MPI_COMM_SIZE(3)

💖 MPI_COMM_SIZE(3) 💖

ok so this is the Open MPI manual, page MPI_COMM_SIZE(3), and bestie we are about to learn how many of your little process babies are in a communicator. it's giving census day. 🎀

---

✨ NAME ✨

MPI_Comm_size — returns the size of the group associated with a communicator. like. how many of us are there. that's it. that's the function. 🫶

---

✨ SYNTAX ✨

ok so depending on which language you're living in, it looks like one of these 👇

🌸 C Syntax

#include <mpi.h>

int MPI_Comm_size(MPI_Comm comm, int *size)

🌸 Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_COMM_SIZE(COMM, SIZE, IERROR)
     INTEGER COMM, SIZE, IERROR

🌸 Fortran 2008 Syntax

USE mpi_f08
MPI_Comm_size(comm, size, ierror)
     TYPE(MPI_Comm), INTENT(IN) :: comm
     INTEGER, INTENT(OUT) :: size
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

---

✨ INPUT PARAMETER ✨

• comm — the communicator, as a handle. this is the friend group we're counting. no cap, that's literally all you hand in. 💅

✨ OUTPUT PARAMETERS ✨

• size — the number of processes in the group of comm, as an integer. the headcount. the roster. the vibe. 🔥
• ierror — Fortran only: error status, an integer. C girlies don't get one because C returns the error value itself, slay.

---

✨ DESCRIPTION ✨

ok so here's the tea ☕ — this function tells you the number of processes involved in a communicator.

for MPI_COMM_WORLD, it tells you the total number of processes available. the whole population. everybody. 🌍

it's equivalent to accessing the communicator's group with MPI_Comm_group, computing the size using MPI_Group_size, and then freeing the temporary group via MPI_Group_free. so like, you could do all three of those steps yourself, but that's extra and we don't do extra here. this function ate. 👑

if the communicator is an inter-communicator (which enables communication between two groups), this function returns the size of the local group. 🫶

to return the size of the remote group, use the MPI_Comm_remote_size function instead. different function, different energy, iykyk.

this call is often used with MPI_Comm_rank to figure out the amount of concurrency available for a specific library or program. ✨

and MPI_Comm_rank indicates the rank of the process that calls it, in the range from 0 . . . size-1, where size is the return value of MPI_Comm_size.

so like. size says "there are this many of us" and MPI_Comm_rank says "and you, specifically, are number this." iconic duo. 💖

---

✨ NOTE ✨

MPI_COMM_NULL is not considered a valid argument to this function. don't do it. not the move. 🙈

---

✨ 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. so like, pay attention to where your error is hiding, it's different per language. 💀

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. 💀
• 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. so like. handle it. for real. ✋

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_group
• MPI_Comm_rank
• MPI_Comm_compare

---

💜 Copyright

2003-2026, The Open MPI Community

---

ok bestie that's the whole page!! 🦋 thanks for reading, go forth and count your processes, you absolute main character. this was the Open MPI manual page MPI_COMM_SIZE(3) and it was last updated Sep 10, 2026 — periodt. 💅✨