CODEWITHSUNDEEP
c++cmpi
John
John

Reputation: 383

How to send and receive string using MPI

I am trying to send and recieve string using MPI but results are hopless.

Send function:

MPI_Send(&result, result.size(), MPI_CHAR, 0, 0, MPI_COMM_WORLD);

And the recv function:

    MPI_Recv(&result,      /* message buffer */
        128,                 /* one data item */
        MPI_CHAR,        /* of type char real */
        MPI_ANY_SOURCE,    /* receive from any sender */
        MPI_ANY_TAG,       /* any type of message */
        MPI_COMM_WORLD,    /* default communicator */
        &status);          /* info about the received message */

Where result is a string.

I didn't get any error but program doesn't want to finish.

Upvotes: 8

Views: 24041

Answers (2)

Ahsan Ayub
Ahsan Ayub

Reputation: 1

I was having similar problem where I had to pass a string between two processes in MPI C. My solution, aligned to your problem, is the following:

MPI_Send(&result, strlen(result)+1, MPI_CHAR, 1, 0, MPI_COMM_WORLD); // Destination: Process 1

On the other side, process 0 will have the following MPI_Recv function:

MPI_Recv(&result, 100, MPI_CHAR, 0, 0, MPI_COMM_WORLD, MPI_STATUS_IGNORE); // Source: Process 0 and considering 100 as the maximum string length

<string.h> has to be added in the header.

Upvotes: -1

piokuc
piokuc

Reputation: 26184

The address of a std::string is not the same as address of the underlying C string, so sending should be fixed like this:

MPI_Send(result.c_str(), result.size(), MPI_CHAR, 0, 0, MPI_COMM_WORLD);

Receiving cannot be done as you tried to do it. You need a buffer (an array of char), which you will pass to MPI_Recv and use it later to create a std::string instance. To get a length of the string received use MPI_Get_count - this you should pass along the pointer to buffer to std::string constructor.

In order to avoid preallocating the buffer and allow it to receive a string of any size you should call MPI_Probe and MPI_Get_count to get the length first. Knowing the length you can allocate the buffer with the exact size which is necessary, and only then call MPI_Recv.

If this sounds a bit complicated then you can consider using BOOST MPI wrappers.

Upvotes: 19

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