# 17.2.169. MPI_Get_count

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

## 17.2.169.1. SYNTAX

### 17.2.169.1.1. C Syntax

    #include <mpi.h>

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

### 17.2.169.1.2. 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

### 17.2.169.1.3. 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

## 17.2.169.2. INPUT PARAMETERS

- `status` : Return status of receive operation (status).

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

## 17.2.169.3. OUTPUT PARAMETERS

- `count` : Number of received elements (integer).

- `ierror` : Fortran only: Error status (integer).

## 17.2.169.4. 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. 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.

## 17.2.169.5. 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.

## 17.2.169.6. 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_thread, after 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 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_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 was called on that communicator. If called on a window or file, acts as if 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 then MPI_Comm_set_errhandler

- MPI_File_create_errhandler then MPI_File_set_errhandler

- MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init

- MPI_Win_create_errhandler then MPI_Win_set_errhandler

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

See the MPI man page for a full list of MPI error codes.

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