C：获取最高有效位和最低有效位

Question

I am given a 32-bit unsigned integer and I am trying to save the 8 most significant bits from that unsigned integer into an unsigned char. I then want to clear those bits to 0s. That last part can be done with a mask but I am unsure of how exactly to find the 8 most significant bits and then having the mask know exactly which bits to clear.

Another related problem is that I am trying to find the least significant bit and shift it to the left to make it the most significant and return that value. Like above I am unsure on how to find the least significant bit and then know how far to shift it to the left

Answer 1

You can write two general functions one of which return the specified byte of an object of the type unsigned character and other returns a number with cleared bits of the number used as an argument.

Here is a demonstrative program.

#include <stdio.h>
#include <limits.h>

unsigned char extract_byte( unsigned int x, size_t n )
{
    n %= sizeof( x );

    return  x >> CHAR_BIT * n & 0xFF;
}

unsigned int clear_byte( unsigned int x, size_t n )
{
    n %= sizeof( x );

    return x & ~( 0xFF << n * CHAR_BIT );
}

int main(void) 
{
    unsigned int x = 0x04030201;

    for ( size_t i = 0; i < sizeof( x ); i++ )
    {
        printf( "%hhu ", extract_byte( x, i ) );
    }
    putchar( '\n' );

    for ( size_t i = 0; i < sizeof( x ); i++ )
    {
        printf( "%#x ", x = clear_byte( x, i ) );
    }
    putchar( '\n' );

    return 0;
}

Its output is

1 2 3 4 
0x4030200 0x4030000 0x4000000 0