Every call to MPI_RECV includes a status argument, wherein the system can return details about the message received. There are also a number of other MPI calls where status is returned. An object of type MPI_Statusis not an MPI opaque object; its structure is declared in mpi.h and (deprecated) mpif.h, and it exists in the user's program. In many cases, application programs are constructed so that it is unnecessary for them to examine the status fields. In these cases, it is a waste for the user to allocate a status object, and it is particularly wasteful for the MPI implementation to fill in fields in this object.
To cope with this problem, there are two predefined constants, MPI_STATUS_IGNORE and MPI_STATUSES_IGNORE, which when passed to a receive, probe, wait, or test function, inform the implementation that the status fields are not to be filled in. Note that MPI_STATUS_IGNORE is not a special type of MPI_Status object; rather, it is a special value for the argument. In C one would expect it to be NULL, not the address of a special MPI_Status.
MPI_STATUS_IGNORE, and the array version MPI_STATUSES_IGNORE, can be used everywhere a status argument is passed to a receive, wait, or test function. MPI_STATUS_IGNORE cannot be used when status is an IN argument. Note that in Fortran MPI_STATUS_IGNORE and MPI_STATUSES_IGNORE are objects like MPI_BOTTOM (not usable for initialization or assignment), see Section Named Constants.
In general, this optimization can apply to all functions for which status or an array of statuses is an OUT argument. Note that this converts status into an INOUT argument. The functions that can be passed MPI_STATUS_IGNORE are all the various forms of MPI_RECV, MPI_PROBE, MPI_TEST, and MPI_WAIT, as well as MPI_REQUEST_GET_STATUS. When an array is passed, as in the MPI_{ TEST|WAIT}{ ALL|SOME} functions, a separate constant, MPI_STATUSES_IGNORE, is passed for the array argument. It is possible for an MPI function to return MPI_ERR_IN_STATUS even when MPI_STATUS_IGNORE or MPI_STATUSES_IGNORE has been passed to that function.
MPI_STATUS_IGNORE and MPI_STATUSES_IGNORE are not required to have the same values in C and Fortran.
It is not allowed to have some of the statuses in an array of statuses for MPI_{ TEST|WAIT}{ ALL|SOME} functions set to MPI_STATUS_IGNORE; one either specifies ignoring all of the statuses in such a call with MPI_STATUSES_IGNORE, or none of them by passing normal statuses in all positions in the array of statuses.