source - source rank (integer)
tag - communication tag (integer)
comm - communicator for message (handle)
status - status information about the message (structure)
The MPI_Probe() function is used to wait until a message is available that matches a given source , tag , and communicator, comm . A message is "available" if a function like MPI_Recv() would immediately return a result without blocking. If such a message is found, then information about it will be stored inside status . However, note that if the message is subsequently cancelled via MPI_Cancel() then a following receive will not match that message. (Probing does not prevent cancellation.)
If tag is MPI_ANY_TAG then any tag will match. Similarly, if source is MPI_ANY_SOURCE then so will a message from any source. The original tag and source are stored in the MPI_TAG and MPI_SOURCE fields of the status respectively. The MPI_ERROR field will be unconditionally set to be MPI_SUCCESS. One additional field holds the total size of the message in bytes. If status is MPI_STATUS_IGNORE then this information will not be stored.
The communicator must be a valid one (not MPI_COMM_NULL). PMPI_Probe() is the profiling version of this function.
If the source is MPI_PROC_NULL then no message is searched for, and MPI_SUCCESS is immediately returned. In this case, the status is filled with MPI_ANY_TAG MPI_PROC_NULL MPI_SUCCESS and zero size in bytes.
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_COMM - Invalid communicator;
MPI_ERR_TAG - Invalid tag;
MPI_ERR_RANK - Invalid destination.
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