problem using bit-fields

[sachinkp21587]

according to me the following code structure should take 6 byte i.e. (3bits *16variable)/8bits but in practical case its taking 8 bytes.
how?
i am using mplab c18

struct {
unsigned int audio1 : 3;
unsigned int audio2 : 3;
unsigned int audio3 : 3;
unsigned int audio4 : 3;
unsigned int audio5 : 3;
unsigned int audio6 : 3;
unsigned int audio7 : 3;
unsigned int audio8 : 3;
unsigned int audio9 : 3;
unsigned int audio10 : 3;
unsigned int audio11 : 3;
unsigned int audio12 : 3;
unsigned int audio13 : 3;
unsigned int audio14 : 3;
unsigned int audio15 : 3;
unsigned int audio16 : 3;
} MutingDelay;

even if i am trying

struct {
unsigned int audio1 : 3;
unsigned int audio2 : 3;
unsigned int audio3 : 3;
unsigned int audio4 : 3;
unsigned int audio5 : 3;
}MutingDelay;

Then if unsigned int is 16 bits thn it should take 2 bytes, but its taking 3 bytes. Why?

 

[Venkadesh_M]

1, Hi what is the size of int?? I think its not 8 may be 16 bit check your compiler manual...
2, when you using struct the first two variables will get allocated 6 bits but the next variable cant be allocated in remaining 2 bits so it will go for next byte thats why your are getting 8 instead of 6...

 

[alexan_e]

unsigned int is (normally) a 16bit type and seems to fit the size you describe

  unsigned int audio1 : 3;
    unsigned int audio2 : 3;
    unsigned int audio3 : 3;
    unsigned int audio4 : 3;
    unsigned int audio5 : 3;
  // 15 bit have been filled, the next three bits can't fit in the one bit left so they are allocated in the next integer space
    unsigned int audio6 : 3;
    unsigned int audio7 : 3;
    unsigned int audio8 : 3;
    unsigned int audio9 : 3;
    unsigned int audio10 : 3;
  // 15 bit have been filled, the next three bits can't fit in the one bit left so they are allocated in the next integer space
    unsigned int audio11 : 3;
    unsigned int audio12 : 3;
    unsigned int audio13 : 3;
    unsigned int audio14 : 3;
    unsigned int audio15 : 3;
  // 15 bit have been filled, the next three bits can't fit in the one bit left so they are allocated in the next integer space
    unsigned int audio16 : 3;

so it occupies four 16bit integers

 

[sachinkp21587]

it says 16bits

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you are right alexan_e that what i think the compiler should work as per my knowledge. bt m not getting why is it taking 3 bytes for even 15 bits...

 

[Venkadesh_M]

Then if unsigned int is 16 bits thn it should take 2 bytes, but its taking 3 bytes. Why?

Hi there is no meaning with unsigned int or char whtever its going to be 3 bits and going to be allocated in byte architecture so the structure padding will be
so first two values two bits remaining
second two variables two bits remaining
last variable

 

[nick703]

according to me the following code structure should take 6 byte i.e. (3bits *16variable)/8bits but in practical case its taking 8 bytes.
how?
i am using mplab c18


even if i am trying


Then if unsigned int is 16 bits thn it should take 2 bytes, but its taking 3 bytes. Why?

in first your code taking 3 byte and second code is taking only 1 byte as alexan_e mentioned .

 

[Venkadesh_M]

whatever the data type you mentioned will be only used when using the variable when allocation every variable will be allocated byte wise....

 

[mathespbe]

Your compiler allocates the memory interms of 8 bits...

//1st 8 bit
unsigned int audio1 : 3;
unsigned int audio2 : 3;
//2nd 8 bit
unsigned int audio3 : 3;
unsigned int audio4 : 3;
//3rd 8 bit
unsigned int audio5 : 3;
unsigned int audio6 : 3;
//4th 8 bit
unsigned int audio7 : 3;
unsigned int audio8 : 3;
//5th 8 bit
unsigned int audio9 : 3;
unsigned int audio10 : 3;
//6th 8 bit
unsigned int audio11 : 3;
unsigned int audio12 : 3;
//7th 8 bit
unsigned int audio13 : 3;
unsigned int audio14 : 3;
//8th 8 bit
unsigned int audio15 : 3;
unsigned int audio16 : 3;

So 8 bytes...

for 2nd the same procedure is followed...

//1st 8 bit
unsigned int audio1 : 3;
unsigned int audio2 : 3;
//2nd 8 bit
unsigned int audio3 : 3;
unsigned int audio4 : 3;
//3rd 8 bit
unsigned int audio5 : 3;

So it takes 3 byte...

 

[jayanth.devarayanadurga]

Struct MutingDelay takes 512 bits or 32 bytes.

 

[nick703]

hey mathespbe why second code also take 8 byte i don't understand please tell me?

and jayanth also tell me how take 512 bit?

 

[jayanth.devarayanadurga]

Second code takes 10 bytes.

 

[Venkadesh_M]

Second code takes 10 bytes.

In which compiler?? there is no structure padding??!!

 

[jayanth.devarayanadurga]

https://publib.boulder.ibm.com/info...bm.vacpp6m.doc/language/ref/clrc03defbitf.htm

https://www.tutorialspoint.com/cprogramming/c_bit_fields.htm

https://publications.gbdirect.co.uk/c_book/chapter6/bitfields.html

https://stackoverflow.com/questions...-a-struct-with-bit-fields-determined-measured

 

[Venkadesh_M]

Hi look at the results

compiled using dev c++

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The problem for stack overflow discussion may be the size of int is 16 bit...

 

[mathespbe]

hey mathespbe why second code also take 8 byte i don't understand please tell me?

Even though you are using 16 bit compiler your memory allocation takes as 8 bit (1 Byte) pattern...
If it doesn't 2nd should only take 2 bytes. Due to this reason only it took 3 bytes...
on the 1st 8 bit (1st BYTE) 3+3 =6 bits allocated... (2 bits not enoug to allocate 3 bit of memory)
on the 2nd 8 bit (2nd BYTE) 3+3 = 6 bits allocated... (2 bits not enoug to allocate 3 bit of memory)
on the 3rd 8 bit (3rd BYTE) 3 bits allocated.... (5 free bits)

if you allocate another variable of 5 bit wide also it'll allocated on the 3 BYTE itself...

 

[nick703]

i understand that 's a reason but in post #8 u posted both 8 byte but second one is only took 3 byte right ?

 

[mathespbe]

i understand that 's a reason but in post #8 u posted both 8 byte but second one is only took 3 byte right ?

sorry i was typed mistakenly... Now i've corrected...