多线程编程基础（线程创建）

1、多线程的创建

线程的创建比较简单，先举一个例子热热身：

#include <stdio.h>
#include <pthread.h>
void* Handler(void* param) {
char* message = static_cast<char*>(param);
printf("%s\n", message);
}
int main(int argc, char** argv) {
pthread_t thread_id;
char message[] = "hello world";
pthread_create(&thread_id, NULL, Handler, message);
void* ret = NULL;
pthread_join(thread_id, &ret);
}

有时候，我们需要将以个类的成员函数作为线程的routine函数使用，但pthread_create不能接收一个非静态的成员函数作为参数，因此类的成员函数必须是静态的，但这样的静态成员函数使得访问类的非静态数据成员会有问题，因此这不是一个很好的解决方案，例如下面的程序，ThreadBase类的设计和实现会有很多不便。

#include <stdio.h>
#include <pthread.h>
class ThreadBase {
public:
static void* Handler(void* param) {
char* message = static_cast<char*>(param);
printf("%s\n", message);
}
};
int main(int argc, char** argv) {
pthread_t thread_id;
char message[] = "hello world";
pthread_create(&thread_id, NULL, ThreadBase::Handler, message);
void* ret = NULL;
pthread_join(thread_id, &ret);
}

解决方案有三个：

1）通过一个非成员函数包装一下这个类，非成员函数的参数是类的对象，有了类的对象就可一调用类的成员函数了，示例代码如下：

#include <stdio.h>
#include <pthread.h>
#include <string>
class ThreadBase {
public:
void SetMessage(const char* message) {
message_ = message;
}
void Handler() {
printf("%s\n", message_.c_str());
}
private:
std::string message_;
};

void* ThreadCall(void* object) {
ThreadBase* thread_base = static_cast<ThreadBase*>(object);
thread_base->Handler();
}
int main(int argc, char** argv) {
pthread_t thread_id;
char message[] = "hello world";
ThreadBase thread_base;
thread_base.SetMessage(message);
pthread_create(&thread_id, NULL, ThreadCall, &thread_base);
void* ret = NULL;
pthread_join(thread_id, &ret);
}

2）当然这个非成员函数也可以是类的静态成员函数，那么结果将是下面这个样式：

#include <stdio.h>
#include <pthread.h>
#include <string>
class ThreadBase {
public:
void SetMessage(const char* message) {
message_ = message;
}
void Handler() {
printf("%s\n", message_.c_str());
}
static void* ThreadCall(void* object) {
ThreadBase* thread_base = static_cast<ThreadBase*>(object);
thread_base->Handler();
}
private:
std::string message_;
};

int main(int argc, char** argv) {
pthread_t thread_id;
char message[] = "hello world";
ThreadBase thread_base;
thread_base.SetMessage(message);
pthread_create(&thread_id, NULL, ThreadBase::ThreadCall, &thread_base);
void* ret = NULL;
pthread_join(thread_id, &ret);
}

3）另线程创建的工作在类的内部完成，使得类具有对立运行的性质，熟悉java的同学对下面的实现一定非常熟悉：

#include <stdio.h>
#include <pthread.h>
#include <string>
class ThreadBase {
public:
virtual ~ThreadBase() {}
void SetMessage(const char* message) {
message_ = message;
}
void Start() {
pthread_create(&thread_id_, NULL, Hook, this);
}
void* Join() {
void* ret = NULL;
pthread_join(thread_id_, &ret);
return ret;
}
virtual void Run() {
printf("%s\n", message_.c_str());
}
private:
static void* Hook(void* object) {
ThreadBase* thread_base= static_cast<ThreadBase*>(object);
thread_base->Run();
}
pthread_t thread_id_;
std::string message_;
};
class ThreadDerived : public ThreadBase {
public:
virtual void Run() {
printf("a new derived multi-thread object is running\n");
}
};
int main(int argc, char** argv) {
pthread_t thread_id;
char message[] = "hello world";
ThreadBase thread_base;
thread_base.SetMessage(message);
thread_base.Start();
thread_base.Join();
ThreadDerived thread_derived;
thread_derived.Start();
thread_derived.Join();
}

有了这个ThreadBase基类，再创建支持多线程的类就非常容易了，只要继承并实现Run函数就可以了。

参考文献：

https://computing.llnl.gov/tutorials/pthreads/

http://www.dutor.net/index.php/2011/11/pthread-in-cpp-class/

http://stackoverflow.com/questions/1151582/pthread-function-from-a-class