MPI_Neighbor_alltoallv_init(3)

💖 MPI_Neighbor_alltoallv_init(3) 💖

okay bestie, so this one lives in the MPI manual, section 3, and it's called MPI_Neighbor_alltoallv_init — say that five times fast, I dare you 🫶

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✨ NAME ✨

MPI_Neighbor_alltoallv_init — bestie, this literally creates a persistent request for neighbor_alltoallv. That's it. That's the whole job. 💅

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🌸 SYNOPSIS 🌸

int MPI_Neighbor_alltoallv_init(const void *sendbuf, const int sendcounts[],
const int sdispls[], MPI_Datatype sendtype,
void *recvbuf, const int recvcounts[],
const int rdispls[], MPI_Datatype recvtype,
MPI_Comm comm, MPI_Info info,
MPI_Request *request)

int MPI_Neighbor_alltoallv_init_c(const void *sendbuf,
const MPI_Count sendcounts[],
const MPI_Aint sdispls[],
MPI_Datatype sendtype, void *recvbuf,
const MPI_Count recvcounts[],
const MPI_Aint rdispls[],
MPI_Datatype recvtype, MPI_Comm comm,
MPI_Info info, MPI_Request *request)

two flavors, same energy: the classic one and the _c one that rolls with MPI_Count and MPI_Aint ✨

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💌 INPUT PARAMETERS 💌

• sendbuf — starting address of send buffer (choice). where your lil outgoing data hangs out 💖
• sendcounts — non-negative integer array (of length outdegree) specifying the number of elements to send to each neighbor (non-negative integer). one entry per neighbor you're pushing data to 🫶
• sdispls — integer array (of length outdegree). Entry j specifies the displacement (relative to sendbuf) from which send the outgoing data to neighbor j (integer). basically the map of "where in the send buffer does neighbor j's stuff start" 🗺️
• sendtype — data type of send buffer elements (handle) ✨
• recvcounts — non-negative integer array (of length indegree) specifying the number of elements that are received from each neighbor (non-negative integer) 🎀
• rdispls — integer array (of length indegree). Entry i specifies the displacement (relative to recvbuf) at which to place the incoming data from neighbor i (integer) 💐
• recvtype — data type of receive buffer elements (handle) 🌸
• comm — communicator with topology structure (handle). it's giving topology, it's giving structure 🦋
• info — info argument (handle) ℹ️

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👑 OUTPUT PARAMETERS 👑

• recvbuf — starting address of receive buffer (choice). the lil landing pad for everything coming in 💖
• request — communication request (handle). this is the persistent one, hold onto it like it's your iced coffee 🧋

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🔥 THREAD AND INTERRUPT SAFETY 🔥

good news: this routine is thread-safe. that means multiple threads can use it with zero user-provided thread locks, no cap. you don't gotta babysit it 🫶

but — and this is the part where we get serious — it is not interrupt safe. typically that's because of memory allocation routines like malloc, or other non-MPICH runtime routines that are themselves not interrupt-safe. so like, don't go interrupting it mid-vibe 😭

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🌈 NOTES FOR FORTRAN 🌈

• every MPI routine in Fortran (except for MPI_WTIME and MPI_WTICK) has an extra argument ierr at the end of the argument list. ierr is an integer and means the exact same thing as the return value of the routine in C 📝
• in Fortran, MPI routines are subroutines and you invoke them with the call statement ✨
• all MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran 💅

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💀 ERRORS 💀

so every MPI routine (except MPI_Wtime and MPI_Wtick) returns an error value — C routines as the value of the function, Fortran routines in that last argument. before the value comes back, the current MPI error handler gets called. by default, this error handler aborts the MPI job 😭 so yeah, default behavior is dramatic.

the error handler can be changed with MPI_Comm_set_errhandler (for communicators), MPI_File_set_errhandler (for files), and MPI_Win_set_errhandler (for RMA windows). the MPI-1 routine MPI_Errhandler_set may be used but its use is deprecated (iykyk, don't).

the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. also: MPI does not guarantee that an MPI program can continue past an error — however, MPI implementations will attempt to continue whenever possible 🙏

here's the full roster of error codes, bestie:

• MPI_SUCCESS — No error; MPI routine completed successfully 🎉
• MPI_ERR_ARG — Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., MPI_ERR_RANK) 🤨
• MPI_ERR_BUFFER — Invalid buffer pointer. Usually a null buffer where one is not valid 💀
• MPI_ERR_COMM — Invalid communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank) 🚫
• MPI_ERR_COUNT — Invalid count argument. Count arguments must be non-negative; a count of zero is often valid 🧮
• MPI_ERR_INFO — Invalid Info 📛
• MPI_ERR_TYPE — Invalid datatype argument. Additionally, this error can occur if an uncommitted MPI_Datatype (see MPI_Type_commit) is used in a communication call ⚠️
• MPI_ERR_OTHER — Other error; use MPI_Error_string to get more information about this error code 🔍

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that's the whole page, periodt 💅 go forth and Neighbor_alltoallv_init responsibly, bestie 🫶

sign-off time: this was served fresh from the MPI source, dated 4/10/2026, and it's MPI_Neighbor_alltoallv_init(3) and proud 💖 mwah, go slay ✨🎀🦋