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SYNTAX
C Syntax
#include <mpi.h>
int MPI_Wait(MPI_Request *request, MPI_Status *status)
Fortran Syntax
USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_WAIT(REQUEST, STATUS, IERROR)
INTEGER REQUEST, STATUS(MPI_STATUS_SIZE), IERROR
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
INPUT PARAMETER
• request: Request (handle).
OUTPUT PARAMETERS
• status: Status object (status).
• ierror: Fortran only: Error status (integer).
DESCRIPTION
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 MPI_REQUEST_NULL.
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 the “Return Status” subsection of the “Point-to-Point
Communication” chapter in the MPI Standard. The status object for a send operation may be queried by a
call to MPI_Test_cancelled (see the “Probe and Cancel” section in the MPI Standard).
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.
NOTES
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 the “Probe and Cancel” section in the MPI
Standard). 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
ERRORS
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_FATAL Causes the program to abort all connected MPI processes.
• MPI_ERRORS_ABORT An 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_RETURN Returns an error code to the application.
MPI applications can also implement their own error handlers by calling:
• MPI_Comm_create_errhandler then MPI_Comm_set_errhandler
• MPI_File_create_errhandler then MPI_File_set_errhandler
• MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init
• MPI_Win_create_errhandler then MPI_Win_set_errhandler
Note that MPI does not guarantee that an MPI program can continue past an error.
See the MPI man page for a full list of MPI error codes.
See the Error Handling section of the MPI-3.1 standard for more information.
Note that per the “Return Status” section in the “Point-to-Point Communication” chapter in the MPI
Standard, MPI errors on requests passed to MPI_Wait do not set the status.MPI_ERROR field in the returned
status. The error code is always 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.
SEE ALSO:
• MPI_Comm_set_errhandler
• MPI_File_set_errhandler
• MPI_Test
• MPI_Testall
• MPI_Testany
• MPI_Testsome
• MPI_Waitall
• MPI_Waitany
• MPI_Waitsome
• MPI_Win_set_errhandler
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Feb 17, 2025 MPI_WAIT(3)