MPI_RECV(3)

📦
version 5.0.10 · 7 versions
, in ubuntu stonking

💖 NAME 💖

MPI_Recv — the standard-mode blocking receive, bestie. This is page 3 of the Open MPI manual pages (so, MPI_RECV(3), iykyk), and it's literally the moment your little process sits there and waits for its mail to arrive. 🎀✨

🌈 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 — the maximum number of elements you're willing to receive (an integer, bestie, it's giving "set a limit" energy). 🍓
  • datatype — the datatype of each entry in the receive buffer (a handle).
  • source — the rank of whoever's sending (an integer).
  • tag — the message tag (an integer). This is how you tell your messages apart, no cap.
  • comm — the communicator (a handle).

📤 OUTPUT PARAMETERS 📤

  • buf — the initial address of the receive buffer (a choice).
  • status — the status object (status).
  • ierror — Fortran only: the error status (an integer).

🌸 DESCRIPTION 🌸

So MPI_Recv, this basic receive operation, is blocking: it does not come back until the receive buffer actually contains the newly received message. Periodt. 💅

Fun lil fact though: a receive can complete before the matching send has completed — of course, it can only complete after the matching send has started. Timing is everything, bestie. 😭

The blocking semantics of this call are described in the “Communication Modes” section of the MPI Standard at https://www.mpi-forum.org/docs/.

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

Heads up: the length of the received message must be less than or equal to the length of the receive buffer. If you overflow? An MPI_ERR_TRUNCATE is returned upon the overflow condition. Not the vibe. 💀

If a message that's shorter than the length of the receive buffer arrives, then only those locations corresponding to the (shorter) received message are modified. Iconic, actually — it minds its own business.

🎀 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 MPI_Get_count to find out the actual number of entries inside that message. 🕵️

To receive messages of unknown length, use MPI_Probe. For more info about MPI_Probe and MPI_Cancel, go read their respective man pages and the “Probe and Cancel” section of the MPI Standard at https://www.mpi-forum.org/docs/.

A message can be received by a receive operation only if it's addressed to the receiving process, and if its source, tag, and communicator (comm) values match the source, tag, and comm values the receive operation specified. The receive operation may specify a wildcard value for source and/or tag, meaning any source and/or any tag is 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 it's different from MPI_ANY_SOURCE, is specified as a rank within the process group associated with that same communicator (remote process group, for intercommunicators). So 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.

Now note the asymmetry between send and receive operations, because bestie this is the whole plot twist: a receive operation may accept messages from an arbitrary sender; but a send operation must specify a unique receiver. This matches a “push” communication mechanism, where the data transfer is effected by the sender (rather than a “pull” mechanism, where data transfer is effected by the receiver). The sender is giving, the receiver is receiving. That's the dynamic. 💅

Source = destination is allowed, so yes, a process can send a message to itself. BUT — and this is a big but — 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. 😭 Please don't. See the “Semantics of Point-to-Point Communication” of the MPI Standard at https://www.mpi-forum.org/docs/ 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. Lowkey the move. 💖

💀 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. 🫠 Just so you know, bestie.

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 at https://www.mpi-forum.org/docs/, 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

That's the whole page, bestie! This was MPI_RECV(3) from the Open MPI manual pages, copyright 2003-2026, The Open MPI Community, dated Sep 10, 2026. Slay, go receive your messages. 🦋✨🎀

📦 where this page comes from