Thread: advanced bit fields

here i have used a char * and have malloc ed an arbitrary number of bytes to it. now what i need to do is fill all the 5 bytes with 0(that is taken care of by the *ch=0 , no problem). now after this i or ch with 1 and shift it left 1 time. if u see the output at this time u will notice that not only is the 2nd bit (from right) is set to 1 but also another bit in the 5th byte. how do u correct this? or is such thing not possible. the code is like this.can be run in TC. thanx in advance.

Code:

#include<stdio.h>
#include<conio.h>
#include<alloc.h>

void showbits(char *n,int b)
{
	int i,k;
	int *mask;
	mask=(int *)malloc(b);
	int count=0;
	for(i=(8*b)-1;i>=0;i--)
	{
		*mask=1<<i;
		k=*n & *mask;
		k==0 ? printf("0"):printf("1");
		count++;
	}
	printf("\n%d\n",count);
}

int main()
{
//	clrscr();
	char *ch;
	ch=(char *)malloc(5);
	*ch=0;
	for(int i=0;i<8;i++)
	{
		*ch=*ch | 1;
		*ch=*ch<<1;
	}
 	*ch=*ch | 1;
	*ch=*ch<<1;
	showbits(ch,5);
	getch();
	return 0;
}

Code:

for(int i=0;i<8;i++)
{
	*ch=*ch | 1;
	*ch=*ch<<1;
}
*ch=*ch | 1;
*ch=*ch<<1;

All this code does is set the value to 254. The last bit (1s) doesn't get set, because you shift at the end. The last two lines server no purpose at all, because that's already been done inside the loop. They're pointless.

Furtheremore, there is no reason to malloc five bytes, because you only ever use one of them.

Finally, your showbits function is well, bad. It doesn't show the bits the way they are. It ... It's wrong. It doesn't just show all set bits. If that is what it was supposed to do, it doesn't. While we're at it, you never free anything you malloc. Not to mention the fact that you use C99 variable declaration some of the time, but not all the time. The idea is to be consistant in what you do.

Consider the following replacement:

Code:

void showbits( unsigned char *n, int b )
{
        int i,j;

        for( j = 0; j < b; j++, n++ )
        {
                for( i = 0; i < 8; i++ )
                        printf("%d", *n & (1<<i) ? 1 : 0 );
                printf("\n");
        }
}

Which is what I assume you were attempting to do. I agree with Kip, it's "interesting".

Quzah.