c/c++ __thread

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本文通过实验演示了__thread关键字在C++中的使用方法及效果，展示了如何利用该关键字实现线程间的变量隔离，确保每个线程拥有独立的变量副本。

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1. __thread 关键字表示每一个线程有一份独立的实体，每一个线程都不会干扰。

2. __thread 只能修饰POD变量，简单的来说可以是如下几种变量

(1) 基本类型 (int , float 等等)

(2)指针类型

(3) 不带自定义构造函数和析构函数的类，如果希望修饰带自定义构造和析构函数的类，需要用到指针。

3. 实验代码

(1)__thread 可以修饰stu_t, 因为stu_t没有自定义的构造和析构函数

(2)__thread 可以修饰teacher_t*，因为__thread可以修饰指针

(3)实验结果表示在不同的线程中，变量不会相互影响。

#include <iostream>
#include <pthread.h>
#include <stdio.h>
#include <unistd.h>
#include <assert.h>
#include <string>

class stu_t {
public:
   int num;
   int age;
};

class teacher_t {
public:
   int num;
   int age;
   teacher_t (int num, int age) {
      this->num = num;
      this->age = age;
   }
};


static __thread int count;
static __thread stu_t stu;
static __thread teacher_t* teacher;

void print(std::string description, int num, stu_t stu, teacher_t* teacher) {
    std::cout << description << ", before======>" << "pid:" << getpid() << ", pthread id:" << pthread_self() << ", count:" << count << std::endl;
    std::cout << description << ", before======>" << "pid:" << getpid() << ", pthread id:" << pthread_self() << ", snum:" << stu.num << ", sage:"<< stu.age << std::endl;
    std::cout << description << ", before======>" << "pid:" << getpid() << ", pthread id:" << pthread_self() << ", snum:" << teacher->num << ", tage:"<< teacher->age << std::endl;
}


void *function1(void *argc)
{
    count = 2;
    stu.num = 2;
    stu.age = 2;
    teacher = new teacher_t(2,2);
    sleep(1);
    print("thread1", count, stu, teacher);
    return 0;
}

void *function2(void *argc)
{
    stu.num = 3;
    stu.age = 3;
    count = 3;
    teacher = new teacher_t(3,3);
    sleep(2);
    print("thread2", count, stu, teacher);
    return 0;
}

int main()
{
    pthread_t  thread_id[2];
    int ret;

    stu.num = 1;
    stu.age = 1;
    count = 1;
    teacher = new teacher_t(1,1);

    print("main", count, stu, teacher);
    pthread_create(thread_id, NULL, function1, NULL);
    pthread_create(thread_id + 1, NULL, function2, NULL);
    pthread_join(thread_id[0], NULL);
    pthread_join(thread_id[1], NULL);
    print("main", count, stu, teacher);

    return 0;
}

4. 实验结果

main, before======>pid:697, pthread id:0x7fff91ca0380, count:1
main, before======>pid:697, pthread id:0x7fff91ca0380, snum:1, sage:1
main, before======>pid:697, pthread id:0x7fff91ca0380, snum:1, tage:1
thread1, before======>pid:697, pthread id:0x700002525000, count:2
thread1, before======>pid:697, pthread id:0x700002525000, snum:2, sage:2
thread1, before======>pid:697, pthread id:0x700002525000, snum:2, tage:2
thread2, before======>pid:697, pthread id:0x7000025a8000, count:3
thread2, before======>pid:697, pthread id:0x7000025a8000, snum:3, sage:3
thread2, before======>pid:697, pthread id:0x7000025a8000, snum:3, tage:3
main, before======>pid:697, pthread id:0x7fff91ca0380, count:1
main, before======>pid:697, pthread id:0x7fff91ca0380, snum:1, sage:1
main, before======>pid:697, pthread id:0x7fff91ca0380, snum:1, tage:1