MPI_Iscatter(3)

💫 MPI_Iscatter — from the MPI manual, section 3 (3) 💫

okay bestie, gather round 🫶 this is MPI_Iscatter, straight outta the MPI manual (section 3, the one you flip to when you're feeling fancy). It's the ✨nonblocking✨ sibling of scatter: one process yeets data out to all the other processes in a communicator, but it does NOT make you sit there and wait. Iconic. 👑

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🌸 NAME 🌸

MPI_Iscatter — Sends data from one process to all other processes in a communicator in a nonblocking way 💅

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✨ SYNOPSIS ✨

int MPI_Iscatter(const void *sendbuf, int sendcount, MPI_Datatype sendtype,
void *recvbuf, int recvcount, MPI_Datatype recvtype, int root,
MPI_Comm comm, MPI_Request *request)

int MPI_Iscatter_c(const void *sendbuf, MPI_Count sendcount,
MPI_Datatype sendtype, void *recvbuf, MPI_Count recvcount,
MPI_Datatype recvtype, int root, MPI_Comm comm,
MPI_Request *request)

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🌈 INPUT PARAMETERS 🌈

like, everything you gotta hand over before the magic happens:

• sendbuf — address of send buffer (choice). this is the pile of stuff being handed out 📦
• sendcount — number of elements sent to each process (non-negative integer). so like, per-process portion, bestie.
• sendtype — data type of send buffer elements (handle)
• recvcount — number of elements in receive buffer (non-negative integer)
• recvtype — data type of receive buffer elements (handle)
• root — rank of sending process (integer). the main character of this operation 👑
• comm — communicator (handle)

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🎀 OUTPUT PARAMETERS 🎀

what you get back, no cap:

• recvbuf — address of receive buffer (choice)
• request — communication request (handle). hold onto this one, you'll need it later fr fr 🫶

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💜 THREAD AND INTERRUPT SAFETY 💜

okay so the tea: this routine is thread-safe ✨. That means multiple threads can use it at the same time and you do not have to bring your own thread locks — it handles all that. slay.

BUT. and this is a big but 😭 — the routine is not interrupt safe. Typically that's because it uses memory allocation routines like malloc or other non-MPICH runtime routines, and those routines are themselves not interrupt-safe. So like… it's giving "trust me but only so far". 💀

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🦋 NOTES FOR FORTRAN 🦋

• Every MPI routine in Fortran has an extra argument ierr tacked onto the end of the argument list 🫶 — the only exceptions are MPI_WTIME and MPI_WTICK. ierr is an integer and it 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. no notes, that's just how it is 💅

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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 that last argument. Before the value comes back, the current MPI error handler gets called.

⚠️ By default, this error handler aborts the MPI job. Like, the whole thing. gone. 💀

You can swap the error handler 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 🙈 — don't do it, bestie.

The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned instead.

📌 Note: MPI does not guarantee that an MPI program can continue past an error; however, MPI implementations will attempt to continue whenever possible.

🌟 the error codes, one by one 🌟

• MPI_SUCCESS — No error; MPI routine completed successfully. we love to see it 🎉
• 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_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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and that's the whole page, bestie 🌈 every flag, every handle, every error code, every caveat — all of it, preserved like a lil museum exhibit 💖 go forth and scatter (nonblockingly) 🧚

signing off from the MPI manual, this was MPI_Iscatter(3), dated 4/10/2026 — periodt 💅⭐