MPI_WIN_ALLOCATE(3)				     Open MPI				       MPI_WIN_ALLOCATE(3)

MPI_Win_allocate — One-sided MPI call that allocates memory and returns a window object for RMA operations.

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

INPUT PARAMETERS
       • size: Size of window in bytes (nonnegative integer).

       • disp_unit: Local unit size for displacements, in bytes (positive integer).

       • info: Info argument (handle).

       • comm: Communicator (handle).

OUTPUT PARAMETERS
       • baseptr: Initial address of window.

       • win: Window object returned by the call (handle).

       • ierror: Fortran only: Error status (integer).

DESCRIPTION
       MPI_Win_allocate	 is  a collective call executed by all processes in the group of comm. On each process, it
       allocates memory of at least size bytes, returns a pointer to it, and returns a window object that  can	be
       used  by	 all processes in comm to perform RMA operations. The returned memory consists of size bytes local
       to each process, starting at address baseptr and is associated with  the	 window	 as  if	 the  user  called
       MPI_Win_create <#mpi-win-create> on existing memory. The size argument may be different at each process and
       size  =	0  is  valid;  however, a library might allocate and expose more memory in order to create a fast,
       globally symmetric allocation. The discussion of and  rationales	 for  MPI_Alloc_mem  <#mpi-alloc-mem>  and
       MPI_Free_mem  <#mpi-free-mem> in MPI-3.1 section 8.2 also apply to MPI_Win_allocate; in particular, see the
       rationale in MPI-3.1 section 8.2 for an explanation of the type used for baseptr.

       The displacement unit argument is provided to facilitate address arithmetic in RMA operations:  the  target
       displacement  argument  of  an  RMA  operation  is  scaled  by the factor disp_unit specified by the target
       process, at window creation.

       For supported info keys see MPI_Win_create <#mpi-win-create>.

NOTES
       Common choices for disp_unit are 1 (no scaling), and (in C syntax) sizeof(type), for a window that consists
       of an array of elements of type type. The later choice will allow one to use array indices  in  RMA  calls,
       and have those scaled correctly to byte displacements, even in a heterogeneous environment.

       Calling	MPI_Win_free  <#mpi-win-free> will deallocate the memory allocated by MPI_Win_allocate. It is thus
       erroneous to manually free baseptr.

C NOTES
       While baseptr is a void * type, this is to allow easy use of any pointer object for  this  parameter.  This
       argument is really a void ** type.

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.

       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>/MPI_Init_thread <#mpi-init-thread>,
       after MPI_Finalize <#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 <#
       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>/MPI_Comm_spawn_multiple	<#
       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.

       • 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 <#mpi-abort> was called on that communicator. If called
	 on  a window or file, acts as if MPI_Abort <#mpi-abort> was called on a communicator containing the group
	 of processes in the corresponding window or file. If called on a session, aborts only the local process.

       • MPI_ERRORS_RETURN Returns an error code to the application.

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

       • MPI_Comm_create_errhandler	<#mpi-comm-create-errhandler>	  then	   MPI_Comm_set_errhandler	<#
	 mpi-comm-set-errhandler>

       • MPI_File_create_errhandler	 <#mpi-file-create-errhandler>	   then	    MPI_File_set_errhandler	<#
	 mpi-file-set-errhandler>

       • MPI_Session_create_errhandler	 <#mpi-session-create-errhandler>   then   MPI_Session_set_errhandler	<#
	 mpi-session-set-errhandler> or at MPI_Session_init <#mpi-session-init>

       • MPI_Win_create_errhandler	<#mpi-win-create-errhandler>	  then	    MPI_Win_set_errhandler	<#
	 mpi-win-set-errhandler>

       Note that MPI does not guarantee that an MPI program can continue past an error.

       See the MPI man page <#open-mpi> for a full list of MPI error codes <#open-mpi-errors>.

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

       See also:

	  • MPI_Alloc_mem <#mpi-alloc-mem>

	  • MPI_Free_mem <#mpi-free-mem>

	  • MPI_Win_create <#mpi-win-create>

	  • MPI_Win_allocate_shared <#mpi-win-allocate-shared>

	  • MPI_Win_free <#mpi-win-free>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026				       MPI_WIN_ALLOCATE(3)
