MPI_Alltoall(3)

✨ NAME ✨

MPI_Alltoall — sends data from all to all processes 💅

okay so bestie, quick vibe check: this is a page from the MPI manual, section 3, and it's about the moment when everybody in the communicator needs to talk to everybody else. it's giving group chat energy, but for processes. 🫶

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🌸 SYNOPSIS 🌸

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

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

two flavors, both iconic 😌 the _c one is the same deal but it takes MPI_Count for the counts instead of plain int. no cap.

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💖 INPUT PARAMETERS 💖

• sendbuf — starting address of send buffer (choice)
• sendcount — number of elements sent to each process (non-negative integer)
• sendtype — data type of send buffer elements (handle)
• recvcount — number of elements received from any process (non-negative integer)
• recvtype — data type of receive buffer elements (handle)
• comm — communicator (handle)

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🎀 OUTPUT PARAMETERS 🎀

• recvbuf — address of receive buffer (choice)

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🔒 THREAD AND INTERRUPT SAFETY 🔒

This routine is thread-safe. ✨ That means it may be safely used by multiple threads without needing any user-provided thread locks — no extra guarding required, she handles herself.

However… and this is the important part, bestie — 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.

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🧚 NOTES FOR FORTRAN 🧚

All MPI routines in Fortran (except for MPI_WTIME and MPI_WTICK) have an additional argument ierr tacked onto 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 get invoked with the call statement.

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 that value comes back, the current MPI error handler gets called. By default, this error handler aborts the MPI job — dramatic, yes, but that's literally the default. 🚨

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 — she's old news, love her but leave her in 1.1. 💅 The predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned.

not me reminding you: 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, in list form because we love organisation:

• MPI_SUCCESS — No error; MPI routine completed successfully. 🎉
• 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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nm bestie, that's the whole page 🦋 this was the MPI MPI_Alltoall(3) manual page, dated 4/10/2026 — go forth and all-to-all, you've got this, no cap 💖✨👑