MPI_Iallgatherv(3)

💖 MPI_Iallgatherv — the tea ☕

okay so bestie, welcome to MPI_Iallgatherv(3), an MPI manpage, and honestly? she's iconic. 🎀

✨ NAME ✨

MPI_Iallgatherv — gathers data from all the tasks and delivers the combined data to all the tasks, but in a nonblocking way. yeah. nonblocking. she does not wait, she does not text back, she hands you a request handle and moves on. 💅

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🌈 SYNOPSIS 🌈

C, the classic:

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

and then the big-count glow-up version, for when your counts are just, like, a lot:

int MPI_Iallgatherv_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,
MPI_Request *request)

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🫶 INPUT PARAMETERS 🫶

the stuff you hand her on the way in:

• sendbuf — starting address of send buffer (choice). where your lil data lives before she takes it out into the world.
• sendcount — number of elements in send buffer (non-negative integer). no negatives, we don't do that here 🙈
• sendtype — data type of send buffer elements (handle)
• recvcounts — non-negative integer array (of length group size) containing the number of elements that are received from each process (non-negative integer). so it's an array, one entry per process, and it's literally of length group size. one per bestie. 💖
• displs — integer array (of length group size). entry i specifies the displacement (relative to recvbuf) at which to place the incoming data from process i (integer). so entry i = where process i's stuff gets dropped off, offset from recvbuf. coordinates, basically. 🗺️
• recvtype — data type of receive buffer elements (handle)
• comm — communicator (handle). the group chat, if you will.

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👑 OUTPUT PARAMETERS 👑

the stuff she hands back to you:

• recvbuf — address of receive buffer (choice). where everybody's data ends up chillin' together.
• request — communication request (handle). this is your lil receipt 🧾 — hold onto it, you'll need it later to check on her.

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🧚 THREAD AND INTERRUPT SAFETY 🧚

This routine is thread-safe. periodt. what that means is you can have multiple threads using it at the same time and you do NOT need to build your own user-provided thread locks. she's self-sufficient, she's got her own lil locks, she's fine. 🫶

However, she is not interrupt safe. and why, you ask? typically it's because of memory allocation routines like malloc or other non-MPICH runtime routines that are themselves not interrupt-safe. so. threads: yes. interrupts: absolutely not, bestie. 💀

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🎀 NOTES FOR FORTRAN 🎀

real talk for the Fortran girlies:

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're invoked with the call statement.

also: all MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran. so no fancy types, just INTEGER. okay? okay. ✨

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🚨 ERRORS 🚨

okay so this part is important, don't skip it, i'm serious 😭

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 that value comes back to you, the current MPI error handler gets called. and by default, this error handler aborts the MPI job. like full stop, whole program, gone. 💀 so if you don't want that, you gotta do something about it.

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 — she's old, we don't really do that anymore, lowkey retire her.

also there's a predefined error handler MPI_ERRORS_RETURN that you can use to cause error values to be returned instead. note that MPI does not guarantee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible. so like. best effort. she tries. 🫶

the actual error codes, bestie:

• MPI_SUCCESS — no error; MPI routine completed successfully. slay. 🎉
• 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. girl where is your buffer 😭
• 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. zero is fine, negative is not. 🚫
• 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 first!! i beg 😭
• MPI_ERR_OTHER — other error; use MPI_Error_string to get more information about this error code. the catch-all, the "idk what happened but something did" 💀

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tl;dr — she gathers from everyone, delivers to everyone, all without blocking, and hands you a request handle so you can vibe check it later. just don't pass her anything null and don't expect interrupts to be safe. that's the whole tea. ☕💅

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the vibe check is complete, bestie 💖 this was the MPI_Iallgatherv(3) page from the MPI manual, dated 4/10/2026. go forth and gather (nonblockingly). 🦋✨🌸