MPI_INTERCOMM_CREATE_FROM_GROUPS.OPENMPI(3)

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💖 MPI_INTERCOMM_CREATE_FROM_GROUPS 💖

okay bestie, welcome to the Open MPI manual, and today's main character is MPI_Intercomm_create_from_groups — the lil function that Creates a new inter-communicator from a local and remote group and stringtag ✨

🌸 SYNTAX 🌸

🎀 C Syntax

#include <mpi.h>

int MPI_Intercomm_create_from_groups(MPI_Group local_group, int local_leader, MPI_Group remote_group, int remote_leader, const char *stringtag, MPI_Info info, MPI_Errhandler errhandler, MPI_Comm *newintercomm)

🎀 Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'

MPI_INTERCOMM_CREATE_FROM_GROUPS(LOCAL_GROUP, LOCAL_LEADER, REMOTE_GROUP, REMOTE_LEADER, STRINGTAG, INFO, ERRHANDLER, NEWINTERCOMM, IERROR)
    INTEGER LOCAL_GROUP, LOCAL_LEADER, REMOTE_GROUP, REMOTE_LEADER, INFO, ERRHANDLER, NEWINTERCOMM, IERROR
    CHARACTER*(*) STRINGTAG

🎀 Fortran 2008 Syntax

USE mpi_f08

MPI_Intercomm_create_from_groups(local_group, local_leader, remote_group, remote_leader, stringtag, info, errhandler, newintercomm, ierror)
    TYPE(MPI_Group), INTENT(IN) :: local_group, remote_group
    INTEGER, INTENT(IN) :: local_leader, remote_leader
    CHARACTER(LEN=*), INTENT(IN) :: stringtag
    TYPE(MPI_Info), INTENT(IN) :: info
    TYPE(MPI_Errhandler), INTENT(IN) :: errhandler
    TYPE(MPI_Comm), INTENT(OUT) :: newintercomm
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

✨ INPUT PARAMETERS ✨

  • local_group — the local group, a handler 🫶
  • local_leader — the rank of the local group leader in local_group, an integer 💅
  • remote_group — the remote group, a handler 🌸
  • remote_leader — the rank of the remote leader in remote_group, significant only at local_leader, an integer. yes bestie, only that one process cares 👀
  • stringtag — a unique identifier for this operation, a string ⭐
  • info — an info object, a handler 🎀
  • errhandler — the error handler to be attached to the new inter-communicator, a handle 💖

💜 OUTPUT PARAMETERS 💜

  • newintercomm — the new inter-communicator, a handle ✨
  • ierror — Fortran only: the error status, an integer 🌈

🦋 DESCRIPTION 🦋

MPI_Intercomm_create_from_groups creates an inter-communicator, periodt. and like… unlike MPI_Intercomm_create, this function takes as input previously defined, disjoint local and remote groups. the calling MPI process must be a member of the local group, no exceptions 🫶. the call is collective over the union of the local and remote groups.

all involved MPI processes have to provide an identical value for the stringtag argument, fr fr. and within each group, all MPI processes have to provide identical local_group and local_leader arguments. Wildcards are NOT permitted for the remote_leader or local_leader arguments — not even a little bit, bestie 💀.

the stringtag argument serves the exact same purpose as the stringtag used in the MPI_Comm_create_from_group function: it differentiates concurrent calls in a multithreaded environment. iconic behavior honestly 👑

the stringtag shall not exceed MPI_MAX_STRINGTAG_LEN characters in length. and for C, that length includes space for a null terminating character, so budget accordingly.

in the event that MPI_GROUP_EMPTY is supplied as the local_group or remote_group1 or both — wait, yes, that's remote_group1 in the original, we keep it exactly as written, no cap — then the call is a local operation and MPI_COMM_NULL is returned as the newintercomm. cute, low-effort, slay.

🌟 NOTES 🌟

the errhandler argument specifies an error handler to be attached to the new inter-communicator. the info argument provides hints and assertions — possibly MPI implementation dependent — which indicate desired characteristics and guide communicator creation. MPI_MAX_STRINGTAG_LEN shall have a value of at least 63. yes, at least. 63 or more. write that down 📝

💀 ERRORS 💀

Important in Open MPI, this function only works properly when the MPI job is launched via mpirun / mpiexec. using this function in other run-time environments is currently unsupported, and may result in undefined behavior (e.g., the MPI job may hang). not a vibe. do not do it 🙈

almost all MPI routines return an error value; C routines as the return result of the function, and Fortran routines in the last argument. iykyk.

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 but effective 💅
  • 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, it aborts only the local process. ✨
  • MPI_ERRORS_RETURN — returns an error code to the application. the chill one 🌈

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. so like… be careful, 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. 📚

See also:

  • MPI_Comm_create_from_group 💖

2003-2026, The Open MPI Community

okay that's the whole page, you ate that fr fr 🍓 thank you for reading the Open MPI manual with me, this was the MPI_INTERCOMM_CREATE_FROM_GROUPS(3) page, dated Sep 10, from the Open MPI crew — go forth and create those inter-communicators, bestie 💅✨🦋

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