MPI_GROUP_F2C(3)

🎀 MPI_GROUP_F2C(3) 🎀

okay bestie, so this is the manual page for a whole little family of Open MPI functions 💅 and it's giving "translator" energy, because that's literally what they do. Folding the manual name in right here so you know what you're reading: MPI_GROUP_F2C(3) — iconic, we stan a page that does the most with sixteen functions and one job.

Here's the whole girlgang of them ✨:

• MPI_Comm_f2c <#mpi-comm-f2c>
• MPI_Comm_c2f <#mpi-comm-c2f>
• MPI_File_f2c <#mpi-file-f2c>
• MPI_File_c2f <#mpi-file-c2f>
• MPI_Info_f2c <#mpi-info-f2c>
• MPI_Info_c2f <#mpi-info-c2f>
• MPI_Message_f2c <#mpi-message-f2c>
• MPI_Message_c2f <#mpi-message-c2f>
• MPI_Op_f2c <#mpi-op-f2c>
• MPI_Op_c2f <#mpi-op-c2f>
• MPI_Request_f2c <#mpi-request-f2c>
• MPI_Request_c2f <#mpi-request-c2f>
• MPI_Type_f2c <#mpi-type-f2c>
• MPI_Type_c2f <#mpi-type-c2f>
• MPI_Win_f2c <#mpi-win-f2c>
• MPI_Win_c2f <#mpi-win-c2f>

…and what do they do? They translate a C handle into a Fortran handle, or the other way around. That's it, that's the whole vibe. 🔁

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✨ SYNTAX ✨

🫶 C Syntax 🫶

#include <mpi.h>

MPI_Comm MPI_Comm_f2c(MPI_Fint comm)
MPI_Fint MPI_Comm_c2f(MPI_Comm comm)

MPI_File MPI_File_f2c(MPI_Fint file)
MPI_Fint MPI_File_c2f(MPI_File file)

MPI_Group MPI_Group_f2c(MPI Fint group)
MPI_Fint MPI_Group_c2f(MPI Group group)

MPI_Info MPI_Info_f2c(MPI_Fint info)
MPI_Fint MPI_Info_c2f(MPI_Info info)

MPI_Message MPI_Message_f2c(MPI_Fint message)
MPI_Fint MPI_Message_c2f(MPI_Message message)

MPI_Op MPI_Op_f2c(MPI_Fint op)
MPI_Fint MPI_Op_c2f(MPI_Op op)

MPI_Request MPI_Request_f2c(MPI_Fint request)
MPI_Fint MPI_Request_c2f(MPI_Request request)

MPI_Datatype MPI_Type_f2c(MPI_Fint datatype)
MPI_Fint MPI_Type_c2f(MPI_Datatype datatype)

MPI_Win MPI_Win_f2c(MPI_Fint win)
MPI_Fint MPI_Win_c2f(MPI_Win win)

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📖 DESCRIPTION 📖

okay so here's the tea ☕: handles get passed between Fortran and C by using an explicit C wrapper to convert Fortran handles into C handles. Periodt. There is no direct access to C handles in Fortran. None. Don't even try it.

The type definition MPI_Fint is provided in C for an integer of the size that matches a Fortran INTEGER; usually, MPI_Fint will be equivalent to int. (yeah, "usually" — it's giving mostly, and that's the note we're working with 🫠)

And the handle translation functions? They're provided in C to convert from a Fortran handle (which is literally just an integer, bestie) to a C handle, and vice versa. 🔄

for example — and I love a good example, it's the main character of this section:

• if comm is a valid Fortran handle to a communicator, then MPI_Comm_f2c <#mpi-comm-f2c> returns a valid C handle to that same communicator ✅
• if comm = MPI_COMM_NULL (that's the Fortran value, ok), then MPI_Comm_f2c <#mpi-comm-f2c> returns a null C handle 🫥
• if comm is an invalid Fortran handle, then MPI_Comm_f2c <#mpi-comm-f2c> returns an invalid C handle 💀

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💀 NOTE 💀

okay this part is important, no cap:

This function does not return an error value. Consequently, the result of calling it before MPI_Init <#mpi-init> or after MPI_Finalize <#mpi-finalize> is undefined.

so like… don't. seriously. undefined behaviour is not the vibe check you want to fail. 🙈

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👑 Copyright 👑

2003-2026, The Open MPI Community

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and that's the whole page bestie, you ate it up 💅✨ it's from the Open MPI manual, page MPI_GROUP_F2C(3), dated Sep 10, 2026 — go forth and translate those handles, you've got this 🫶🦋