need help in bit field

**lovesunset21** · 10-29-2010

Hi everyone, I have a program with bit field. However, it displays error when I try to compile. Please show me why it displays error. Thank you so much.

#include <stdio.h>
#include <stdlib.h>
typedef struct
{
unsigned b1:1;
unsigned b2:1;
unsigned b3:1;
unsigned b4:1;
unsigned b5:1;
unsigned b6:1;
unsigned b7:1;
unsigned b8:1;
} byte_s;

int main(void) {
byte_s bfbyte;
unsigned char a[]={0x11,0xff};

bfbyte= (byte_s) a[0]; //displays error: conversion to non-scalar type requested

printf ("%d %d %d %d : %d %d %d %d \n",
bfbyte.b1, bfbyte.b2, bfbyte.b3, bfbyte.b4,
bfbyte.b5, bfbyte.b6, bfbyte.b7, bfbyte.b8);

printf ("%d %d %d %d : %d %d %d %d \n",
bfbyte.b1, bfbyte.b2, bfbyte.b3, bfbyte.b4,
bfbyte.b5, bfbyte.b6, bfbyte.b7, bfbyte.b8);

return EXIT_SUCCESS;
}

**Salem** · 10-29-2010

Guess what you've ignored 11 times now?

And no, that isn't binary, it's DECIMAL
<< !! Posting Code? Read this First !! >>

**~~CommonTater~~** · 10-29-2010

Originally Posted by lovesunset21

Hi everyone, I have a program with bit field. However, it displays error when I try to compile. Please show me why it displays error. Thank you so much.

I take it you are trying to use a struct to extract the bits from a byte?

You need to use a union, not a struct.
Also your bitfield definition is wrong.

Check your docs, try again... and this time please post your code inside the appropriate tags.

**lovesunset21** · 10-30-2010

Thanks for your quick reply. I am working with memcopy function, but it displays a warning "implicit declaration of function ‘memcopy’". I don't know why

**GReaper** · 10-30-2010

Have you included "memory.h"?

**R.Stiltskin** · 10-30-2010

The function is memcpy and it's in string.h.

**GReaper** · 10-30-2010

Originally Posted by R.Stiltskin

The function is memcpy and it's in string.h.

"memory.h" maps to "string.h" ...

**laserlight** · 10-30-2010

Originally Posted by Sipher

"memory.h" maps to "string.h" ...

That may be true, but it is non-standard. Furthermore, unless it is guaranteed that that header will always include <string.h>, you should also include <string.h> for memcpy.

**GReaper** · 10-30-2010

Oh, ok. I didn't know that.

**lovesunset21** · 10-30-2010

Thanks a lot. I have solved my problem. However, I still have question, please help me.

I am working with bit field. I have

Code:

typedef struct bytes{
  unsigned char b8:1;
  unsigned char b7:1;
  unsigned char b6:1;
  unsigned char b5:1;
  unsigned char b4:1;
  unsigned char b3:1;
  unsigned char b2:1;
  unsigned char b1:1;
} bytes;

Then I have an array of bytes

bytes s[10];

Then I want to copy 0x00 to s[0]. I did as followed:

Code:

unsigned char *m;
*m= 0x00
s[0]= *((bytes*)(m));

I don't know why it displays segmentation fault. Please help me. Thank you so much.

**~~CommonTater~~** · 10-30-2010

Originally Posted by lovesunset21

Thanks for your quick reply. I am working with memcopy function, but it displays a warning "implicit declaration of function ‘memcopy’". I don't know why

Why would you need memcpy?

First you've got to straighten out your bitfield definition...

Code:

typedef struct
{
unsigned b1:1;
unsigned b2:1;
unsigned b3:1;
unsigned b4:1;
unsigned b5:1;
unsigned b6:1;
unsigned b7:1;
unsigned b8:1;
} byte_s;

Your struct is going to be 32 bytes long.

Try it like this...

