MPI_Win_allocate(3)

💖 MPI_Win_allocate(3) — the MPI one 🫶

okay bestie, so this is a page from the MPI manual, and today we're talking about MPI_Win_allocate — the girlies who create and allocate an MPI Window object for one-sided communication. yes, both. at the same time. she's an overachiever and we love her for it 💅

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✨ NAME ✨

MPI_Win_allocate — Create and allocate an MPI Window object for one-sided communication

🌸 SYNOPSIS 🌸

int MPI_Win_allocate(MPI_Aint size, int disp_unit, MPI_Info info, MPI_Comm comm,
void *baseptr, MPI_Win *win)

int MPI_Win_allocate_c(MPI_Aint size, MPI_Aint disp_unit, MPI_Info info,
MPI_Comm comm, void *baseptr, MPI_Win *win)

bestie the second one has a _c on the end and an MPI_Aint disp_unit instead of an int — do not mix them up, i'm begging 🙈

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🎀 INPUT PARAMETERS 🎀

• size — the size of your window, in bytes (non-negative integer, so like, zero is fine, negative is not, don't try it)
• disp_unit — local unit size for displacements, in bytes (positive integer — positive, bestie, not zero, not negative)
• info — info argument (handle). she's just vibing there carrying your info
• comm — intra-communicator (handle)

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🦋 OUTPUT PARAMETERS 🦋

• baseptr — initial address of window (choice)
• win — window object returned by the call (handle). your brand new baby, freshly allocated 👑

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💀 THREAD AND INTERRUPT SAFETY 💀

okay so good news first: this routine is thread-safe. that means multiple threads can use it safely without you having to bring your own thread locks. no gatekeeping, no user-provided lock drama, she handles it. slay ✨

but — and this is the plot twist 😭 — the routine is not interrupt safe. typically this is because of memory allocation routines like malloc or other non-MPICH runtime routines that are themselves not interrupt-safe. so yeah. she's chill with your threads but do not interrupt her mid-alloc, periodt.

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🌈 NOTES FOR FORTRAN 🌈

• every MPI routine in Fortran (except for MPI_WTIME and MPI_WTICK) has an extra argument ierr tacked on the end of the argument list. ierr is an integer and it means the exact same thing as the return value of the routine in C. so same energy, different syntax 💫
• in Fortran, MPI routines are subroutines, and you invoke them with the call statement. that's just how she is.
• all MPI objects (e.g., MPI_Datatype, MPI_Comm) are of type INTEGER in Fortran. iconic behavior honestly.

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😭 ERRORS 😭

every single MPI routine (except MPI_Wtime and MPI_Wtick) returns an error value — C routines give it back as the value of the function, Fortran routines put it in that last argument. before the value comes back to you, the current MPI error handler gets called. and by default? that error handler aborts the MPI job. yes really. dramatic much 💀

you can swap the error handler out 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 too, but ngl its use is deprecated — so like, don't, bestie. the predefined error handler MPI_ERRORS_RETURN may be used to cause error values to be returned instead.

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 it's giving "we'll try our best" energy 🙏

the error codes, listed so you know exactly what went wrong:

• MPI_SUCCESS — no error; the MPI routine completed successfully. we love a happy ending 🎉
• MPI_ERR_ARG — invalid argument. some argument is invalid and is not identified by a specific error class (e.g., MPI_ERR_RANK). vague, but okay.
• MPI_ERR_COMM — invalid communicator. a super common oopsie is using a null communicator in a call (which is not even allowed in MPI_Comm_rank, so you know it's serious 💅)
• MPI_ERR_DISP — (no description, she's mysterious, she's just a name on the list)
• MPI_ERR_INFO — invalid Info
• MPI_ERR_SIZE — (also no description, equally mysterious 🫥)
• MPI_ERR_OTHER — other error; use MPI_Error_string to get more information about this error code. she's the catch-all, go ask her for the tea.

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👑 SEE ALSO 👑

• MPI_Win_allocate_shared
• MPI_Win_create
• MPI_Win_create_dynamic
• MPI_Win_free

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and that's the whole page bestie, you ate 💖 this was the MPI manual's MPI_Win_allocate(3) page, dated 4/10/2026 — go forth and allocate responsibly, mwah 🎀✨🦋