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MPI_Comm_size(3) man page (version 1.10.0)

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MPI_Comm_size - Returns the size of the group associated with a communicator.


C Syntax

#include <mpi.h>
int MPI_Comm_size(MPI_Comm comm, int *size)

Fortran Syntax

INCLUDE ’mpif.h’

C++ Syntax

#include <mpi.h>
int Comm::Get_size() const

Input Parameter

Communicator (handle).

Output Parameters

Number of processes in the group of comm (integer).
Fortran only: Error status (integer).


This function indicates the number of processes involved in a communicator. For MPI_COMM_WORLD, it indicates the total number of processes available. This function is equivalent to accessing the communicator’s group with MPI_Comm_group, computing the size using MPI_Group_size, and then freeing the temporary group via MPI_Group_free. If the communicator is an inter-communicator (enables communication between two groups), this function returns the size of the local group. To return the size of the remote group, use the MPI_Comm_remote_size function.

This call is often used with MPI_Comm_rank to determine the amount of concurrency available for a specific library or program. MPI_Comm_rank indicates the rank of the process that calls it in the range from 0 . . . size-1, where size is the return value of MPI_Comm_size.


MPI_COMM_NULL is not considered a valid argument to this function.


Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument. C++ functions do not return errors. If the default error handler is set to MPI::ERRORS_THROW_EXCEPTIONS, then on error the C++ exception mechanism will be used to throw an MPI::Exception object.

Before the error value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error.

See Also


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