Code:

typedef struct t_byte_s
{ unsigned char b0:1, 
                b1:1,
                b2:1,
                b3:1,
                b4:1,
                b5:1,
                b6:1,
                b7:1; } 
       byte_s;

This will be only 1 byte long (although not all compilers will do this for a byte)

Now you need some means of putting a value in there in one form and getting it out in another. For that we have to use a union...

Code:

typedef union t_byte_s
{ unsigned char MyByte;
   unsigned char b0:1, 
                b1:1,
                b2:1,
                b3:1,
                b4:1,
                b5:1,
                b6:1,
                b7:1; } 
       byte_s;

Ok, now you can do this...

Code:

byte_s testvar;

testvar.MyByte = 127;

if (testvar.b6)
  puts ("eyup, it's a 1");

testvar.MyByte = 0; // clear the bits

testvar.b2 = 1;
printf("We got %d",testvar.MyByte);  // = 4

You can feed in individual bits and read the byte or vice versa, hand it a byte and read the bits.
The reason this works is that in a union all variables occupy the same space (i.e. start at the same address).

**lovesunset21** · 10-30-2010

Thanks so much. But i don't really understand.

Code:

typedef struct
{
unsigned b1:1;
unsigned b2:1;
unsigned b3:1;
unsigned b4:1;
unsigned b5:1;
unsigned b6:1;
unsigned b7:1;
unsigned b8:1;
} byte_s;
Your struct is going to be 32 bytes long.

I thought that it should be 8 bits long.

And in case I want to copy its value to a element of an array. What should I do? I face error segmentation fault

**~~CommonTater~~** · 10-30-2010

Originally Posted by lovesunset21

Thanks so much. But i don't really understand.

Code:

typedef struct
{
unsigned b1:1;
unsigned b2:1;
unsigned b3:1;
unsigned b4:1;
unsigned b5:1;
unsigned b6:1;
unsigned b7:1;
unsigned b8:1;
} byte_s;
Your struct is going to be 32 bytes long.

I thought that it should be 8 bits long.

And in case I want to copy its value to a element of an array. What should I do? I face error segmentation fault

You are using "unsigned" which is an int (32bits, 4 bytes on most systems)
See the semicolon at the end of each line?
That causes each bit to occupy a separate 4 byte value... 4 * 8 = 32 bytes.

Note that I specified unsigned char ... on most systems that is 8bits.
By terminating each line with a coma, except the last one, I've told it to put all 8 values in the same unsigned char... thus, only 1 byte.

**~~CommonTater~~** · 10-30-2010

Originally Posted by lovesunset21

Thanks so much. But i don't really understand.

Code:

typedef struct
{
unsigned b1:1;
unsigned b2:1;
unsigned b3:1;
unsigned b4:1;
unsigned b5:1;
unsigned b6:1;
unsigned b7:1;
unsigned b8:1;
} byte_s;
Your struct is going to be 32 bytes long.

I thought that it should be 8 bits long.

And in case I want to copy its value to a element of an array. What should I do? I face error segmentation fault

Take a look at the way I showed you to do it....

If you want to copy each bit to an array element you will have to do it manually...

Code:

Int bitarray[8];  // array for bits

testvar.MyByte = x;  // stuff in a value

// copy out the bits
bitarray[0] = testvar.b0;
bitarray[1] = testvar.b1;
...
bitarray[7] = testvar.b7;

// or to get them as ascii 1 and 0 for printing
char txtarray[8];

testvar.MyByte = x; // value in
// ascii out
txtarray[0] = testvar.b0 + '0';
...
txtarray[7] = textvar.b7 + '0';

//or to be extra sneaky
sprintf(txtarray,"%d%d%d%d%d%d%d%d",
             testvar.b0,testvar.b1,testvar.b2,testvar.b3,testvar.b4,testvar.b5,testvar.b6,testvar.b7);

**lovesunset21** · 10-30-2010

Thanks so much. Can you explain the UNION keyword for me?

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