MPI_Wait - Waits for an MPI send or receive to complete.
18.2.423.1.1. C Syntax
#include <mpi.h> int MPI_Wait(MPI_Request *request, MPI_Status *status)
18.2.423.1.2. Fortran Syntax
USE MPI ! or the older form: INCLUDE 'mpif.h' MPI_WAIT(REQUEST, STATUS, IERROR) INTEGER REQUEST, STATUS(MPI_STATUS_SIZE), IERROR
18.2.423.1.3. Fortran 2008 Syntax
USE mpi_f08 MPI_Wait(request, status, ierror) TYPE(MPI_Request), INTENT(INOUT) :: request TYPE(MPI_Status) :: status INTEGER, OPTIONAL, INTENT(OUT) :: ierror
18.2.423.2. INPUT PARAMETER
request: Request (handle).
18.2.423.3. OUTPUT PARAMETERS
status: Status object (status).
ierror: Fortran only: Error status (integer).
A call to MPI_Wait returns when the operation identified by request is
complete. If the communication object associated with this request was
created by a nonblocking send or receive call, then the object is
deallocated by the call to MPI_Wait and the request handle is set to
The call returns, in status, information on the completed operation. The content of the status object for a receive operation can be accessed as described in Section 3.2.5 of the MPI-1 Standard, “Return Status.” The status object for a send operation may be queried by a call to MPI_Test_cancelled (see Section 3.8 of the MPI-1 Standard, “Probe and Cancel”).
If your application does not need to examine the status field, you can
save resources by using the predefined constant
MPI_STATUS_IGNORE as a
special value for the status argument.
One is allowed to call MPI_Wait with a null or inactive request argument. In this case the operation returns immediately with empty status.
Successful return of MPI_Wait after an MPI_Ibsend implies that the user send buffer can be reused i.e., data has been sent out or copied into a buffer attached with MPI_Buffer_attach. Note that, at this point, we can no longer cancel the send (for more information, see Section 3.8 of the MPI-1 Standard, “Probe and Cancel”). If a matching receive is never posted, then the buffer cannot be freed. This runs somewhat counter to the stated goal of MPI_Cancel (always being able to free program space that was committed to the communication subsystem).
Example: Simple usage of nonblocking operations and MPI_Wait.
CALL MPI_COMM_RANK(comm, rank, ierr) IF(rank == 0) THEN CALL MPI_ISEND(a(1), 10, MPI_REAL, 1, tag, comm, request, ierr) **** do some computation **** CALL MPI_WAIT(request, status, ierr) ELSE IF (rank == 1) THEN CALL MPI_IRECV(a(1), 15, MPI_REAL, 0, tag, comm, request, ierr) **** do some computation **** CALL MPI_WAIT(request, status, ierr) END IF
Almost all MPI routines return an error value; C routines as the return result of the function and Fortran routines in the last argument.
Before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. If no communication object is associated with the MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When MPI_COMM_SELF is not initialized (i.e., before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler. The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all other MPI functions.
Open MPI includes three predefined error handlers that can be used:
MPI_ERRORS_ARE_FATALCauses the program to abort all connected MPI processes.
MPI_ERRORS_ABORTAn error handler that can be invoked on a communicator, window, file, or session. When called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a window or file, acts as if MPI_Abort was called on a communicator containing the group of processes in the corresponding window or file. If called on a session, aborts only the local process.
MPI_ERRORS_RETURNReturns an error code to the application.
MPI applications can also implement their own error handlers by calling:
Note that MPI does not guarantee that an MPI program can continue past an error.
See the Error Handling section of the MPI-3.1 standard for more information.
Note that per MPI-1 section 3.2.5, MPI errors on requests passed to
MPI_Wait do not set the
status.MPI_ERROR field in the returned status.
The error code is passed to the back-end error handler and may be passed
back to the caller through the return value of MPI_Wait if the back-end
error handler returns it. The pre-defined MPI error handler
MPI_ERRORS_RETURN exhibits this behavior, for example.