# MPI_Win_allocate_shared

Create an MPI Window object for one-sided communication and shared memory access, and allocate memory at each process

## Synopsis

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

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

## Input Parameters

size
  size of local window in bytes (non-negative integer)
disp_unit
  local unit size for displacements, in bytes (positive integer)
info
  info argument (handle)
comm
  intra-communicator (handle)

## Output Parameters

baseptr
  address of local allocated window segment (choice)
win
  window object returned by the call (handle)

This is a collective call executed by all processes in the group of comm. On each process i, 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 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.

## Thread and Interrupt Safety

This routine is thread-safe. This means that this routine may be safely used by multiple threads without the need for any user-provided thread locks. However, the routine is not interrupt safe. Typically, this is due to the use of memory allocation routines such as `malloc` or other non-MPICH runtime routines that are themselves not interrupt-safe.

## Notes for Fortran

All MPI routines in Fortran (except for `MPI_WTIME` and `MPI_WTICK`) have an additional argument `ierr` at the end of the argument list. `ierr` is an integer and has the same meaning as the return value of the routine in C. In Fortran, MPI routines are subroutines, and are invoked with the `call` statement.

All MPI objects (e.g., `MPI_Datatype`, `MPI_Comm`) are of type `INTEGER` in Fortran.

## Errors

All MPI routines (except `MPI_Wtime` and `MPI_Wtick`) return an error value; C routines as the value of the function and Fortran routines in the last argument. Before the value is returned, the current MPI error handler is called. By default, this error handler aborts the MPI job. The error handler may be changed 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 but its use is deprecated. 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; however, MPI implementations will attempt to continue whenever possible.

MPI_SUCCESS
  No error; MPI routine completed successfully.

MPI_ERR_ARG
  Invalid argument. Some argument is invalid and is not identified by a specific error class (e.g., `MPI_ERR_RANK`).

MPI_ERR_COMM
  Invalid communicator. A common error is to use a null communicator in a call (not even allowed in `MPI_Comm_rank`).

MPI_ERR_DISP

MPI_ERR_INFO
  Invalid Info

MPI_ERR_SIZE

MPI_ERR_OTHER
  Other error; use `MPI_Error_string` to get more information about this error code.

## See Also

MPI_Win_allocate MPI_Win_create MPI_Win_create_dynamic MPI_Win_free MPI_Win_shared_query