MPI_INFO_GET_NKEYS(3)

💅 MPI_INFO_GET_NKEYS — Open MPI vibes, bestie ✨

okay so real talk, this is the MPI_INFO_GET_NKEYS(3) page from Open MPI and it's genuinely so useful. here's the whole thing, translated for us 🫶

✨ NAME ✨

MPI_Info_get_nkeys — gets the number of keys currently defined in an info object. that's it, that's the whole job, and it does it flawlessly 💖

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🌸 SYNTAX 🌸

C Syntax

#include <mpi.h>

int MPI_Info_get_nkeys(MPI_Info info, int *nkeys)

Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_INFO_GET_NKEYS(INFO, NKEYS, IERROR)
     INTEGER         INFO, NKEYS, IERROR

Fortran 2008 Syntax

USE mpi_f08
MPI_Info_get_nkeys(info, nkeys, ierror)
     TYPE(MPI_Info), INTENT(IN) :: info
     INTEGER, INTENT(OUT) :: nkeys
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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🎀 INPUT PARAMETER 🎀

• info — the info object, aka the handle. it's giving "main character," it's the whole reason we're here 👑

🦋 OUTPUT PARAMETERS 🦋

• nkeys — the number of defined keys (an integer). this is the lil number you came for ✨
• ierror — Fortran only: the error status (integer). Fortran girlies get their own error slot, we love consistency 💜

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🌈 DESCRIPTION 🌈

MPI_Info_get_nkeys returns the number of currently defined keys in info. no cap, that's literally it — it's the chillest function in the whole info family 😌

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🔥 ERRORS 🔥

okay buckle up bestie, this part is long but it's important, i'm not skipping a single word of it 🫡

Almost all MPI routines return an error value — C routines hand it back as the return result of the function, and Fortran routines put it in the last argument.

Before that 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.

⚠️ also lowkey important: note that MPI does not guarantee that an MPI program can continue past an error. so like… plan accordingly 💀

Open MPI ships three predefined error handlers you can use, and they are iconic in their own ways:

• MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. yeah. all of them. goodbye 👋
• 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, it 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, it aborts only the local process.
• MPI_ERRORS_RETURN — returns an error code to the application. the gentle one 🥺

MPI applications can also implement their own error handlers, and honestly, DIY queen behavior, 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

See the MPI man page for a full list of MPI error codes (iykyk).

See the Error Handling section of the MPI-3.1 standard for more information. that's the deep lore 📚

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💫 SEE ALSO 💫

• MPI_Info_get
• MPI_Info_get_nthkey
• MPI_Info_get_valuelen

the whole info-object friend group, go say hi 🫶

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👑 COPYRIGHT 👑

2003-2026, The Open MPI Community

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okay that's the whole page, you ate that, go count some keys bestie 🎀✨ and just so you know where this came from: Open MPI, dated Sep 11, 2026. slay 💅