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MPI_Win_allocate_shared(3) man page (version 2.0.4)

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


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

INCLUDE ’mpif.h’

Input Parameters

Size of window in bytes (nonnegative integer).
Local unit size for displacements, in bytes (positive integer).
Info argument (handle).
Communicator (handle).

Output Parameters

Initial address of window.
Window object returned by the call (handle).
Fortran only: Error status (integer).


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. 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 and MPI_Free_mem in MPI-3.1 [char167] 8.2 also apply to MPI_Win_allocate_shared; in particular, see the rationale in MPI-3.1 [char167] 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:

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).

If set to true, the osc/sm component will use MPI_Barrier for 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.


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.


Almost all MPI routines return an error value; C routines as the value of the function and Fortran routines in the last argument.

Before the error value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job, except for I/O function errors. The error handler may be changed with MPI_Comm_set_errhandler; 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.

See Also

MPI_Alloc_mem MPI_Free_mem MPI_Win_allocate MPI_Win_create MPI_Win_shared_query

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