Binary operations can be somewhat confusing at times, though quite simple to understand. It is customary to refer to a digit in binary (either 1 or 0) as a bit, likewise a byte is a set of eight bits. So, for example, 10100110 is a byte. A "nibble" is half of a byte, as it is a binary number with four bits.

A word is a sixteen-bit binary number. It is equivalent to two bytes, or four nibbles. A double word is a 32-bit binary number, so a dword is equivalent to two words, or four bytes, or eight nibbles. Most commonly, the number of bits (or bytes) in a number-type is referred to as that type's width.

This is perhaps a more portable way than my previous post.

(Toggle Plain Text)

 #include <stdio.h>
 #include <limits.h>
 
 void showbits(unsigned char byte)
 {
    unsigned char bit;
    for ( bit = 1 << (CHAR_BIT - 1); bit; bit >>= 1 )
    {
 	  putchar(byte & bit ? '1' : '0');
    }
    putchar('\n');
 }
 
 unsigned char pack(unsigned char a, unsigned char b, unsigned char c)
 {
    return ((a & 0xF) << 4) | ((b & 0x3) << 2) | (c & 0x3);
 }
 
 void unpack(unsigned char value, unsigned char *a, unsigned char *b, unsigned char *c)
 {
    *a = (value >> 4) & 0xF;
    *b = (value >> 2) & 0x3;
    *c =  value	   & 0x3;
 }
 
 int main()
 {
    unsigned char a, b, c, value = pack(4,3,2);
    printf("sizeof value = %d\n", (int)(sizeof value));
    showbits(value);
    unpack(value, &a, &b, &c);
    printf("a = %d, b = %d, c = %d\n", a, b, c);
    return 0;
 }
 
 /* my output
 sizeof value = 1
 01001110
 a = 4, b = 3, c = 2
 */

#include <stdio.h> #include <limits.h> void showbits(unsigned char byte) { unsigned char bit; for ( bit = 1 << (CHAR_BIT - 1); bit; bit >>= 1 ) { putchar(byte & bit ? '1' : '0'); } putchar('\n'); } unsigned char pack(unsigned char a, unsigned char b, unsigned char c) { return ((a & 0xF) << 4) | ((b & 0x3) << 2) | (c & 0x3); } void unpack(unsigned char value, unsigned char *a, unsigned char *b, unsigned char *c) { *a = (value >> 4) & 0xF; *b = (value >> 2) & 0x3; *c = value & 0x3; } int main() { unsigned char a, b, c, value = pack(4,3,2); printf("sizeof value = %d\n", (int)(sizeof value)); showbits(value); unpack(value, &a, &b, &c); printf("a = %d, b = %d, c = %d\n", a, b, c); return 0; } /* my output sizeof value = 1 01001110 a = 4, b = 3, c = 2 */

thx for ur help, but all this is under the assumption that the first char must be packed in 4 bits, the second in 2bits,.. but what if the sizes increase ,what about if i want to write 200,400,3 in bits how can i do so

Well, if you want to store a nine-bit value (400) in an eight-bit object (the common unsigned char), you are going to have issues now, aren't you?

Do your values have a known fixed range? Then the solutions I presented can be modified.

Or are you trying to compress data? Then search for compression algorithms.

Or can each value be of a different size? ASN.1 and BER is a way to handle that one.

What is the "big picture"?