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Bit_test function and pointers

 
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d00dajo



Joined: 20 Jul 2004
Posts: 34

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Bit_test function and pointers
PostPosted: Tue Sep 20, 2005 1:46 am     Reply with quote
Hi,

I have a problem using the bit_test function with pointers.
bit_clear and bit_set will both work perfectly, but the following:

Code:

int8 * myPtr;

myPtr = &(MyStruct[2].MyElement[6]);

if(Alpha)
{
   bit_set(*myPtr,0); //OK
}
else
{
 bit_clear(*myPtr,0); //OK
}

if(bit_test(*myPtr,0)) //NOT OK. Will return TRUE always!!!
{

}



So, it seems that compiler function bit_test will not work with pointers. I dont really see WHY the compiler does not have support for that, since it is stated that bit_test is the same as ((var & (1<<bit)) != 0) only more efficient. (*myPtr & (1 << bit)) != 0 should work perfectly.

Any suggestions?

//Daniel.[/code]
d00dajo



Joined: 20 Jul 2004
Posts: 34

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Addition.
PostPosted: Tue Sep 20, 2005 1:54 am     Reply with quote
Addition:

((*myPtr & (1<<bitnr)) != 0)

This does work!
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Tue Sep 20, 2005 2:03 am     Reply with quote
You didn't give a full test program, so I made one.
This program works OK with PCM vs. 3.233.
It displays:

False

True


Code:

#include <16F877.H>
#fuses XT, NOWDT, NOPROTECT, BROWNOUT, PUT, NOLVP
#use delay(clock = 4000000)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7, ERRORS)

struct
{
int8 MyElement[10];
}MyStruct[4];
   
//==============================
void main()
{
int8 *myPtr;

myPtr = &(MyStruct[2].MyElement[6]);


MyStruct[2].MyElement[6] = 0;

if(bit_test(*myPtr,0))
   printf("True\n\r");
else
   printf("False\n\r");
   

MyStruct[2].MyElement[6] = 0xFF;

if(bit_test(*myPtr,0))
   printf("True\n\r");
else
   printf("False\n\r");

while(1);
}
d00dajo



Joined: 20 Jul 2004
Posts: 34

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Problem resolved
PostPosted: Tue Sep 20, 2005 2:24 am     Reply with quote
PCM programmer wrote:
You didn't give a full test program, so I made one.
This program works OK with PCM vs. 3.233.
It displays:

False

True


Hi "PCM programmer",

Thank you for your assistance,

Yes, of course the bit_test function works perfectly with pointers. The problem was derived from a memory overallocation on my part.

//Daniel.
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