| Re: hooking external 'C' functions to IDL [message #5912] |
Wed, 13 March 1996 00:00 |
rivers
Messages: 228
Registered: March 1991
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Senior Member |
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In article <4i6u40$ivf@peabody.colorado.edu>, sena@spot.Colorado.EDU (Michelle Sena) writes:
> I am currently working on a program that uses an IDL gui interface to access
> an external database (Sybase) that contains various data needed for analysis
> and display. I need to go through 'C' to access the database. I would be
> getting back an array or arrays of data. From what I've seen, CALL_EXTERNAL
> can only return discrete data values. Do I need to use the LINKIMAGE routine
> to get back arrays of data? Or am I missing an option for the CALL_EXTERNAL
> procedure that would allow me to do this?
There is no restriction on CALL_EXTERNAL returning arrays in its function
arguments. I think it can probably only return a scalar as the function value.
IDL essentially passes a pointer to your C routine. You need to make sure that
IDL and C agree on what it is pointing to in terms of data type and number of
elements.
____________________________________________________________
Mark Rivers (312) 702-2279 (office)
CARS (312) 702-9951 (secretary)
Univ. of Chicago (312) 702-5454 (FAX)
5640 S. Ellis Ave. (708) 922-0499 (home)
Chicago, IL 60637 rivers@cars3.uchicago.edu (Internet)
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| Re: hooking external 'C' functions to IDL [message #5915 is a reply to message #5912] |
Wed, 13 March 1996 00:00  |
Ken Knighton
Messages: 44
Registered: May 1995
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Member |
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sena@spot.Colorado.EDU (Michelle Sena) wrote:
> I am currently working on a program that uses an IDL gui interface to access
> an external database (Sybase) that contains various data needed for analysis
> and display. I need to go through 'C' to access the database. I would be
> getting back an array or arrays of data. From what I've seen, CALL_EXTERNAL
> can only return discrete data values. Do I need to use the LINKIMAGE routine
> to get back arrays of data? Or am I missing an option for the CALL_EXTERNAL
> procedure that would allow me to do this?
CALL_EXTERNAL can pass arguments to the C function being called. These
arguments are passed as argc (the number of arguments) and argv (an array
of pointers to the arguments). The memory being addressed by these pointers
can be manipulated at will (keeping in mind that you have to know how
much memory is set aside for each IDL type). An argument could be a scalar
or an array. It is not difficult to copy your Sybase data over into an
array argument that you have passed into your C routine. When CALL_EXTERNAL
returns, the new data is now in the IDL variable used as the parameter in
the CALL_EXTERNAL call. Returning values this way rather than as the return
value of the function means that the return value can be used for something
better, such as an error code.
In order to pass strings back to the calling program, create a byte array
of the appropriate size and pass that into the C routine. The documentation
warns against overwriting string areas that were passed to the C routine via
CALL_EXTERNAL.
One of the drawbacks to this approach is that you have to figure out how
much data is going to be passed back to your routine and allocate an array
big enough to hold it all. If this presents a problem, you may want to set
up some sort of a dual call system in which the first CALL_EXTERNAL returns
the size of the data, you allocate an array that large in IDL, and then call
the same routine (i.e. the routine uses static variables) or another routine
that completes the process. I have never had a need for anything other
than a single call with predefined data, but I don't see why it wouldn't
work.
By the way, you should make sure that you thoroughly read all of the
documentation on CALL_EXTERNAL in the User's Guide and in the Reference
Manual as well as the documentation provided in text files with your IDL
distribution. CALL_EXTERNAL can be a little tricky to use. If you make
a mistake, you will probably crash IDL or corrupt it in some mysterious way.
Another suggestion: Since you are doing I/O from within a CALL_EXTERNAL
routine, look in the IDL Advanced Development Guide for information
about UNIX signals (if applicable). This can save you a lot of heartache.
I hope this helps.
Ken Knighton knighton@gav.gat.com knighton@cts.com
General Atomics
San Diego CA
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