accessing pointed values and increasing pointer, get different results in Visual Studio and CCS3/CCS5

francis

Intellectual 480 points

Hello,
I meet a problem when debugging below code:

/////////////////////////////////////////////////////////////////////

unsigned int sigmaM,sigmaM2;

DSP8u *pTempTpl = (DSP8u *)malloc(64);

sigmaM=0;
sigmaM2=0;

*(pTempTpl+0) = 90;
*(pTempTpl+1) = 80;

sigmaM += *pTempTpl; /* pos1 */
sigmaM2 += (*pTempTpl) * (*pTempTpl++);

// /* pos2 */

/////////////////////////////////////////////////////////////////////

I got different results when running above code in Visual Studio 2008 and CCS3/CCS5:
(1) when stopping at pos1
I see:
*(pTempTpl+0) = 90;
*(pTempTpl+1) = 80;
which is as expected both in VS2008 and CCS.
(2) when stopping at pos2
(a) in VS2008
sigmaM value is 90,
sigmaM2 value is 8100 as expected
(b) in CCS C6000 pjt
sigmaM value is 90,
sigmaM2 value is 7200, not as expected

what's the reason?
thanks!

over 9 years ago

0 Titusrathinaraj Stalin over 9 years ago

TI__Guru** 116100 points

Dear Francis,
Can you please share the complete projects both VS2008 & CCS ?
BTW, which DSP processor are you using?

0 francis over 9 years ago in reply to Titusrathinaraj Stalin

Intellectual 480 points

I can's add attachments here, so I send you an email.thanks!

0 Raja over 9 years ago in reply to francis

TI__Guru* 81335 points

Francis, I think this is not the correct forum for this query. Please provide the CGT version. I will move this to TI compiler forum. Thanks.

0 Archaeologist over 9 years ago

TI__Guru* 84285 points

Your code has undefined behavior because of this line:

sigmaM2 += (*pTempTpl) * (*pTempTpl++);

In the C language, it's not legal to modify pTempTpl and also access it more than once between sequence points. Change it to this:

sigmaM2 += (*pTempTpl) * (*pTempTpl);
pTempTpl++;

0 francis over 9 years ago in reply to Archaeologist

Intellectual 480 points

this will give out same results both in Viasual Studio and CCS.

My question is why the results are different when running the old code?

0 Archaeologist over 9 years ago in reply to francis

TI__Guru* 84285 points

Because you've got undefined behavior. When you have undefined behavior, the results may be different between implementations. It could be different depending on the phase of them moon. See en.wikipedia.org/.../Undefined_behavior and c-faq.com/.../undef.html

0 pf over 9 years ago in reply to Archaeologist

TI__Expert 4930 points

In this specific case, the language does not define the order in which *pTempTpl and *pTempTpl++ happen, as long as both happen before the multiply, and does not define the exact point at which the ++ happens, as long as it happens before the next statement.
Thus you could read the same location twice, or read two consecutive locations, and which one you get will depend on the compiler, the platform, the compilation options, and other environmental factors (possibly including the moon).

0 francis over 9 years ago in reply to pf

Intellectual 480 points

thanks!

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accessing pointed values and increasing pointer, get different results in Visual Studio and CCS3/CCS5