MPI and stdin overheading?

Question

I am coding a program in C++ MPI but when passing a large file as stdin I am facing the problem that the threads are not seeing the same stdin information.

More elaborated, I am passing as standard input a list of input files, which is then stored in a vector<string>:

MPI_Init(NULL,NULL);
int CORES, thread;
MPI_Comm_size(MPI_COMM_WORLD,&CORES);
MPI_Comm_rank(MPI_COMM_WORLD,&thread);
stringstream tline;
int count = 0;
for (std::string line; std::getline(std::cin, line);){
    tline << line << " ";
    count++;
    }

vector<string> args(count,"");
for(int i = 0; i < count; i++)
    tline >> args[i];

cout << thread << " " << count << endl; //each thread outputs the number of input files it received

My problem is that this gives different numbers for different threads. For instance, after passing a file of 10 000 lines, I get:

5 9464
6 9464
3 9464
4 9464
1 9554
2 9554
0 10000
7 9464

Is it because of some overheading? How can I avoid that?

Answer 1

好的，所以基本上你的问题是你所有的线程都在消耗来自 cin 的线并且他们竞争。尽管 cin 通常会为线程安全性提供一些保证，但您并不总是确定自己会得到什么。检查此主题：How do scanf(), std::cin behave on multithreaded environment?

解决方案：不要使用CIN？使用文件并让每个线程使用文件句柄自己打开文件。如果你真的想使用 cin ，那么让一个来自MPI的线程读取 CIN 并将其广播到其他线程，然后他们可以随心所欲地使用它。