# 17.2.211. MPI_Imrecv

MPI_Imrecv — Non-blocking receive for a matched message

## 17.2.211.1. SYNTAX

### 17.2.211.1.1. C Syntax

    #include <mpi.h>

    int MPI_Imrecv(void *buf, int count, MPI_Datatype type,
         MPI_Message *message, MPI_Request *request)

### 17.2.211.1.2. Fortran Syntax

    USE MPI
    ! or the older form: INCLUDE 'mpif.h'
    MPI_IMRECV(BUF, COUNT, DATATYPE, MESSAGE, REQUEST, IERROR)
         <type>  BUF(*)
         INTEGER COUNT, DATATYPE, MESSAGE, REQUEST, IERROR

### 17.2.211.1.3. Fortran 2008 Syntax

    USE mpi_f08
    MPI_Imrecv(buf, count, datatype, message, request, ierror)
         TYPE(*), DIMENSION(..), ASYNCHRONOUS :: buf
         INTEGER, INTENT(IN) :: count
         TYPE(MPI_Datatype), INTENT(IN) :: datatype
         TYPE(MPI_Message), INTENT(INOUT) :: message
         TYPE(MPI_Request), INTENT(OUT) :: request
         INTEGER, OPTIONAL, INTENT(OUT) :: ierror

## 17.2.211.2. INPUT PARAMETERS

- `count`: Number of elements to receive (nonnegative integer).

- `datatype`: Datatype of each send buffer element (handle).

- `message`: Message (handle).

## 17.2.211.3. OUTPUT PARAMETERS

- `buf`: Initial address of receive buffer (choice).

- `request`: Request (handle).

- `ierror`: Fortran only: Error status (integer).

## 17.2.211.4. DESCRIPTION

The functions MPI_Mrecv and MPI_Imrecv receive messages that have been previously matched by a matching probe.

The request returned from MPI_Imrecv can be used with any of the MPI_Test and MPI_Wait variants, like any non-blocking receive request.

If MPI_Imrecv is called with `MPI_MESSAGE_NULL` as the message argument, a call to one of the MPI_Test or MPI_Wait variants will return immediately with the status object set to source = `MPI_PROC_NULL`, tag = `MPI_ANY_TAG`, and count = 0, as if a receive from `MPI_PROC_NULL` was issued.

If reception of a matched message is started with MPI_Imrecv, then it is possible to cancel the returned request with MPI_Cancel. If MPI_Cancel succeeds, the matched message must be found by a subsequent message probe (MPI_Probe, MPI_Iprobe, MPI_Mprobe, or MPI_Improbe), received by a subsequent receive operation or canceled by the sender.

Note, however, that is it possible for the cancellation of operations initiated with MPI_Imrecv to fail. An example of a failing case is when canceling the matched message receive would violate MPI message ordering rules (e.g., if another message matching the same message signature has matched — and possibly received — before this MPI_Imrecv is canceled).

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.

## 17.2.211.5. 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 messages received by MPI_Imrecv 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_Imrecv 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_Mprobe

- MPI_Improbe

- MPI_Probe

- MPI_Iprobe

- MPI_Imrecv

- MPI_Cancel