MPI_COMM_CREATE(3)

💖 NAME 💖

MPI_Comm_create — Creates a new communicator. This is the Open MPI manual, bestie: MPI_COMM_CREATE(3) energy from the jump. ✨

---

🧚 SYNTAX 🧚

C Syntax

#include <mpi.h>

int MPI_Comm_create(MPI_Comm comm, MPI_Group group, MPI_Comm *newcomm)

Fortran Syntax

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

MPI_COMM_CREATE(COMM, GROUP, NEWCOMM, IERROR)
    INTEGER COMM, GROUP, NEWCOMM, IERROR

Fortran 2008 Syntax

USE mpi_f08

MPI_Comm_create(comm, group, newcomm, ierror)
    TYPE(MPI_Comm), INTENT(IN) :: comm
    TYPE(MPI_Group), INTENT(IN) :: group
    TYPE(MPI_Comm), INTENT(OUT) :: newcomm
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

---

🎀 INPUT PARAMETER 🎀

• comm — Communicator (handle). The main character communicator, periodt.
• group — Group, which is a subset of the group of comm (handle). It’s the VIP list, and yes it has to be a subset of comm’s group.

---

🌸 OUTPUT PARAMETERS 🌸

• newcomm — New communicator (handle). This is the new sub-chat handle.
• ierror — Fortran only: Error status (integer).

---

🦋 DESCRIPTION 🦋

Okay so MPI_Comm_create makes a new communicator called newcomm with the communication group defined by group and a brand-new context. It sets newcomm to a new communicator that spans all the processes that are in the group. For processes that are not in the group, it sets newcomm to MPI_COMM_NULL. 💀

Each process must call with a group argument that is a subgroup of the group associated with comm; this could be MPI_GROUP_EMPTY. The processes may specify different values for the group argument. If a process calls with a non-empty group, then all processes in that group must call the function with the same group as argument, that is: the same processes in the same order. Otherwise the call is erroneous. 🚨

---

🌈 NOTES 🌈

MPI_Comm_create provides a means of making a subset of processes for the purpose of separate MIMD computation, with separate communication space. newcomm, which is created by MPI_Comm_create, can be used in subsequent calls to MPI_Comm_create (or other communicator constructors) to further subdivide a computation into parallel sub-computations. A more general service is provided by MPI_Comm_split. So yes, you can keep splitting your friend group into smaller group chats. 💅

---

😭 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. Like, 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, 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. No cap, that’s serious. 🫶

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_split

---

💜 COPYRIGHT 💜

2003-2026, The Open MPI Community

✨ xoxo from Open MPI, dated Sep 11, 2026, manual MPI_COMM_CREATE(3) — bye bestie! 🫶💖