# MPI_Grequest_start

Create and return a user-defined request

## Synopsis

    int MPI_Grequest_start( MPI_Grequest_query_function *query_fn,
                          MPI_Grequest_free_function *free_fn,
                          MPI_Grequest_cancel_function *cancel_fn,
                          void *extra_state, MPI_Request *request )

## Input Parameters

query_fn
  callback function invoked when request status is queried (function)
free_fn
  callback function invoked when request is freed (function)
cancel_fn
  callback function invoked when request is cancelled (function)
extra_state
  Extra state passed to the above functions.

## Output Parameters

request
  Generalized request (handle)

## Notes on the callback functions

The return values from the callback functions must be a valid MPI error code or class. This value may either be the return value from any MPI routine (with one exception noted below) or any of the MPI error classes. For portable programs, `MPI_ERR_OTHER` may be used; to provide more specific information, create a new MPI error class or code with `MPI_Add_error_class` or `MPI_Add_error_code` and return that value.

The MPI standard is not clear on the return values from the callback routines. However, there are notes in the standard that imply that these are MPI error codes. For example, pages 169 line 46 through page 170, line 1 require that the `free_fn` return an MPI error code that may be used in the MPI completion functions when they return `MPI_ERR_IN_STATUS`.

The one special case is the error value returned by `MPI_Comm_dup` when the attribute callback routine returns a failure. The MPI standard is not clear on what values may be used to indicate an error return. Further, the Intel MPI test suite made use of non-zero values to indicate failure, and expected these values to be returned by the `MPI_Comm_dup` when the attribute routines encountered an error. Such error values may not be valid MPI error codes or classes. Because of this, it is the user's responsibility to either use valid MPI error codes in return from the attribute callbacks, if those error codes are to be returned by a generalized request callback, or to detect and convert those error codes to valid MPI error codes (recall that MPI error classes are valid error codes).

## 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`).