MPI_Scatter(3)

💌 MPI_Scatter(3) — the MPI manual, but make it make sense

okay bestie, welcome to the MPI manual section for MPI_Scatter(3) ✨ yes it's a section 3 page, yes it's the MPI docs, yes we're doing this together 💖

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🌸 NAME 🌸

MPI_Scatter — sends data from one process to all other processes in a communicator 📤➡️👯‍♀️

like... one process has the goods, and everybody else gets their lil share. iconic distribution behavior, no cap.

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✨ SYNOPSIS ✨

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

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

two flavors, same vibe 💅 the _c one is the big-count girlie version.

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🧚 INPUT PARAMETERS 🧚

• sendbuf — the address of the send buffer, it's a choice type (handle-ish situation) 🎀
• sendcount — how many elements get sent to each process, non-negative integer so like, zero is fine but negative is NOT the move 😭
• sendtype — the data type of the elements chillin' in the send buffer (handle) 🦋
• recvcount — how many elements live in the receive buffer, also a non-negative integer 💫
• recvtype — the data type of the receive buffer elements (handle) 🌈
• root — the rank of the sending process, an integer. she's the main character feeding everyone else 👑
• comm — the communicator (handle), aka the group chat everyone's in 💬

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💝 OUTPUT PARAMETERS 💝

• recvbuf — the address of the receive buffer (choice) 🫶 this is where your slice lands, bestie

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🔒 THREAD AND INTERRUPT SAFETY 🔒

this routine is thread-safe 🧵✨ which means multiple threads can use it at the same time and you do NOT need to bring your own thread locks. she handles it. periodt.

BUT — and this is the important part — the routine 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 be messy with interrupts, okay? 🙈

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🎀 NOTES FOR FORTRAN 🎀

• every MPI routine in Fortran (except MPI_WTIME and MPI_WTICK) has an extra argument ierr tacked on at the end of the argument list 📎
• ierr is an integer and it means the 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 (like 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 function's return value, Fortran routines in that last argument 🫠 before that value comes back to you, the current MPI error handler gets called. by default, that error handler aborts the MPI job 😭 so yeah, she's dramatic by default.

you can swap the error handler with:

• MPI_Comm_set_errhandler (for communicators)
• MPI_File_set_errhandler (for files)
• MPI_Win_set_errhandler (for RMA windows)

the MPI-1 routine MPI_Errhandler_set also exists but its use is deprecated 🚫 don't do it, that's old news.

the predefined error handler MPI_ERRORS_RETURN can be used to make error values actually get returned.

heads up: MPI does not guarantee that an MPI program can continue past an error 😬 that said, MPI implementations will attempt to continue whenever possible. so like, fingers crossed energy 🤞

here's the error code lineup:

• MPI_SUCCESS — no error, the routine completed successfully. slay 💚
• MPI_ERR_BUFFER — invalid buffer pointer. usually a null buffer where one is not valid 🙃
• MPI_ERR_COMM — invalid communicator. a very common oopsie is using a null communicator in a call — which isn't even allowed in MPI_Comm_rank 💀
• MPI_ERR_COUNT — invalid count argument. counts must be non-negative; a count of zero is often valid 🍓
• MPI_ERR_TYPE — invalid datatype argument. also, this can pop off if an uncommitted MPI_Datatype (see MPI_Type_commit) gets used in a communication call 🦋
• MPI_ERR_OTHER — other error; use MPI_Error_string to get more info about this error code 💫

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and that's the whole page bestie, every flag, every error, every lil Fortran footnote, all of it 🫶 go forth and scatter 💅

this lil masterpiece is from the MPI manual, page MPI_Scatter(3), dated 4/10/2026 📅✨ catch you in the SEE ALSO section of your heart 🌈