MPI_Scatterv(3)

✨ MPI_Scatterv — the "everyone gets a lil piece of the pie" era ✨

okay bestie, quick vibe check: this is a page from the MPI manual, section 3, and it's all about MPI_Scatterv. It lives at MPI_Scatterv(3). Sit down, grab a boba, we're doing distributed memory 💖

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🎀 NAME 🎀

MPI_Scatterv — Scatters a buffer in parts to all processes in a communicator

no cap, that's literally the whole job. one buffer, everybody eats, but not equally (that part is your choice, and it's giving main character energy).

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🌸 SYNOPSIS 🌸

the C girlies get two flavors:

int MPI_Scatterv(const void *sendbuf, const int sendcounts[],
       const int displs[], MPI_Datatype sendtype, void *recvbuf,
       int recvcount, MPI_Datatype recvtype, int root, MPI_Comm comm)

int MPI_Scatterv_c(const void *sendbuf, const MPI_Count sendcounts[],
       const MPI_Aint displs[], MPI_Datatype sendtype,
       void *recvbuf, MPI_Count recvcount, MPI_Datatype recvtype,
       int root, MPI_Comm comm)

yeah, the _c version is just the big-count variant. same energy, bigger numbers 💅

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👑 INPUT PARAMETERS 👑

• sendbuf — address of send buffer (choice). it's choice coded, iconic.
• sendcounts — non-negative integer array (of length group size) specifying the number of elements to send to each rank (non-negative integer). yes bestie, it's an array, it's per-rank, and it's lowkey the whole point of this function.
• displs — integer array (of length group size). Entry i specifies the displacement (relative to sendbuf) from which to take the outgoing data to process i (integer). so like, "where in sendbuf does your lil gift start" 🎁
• 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 one doing the scatter, the giver, the queen 👑
• comm — communicator (handle)

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🦋 OUTPUT PARAMETERS 🦋

• recvbuf — address of receive buffer (choice)

that's your lil piece. enjoy it 🫶

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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. slay, no mutexes needed, we love to see it ✨

HOWEVER (and this is the not-so-slay part) — 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 yes, interrupts no. iykyk 😭

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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.

not me and the Fortran girlies getting a whole extra argument at the end 😌

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

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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. like, the whole thing. gone. so if you were hoping to vibe through an error, you were wrong 💀

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.

here's the roster of things that can go wrong:

• MPI_SUCCESS — No error; MPI routine completed successfully. the bestie outcome, the slay outcome 💖
• 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. go scatter something scatterv-able, text your group chat, hydrate 🌸

also: this was served fresh from the MPI manual, dated 4/10/2026, and it is forever MPI_Scatterv(3). bye pookie, go forth and distribute 🌈✨🦋