| CALL_EXTERNAL puzzle? [message #12680] |
Thu, 03 September 1998 00:00 |
rmlongfield
Messages: 68
Registered: August 1998
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Member |
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Hi All, I've been working with CALL_EXTERNAL and C programs for the
past week or so. I've copied the code concerning my question below this
letter. The problem first appeared when I tried calling CALL_EXTERNAL a
second time (to call another C program). The variable I wanted to pass,
filename, was no longer being passed correctly. After several hours of
searching, I think I have found the problem, although I still do not
understand it. I had an informational PRINT statement on the line just before
the call to the second CALL_EXTERNAL (seemed a harmless thing to do). But...
the variable that I printed (the long integer returned from the first
CALL_EXTERNAL) became the argv[0] value for the call to the C program in the
second CALL_EXTERNAL (Is anyone still following me here?)!
Now, I thought that I didn't tell CALL_EXTERNAL to do this, but when I deleted
the informational PRINT just before the FIRST CALL_EXTERNAL, it didn't work
properly. THAT informational statement printed the name of the file, which
conveniently ended up in my C program where I actually wanted it. :)
So, is this a bug or a feature of CALL_EXTERNAL which I didn't quite grasp
from the excellent documentation ;) I hate to think that my success with
CALL_EXTERNAL was merely accidental.
Rose Dllllllllsky
p.s. Concerning the speed of CALL_EXTERNAL, I've never made a comparison.
I just have a lot of C code that I do not want to convert to IDL.
"It's easy, once you know how" --Miss Hubbard (Postman Pat)
;____ IDL Code_____________________________________________________
;A CALL_EXTERNAL puzzle:
;IDL Version 5.0 (IRIX mipseb)
;------------------------------------------
PRO example
ddyyyhhmm = '961081302'
filename=(PTR_NEW(STRTRIM(STRING(ddyyyhhmm),2)))
print,'de-referenced pointer: ',*filename
result = CALL_EXTERNAL(VALUE=[1B],'example_c.so','example_c', filename)
print,'Result after first call: ',result
result = CALL_EXTERNAL(VALUE=[1B],'example_c.so','example_c', filename)
print,'Result after second call: ',result
print,'Has filename changed? ',*filename
end
;------------------------------------------
; C_ program (commented out for IDL)
;------------------------------------------
; #include <stdio.h>
; #include <stdlib.h>
; long example_c(int argc,void *argv[])
; {
; static char *filename;
; static long return_number;
; filename = (char *)argv[0];
; /* Read number of parameters passed from CALL_EXTERNAL */
; fprintf(stderr,"*IDL____Value of argc: %d \n",argc);
; /* Check values of parameters */
; fprintf(stderr,"*IDL____Value of argv: %s \n",(char *)argv[0]);
; fprintf(stderr,"*IDL____Value of filename: %s \n",filename);
; return_number = 88;
; return(return_number);
; }
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| Re: CALL_EXTERNAL puzzle? [message #12766 is a reply to message #12680] |
Thu, 03 September 1998 00:00  |
steinhh
Messages: 260
Registered: June 1994
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Senior Member |
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rmlongfield@my-dejanews.com wrote:
> print,'de-referenced pointer: ',*filename
> result = CALL_EXTERNAL(VALUE=[1B],'example_c.so','example_c', filename)
Whooa - here you're sending a *pointer* to a *string* variable
into the call_external routine - and it's definitely *not* set
up to receive it! In fact, your routine would *only* work if
you sent it a scalar string that is passed by *value* only.
(E.g. a string literal)
> So, is this a bug or a feature of CALL_EXTERNAL which I didn't quite grasp
> from the excellent documentation ;) I hate to think that my success with
> CALL_EXTERNAL was merely accidental.
Sorry to say it's a feature :-)
Regards,
Stein Vidar
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