8.3. Associating Information with Status

In MPI-1, requests were associated with point-to-point operations. In MPI-2 there are several different types of requests. These range from new MPI calls for I/O to generalized requests. It is desirable to allow these calls use the same request mechanism. This allows one to wait or test on different types of requests. However, MPI_{TEST|WAIT}{ANY|SOME|ALL} returns a status with information about the request. With the generalization of requests, one needs to define what information will be returned in the status object.

In MPI-2, each call fills in the appropriate fields in the status object. Any unused fields will have undefined values. A call to MPI_{TEST|WAIT}{ANY|SOME|ALL} can modify any of the fields in the status object. Specifically, it can modify fields that are undefined. The fields with meaningful value for a given request are defined in the sections with the new request.

Generalized requests raise additional considerations. Here, the user provides the functions to deal with the request. Unlike other MPI calls, the user needs to provide the information to be returned in status. The status argument is provided directly to the callback function where the status needs to be set. Users can directly set the values in 3 of the 5 status values. The count and cancel fields are opaque. To overcome this, new calls are provided:

MPI_STATUS_SET_ELEMENTS(status, datatype, count)
[ INOUT status] status to associate count with (Status)
[ IN datatype] datatype associated with count (handle)
[ IN count] number of elements to associate with status (integer)

int MPI_Status_set_elements(MPI_Status *status, MPI_Datatype datatype, int count)

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

void MPI::Status::Set_elements(const MPI::Datatype& datatype, int count)

This call modifies the opaque part of status so that a call to MPI_GET_ELEMENTS will return count. MPI_GET_COUNT will return a compatible value.

[] Rationale.

The number of elements is set instead of the count because the former can deal with nonintegral number of datatypes. ( End of rationale.)
A subsequent call to MPI_GET_COUNT(status, datatype, count) or to MPI_GET_ELEMENTS(status, datatype, count) must use a datatype argument that has the same type signature as the datatype argument that was used in the call to MPI_STATUS_SET_ELEMENTS.

[] Rationale.

This is similar to the restriction that holds when when count is set by a receive operation: in that case, the calls to MPI_GET_COUNT and MPI_GET_ELEMENTS must use a datatype with the same signature as the datatype used in the receive call. ( End of rationale.)

MPI_STATUS_SET_CANCELLED(status, flag)
[ INOUT status] status to associate cancel flag with (Status)
[ IN flag] if true indicates request was cancelled (logical)

int MPI_Status_set_cancelled(MPI_Status *status, int flag)

MPI_STATUS_SET_CANCELLED(STATUS, FLAG, IERROR)
INTEGER STATUS(MPI_STATUS_SIZE), IERROR
LOGICAL FLAG

void MPI::Status::Set_cancelled(bool flag)

If flag is set to true then a subsequent call to MPI_TEST_CANCELLED(status, flag) will also return flag = true, otherwise it will return false.

[] Advice to users.

Users are advised not to reuse the status fields for values other than those for which they were intended. Doing so may lead to unexpected results when using the status object. For example, calling MPI_GET_ELEMENTS may cause an error if the value is out of range or it may be impossible to detect such an error. The extra_state argument provided with a generalized request can be used to return information that does not logically belong in status. Furthermore, modifying the values in a status set internally by MPI, e.g., MPI_RECV, may lead to unpredictable results and is strongly discouraged. ( End of advice to users.)