {
            cout << "pthread_create error:error_code=" << ret << endl;
        }
    }
    pthread_exit( NULL ); //等待各個線程退出後，進程才結束，否則進程強制結束，線程處於未終止的狀態
}

測試結果：


wq@wq-desktop:~/coding/muti_thread$ ./muti_thread_test_3
hello in main..
Current pthread id = 3078458224
Current pthread id = 3070065520
hello in hello in 2
1
Current pthread id = hello in 2
3061672816
Current pthread id = 3053280112
hello in 4
Current pthread id = hello in 4
3044887408

顯然不是想要的結果，調用順序很亂，這是爲什麼呢？

這是因爲多線程到緣故，主進程還沒開始對i賦值，線程已經開始跑了...?

修改代碼如下：


#include <iostream>
#include <pthread.h> //多線程相關操作頭文件，可移植衆多平臺
 
using namespace std;
 
#define NUM_THREADS 5 //線程數
 
void* say_hello( void* args )
{
    cout << "hello in thread " << *( (int *)args ) <<  endl;
} //函數返回的是函數指針，便於後面作爲參數
 
int main()
{
    pthread_t tids[NUM_THREADS]; //線程id
    int indexes[NUM_THREADS]; //用來保存i的值避免被修改
 
    for( int i = 0; i < NUM_THREADS; ++i )
    {
        indexes[i] = i;
        int ret = pthread_create( &tids[i], NULL, say_hello, (void*)&(indexes[i]) );
        if( ret != 0 ) //創建線程成功返回0
        {
            cout << "pthread_create error:error_code=" << ret << endl;
        }
    }
    for( int i = 0; i < NUM_THREADS; ++i )
        pthread_join( tids[i], NULL ); //pthread_join用來等待一個線程的結束，是一個線程阻塞的函數
}

測試結果：


wq@wq-desktop:~/coding/muti_thread$ ./muti_thread_test_3
hello in thread hello in thread hello in thread hello in thread hello in thread 30124

這是正常的嗎？感覺還是有問題...待續

代碼中如果沒有pthread_join主線程會很快結束從而使整個進程結束，從而使創建的線程沒有機會開始執行就結束了。加入pthread_join後，主線程會一直等待直到等待的線程結束自己才結束，使創建的線程有機會執行。

4.線程創建時屬性參數的設置pthread_attr_t及join功能的使用

線程的屬性由結構體pthread_attr_t進行管理。

typedef struct
{
int detachstate; 線程的分離狀態
int schedpolicy; 線程調度策略
struct sched_param schedparam; 線程的調度參數
int inheritsched; 線程的繼承性
int scope; 線程的作用域
size_t guardsize; 線程棧末尾的警戒緩衝區大小
int stackaddr_set; void * stackaddr; 線程棧的位置
size_t stacksize; 線程棧的大小
}pthread_attr_t;


#include <iostream>
#include <pthread.h>
 
using namespace std;
 
#define NUM_THREADS 5
 
void* say_hello( void* args )
{
    cout << "hello in thread " << *(( int * )args) << endl;
    int status = 10 + *(( int * )args); //線程退出時添加退出的信息，status供主程序提取該線程的結束信息
    pthread_exit( ( void* )status ); 
}
 
int main()
{
    pthread_t tids[NUM_THREADS];
    int indexes[NUM_THREADS];
    
    pthread_attr_t attr; //線程屬性結構體，創建線程時加入的參數
    pthread_attr_init( &attr ); //初始化
    pthread_attr_setdetachstate( &attr, PTHREAD_CREATE_JOINABLE ); //是設置你想要指定線程屬性參數，這個參數表明這個線程是可以join連接的，join功能表示主程序可以等線程結束後再去做某事，實現了主程序和線程同步功能
    for( int i = 0; i < NUM_THREADS; ++i )
    {
        indexes[i] = i;
        int ret = pthread_create( &tids[i], &attr, say_hello, ( void* )&( indexes[i] ) );
        if( ret != 0 )
        {
	    cout << "pthread_create error:error_code=" << ret << endl;
	}
    } 
    pthread_attr_destroy( &attr ); //釋放內存 
    void *status;
    for( int i = 0; i < NUM_THREADS; ++i )
    {
	int ret = pthread_join( tids[i], &status ); //主程序join每個線程後取得每個線程的退出信息status
	if( ret != 0 )
	{
	    cout << "pthread_join error:error_code=" << ret << endl;
	}
	else
	{
	    cout << "pthread_join get status:" << (long)status << endl;
	}
    }
}

