.\" -*- nroff -*-
.\" Copyright 2010 Cisco Systems, Inc.  All rights reserved.
.\" Copyright 2006-2008 Sun Microsystems, Inc.
.\" Copyright (c) 1996 Thinking Machines Corporation
.\" $COPYRIGHT$
.TH MPI_Comm_size 3 "Sep 30, 2023" "4.1.6" "Open MPI"
.SH NAME
\fBMPI_Comm_size \fP \- Returns the size of the group associated with a communicator.

.SH SYNTAX
.ft R
.SH C Syntax
.nf
#include <mpi.h>
int MPI_Comm_size(MPI_Comm \fIcomm\fP, int *\fIsize\fP)

.fi
.SH Fortran Syntax
.nf
USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_COMM_SIZE(\fICOMM, SIZE, IERROR\fP)
	INTEGER	\fICOMM, SIZE, IERROR\fP

.fi
.SH Fortran 2008 Syntax
.nf
USE mpi_f08
MPI_Comm_size(\fIcomm\fP, \fIsize\fP, \fIierror\fP)
	TYPE(MPI_Comm), INTENT(IN) :: \fIcomm\fP
	INTEGER, INTENT(OUT) :: \fIsize\fP
	INTEGER, OPTIONAL, INTENT(OUT) :: \fIierror\fP

.fi
.SH C++ Syntax
.nf
#include <mpi.h>
int Comm::Get_size() const

.fi
.SH INPUT PARAMETER
.ft R
.TP 1i
comm
Communicator (handle).

.SH OUTPUT PARAMETERS
.ft R
.TP 1i
size
Number of processes in the group of comm (integer).
.ft R
.TP 1i
IERROR
Fortran only: Error status (integer).

.SH DESCRIPTION
.ft R
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.
.sp
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.

.SH NOTE
.ft R
MPI_COMM_NULL is not considered a valid argument to this function.

.SH ERRORS
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.
.sp
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.

.SH SEE ALSO
.ft R
.sp
.nf
MPI_Comm_group
MPI_Comm_rank
MPI_Comm_compare