MPI_COMM_SPLIT(3)

💖 MPI_Comm_split 💖

okay bestie, this one is from the Open MPI manual, page MPI_COMM_SPLIT(3), and it's literally one of the most useful little spells in the whole MPI grimoire. it says: Creates new communicators based on colors and keys. yes. colors and keys. like a friendship group sorting algorithm but for processes. 🎀

---

✨ SYNTAX ✨

🩷 C Syntax

#include <mpi.h>

int MPI_Comm_split(MPI_Comm comm, int color, int key,
     MPI_Comm *newcomm)

🩷 Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_COMM_SPLIT(COMM, COLOR, KEY, NEWCOMM, IERROR)
     INTEGER COMM, COLOR, KEY, NEWCOMM, IERROR

🩷 Fortran 2008 Syntax

USE mpi_f08
MPI_Comm_split(comm, color, key, newcomm, ierror)
     TYPE(MPI_Comm), INTENT(IN) :: comm
     INTEGER, INTENT(IN) :: color, key
     TYPE(MPI_Comm), INTENT(OUT) :: newcomm
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

---

🌸 INPUT PARAMETERS 🌸

• comm — the communicator you're starting from 💜 (a handle).
• color — this is what decides which subset you land in bestie. it's a nonnegative integer — nonnegative, so don't be out here passing anything negative 💀
• key — this one controls the rank assignment, it's an integer, and it's how the group gets ordered ✨

---

🦋 OUTPUT PARAMETERS 🦋

• newcomm — the brand new communicator, fresh out the oven, a handle 🫶
• ierror — Fortran only: error status, an integer. the Fortran girlies get their errors in an argument, iconic of them.

---

👑 DESCRIPTION 👑

so here's the tea, bestie. This function partitions the group associated with comm into disjoint subgroups, one for each value of color. every subgroup contains all the processes of the same color — yes literally, it's a sorting-by-vibe moment and the vibe is color 🌈

Within each subgroup, the processes are ranked in the order defined by the value of the argument key, and any ties get broken according to their rank in the old group. so like, if two of y'all have the same key, the tiny detail of where you were in the original group decides. no drama, just rank.

A new communicator is created for each subgroup and returned in newcomm. 🎀

A process may supply the color value MPI_UNDEFINED, in which case newcomm returns MPI_COMM_NULL. yes, that's a thing — you can opt out of the whole situation entirely and get NULL back. respectfully exiting the group chat.

This is a collective call, but each process is permitted to provide different values for color and key. so like, everybody has to show up, but everyone can bring their own opinion. periodt 💅

---

okay now, inter-communicator tea ☕ When you call MPI_Comm_split on an inter-communicator, the processes on the left with the same color as those on the right combine to create a new inter-communicator. The key argument describes the relative rank of processes on each side of the inter-communicator.

And the function returns MPI_COMM_NULL for those colors that are specified on only one side of the inter-communicator, or for those that specify MPI_UNDEFINED as the color. so if your color is one-sided, you get nothing back. it's giving unmatched energy, but it's on purpose 🙈

---

💜 the equivalence lore 💜

A call to MPI_Comm_create(comm, group, newcomm) is equivalent to a call to MPI_Comm_split(comm, color,key, newcomm), where all members of group provide color = 0 and key = rank in group, and all processes that are not members of group provide color = MPI_UNDEFINED.

So MPI_Comm_split is basically MPI_Comm_create with way more range — it allows more general partitioning of a group into one or more subgroups with optional reordering. glow up fr fr ✨

⚠️ important: The value of color must be nonnegative or MPI_UNDEFINED. no in-between, no chaos, no guessing.

---

🧚 NOTES 🧚

This is an extremely powerful mechanism for dividing a single communicating group of processes into k subgroups, with k chosen implicitly by the user — meaning by the number of colors asserted over all the processes. Each resulting communicator will be nonoverlapping. 👑

Such a division could be useful for defining a hierarchy of computations, such as for multigrid or linear algebra. math girlies, this one's for you 🍓

Multiple calls to MPI_Comm_split can be used to overcome the requirement that any call have no overlap of the resulting communicators (each process is of only one color per call). In this way, multiple overlapping communication structures can be created. Creative use of the color and key in such splitting operations is encouraged — go off, honestly, be unhinged with it 💅

---

Note that, for a fixed color, the keys need not be unique. It is MPI_Comm_split's responsibility to sort processes in ascending order according to this key, and to break ties in a consistent way.

If all the keys are specified in the same way, then all the processes in a given color will have the relative rank order as they did in their parent group. (In general, they will have different ranks.) 🙈

Essentially, making the key value zero for all processes of a given color means that one needn't really pay attention to the rank-order of the processes in the new communicator. just zero it out and vibe 😌

---

⭐ 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. classic.

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. 😭

---

🌟 the three predefined error handlers 🌟

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, 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. the chill one 🫶

---

🌈 DIY error handlers, because you're a main character 🌈

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, no promises bestie, that's just how it is 😭

See the MPI man page for a full list of MPI error codes. And see the Error Handling section of the MPI-3.1 standard for more information. 📚

---

🦋 See also 🦋

• MPI_Comm_create
• MPI_Intercomm_create
• MPI_Comm_dup
• MPI_Comm_free

---

💅 Copyright

2003-2026, The Open MPI Community

---

okay that's the whole page bestie, and final housekeeping: this was Open MPI's MPI_COMM_SPLIT(3) page, dated Sep 10, 2026. go split some communicators, you absolute icon 💖👑✨