MPI_WIN_ALLOCATE(3)

💅 MPI_Win_allocate — Open MPI's iconic little memory-and-window two-for-one

okay so bestie, you're reading the MPI_Win_allocate(3) page from Open MPI, and honestly? It's giving "I do two things and I do them flawlessly" main character energy. Let's get into it. ✨

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🎀 NAME 🎀

MPI_Win_allocate — a one-sided MPI call that allocates memory and hands you back a window object for RMA operations. Two gifts in one call, we love a multitasker. 🫶

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✨ SYNTAX ✨

💖 C Syntax

#include <mpi.h>

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

💖 Fortran Syntax

USE MPI
! or the older form: INCLUDE 'mpif.h'
MPI_WIN_ALLOCATE(SIZE, DISP_UNIT, INFO, COMM, BASEPTR, WIN, IERROR)
     INTEGER(KIND=MPI_ADDRESS_KIND) SIZE, BASEPTR
     INTEGER DISP_UNIT, INFO, COMM, WIN, IERROR

💖 Fortran 2008 Syntax

USE mpi_f08
MPI_Win_allocate(size, disp_unit, info, comm, baseptr, win, ierror)
     USE, INTRINSIC :: ISO_C_BINDING, ONLY : C_PTR
     INTEGER(KIND=MPI_ADDRESS_KIND), INTENT(IN) :: size
     INTEGER, INTENT(IN) :: disp_unit
     TYPE(MPI_Info), INTENT(IN) :: info
     TYPE(MPI_Comm), INTENT(IN) :: comm
     TYPE(C_PTR), INTENT(OUT) :: baseptr
     TYPE(MPI_Win), INTENT(OUT) :: win
     INTEGER, OPTIONAL, INTENT(OUT) :: ierror

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🌸 INPUT PARAMETERS 🌸

• size — how big the window is, in bytes. Nonnegative integer, so zero is allowed and lowkey valid (more on that later 👀).
• disp_unit — the local unit size for displacements, in bytes. Positive integer, bestie — no negatives, no zeroes.
• info — the info argument, a handle. 🎀
• comm — the communicator, also a handle. 💖

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🦋 OUTPUT PARAMETERS 🦋

• baseptr — the initial address of the window. Your memory's home address, basically.
• win — the window object the call returns (handle). This is the whole point, purr. 👑
• ierror — Fortran only, bestie: error status (integer).

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🔥 DESCRIPTION 🔥

MPI_Win_allocate is a collective call, meaning every single process in the group of comm executes it together — no lone wolves here, it's a group project and everyone's contributing. ✨

On each process, it:

1. allocates memory of at least size bytes
2. returns a pointer to it
3. returns a window object that all processes in comm can use to perform RMA operations

The memory you get back consists of size bytes local to each process, starting at address baseptr, and it's associated with the window as if you'd called MPI_Win_create on existing memory. The size argument can be different at each process (iconic, honestly) and size = 0 is fully valid. That said — and this is the tea ☕ — a library might allocate and expose more memory than you asked for, in order to create a fast, globally symmetric allocation. So don't be shocked if there's extra. 🙈

The discussion of and rationales for MPI_Alloc_mem and MPI_Free_mem in MPI-3.1 section 8.2 also apply to MPI_Win_allocate; in particular, peep the rationale in MPI-3.1 section 8.2 for an explanation of the type used for baseptr. iykyk 📚

Now, the displacement unit argument exists to make address arithmetic in RMA operations easier: the target displacement argument of an RMA operation gets scaled by the factor disp_unit specified by the target process, at window creation. Math but make it cute. 🧮💅

For supported info keys, see MPI_Win_create.

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⭐ NOTES ⭐

Common choices for disp_unit are:

• 1 — no scaling, simple, clean, we stan
• (in C syntax) sizeof(type) — for a window that consists of an array of elements of type type

That second choice is the big-brain move: it lets you use array indices in RMA calls and have those scaled correctly to byte displacements — even in a heterogeneous environment. Slay. 🌈

⚠️ Calling MPI_Win_free will deallocate the memory allocated by MPI_Win_allocate. It is thus erroneous to manually free baseptr. Do not do it. I'm begging. 😭

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💀 C NOTES 💀

So baseptr is a void * type — but that's just to allow easy use of any pointer object for this parameter. This argument is really a void ** type. It's giving disguise. 🕵️‍♀️

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🍓 ERRORS 🍓

Almost all MPI routines return an error value: C routines as the return result of the function, and Fortran routines in the last argument. Classic. 🎀

Before the error value is returned, the current MPI error handler associated with the communication object (e.g., communicator, window, file) is called. If no communication object is associated with the MPI call, then the call is considered attached to MPI_COMM_SELF and will call the associated MPI error handler. When MPI_COMM_SELF is not initialized (i.e., before MPI_Init/MPI_Init_thread, after MPI_Finalize, or when using the Sessions Model exclusively) the error raises the initial error handler. The initial error handler can be changed by calling MPI_Comm_set_errhandler on MPI_COMM_SELF when using the World model, or the mpi_initial_errhandler CLI argument to mpiexec or info key to MPI_Comm_spawn/MPI_Comm_spawn_multiple. If no other appropriate error handler has been set, then the MPI_ERRORS_RETURN error handler is called for MPI I/O functions and the MPI_ERRORS_ABORT error handler is called for all other MPI functions. 😤

Open MPI includes three predefined error handlers that can be used:

• MPI_ERRORS_ARE_FATAL — causes the program to abort all connected MPI processes. dramatic but effective 💀
• MPI_ERRORS_ABORT — an error handler that can be invoked on a communicator, window, file, or session. When called on a communicator, it acts as if MPI_Abort was called on that communicator. If called on a window or file, it acts as if MPI_Abort was called on a communicator containing the group of processes in the corresponding window or file. If called on a session, it aborts only the local process.
• MPI_ERRORS_RETURN — returns an error code to the application. polite king 🫶

MPI applications can also implement their own error handlers by calling:

• MPI_Comm_create_errhandler then MPI_Comm_set_errhandler
• MPI_File_create_errhandler then MPI_File_set_errhandler
• MPI_Session_create_errhandler then MPI_Session_set_errhandler or at MPI_Session_init
• MPI_Win_create_errhandler then MPI_Win_set_errhandler

⚠️ Note that MPI does not guarantee that an MPI program can continue past an error. Like, at all. 😬

See the MPI man page for a full list of MPI error codes.

See the Error Handling section of the MPI-3.1 standard for more information.

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🌟 SEE ALSO 🌟

• MPI_Alloc_mem
• MPI_Free_mem
• MPI_Win_create
• MPI_Win_allocate_shared
• MPI_Win_free

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👑 COPYRIGHT 👑

2003-2026, The Open MPI Community

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and that's the whole page, bestie — no cap, every flag, every handle, every warning, all of it. 💜

✧･ﾟ: * MPI_WIN_ALLOCATE(3) — Open MPI — Sep 11, 2026 * :･ﾟ✧ go forth and allocate responsibly 🦋🎀✨