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Groups > comp.soft-sys.math.mathematica > #2669

Re: Problem with MLGetReal(Double) in Mathlink

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addtwo isn't the program you should be changing.  factor is the example you want
to start with because it's the closest to doing exactly what it is you want to
do.  And if you look at factor.c, you'll notice that you are missing a critical
step.  You didn't send an EvaluatePacket.  The Mathematica kernel has a protocol
for how it receives information from other applications, and you need to follow
it.  That means, in this case, using EvaluatePacket (or one of its compatriots,
depending upon your needs) to wrap the evaluation you wish to perform.

The reason for the wild answer, incidentally, is because MLGetDouble() failed. 
You didn't realize this because you didn't check the return value to determine
whether there was an error.  The documentation for MLGetDouble() does not define
any behavior for how MLGetDouble will treat its double pointer argument when it
fails.  I suspect, in this case, that it merely left untouched whatever random
bits happen to be in the uninitialized double.

Sincerely,

John Fultz
jfultz@wolfram.com
User Interface Group
Wolfram Research, Inc.


On Sat, 21 May 2011 06:48:17 -0400 (EDT), Soroush Heidari wrote:
> When I change the simple addtwo program in order to get 2 Real
> (Double) numbers and get a Real result, the result is wrong (5+4 => 97986213 !) What is the problem.
> Thanks...
> ***********************************************************************
> #include <stdio.h>
> #include "mathlink.h"
>
> int main( int argc, char* argv[])
>
> {
> double a, b;
> double ans;
>
> MLINK lp;
> MLEnvironment env;
> printf( "Enter two integers: a and b\n\t" );
> scanf( "%d %d", &a, &b);
> env = MLInitialize(NULL);
> if(env == NULL) return 1;
> lp = MLOpen(argc, argv);
> if(lp == NULL) return 1;
>
> MLPutFunction(lp, "Plus", 2);
> MLPutDouble(lp, a);
> MLPutDouble(lp, b);
> MLEndPacket(lp);
>
> /* skip any packets before the first ReturnPacket */
> while (MLNextPacket(lp) != RETURNPKT) MLNewPacket(lp);
>
> MLGetDouble(lp, &ans);
> printf( "ans = %d\n", ans);
> MLClose(lp);
> MLDeinitialize(env);
> return 0;
> }

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Re: Problem with MLGetReal(Double) in Mathlink John Fultz <jfultz@wolfram.com> - 2011-05-24 09:58 +0000

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