MPI_BCAST(3)

💅 MPI_BCAST(3) — the Open MPI glow-up

okay so bestie, picture this: MPI_Bcast and MPI_Ibcast — the broadcast girlies of the Open MPI fam. One process (the queen, aka root) yells something, and literally every other process in the group hears it. It's giving group-chat announcement energy, honestly. 👑

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✨ SYNTAX ✨

🌸 C Syntax

#include <mpi.h>

int MPI_Bcast(void *buffer, int count, MPI_Datatype datatype,
    int root, MPI_Comm comm)

int MPI_Ibcast(void *buffer, int count, MPI_Datatype datatype,
    int root, MPI_Comm comm, MPI_Request *request)

🎀 Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_BCAST(BUFFER, COUNT, DATATYPE, ROOT, COMM, IERROR)
    <type>  BUFFER(*)
    INTEGER COUNT, DATATYPE, ROOT, COMM, IERROR

MPI_IBCAST(BUFFER, COUNT, DATATYPE, ROOT, COMM, REQUEST, IERROR)
    <type>  BUFFER(*)
    INTEGER COUNT, DATATYPE, ROOT, COMM, REQUEST, IERROR

🦋 Fortran 2008 Syntax

USE mpi_f08
MPI_Bcast(buffer, count, datatype, root, comm, ierror)
    TYPE(*), DIMENSION(..) :: buffer
    INTEGER, INTENT(IN) :: count, root
    TYPE(MPI_Datatype), INTENT(IN) :: datatype
    TYPE(MPI_Comm), INTENT(IN) :: comm
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

MPI_Ibcast(buffer, count, datatype, root, comm, request, ierror)
    TYPE(*), DIMENSION(..), ASYNCHRONOUS :: buffer
    INTEGER, INTENT(IN) :: count, root
    TYPE(MPI_Datatype), INTENT(IN) :: datatype
    TYPE(MPI_Comm), INTENT(IN) :: comm
    TYPE(MPI_Request), INTENT(OUT) :: request
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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💖 INPUT/OUTPUT PARAMETERS 💖

• buffer — starting address of the buffer (choice). yeah that's the "choice" one, iconic of the MPI spec ngl.
• count — number of entries in the buffer (integer). how many thingies we're moving, basically.
• datatype — data type of the buffer (handle).
• root — rank of the broadcast root (integer). this is the main character of the operation 🫶
• comm — communicator (handle).

🌈 OUTPUT PARAMETERS 🌈

• request — request (handle, non-blocking only). only the I-version hands you one of these, bestie.
• ierror — Fortran only: error status (integer).

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⭐ DESCRIPTION ⭐

So MPI_Bcast broadcasts a message from the process with rank root to all processes of the group — including itself, honestly, she's not gonna leave herself out of the conversation 💅. It's called by all members of the group using the same arguments for comm and root. When it returns, the contents of root's communication buffer have been copied to all processes. periodt.

General, derived datatypes are allowed for datatype. But here's the vibe check: the type signature of count, datatype on any process must be equal to the type signature of count, datatype at the root. Translation? The amount of data sent must equal the amount received, pairwise between each process and the root. MPI_Bcast and literally all the other data-movement collective routines have this same restriction, no cap. Distinct type maps between sender and receiver are still allowed though, so you've got some wiggle room 🧚

Example: broadcast 100 ints from process 0 to every process in the group.

MPI_Comm comm;
int array[100];
int root=0;
//...
MPI_Bcast( array, 100, MPI_INT, root, comm);

And as in many of the sample code fragments, we're assuming some of the variables (like comm up there) have already been assigned appropriate values. don't come for me about the uninitialized ones 🙈

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🦋 WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR 🦋

Okay this part is lowkey the plot twist. When the communicator is an inter-communicator, the root process in the first group broadcasts data to all the processes in the second group.

• The first group defines the root process. That process uses MPI_ROOT as the value of its root argument.
• The remaining processes use MPI_PROC_NULL as the value of their root argument.
• All processes in the second group use the rank of that root process in the first group as the value of their root argument.
• The receive buffer arguments of the processes in the second group must be consistent with the send buffer argument of the root process in the first group.

like, everyone has a different role and everyone's valid, it's very ensemble cast 🔥

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🎀 NOTES 🎀

This function does not support the in-place option. sorry bestie, no shortcuts here 😭

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💀 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. yeah. all of them. 😭
• 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, it 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. like, it might just be over. 💀

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.

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👑 COPYRIGHT 👑

2003-2026, The Open MPI Community

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and that's the tea, bestie 🍓 — this page is brought to you by Open MPI, dated Sep 11, 2026. go forth and broadcast responsibly 💜✨🦋