# MPI_Win_shared_query

Query the size and base pointer for a patch of a shared memory window.

## Synopsis

    int MPI_Win_shared_query(MPI_Win win, int rank, MPI_Aint *size, int *disp_unit, void *baseptr)

This function queries the process-local address for remote memory segments created with `MPI_Win_allocate_shared`. This function can return different process-local addresses for the same physical memory on different processes.

The returned memory can be used for load/store accesses subject to the constraints defined in MPI 3.0, Section 11.7. This function can only be called with windows of type `MPI_Win_flavor_shared`. If the passed window is not of flavor `MPI_Win_flavor_shared`, the error `MPI_ERR_RMA_FLAVOR` is raised. When rank is `MPI_PROC_NULL`, the pointer, disp_unit, and size returned are the pointer, disp_unit, and size of the memory segment belonging the lowest rank that specified size > 0. If all processes in the group attached to the window specified size = 0, then the call returns size = 0 and a baseptr as if `MPI_Alloc_mem` was called with size = 0.

## Input Parameters

win
  window object used for communication (handle)
rank
  target rank

## Output Parameters

size
  size of the segment at the given rank
disp_unit
  local unit size for displacements, in bytes (positive integer)
baseptr
  base pointer in the calling process' address space of the shared segment belonging to the target rank.

## 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 guarentee 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_RANK
  Invalid source or destination rank. Ranks must be between zero and the size of the communicator minus one; ranks in a receive (`MPI_Recv`, `MPI_Irecv`, `MPI_Sendrecv`, etc.) may also be `MPI_ANY_SOURCE`.

MPI_ERR_WIN
  Invalid MPI window object

## See Also

MPI_Win_allocate_shared