MPI_GET_COUNT(3)				     Open MPI					  MPI_GET_COUNT(3)

MPI_Get_count — Gets the number of top-level elements received.

SYNTAX
   C Syntax
	  #include <mpi.h>

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

   Fortran Syntax
	  USE MPI
	  ! or the older form: INCLUDE 'mpif.h'

	  MPI_GET_COUNT(STATUS, DATATYPE, COUNT, IERROR)
	      INTEGER STATUS(MPI_STATUS_SIZE), DATATYPE, COUNT, IERROR

   Fortran 2008 Syntax
	  USE mpi_f08

	  MPI_Get_count(status, datatype, count, ierror)
	      TYPE(MPI_Status), INTENT(IN) :: status
	      TYPE(MPI_Datatype), INTENT(IN) :: datatype
	      INTEGER, INTENT(OUT) :: count
	      INTEGER, OPTIONAL, INTENT(OUT) :: ierror

INPUT PARAMETERS
       • status : Return status of receive operation (status).

       • datatype : Datatype of each receive buffer element (handle).

OUTPUT PARAMETERS
       • count : Number of received elements (integer).

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

DESCRIPTION
       Returns	the number of entries received. (We count entries, each of type datatype, not bytes.) The datatype
       argument should match the argument provided by the receive call that set the status variable. As	 explained
       in  the “Use of General Datatypes in Communication” section of the MPI Standard <https://www.mpi-forum.org/
       docs/>.	MPI_Get_count may, in certain situations, return the value MPI_UNDEFINED.

       The datatype argument is passed to MPI_Get_count to improve  performance.   A  message  might  be  received
       without	counting  the  number of elements it contains, and the count value is often not needed. Also, this
       allows the same function to be used after a call to MPI_Probe <#mpi-probe>.

NOTES
       If the size of the datatype is zero, this routine will return a count of zero. If the  amount  of  data	in
       status  is  not an exact multiple of the size of datatype (so that count would not be integral), a count of
       MPI_UNDEFINED is returned instead.

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.

       If  the	value to be returned is larger than can fit into the count parameter, an MPI_ERR_TRUNCATE error is
       raised.

       See also:
	  MPI_Get_elements <#mpi-get-elements>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026					  MPI_GET_COUNT(3)
