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Some Casting question

 
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kmp84



Joined: 02 Feb 2010
Posts: 354

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Some Casting question
PostPosted: Wed Jul 06, 2016 1:27 pm     Reply with quote

Hello All,
Is there any problem on this code if not do some cast manipulation?
(Store data as int16 and return only LSB Byte):
Code:

int8 TestFunc(int8 *data){
   int8 i,len;
   int16 tmp;
   for(i=0;i<len;i++){
      tmp+=data[i];
   }
   return tmp;// return only LSB Byte
}


Thanks.
Ttelmah



Joined: 11 Mar 2010
Posts: 19515

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PostPosted: Wed Jul 06, 2016 2:22 pm     Reply with quote

Obvious thing is that len is not initialised, and could contain anything. You might sum one byte, or 255.....

You are actually performing teo implicit casts. C does this all the time. If you pass any variable of the 'wrong' type, it'll attempt to cast it to the target type.

So the arithmetic when you add data[i], is 16bit, and this is converted to a 16bit value when used. Then tmp is cast to an 8bit value when passed to the return.
kmp84



Joined: 02 Feb 2010
Posts: 354

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PostPosted: Wed Jul 06, 2016 3:14 pm     Reply with quote

Hello,
Yes "len" and "tmp" has to be initialized. It's my mistake. Main focus was typecasting.
Thanks Mr.Telmah!
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