MPI_COMM_SIZE(3)				     Open MPI					  MPI_COMM_SIZE(3)

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

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Comm_size(MPI_Comm comm, int *size)

   Fortran Syntax
	  USE MPI
	  ! or the older form: INCLUDE 'mpif.h'
	  MPI_COMM_SIZE(COMM, SIZE, IERROR)
	       INTEGER COMM, SIZE, IERROR

   Fortran 2008 Syntax
	  USE mpi_f08
	  MPI_Comm_size(comm, size, ierror)
	       TYPE(MPI_Comm), INTENT(IN) :: comm
	       INTEGER, INTENT(OUT) :: size
	       INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETER
       • comm: Communicator (handle).

OUTPUT PARAMETERS
       • size: Number of processes in the group of comm (integer).

       • ierror: Fortran only: Error status (integer).

DESCRIPTION
       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  <#mpi-comm-group>,	computing the size using MPI_Group_size <#
       mpi-group-size>, and then freeing  the  temporary  group	 via  MPI_Group_free  <#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 <#
       mpi-comm-remote-size> function.

       This call is often used	with  MPI_Comm_rank  <#mpi-comm-rank>  to  determine  the  amount  of  concurrency
       available  for  a  specific  library  or	 program. MPI_Comm_rank <#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.

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

ERRORS
       Almost all MPI routines return an error value; C routines as the return result of the function and  Fortran
       routines in the last argument.

       Before  the error value is returned, the current MPI error handler associated with the communication object
       (e.g., communicator, window, file) is called.  If no communication object is associated with the MPI  call,
       then  the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When
       MPI_COMM_SELF is not initialized (i.e.,	before	MPI_Init  <#mpi-init>/MPI_Init_thread  <#mpi-init-thread>,
       after  MPI_Finalize  <#mpi-finalize>,  or  when	using the Sessions Model exclusively) the error raises the
       initial error handler. The initial error handler can  be	 changed  by  calling  MPI_Comm_set_errhandler	<#
       mpi-comm-set-errhandler>	 on  MPI_COMM_SELF  when  using the World model, or the mpi_initial_errhandler CLI
       argument	 to  mpiexec  or  info	key   to   MPI_Comm_spawn   <#mpi-comm-spawn>/MPI_Comm_spawn_multiple	<#
       mpi-comm-spawn-multiple>.   If  no other appropriate error handler has been set, then the MPI_ERRORS_RETURN
       error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error  handler  is  called  for  all
       other MPI functions.

       Open MPI includes three predefined error handlers that can be used:

       • MPI_ERRORS_ARE_FATAL Causes the program to abort all connected MPI processes.

       • MPI_ERRORS_ABORT  An  error handler that can be invoked on a communicator, window, file, or session. When
	 called on a communicator, it acts as if MPI_Abort <#mpi-abort> was called on that communicator. If called
	 on a window or file, acts as if MPI_Abort <#mpi-abort> was called on a communicator containing the  group
	 of processes in the corresponding window or file. If called on a session, aborts only the local process.

       • MPI_ERRORS_RETURN Returns an error code to the application.

       MPI applications can also implement their own error handlers by calling:

       • MPI_Comm_create_errhandler	 <#mpi-comm-create-errhandler>	   then	    MPI_Comm_set_errhandler	<#
	 mpi-comm-set-errhandler>

       • MPI_File_create_errhandler	<#mpi-file-create-errhandler>	  then	   MPI_File_set_errhandler	<#
	 mpi-file-set-errhandler>

       • MPI_Session_create_errhandler	 <#mpi-session-create-errhandler>   then   MPI_Session_set_errhandler	<#
	 mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>

       • MPI_Win_create_errhandler	<#mpi-win-create-errhandler>	  then	    MPI_Win_set_errhandler	<#
	 mpi-win-set-errhandler>

       Note that MPI does not guarantee that an MPI program can continue past an error.

       See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

       See the Error Handling section of the MPI-3.1 standard for more information.

       See also:

	  • MPI_Comm_group <#mpi-comm-group>

	  • MPI_Comm_rank <#mpi-comm-rank>

	  • MPI_Comm_compare <#mpi-comm-compare>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026					  MPI_COMM_SIZE(3)
