MPI_IBCAST(3)

🦋 MPI_IBCAST 💅

okay so bestie, we're in the Open MPI manual, section 3, and this page is literally about MPI_Bcast and MPI_Ibcast — the two glow-up versions of "one process yells, everybody hears it." ✨

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🌸 NAME 🌸

MPI_Bcast, MPI_Ibcast — Broadcasts a message from the process with rank root to all other processes of the group. 📣

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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 buffer (choice). it's giving "the goods, going both ways" 🫶
• count — Number of entries in buffer (integer). count them, bestie 🧮
• datatype — Data type of buffer (handle).
• root — Rank of broadcast root (integer). the main character of this whole situation 👑
• comm — Communicator (handle).

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💜 OUTPUT PARAMETERS 💜

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

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🌈 DESCRIPTION 🌈

MPI_Bcast broadcasts a message from the process with rank root to all processes of the group, itself included. So yes, the root gets its own message back. no FOMO in this group chat 💅 Every member of the group calls it, and they all use the same arguments for comm and root. On return, the contents of root's communication buffer have been copied to all processes. slay. ✨

General, derived datatypes are allowed for datatype. The type signature of count, datatype on any process must be equal to the type signature of count, datatype at the root. Which implies the amount of data sent must equal the amount received, pairwise between each process and the root. MPI_Bcast and all other data-movement collective routines make this restriction — literally all of them, no exceptions. Distinct type maps between sender and receiver are still allowed, though, which is kind of iconic of them. 🦋

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);

As in many of our sample code fragments, we assume that some of the variables (such as comm in the example above) have been assigned appropriate values. (bestie we are NOT declaring everything, we're keeping it cute 😭)

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🌟 WHEN COMMUNICATOR IS AN INTER-COMMUNICATOR 🌟

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. three different root values, three different roles — vibe check passed ✅

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💀 NOTES 💀

This function does not support the in-place option. nope. not a thing. don't try it 🚫

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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. dramatic much? 💀
• 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. fr fr, this is the real warning ⚠️

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 whole page, bestie! 🫶 this dropped Sep 10, 2026, straight from the Open MPI squad — go forth and broadcast responsibly, MPI_IBCAST(3) out 💖✨🎀