MPI_Alltoallw_init(3)

💖 MPI_Alltoallw_init(3) 💖

okay bestie, welcome to the MPI manual, section 3, and today's main character is MPI_Alltoallw_init 🎀

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✨ NAME ✨

MPI_Alltoallw_init — Create a persistent request for alltoallw.

like, it makes you a persistent request. that's the whole vibe. periodt. 💅

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🌸 SYNOPSIS 🌸

int MPI_Alltoallw_init(const void *sendbuf, const int sendcounts[],
const int sdispls[], const MPI_Datatype sendtypes[],
void *recvbuf, const int recvcounts[],
const int rdispls[], const MPI_Datatype recvtypes[],
MPI_Comm comm, MPI_Info info, MPI_Request *request)

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

yes bestie, there are two of them — the regular one and the _c one (that's the big-count girlie 💅). both are giving exactly what you asked for.

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💜 INPUT PARAMETERS 💜

• sendbuf — starting address of send buffer (choice). that's where the stuff you're shipping out lives, bestie 📦
• sendcounts — integer array (of length group size) specifying the number of elements to send to each rank (non-negative integer). one count per rank, no cap.
• sdispls — integer array (of length group size). Entry j specifies the displacement in bytes (relative to sendbuf) from which to take the outgoing data destined for process j (integer). it's giving "where TF do I grab rank j's gift from" 🎁
• sendtypes — Array of datatypes (of length group size). Entry j specifies the type of data to send to process j (handle). each rank gets its own type, obsessed.
• recvcounts — integer array (of length group size) specifying the number of elements that can be received from each rank (non-negative integer). capacity check, lowkey important.
• rdispls — integer array (of length group size). Entry i specifies the displacement in bytes (relative to recvbuf) at which to place the incoming data from process i (integer). where you're dropping rank i's incoming package 📬
• recvtypes — array of datatypes (of length group size). Entry i specifies the type of data received from process i (handle). incoming type per rank, slay.
• comm — communicator (handle). the group chat you're all in 💬
• info — info argument (handle). the little notes you're passing along.

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🫶 OUTPUT PARAMETERS 🫶

• recvbuf — address of receive buffer (choice). where all the incoming goodies land 🌈
• request — communication request (handle). your shiny new persistent request, congrats you're a parent now 👑

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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. no extra locks needed, she's low-maintenance fr fr 🧚

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.

💀 not me catching a routine that's thread-safe but NOT interrupt-safe. the duality. know the difference bestie, it matters.

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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 they are invoked with the call statement.

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

so if you're a Fortran girlie: yes, tack ierr on the end. yes, everything is INTEGER. it's iconic behavior honestly. ✨

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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.

😭 bestie the DEFAULT error handler literally kills your whole job. not dramatic, just facts. if you want error values back instead of a corpse of a program, you gotta use MPI_ERRORS_RETURN.

and here are the error codes, the full lineup:

• MPI_SUCCESS — No error; MPI routine completed successfully. we love a happy ending 🥳
• MPI_ERR_ARG — Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., MPI_ERR_RANK). it's giving "something's off but I won't tell you which one" 🙈
• MPI_ERR_BUFFER — Invalid buffer pointer. Usually a null buffer where one is not valid. bestie please don't pass null, the pointer wants to point 🫠
• MPI_ERR_COMM — Invalid communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank). yo that's a null communicator, not even MPI_Comm_rank lets that slide 💀
• MPI_ERR_COUNT — Invalid count argument. Count arguments must be non-negative; a count of zero is often valid. negative counts = not the vibe, zero is fine though.
• MPI_ERR_INFO — Invalid Info. that info handle is sus.
• 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. commit your types bestie, fr fr — uncommitted datatypes are a walking red flag 🚩
• MPI_ERR_OTHER — Other error; use MPI_Error_string to get more information about this error code. the catch-all girlie, go ask MPI_Error_string for the tea ☕

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and that's the whole page, bestie ✨ thanks for reading to the very bottom, ily. this page came from the MPI manual and is dated 4/10/2026 — MPI_Alltoallw_init(3), page done and dusted. go make some persistent requests, you're gonna ate. 🎀💅🌈👑