MPI_WIN_ALLOCATE_SHARED(3)			     Open MPI				MPI_WIN_ALLOCATE_SHARED(3)

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

SYNTAX
   C Syntax
	  #include <mpi.h>

	  int MPI_Win_allocate_shared (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_SHARED(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_shared(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_shared is a collective call executed by all processes  in  the	group  of  comm.  On  each
       process, it allocates memory of at least size bytes that is shared among all processes in comm, and returns
       a  pointer  to  the  locally  allocated	segment in baseptr that can be used for load/store accesses on the
       calling process. The locally allocated memory can be the target of load/store accesses by remote processes;
       the base	 pointers  for	other  processes  can  be  queried  using  the	function  MPI_Win_shared_query	<#
       mpi-win-shared-query>.  The  call also returns a window object that can be used by all processes in comm to
       perform RMA operations. The size argument may be different at each process and size = 0 is valid. It is the
       user’s responsibility to ensure that the communicator comm represents a group of processes that can  create
       a  shared  memory segment that can be accessed by all processes in the group. The discussions of 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_shared;	 in particular, see the rationale in MPI-3.1 section 8.2 for an explanation of the
       type used for baseptr. The allocated memory  is	contiguous  across  process  ranks  unless  the	 info  key
       alloc_shared_noncontig  is  specified.  Contiguous across process ranks means that the first address in the
       memory segment of process i is consecutive with the last address in the memory segment of process  i  -	1.
       This may enable the user to calculate remote address offsets with local information only.

       The following info keys are supported:

       alloc_shared_noncontig
	      If  not  set to true, the allocation strategy is to allocate contiguous memory across process ranks.
	      This may limit the performance on some architectures because it does not allow the implementation to
	      modify the data layout (e.g., padding to reduce access latency).

       blocking_fence
	      If set to true, the osc/sm component  will  use  MPI_Barrier  <#mpi-barrier>  for	 MPI_Win_fence	<#
	      mpi-win-fence>.  If  set to false a condition variable and counter will be used instead. The default
	      value is false. This info key is Open MPI specific.

       For additional 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_shared. 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_allocate <#mpi-win-allocate>

	  • MPI_Win_create <#mpi-win-create>

	  • MPI_Win_shared_query <#mpi-win-shared-query>

	  • MPI_Win_free <#mpi-win-free>

Copyright
       2003-2026, The Open MPI Community

						   Mar 05, 2026				MPI_WIN_ALLOCATE_SHARED(3)
