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SYNTAX
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)
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
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
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).
OUTPUT PARAMETERS
• buf: Initial address of receive buffer (choice).
• status: Status object (status).
• ierror: Fortran only: Error status (integer).
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.
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.
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
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2003-2025, The Open MPI Community
Feb 17, 2025 MPI_RECV(3)