MPI\_WAIT(3) Open MPI MPI\_WAIT(3) MPI\_Wait — Waits for an MPI send or receive to complete. # [SYNTAX](#SYNTAX) ## [C Syntax](#C_Syntax) ``` #include int MPI_Wait(MPI_Request *request, MPI_Status *status) ``` ## [Fortran Syntax](#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](#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](#INPUT_PARAMETER) • **request**: Request (handle). # [OUTPUT PARAMETERS](#OUTPUT_PARAMETERS) - **status**: Status object (status). - **ierror**: Fortran only: Error status (integer). # [DESCRIPTION](#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 <https://www.mpi-forum.org/docs/>. The status object for a send operation may be queried by a call to MPI\_Test\_cancelled <#mpi-test-cancelled> (see the “Probe and Cancel” section in the MPI Standard <https://www.mpi-forum.org/docs/>). 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](#NOTES) Successful return of MPI\_Wait after an MPI\_Ibsend <#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 <#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 <https://www.mpi-forum.org/docs/>). If a matching receive is never posted, then the buffer cannot be freed. This runs somewhat counter to the stated goal of MPI\_Cancel <#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](#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>/MPI\_Init\_thread <#mpi-init-thread>, after MPI\_Finalize <#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 <#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>/MPI\_Comm\_spawn\_multiple <#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 <#mpi-abort> was called on that communicator. If called on a window or file, acts as if MPI\_Abort <#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 <#mpi-comm-create-errhandler> then MPI\_Comm\_set\_errhandler <#mpi-comm-set-errhandler> - MPI\_File\_create\_errhandler <#mpi-file-create-errhandler> then MPI\_File\_set\_errhandler <#mpi-file-set-errhandler> - MPI\_Session\_create\_errhandler <#mpi-session-create-errhandler> then MPI\_Session\_set\_errhandler <#mpi-session-set-errhandler> or at MPI\_Session\_init <#mpi-session-init> - MPI\_Win\_create\_errhandler <#mpi-win-create-errhandler> then MPI\_Win\_set\_errhandler <#mpi-win-set-errhandler> Note that MPI does not guarantee that an MPI program can continue past an error. See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>. 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 <https://www.mpi-forum.org/docs/>, 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-comm-set-errhandler> - MPI\_File\_set\_errhandler <#mpi-file-set-errhandler> - MPI\_Test <#mpi-test> - MPI\_Testall <#mpi-testall> - MPI\_Testany <#mpi-testany> - MPI\_Testsome <#mpi-testsome> - MPI\_Waitall <#mpi-waitall> - MPI\_Waitany <#mpi-waitany> - MPI\_Waitsome <#mpi-waitsome> - MPI\_Win\_set\_errhandler <#mpi-win-set-errhandler> # [Copyright](#Copyright) 2003-2026, The Open MPI Community November 1, 2026 --- Document: MPI_Wait(3) from openmpi-cuda-12.9 5.0.11-r1 (Chainguard OS) Source: https://github.com/open-mpi/ompi