[gradsusr] UDF in open grads 2

[Arlindo da Silva]

On Thu, Dec 1, 2011 at 5:01 PM, Arlindo da Silva
<arlindo.dasilva at gmail.com>wrote:

> Henrique,
>
>   This is a proposed API which has not been released. Your best bet is to
> download the opengrads sources and look at all the examples under
> extensions/. Start with the "hello, world" example under hello/. You can
> write extensions (UDFs or UDCs) in C, Fortran or even gs scripts.
>
>
At this point only the low level API is available. You can find some
information about this API here:


http://opengrads.org/wiki/index.php?title=Low-level_API_for_Writing_Extensions_in_GrADS_v2.0

For example, you could use this interface to re-implement any of the
regular grads intrinsic functions in src/gafunc.c.  As I said, I have
purposedely not documented the grads internal data structures involved, you
need to know what you are doing to use this API. But with just a bit of
"monkey see, monkey do" you should be able to create meaningful extensions
with relatively minimum effort.

I should also mention that the IPC
extensions<http://opengrads.org/doc/udxt/libipc/>include some
functions

  IPC_save()
  IPC_load()
  IPC_pipe()

that could be used to emulate the functionality of the older UDF in GrADS
v1.9. For example, IPC_save() can write any grads expression to a file
(using a header very similar to the header in traditional UDFs), and
IPC_load() can read similar files returning a grads expression. So, you can
use IPC_save() to write to disk the input data for your UDF; you can use
"!" to run you UDF applicatication, and IPC_load() to read the results of
your UDF application. IPC_pipe() is a bit trickier to use, but it is very
powerful (I used it to implement the GRIB-2 interface in GrADS v1.9).
Combine this with the gsUDF
capability<http://opengrads.org/doc/udxt/gsudf/>and you got yourself a
bonafide UDF as in the good old days. Read the
documentation above for more information.

  Good Luck,

-- 
Arlindo da Silva
dasilva at alum.mit.edu
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