R. Necaise | ParallelComputing / MPI Function Reference

<< Basic Message Passing | Table of Contents | >>
The Message Passing Interface library contains many functions and data types for use in creating parallel programs. This section provides a reference for the more common functions and data types. The reference is divided into the following groups.

Preliminaries
Message Passing
Data Types

Preliminaries

(:begfnct mpi_init:)

int MPI_Init( int *argc, char **argv[] )

Initializes the MPI environment. This routine must be the first MPI function called by each process in the virtual computer.

argc: the value of argc from main().
argv: the value of argv from main().

(:endfnct:)

(:begfnct mpi_finalize:)

int MPI_Finalize()

Terminates the MPI environment and should be called by at the end of the program by every process in the virtual computer. (:endfnct:)

(:begfnct mpi_rank:)

int MPI_Comm_rank( MPI_Comm comm, int *pid )

Used by a process to determine its rank or process id number within a communicator.

comm: the communicator to be examined.
pid: the process id number is stored in the location pointed to by pid.

(:endfnct:)

(:begfnct mpi_size:)

int MPI_Comm_size( MPI_Comm comm, int *size )

Used by a process to determine the total number of processes associated with a given communicator.

comm: the communicator to be examined.
size: the number of processes is stored in the integer variable at the given reference.

(:endfnct:)

(:begfnct mpi_barrier:)

int MPI_Barrier( MPI_Comm comm )

Blocks all processes in the given communicator group until all processes have executed the function call. (:endfnct:)

(:begfnct mpi_wtime:)

double MPI_Wtime()

Returns the elapsed time, in seconds, from some point in the past. (:endfnct:)

Message Passing

(:begfnct mpi_bcast:)

int MPI_Bcast( void *buf, int count, MPI_Datatype datatype,

int srcPid, MPI_Comm comm )

(:endfnct:)

(:begfnct mpi_send:)

int MPI_Send( void *buf, int count, MPI_Datatype datatype,

int destPid, int tag, MPI_Comm comm )

(:endfnct:)

(:begfnct mpi_recv:)

int MPI_Recv( void *buf, int count, MPI_Datatype datatype,

int srcPid, int tag, MPI_Comm comm, MPI_Status *status )

srcPid: The id of the process from whom you want to receive a message. This can be set to MPI_ANY_SOURCE to receive a message from any process within the give communications group.
tag: A code indicating the type of message to be received. This can be set to MPI_ANY_TAG to receive a message with any type of tag associated with it.

(:endfnct:)

(:begfnct mpi_reduce:)

int MPI_Reduce( void *sendbuf, void *recvbuf, int count,

MPI_Datatype datatype, MPI_Op op, int destPid, MPI_Comm comm )

(:endfnct:)

(:begfnct mpi_probe:)

int MPI_Probe( int srcPid, int tag, MPI_Comm comm, MPI_Status *status );

Blocks and waits for a message with the given data tag from the indicated process. The function does not actually receive the message. It can be used in conjunction with MPI_Get_count().

srcPid: the id of the process from which you are waiting for a message.
tag: the tag associated with the type of message for which you are waiting.
comm: the communications group being probed.
status: address of a struct into which information related to the message can be stored.

(:endfnct:)

(:begfnct mpi_getcount:)

int MPI_Get_count( MPI_Status *status, MPI_Datatype datatype, int *count )

Determines the number of data items contained in a message just received or the next message to be received. This function is used with either MPI_Recv() or MPI_Probe().

status: the address of the MPI_Status struct filled in by either the MPI_Recv() or MPI_Probe() function.
datatype: the type of data items being received.
count: the address of an integer variable into which the number of items received is stored.

The following code segment illustrates the use of this function

(:source lang=cpp:)

MPI_Status msgStatus;
int numItems;

MPI_Probe( ROOT_PROC, DATA_TAG, MPI_COMM_WORLD, &msgStatus );
MPI_Get_count( &msgStatus, MPI_DOUBLE, &numItems );

MPI Data Types

(:begfnct mpi_comm:)

MPI_Comm

Indicates the process communicator group. The most common entry for a parameter of this type is MPI_COMM_WORLD. (:endfnct:)

(:begfnct mpi_datatype:)

MPI_Datatype

Indicates the type of data being transmitted using one of the MPI message passing routines. An MPI_Datatype argument should be one of the following which represents a common C-language data type.

  MPI_CHAR
  MPI_DOUBLE
  MPI_FLOAT
  MPI_INT
  MPI_LONG
  MPI_LONG_DOUBLE
  MPI_LONG_LONG
  MPI_SHORT
  MPI_UNSIGNED_CHAR
  MPI_UNSIGNED_INT
  MPI_UNSIGNED_LONG
  MPI_UNSIGNED_SHORT
  MPI_UNSIGNED_LONG_LONG

(:endfnct:)

(:begfnct mpi_op:)

MPI_OP

Specifies the type of reduce operation to be performed by the MPI_Reduce() function. Legal values for this data type include

`MPI_MAX`	Select the maximum value.
`MPI_MIN`	Select the minimum value.
`MPI_PROD`	Compute the product of the values.
`MPI_SUM`	Compute the sum of the values.

(:endfnct:)
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