18.2.290. MPI_Recv — Open MPI 5.0.1 documentation

# 18.2.290. MPI_Recv

MPI_Recv — Performs a standard-mode blocking receive.

## 18.2.290.1. SYNTAX

### 18.2.290.1.1. C Syntax

    #include <mpi.h>

    int MPI_Recv(void *buf, int count, MPI_Datatype datatype,
         int source, int tag, MPI_Comm comm, MPI_Status *status)

### 18.2.290.1.2. Fortran Syntax

    USE MPI
    ! or the older form: INCLUDE 'mpif.h'
    MPI_RECV(BUF, COUNT, DATATYPE, SOURCE, TAG, COMM, STATUS, IERROR)
         <type>  BUF(*)
         INTEGER COUNT, DATATYPE, SOURCE, TAG, COMM
         INTEGER STATUS(MPI_STATUS_SIZE), IERROR

### 18.2.290.1.3. Fortran 2008 Syntax

    USE mpi_f08
    MPI_Recv(buf, count, datatype, source, tag, comm, status, ierror)
         TYPE(*), DIMENSION(..) :: buf
         INTEGER, INTENT(IN) :: count, source, tag
         TYPE(MPI_Datatype), INTENT(IN) :: datatype
         TYPE(MPI_Comm), INTENT(IN) :: comm
         TYPE(MPI_Status) :: status
         INTEGER, OPTIONAL, INTENT(OUT) :: ierror

## 18.2.290.2. INPUT PARAMETERS

- `count`: Maximum number of elements to receive (integer).

- `datatype`: Datatype of each receive buffer entry (handle).

- `source`: Rank of source (integer).

- `tag`: Message tag (integer).

- `comm`: Communicator (handle).

## 18.2.290.3. OUTPUT PARAMETERS

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

- `status`: Status object (status).

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

## 18.2.290.4. DESCRIPTION

This basic receive operation, MPI_Recv, is blocking: it returns only after the receive buffer contains the newly received message. A receive can complete before the matching send has completed (of course, it can complete only after the matching send has started).

The blocking semantics of this call are described in the “Communication Modes” section of the MPI Standard.

The receive buffer contains a number (defined by the value of count) of consecutive elements. The first element in the set of elements is located at address_buf. The type of each of these elements is specified by datatype.

The length of the received message must be less than or equal to the length of the receive buffer. An MPI_ERR_TRUNCATE is returned upon the overflow condition.

If a message that is shorter than the length of the receive buffer arrives, then only those locations corresponding to the (shorter) received message are modified.

## 18.2.290.5. NOTES

The count argument indicates the maximum number of entries of type datatype that can be received in a message. Once a message is received, use the MPI_Get_count function to determine the actual number of entries within that message.

To receive messages of unknown length, use the MPI_Probe function. For more information about MPI_Probe and MPI_Cancel, see their respective man pages and the “Probe and Cancel” section of the MPI Standard.

A message can be received by a receive operation only if it is addressed to the receiving process, and if its source, tag, and communicator (comm) values match the source, tag, and comm values specified by the receive operation. The receive operation may specify a wildcard value for source and/or tag, indicating that any source and/or tag are acceptable. The wildcard value for source is source = `MPI_ANY_SOURCE`. The wildcard value for tag is tag = `MPI_ANY_TAG`. There is no wildcard value for comm. The scope of these wildcards is limited to the processes in the group of the specified communicator.

The message tag is specified by the tag argument of the receive operation.

The argument source, if different from `MPI_ANY_SOURCE`, is specified as a rank within the process group associated with that same communicator (remote process group, for intercommunicators). Thus, the range of valid values for the source argument is {0,…,n-1} {`MPI_ANY_SOURCE`}, where n is the number of processes in this group.

Note the asymmetry between send and receive operations: A receive operation may accept messages from an arbitrary sender; on the other hand, a send operation must specify a unique receiver. This matches a “push” communication mechanism, where data transfer is effected by the sender (rather than a “pull” mechanism, where data transfer is effected by the receiver).

Source = destination is allowed, that is, a process can send a message to itself. However, it is not recommended for a process to send messages to itself using the blocking send and receive operations described above, since this may lead to deadlock. See the “Semantics of Point-to-Point Communication” of the MPI Standard for more details.

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.

## 18.2.290.6. 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_Recv 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_Recv 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_Irecv

- MPI_Probe