[英]Shared Variable in pthread
#include<iostream>
#include<pthread.h>
#include<chrono>
struct Arg1{
int x,y;
};
void * process(void * threadarg){
int x,y;
Arg1 * myArg1;
myArg1 = (Arg1 *) threadarg;
x = myArg1->x;
y=myArg1->y;
int i=0;
while(i<500){
x+=1;
y+=2;
i++;
}
myArg1->x=x;
myArg1->y=y;
pthread_exit(NULL);
}
using namespace std;
int main(){
int x = 0;
int y = 0;
int n;
cin>>n;
Arg1 sharedObj;
sharedObj.x=x;
sharedObj.y=y;
auto start = chrono::high_resolution_clock::now();
pthread_t *threads = new pthread_t[n];
for(int i=0;i<n;++i){
pthread_create(&threads[i],NULL,process,(void *)&sharedObj);
}
for(int i=0;i<n;++i){
pthread_join(threads[i],NULL);
}
cout<<sharedObj.x<<" "<<sharedObj.y<<"\n";
auto stop = chrono::high_resolution_clock::now();
auto duration = chrono::duration_cast<chrono::microseconds>(stop-start);
cout<<duration.count()<<"\n";
}
I wrote this code to see if using shared variables cause problems in the result.我编写了这段代码来查看使用共享变量是否会导致结果出现问题。
What it does is this- It first asks the user for number of threads n
.它的作用是 - 它首先询问用户线程数n
。 Each thread increments shared variables x and y by 1 and 2 respectively.每个线程将共享变量 x 和 y 分别增加 1 和 2。 Variables are shared via the use of the same struct object.通过使用相同的结构 object 来共享变量。 In the end I print the values of x and y from the struct object.最后,我从结构 object 中打印 x 和 y 的值。
I saw that for a small while loop in the function process
, I am able to get correct results, regardless the number of threads n.我在 function process
中看到了一个小的 while 循环,无论线程数 n 多少,我都能得到正确的结果。 However, for large values, about 5000, I get wrong results sometimes.但是,对于较大的值,大约 5000,我有时会得到错误的结果。
Also, the time of execution increases with the number of threads, and I don't understand whether this is because of improper multithreading or because of overhead in creating more number of threads.此外,执行时间随着线程数的增加而增加,我不明白这是因为不正确的多线程还是因为创建更多线程的开销。
Thanks for any help in advance.感谢您提前提供任何帮助。
Unless we use a mutex or some method to ensure atomicity of write, we can not guarantee that we would get a correct value in pthreaded applications.除非我们使用互斥锁或某种方法来确保写入的原子性,否则我们不能保证我们会在 pthread 应用程序中获得正确的值。 This is because, simultaneous writes can lead to a race condition, with only one of the write affecting the shared variable.这是因为,同时写入会导致竞争条件,只有一个写入会影响共享变量。
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