MPI_Exscan_init(3)

💖 MPI_Exscan_init(3) — the MPI manual, but make it slay 💖

okay bestie, welcome to the MPI manual page for MPI_Exscan_init, and yes we're doing this whole thing together, hand in hand, no cap 🫶

So what's the vibe? MPI_Exscan_init Creates a persistent request for exscan. That's the whole NAME section energy right there. Short, sweet, iconic ✨

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✨ SYNOPSIS ✨

Like, here's the literal shape of the function calls, copied character for character because I would NEVER mess with your code 🎀

int MPI_Exscan_init(const void *sendbuf, void *recvbuf, int count,
MPI_Datatype datatype, MPI_Op op, MPI_Comm comm,
MPI_Info info, MPI_Request *request)

int MPI_Exscan_init_c(const void *sendbuf, void *recvbuf, MPI_Count count,
MPI_Datatype datatype, MPI_Op op, MPI_Comm comm,
MPI_Info info, MPI_Request *request)

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🌸 INPUT PARAMETERS 🌸

the stuff you hand over to it 💅

• sendbuf — starting address of send buffer (choice). where your data starts its journey, bestie
• count — number of elements in input buffer (non-negative integer). lowkey important that it's non-negative, just saying 🙈
• datatype — data type of elements of input buffer (handle)
• op — operation (handle)
• comm — intra-communicator (handle)
• info — info argument (handle)

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💜 OUTPUT PARAMETERS 💜

the stuff it hands back to you 👑

• recvbuf — starting address of receive buffer (choice)
• request — communication request (handle)

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🧵 THREAD AND INTERRUPT SAFETY 🧵

okay so this one is thread-safe, which means: this routine may be safely used by multiple threads without the need for any user-provided thread locks. slay 💅

BUT — and this is the plot twist 😭 — 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? absolutely not the moment, babe 💀

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🎀 NOTES FOR FORTRAN 🎀

Fortran girlies, this part is for you 🫶

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 ⚠️

bestie. listen. 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, fr fr, it just ends everything 😭)

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.

and now, the error code menu 🍓:

• MPI_SUCCESS — No error; MPI routine completed successfully. the dream ✨
• 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_INFO — Invalid Info
• MPI_ERR_OP — Invalid operation. MPI operations (objects of type MPI_Op) must either be one of the predefined operations (e.g., MPI_SUM) or created with MPI_Op_create.
• 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, you ate it 🍽️💅

this little masterpiece is from the MPI manual, MPI_Exscan_init(3), dated 4/10/2026 — so when you're out here scanning exclusively, remember who taught you 🫶✨ go be the main character of your communicator 🦋👑