c ++中MPI线程的示例

Question

我已经安装了MPI，并找到了我不完全理解的简单线程示例。我可以得到一个打印东西的MPI线程的简单示例。感谢

MPI_Init(&argc, &argv);
int errs = 0;
int provided, flag, claimed;

MPI_Init_thread(0, 0, MPI_THREAD_MULTIPLE, &provided); //what does it MPI_THREAD_MULTIPLE mean

MPI_Is_thread_main(&flag); //why there is flag here
if (!flag) {
    errs++;  //why there is counter for errs
    printf("..."); fflush(stdout); // doesn't print anything
}
MPI_Finalize();
//return errs;  I get error here