測試結果：


wq@wq-desktop:~/coding/muti_thread$ ./muti_thread_test_4
hello in thread hello in thread hello in thread hello in thread 0hello in thread 321
 
 
 
4
pthread_join get status:10
pthread_join get status:11
pthread_join get status:12
pthread_join get status:13
pthread_join get status:14

5.互斥鎖的實現
互斥鎖是實現線程同步的一種機制，只要在臨界區前後對資源加鎖就能阻塞其他進程的訪問。


#include <iostream>
#include <pthread.h>
 
using namespace std;
 
#define NUM_THREADS 5
 
int sum = 0; //定義全局變量，讓所有線程同時寫，這樣就需要鎖機制
pthread_mutex_t sum_mutex; //互斥鎖
 
void* say_hello( void* args )
{
    cout << "hello in thread " << *(( int * )args) << endl;
    pthread_mutex_lock( &sum_mutex ); //先加鎖，再修改sum的值，鎖被佔用就阻塞，直到拿到鎖再修改sum;
    cout << "before sum is " << sum << " in thread " << *( ( int* )args ) << endl;
    sum += *( ( int* )args );
    cout << "after sum is " << sum << " in thread " << *( ( int* )args ) << endl;
    pthread_mutex_unlock( &sum_mutex ); //釋放鎖，供其他線程使用
    pthread_exit( 0 ); 
}
 
int main()
{
    pthread_t tids[NUM_THREADS];
    int indexes[NUM_THREADS];
    
    pthread_attr_t attr; //線程屬性結構體，創建線程時加入的參數
    pthread_attr_init( &attr ); //初始化
    pthread_attr_setdetachstate( &attr, PTHREAD_CREATE_JOINABLE ); //是設置你想要指定線程屬性參數，這個參數表明這個線程是可以join連接的，join功能表示主程序可以等線程結束後再去做某事，實現了主程序和線程同步功能
    pthread_mutex_init( &sum_mutex, NULL ); //對鎖進行初始化    
 
    for( int i = 0; i < NUM_THREADS; ++i )
    {
        indexes[i] = i;
        int ret = pthread_create( &tids[i], &attr, say_hello, ( void* )&( indexes[i] ) ); //5個進程同時去修改sum
        if( ret != 0 )
        {
	    cout << "pthread_create error:error_code=" << ret << endl;
	}
    } 
    pthread_attr_destroy( &attr ); //釋放內存 
    void *status;
    for( int i = 0; i < NUM_THREADS; ++i )
    {
	int ret = pthread_join( tids[i], &status ); //主程序join每個線程後取得每個線程的退出信息status
	if( ret != 0 )
	{
	    cout << "pthread_join error:error_code=" << ret << endl;
	}
    }
    cout << "finally sum is " << sum << endl;
    pthread_mutex_destroy( &sum_mutex ); //註銷鎖
}

測試結果：


wq@wq-desktop:~/coding/muti_thread$ ./muti_thread_test_5
hello in thread hello in thread hello in thread 410
before sum is hello in thread 0 in thread 4
after sum is 4 in thread 4hello in thread 
 
 
2
3
before sum is 4 in thread 1
after sum is 5 in thread 1
before sum is 5 in thread 0
after sum is 5 in thread 0
before sum is 5 in thread 2
after sum is 7 in thread 2
before sum is 7 in thread 3
after sum is 10 in thread 3
finally sum is 10

可知，sum的訪問和修改順序是正常的，這就達到了多線程的目的了，但是線程的運行順序是混亂的，混亂就是正常？

6.信號量的實現
信號量是線程同步的另一種實現機制，信號量的操作有signal和wait，本例子採用條件信號變量pthread_cond_t tasks_cond;
信號量的實現也要給予鎖機制。


#include <iostream>
#include <pthread.h>
#include <stdio.h>
 
using namespace std;
 
