MPI_GRAPH_GET(3)

💅 MPI_Graph_get — Open MPI (3)

okay so bestie, welcome to the MPI_GRAPH_GET(3) page 💖 this is the Open MPI one, and it's literally about pulling the graph topology info back out of a communicator. let me walk you through it 🫶

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✨ SYNTAX ✨

🌸 C Syntax

#include <mpi.h>

int MPI_Graph_get(MPI_Comm comm, int maxindex, int maxedges,
    int index[], int edges[])

🎀 Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'

MPI_GRAPH_GET(COMM, MAXINDEX, MAXEDGES, INDEX, EDGES, IERROR)
    INTEGER COMM, MAXINDEX, MAXEDGES, INDEX(*)
    INTEGER EDGES(*), IERROR

🦋 Fortran 2008 Syntax

USE mpi_f08

MPI_Graph_get(comm, maxindex, maxedges, index, edges, ierror)
    TYPE(MPI_Comm), INTENT(IN) :: comm
    INTEGER, INTENT(IN) :: maxindex, maxedges
    INTEGER, INTENT(OUT) :: index(maxindex), edges(maxedges)
    INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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💜 INPUT PARAMETERS 💜

• comm — the communicator that's got the graph structure, it's a handle, main character honestly 👑
• maxindex — the length of the index vector in the calling program, integer bestie, don't fumble this one 🙈
• maxedges — the length of the edges vector in the calling program, also an integer, also don't fumble it 💀

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⭐ OUTPUT PARAMETERS ⭐

• index — an array of integers holding the graph structure (for the full details, go peek at the definition of MPI_Graph_create, iykyk) ✨
• edges — an array of integers holding the graph structure, yes again, both of them carry graph structure 💅
• ierror — Fortran only, boo: the error status, an integer 🌸

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🔥 DESCRIPTION 🔥

so the functions MPI_Graphdims_get and MPI_Graph_get retrieve the graph-topology information that was associated with a communicator by MPI_Graph_create. like, it's giving "who did this communicator get matched with" energy 🌈

and the information that MPI_Graphdims_get hands you can be used to dimension the vectors index and edges correctly for a call to MPI_Graph_get. literally size them right first, then go get them, periodt 💖

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🧚 ERRORS 🧚

almost all MPI routines return an error value bestie — C routines give it back as the return result of the function, and Fortran routines put it in the last argument 😭

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. ngl that's a lot but it all matters 🫶

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, cute and polite ✨

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... don't count on it bestie 😭

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_Graph_create

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🎀 COPYRIGHT 🎀

2003-2026, The Open MPI Community

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and that's the whole page bestie, slay 💅✨ the footer says 🦋 Sep 11, 2026 🦋 and it's all from the MPI_GRAPH_GET(3) page of Open MPI — go forth and get those graphs 👑💖