Re: Help on bit operation

[Ben Rosenberg <scdlbx@xxxxxxxxx>]

On Wed, Aug 26, 2009 at 4:02 PM, Randi Botse<nightdecoder@xxxxxxxxx> wrote:
> Hi, I'm beginner C programmer, i have a problem, i want to store some
> information in a integer, a integer will be 32 bit on my machine, i
> want to have as follow:
>
> 6 bit (information 1)   MSB
> 4 bit (information 2)
> 8 bit (information 3)
> 5 bit (information 4)
> 9 bit (information 5)   LSB
>
> For example i set the informations as follow (in decimal):
>
> information 1 = 43        or 101011
> information 2 = 11        or 1011
> information 3 = 120      or 1111000
> information 3 = 30        or 11110
> information 4 = 418      or 110100010
>
> if i join all informations i should get a 32 bit integer valued
> 2935782212 or 01010111011111100011110110100010, then my problem is how
> to retrieve these informations on bit operation? i want to know what's
> the value of information-2 or information-3, etc directly.  And, is
> there any good way to join these informations to be an 32 bit integer?
>
> at this time i convert the 32bit integer into binary string, process
> it's with array segment to get all informations then convert them to
> integer,
> to build the 32bit integer, i join all information value into binary
> string (yes, 32 bit of char ;p) join all of them then convert to
> integer.
>
> I know my way is sucks and too far away from COOL thing ;p,  i think
> there are cool way to do this!.
>
> Thanks before!
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>

A quick, dirty way of doing it without structs:

#include <stdio.h>

void store_in_int( unsigned int *target, unsigned int offset, size_t
field_sz, unsigned int value ) {
    unsigned int mask = ((1<<(field_sz))-1)<<offset;
    /* Clear field */
    *target |= mask;
    *target ^= mask;
    /* Load field */
    *target |= value<<offset;
}

unsigned int retrieve_from_int( unsigned int target, unsigned int
offset, size_t field_sz ) {
    unsigned int mask = ((1<<(field_sz))-1);
    return (target>>offset)&mask;
}

int main ( void ) {
    unsigned int info=0;
    store_in_int(&info, 0, 6, 43);
    store_in_int(&info, 6, 4, 11);
    store_in_int(&info,10, 8,120);
    store_in_int(&info,18, 5, 30);
    store_in_int(&info,23, 9,418);
    printf("info1: %u\n",retrieve_from_int( info, 0, 6 ));
    printf("info2: %u\n",retrieve_from_int( info, 6, 4 ));
    printf("info3: %u\n",retrieve_from_int( info,10, 8 ));
    printf("info4: %u\n",retrieve_from_int( info,18, 5 ));
    printf("info5: %u\n",retrieve_from_int( info,23, 9 ));
    return 0;
}
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