Basics of MPI Programming: Discussion: MPI

CTC Discussion on MPI Basics: Six Basic Calls

1.2.4 Sending a message

In MPI, there are many flavors of sends and receives. This is one reason why there are over 125 routines provided in MPI. In our basic set of six calls, we will look at just one type of send and one type of receive.

MPI_SEND is a blocking send. This means the call does not return control to your program until the data have been copied from the location you specify in the parameter list. Because of this, you can change the data after the call and not affect the original message. (There are non-blocking sends where this is not the case.)

C
- int MPI_Send(void *buf, int count, MPI_Datatype datatype, int dest, int tag, MPI_Comm comm)
Fortran
- MPI_SEND(buf, count, datatype, dest, tag, comm, ierror)
  <type> buf(*)
  integer count, datatype, dest, tag, comm, ierror

The parameters:

buf is the beginning of the buffer containing the data to be sent. For Fortran, this is often the name of an array in your program. For C, it is an address.
count is the number of elements to be sent (not bytes)
datatype is the type of data
dest is the rank of the process which is the destination for the message
tag is an arbitrary number which can be used to distinguish among messages
comm is the communicator
ierror is a return error code

These will be discussed in more detail later in the module.