#define BOUNDARY 5
 
int tasks = 10;
pthread_mutex_t tasks_mutex; //互斥鎖
pthread_cond_t tasks_cond; //條件信號變量，處理兩個線程間的條件關係，當task>5，hello2處理，反之hello1處理，直到task減爲0
 
void* say_hello2( void* args )
{
    pthread_t pid = pthread_self(); //獲取當前線程id
    cout << "[" << pid << "] hello in thread " <<  *( ( int* )args ) << endl;
    
    bool is_signaled = false; //sign
    while(1)
    {
	pthread_mutex_lock( &tasks_mutex ); //加鎖
	if( tasks > BOUNDARY )
	{
	    cout << "[" << pid << "] take task: " << tasks << " in thread " << *( (int*)args ) << endl;
 	    --tasks; //modify
	}
	else if( !is_signaled )
	{
	    cout << "[" << pid << "] pthread_cond_signal in thread " << *( ( int* )args ) << endl;
	    pthread_cond_signal( &tasks_cond ); //signal:向hello1發送信號，表明已經>5
	    is_signaled = true; //表明信號已發送，退出此線程
	}
	pthread_mutex_unlock( &tasks_mutex ); //解鎖
	if( tasks == 0 )
	    break;
    }    
}
 
void* say_hello1( void* args )
{
    pthread_t pid = pthread_self(); //獲取當前線程id
    cout << "[" << pid << "] hello in thread " <<  *( ( int* )args ) << endl;
 
    while(1)
    {
        pthread_mutex_lock( &tasks_mutex ); //加鎖
        if( tasks > BOUNDARY )
        {
	    cout << "[" << pid << "] pthread_cond_signal in thread " << *( ( int* )args ) << endl;
	    pthread_cond_wait( &tasks_cond, &tasks_mutex ); //wait:等待信號量生效，接收到信號，向hello2發出信號，跳出wait,執行後續 
        }
        else
        {
	    cout << "[" << pid << "] take task: " << tasks << " in thread " << *( (int*)args ) << endl;
            --tasks;
	}
        pthread_mutex_unlock( &tasks_mutex ); //解鎖
        if( tasks == 0 )
            break;
    } 
}
 
 
int main()
{
    pthread_attr_t attr; //線程屬性結構體，創建線程時加入的參數
    pthread_attr_init( &attr ); //初始化
    pthread_attr_setdetachstate( &attr, PTHREAD_CREATE_JOINABLE ); //是設置你想要指定線程屬性參數，這個參數表明這個線程是可以join連接的，join功能表示主程序可以等線程結束後再去做某事，實現了主程序和線程同步功能
    pthread_cond_init( &tasks_cond, NULL ); //初始化條件信號量
    pthread_mutex_init( &tasks_mutex, NULL ); //初始化互斥量
    pthread_t tid1, tid2; //保存兩個線程id
    int index1 = 1;
    int ret = pthread_create( &tid1, &attr, say_hello1, ( void* )&index1 );
    if( ret != 0 )
    {
        cout << "pthread_create error:error_code=" << ret << endl;
    }
    int index2 = 2;
    ret = pthread_create( &tid2, &attr, say_hello2, ( void* )&index2 );
    if( ret != 0 )
    {
        cout << "pthread_create error:error_code=" << ret << endl;
    }
    pthread_join( tid1, NULL ); //連接兩個線程
    pthread_join( tid2, NULL ); 
 
    pthread_attr_destroy( &attr ); //釋放內存 
    pthread_mutex_destroy( &tasks_mutex ); //註銷鎖
    pthread_cond_destroy( &tasks_cond ); //正常退出
}

測試結果：
先在線程2中執行say_hello2，再跳轉到線程1中執行say_hello1，直到tasks減到0爲止。


wq@wq-desktop:~/coding/muti_thread$ ./muti_thread_test_6
[3069823856] hello in thread 2
[3078216560] hello in thread 1[3069823856] take task: 10 in thread 2
 
[3069823856] take task: 9 in thread 2
[3069823856] take task: 8 in thread 2
[3069823856] take task: 7 in thread 2
[3069823856] take task: 6 in thread 2
[3069823856] pthread_cond_signal in thread 2
[3078216560] take task: 5 in thread 1
[3078216560] take task: 4 in thread 1
[3078216560] take task: 3 in thread 1
[3078216560] take task: 2 in thread 1
[3078216560] take task: 1 in thread 1

到此，對多線程編程有了一個初步的瞭解，當然還有其他實現線程同步的機制，有待進一步探索。

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