MPI_MESSAGE_C2F(3)

💅 MPI_MESSAGE_C2F(3) — the girl's guide to handle glow-ups

okay bestie, so this is a page out of the Open MPI manual, and it's lowkey one of the most useful little pages ever, because it's all about making C handles and Fortran handles ✨understand each other✨. iykyk 💖

The full roster of legends on this page is: MPI_Comm_f2c, MPI_Comm_c2f, MPI_File_f2c, MPI_File_c2f, MPI_Info_f2c, MPI_Info_c2f, MPI_Message_f2c, MPI_Message_c2f, MPI_Op_f2c, MPI_Op_c2f, MPI_Request_f2c, MPI_Request_c2f, MPI_Type_f2c, MPI_Type_c2f, MPI_Win_f2c, MPI_Win_c2f — and their whole job is translating a C handle into a Fortran handle, or vice versa. bilingual icons fr fr 🌈

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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)

(not me copying those two MPI Group / MPI Fint lines exactly as they appear — yes bestie, that's how they're printed, i'm not touching the source, that's the whole point 💅)

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🦋 DESCRIPTION 🦋

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. there is no direct access to C handles in Fortran — none, zero, 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. and the handle translation functions are provided in C to convert from a Fortran handle (which is an integer) to a C handle, and vice versa. it's giving translator at the group chat, honestly 🫶

for example 🌸 — if comm is a valid Fortran handle to a communicator, then MPI_Comm_f2c returns a valid C handle to that same communicator; if comm = MPI_COMM_NULL (Fortran value), then MPI_Comm_f2c returns a null C handle; if comm is an invalid Fortran handle, then MPI_Comm_f2c returns an invalid C handle. garbage in, garbage out, but make it multilingual ✨

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💀 NOTE 💀

okay this is the important one so listen up 📢 — this function does not return an error value. consequently, the result of calling it before MPI_Init or after MPI_Finalize is undefined. like, undefined undefined. don't do it, bestie, i'm begging 😭

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👑 COPYRIGHT 👑

2003-2026, The Open MPI Community

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and that's the whole page, no cap 🧚✨ mwah, go convert some handles — sourced fresh from the Open MPI crew, dated Sep 11, 2026, MPI_MESSAGE_C2F(3) and out 💜🎀