MPIX_Comm_agree(3)

💖 MPIX_Comm_agree — bestie we're doing a group project agreement 💖

okay so this page is from the MPI manual, section 3 (the library-call girlies, iykyk ✨). Welcome to MPIX_Comm_agree(3) 🫶

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✨ NAME ✨

MPIX_Comm_agree — Performs agreement operation on comm 🎀

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✨ SYNOPSIS ✨

int MPIX_Comm_agree(MPI_Comm comm, int *flag)

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🌸 INPUT PARAMETERS 🌸

• comm — communicator (handle) 💫

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🌸 OUTPUT PARAMETERS 🌸

• flag — new communicator (logical) 👑

yes bestie, that's what it says, that's what it is — copy it exactly, don't come for me 💅

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💜 NOTES 💜

The whole purpose of this collective communication is to agree on the integer value flag and on the group of failed processes in comm. like a group chat where everyone has to heart-react to the same thing, no cap 🌈

On completion, all non-failed MPI processes have agreed to set the output integer value of flag to the result of a bitwise AND operation over the contributed input values of flag. If comm is an intercommunicator, the value of flag is a bitwise AND operation over the values contributed by the remote group. 🍓

When an MPI process fails before contributing to the operation, the flag is computed ignoring its contribution, and MPIX_Comm_agree raises an error of class MPI_ERR_PROC_FAILED. However, if all MPI processes have acknowledged this failure prior to the call to MPIX_Comm_agree, using MPIX_Comm_failure_ack, the error related to this failure is not raised. When an error of class MPI_ERR_PROC_FAILED is raised, it is consistently raised at all MPI processes, in both the local and remote groups (if applicable). 💀 so like, no sneaky one-sided drama

After MPIX_Comm_agree raised an error of class MPI_ERR_PROC_FAILED, a subsequent call to MPIX_Comm_failure_ack on comm acknowledges the failure of every MPI process that didn't contribute to the computation of flag. 💖

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🦋 THREAD AND INTERRUPT SAFETY 🦋

This routine is thread-safe. That means it may be safely used by multiple threads without the need for any user-provided thread locks. 🔥

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 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. ⭐

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. 😭 (yes, dramatic, but that's literally what it does.)

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 🚫 — lowkey don't, fr fr.

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. 🤞

• 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_COMM — Invalid communicator. A common error is to use a null communicator in a call (not even allowed in MPI_Comm_rank). 💀 iconic mistake tbh
• MPI_ERR_OTHER — Other error; use MPI_Error_string to get more information about this error code. 🔍

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mwah, that's the whole page, no notes skipped, nothing left out 💜 This was MPIX_Comm_agree(3), dated 4/10/2026 — go forth and agree on things, bestie 🌸💖✨