status - request status (structure)
datatype - type of data in message (handle)
count - number of elements transferred (integer).
The MPI_Get_count() function is used to determine how many elements of type datatype were transferred in a message using a successfully completed message status .
If the datatype is "empty", with no data such as MPI_UB then this function will always return a count of zero.
Otherwise the value returned will be equal to the size of the message transferred in bytes ( status->size ) divided by the size of the datatype in bytes. If this does not work out to be a whole number, then count is set to be MPI_UNDEFINED.
The C++ version of this function will pass the count as the return value.
PMPI_Get_count() is the profiling version of this function.
All MPI routines except for MPI_Wtime and MPI_Wtick return an error code. The the current MPI error handler is invoked if the return value is not MPI_SUCCESS. The default error handler aborts, but this may be changed with by using the MPI_Errhandler_set() function. The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned instead. Note that MPI does not guarentee that an MPI program can continue past an error. In this implementation, all errors except MPI_ERR_INTERN or MPI_ERR_OTHER should always be recoverable.
In C, the error code is passed as the return value. In FORTRAN, all functions have an parameter ierr which returns the error code. MPI C++ functions do not directly return an error code. However, C++ users may want to use the MPI::ERRORS_THROW_EXCEPTIONS handler. This will throw an MPI::Exception with the corresponding error code. To prevent exceptions from being raised from within C and Fortran code, they will see all error return values as MPI_ERR_PENDING when this handler is chosen. In this implementation, call MPI::throw_exception() to throw the correct exception if this occurs.
MPI_SUCCESS - No error;
MPI_ERR_PENDING - Pending exception;
MPI_ERR_TYPE - Invalid data type;
MPI_ERR_ARG - Invalid status pointer;
MPI_ERR_COUNT - Invalid count pointer.
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