MPI_Neighbor_allgatherv(3)

💖 MPI_Neighbor_allgatherv(3) 💖

okay bestie, welcome to the MPI manual, and today's main character is MPI_Neighbor_allgatherv — the vector variant of MPI_Neighbor_allgather. yes, it's giving "everyone gets their own custom amount of stuff from each neighbor" energy. iconic. periodt.

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✨ NAME ✨

MPI_Neighbor_allgatherv — The vector variant of MPI_Neighbor_allgather.

(so like… the regular one sends the same count to everybody. this one? it lets every neighbor get a different amount. slay.)

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🦋 SYNOPSIS 🦋

int MPI_Neighbor_allgatherv(const void *sendbuf, int sendcount,
MPI_Datatype sendtype, void *recvbuf,
const int recvcounts[], const int displs[],
MPI_Datatype recvtype, MPI_Comm comm)

int MPI_Neighbor_allgatherv_c(const void *sendbuf, MPI_Count sendcount,
MPI_Datatype sendtype, void *recvbuf,
const MPI_Count recvcounts[],
const MPI_Aint displs[], MPI_Datatype recvtype,
MPI_Comm comm)

two flavors, both valid, pick your fighter 💅

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🌸 INPUT PARAMETERS 🌸

• sendbuf — starting address of send buffer (choice)
• sendcount — number of elements sent to each neighbor (non-negative integer)
• sendtype — data type of send buffer elements (handle)
• recvcounts — non-negative integer array (of length indegree) containing the number of elements that are received from each neighbor (non-negative integer)
• displs — 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)

ngl that's a lot of args but every single one is doing its little job 🫶

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🎀 OUTPUT PARAMETERS 🎀

• recvbuf — starting address of receive buffer (choice)

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🔥 THREAD AND INTERRUPT SAFETY 🔥

This routine is thread-safe. This means that this routine may be safely used by multiple threads without the need for any user-provided thread locks. 💪

However, the routine is not interrupt safe. Typically, this is due to the use of memory allocation routines such as malloc or other non-MPICH runtime routines that are themselves not interrupt-safe. so like… threads? fine. signals mid-call? 💀 no thank you.

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🌈 NOTES FOR FORTRAN 🌈

All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK) have an additional argument ierr at the end of the argument list. ierr is an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and are invoked with the call statement.

All MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran.

(so yes, Fortran girlies, you get a bonus ierr at the end of everything. it's a whole thing. iykyk 💜)

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⭐ ERRORS ⭐

All MPI routines (except MPI_Wtime and MPI_Wtick) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. 😭

The error handler may 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. The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned. Note that MPI does not guarantee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible. 🤞

• MPI_SUCCESS — No error; MPI routine completed successfully. 🎉
• 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_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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real talk: check your return codes bestie. the default handler literally kills the whole job and that is not the vibe. 🕯️

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okay that's the whole page, no cap. this was MPI_Neighbor_allgatherv(3), from the MPI manual, dated 4/10/2026 💖✨ stay gathered, stay vectorized, and don't forget your displacements. mwah! 🎀🦋💅