问题
I am interested in whether there is a command or a technique within OpenMPI to have an atomic call to write to stdout (or, for that matter, any stream).
What I have noticed is that during the execution of MPI programs, calls to write to cout (or other streams) can become confusing, as each proc may write whenever it gets to a certain section of code. When reporting results, a line can be written to by several procs, confusing the issue. So 2 different procs might do something like this:
//Proc 10 - results calculated somewhere above
// result1 = 10
// result2 = 11
cout << "My results are: " << result1 << " " << resul2 << endl;
and:
//Proc 20 - results calculated somewhere above
// result1 = 20
// result2 = 21
cout << "My results are: " << result1 << " " << resul2 << endl;
But the result might be:
My results are: 20 My results are: 10 11 21
What I'm looking for is something like a "blocking" or atomic cout (as well as possibly writing to other streams, such as file streams). So once I start writing a to cout, it blocks until the end of the statement, or until endl or a flush to the stream is issued. If this were the case, I'd be guaranteed that the 2 lines would be separate (but, of course, I still wouldn't know which line would come first):
My results are: 20 21
My results are: 10 11
回答1:
Working with standard i/o streams (stdout and stdin in terms of C language or cout and cin in C++) is not the best part of MPI (OpenMPI implements MPI). There are some slides about this: http://www.csee.wvu.edu/~donm/classes/cs591x/notes/io1.ppt and Slide 10 has a solution. (or chapter 8.1 of the book "Parallel Programming With Mpi By P. S. Pacheco")
Do all your I/O with cout in the process with rank 0. If you want to output some data from other processes, just send MPI message with this data to rank 0.
来源：https://stackoverflow.com/questions/15901226/an-atomic-call-to-cout-in-mpi