MPIGetelements

MPI_Get_elements is a function in the MPI (Message Passing Interface) standard used to determine the number of basic elements received in a message, according to a specified datatype. It is typically called after a successful probe operation (MPI_Probe or MPI_Iprobe) to learn how much data was received without performing the actual receive.

In C, the function signature is: int MPI_Get_elements(MPI_Status *status, MPI_Datatype datatype, int *elements). The status argument is the status object from a prior probe or nonblocking probe, datatype is the datatype used in the corresponding receive, and elements is an output parameter that receives the number of basic elements in the message. For Fortran, the interface is MPI_GET_ELEMENTS(STATUS, DATATYPE, ELEMENTS, IERROR).

The value returned in elements represents the number of basic elements contained in the received message, not the number of datatype instances. For example, receiving five doubles yields 5; receiving three blocks of MPI_2INT yields 6 basic elements (two integers per block). If a derived datatype is used, the count reflects the total count of its basic elements.

A variant for large counts is MPI_Get_elements_x, which uses MPI_Count for the elements parameter to avoid overflow on very large messages; this is available in MPI-3 and later.

See also MPI_Probe, MPI_Iprobe, and MPI_Get_count for related mechanisms to query message size and contents before or during receipt.