gistfile1.c

#include <stdio.h>
#include <stdint.h>
#include <string.h>

#define CHAR_BIT 8
#define MAXBITS (CHAR_BIT*sizeof(uintmax_t))
#define FMT_BUF_SIZE (MAXBITS+1)

char *binary_fmt(uintmax_t x, char buf[static FMT_BUF_SIZE])
{
    char *s = buf + FMT_BUF_SIZE;
    memset(buf, '0', FMT_BUF_SIZE);
    *--s = 0;
//    printf("buf: %s", buf);
//    if (!x) *--s = '0';
    for (; x; x/=2) *--s = '0' + x%2;
    return buf;
}

uintmax_t createmask(int bits) {
    uintmax_t x = ~0;
    // why is GCC using a shift that respects sign bit on an unsigned variable?
    if (bits) {
        return x >> (MAXBITS - bits);
    } else {
        return 0;
    }
}

uintmax_t buttface(int startbit, int endbit) {
    return createmask(startbit - endbit) << endbit;
}

main() {
    char buf[FMT_BUF_SIZE];

    uintmax_t m;

    printf("%s\n", binary_fmt(buttface(4,2), buf));
    printf("%s\n", binary_fmt(buttface(64,32), buf));
    printf("%s\n", binary_fmt(buttface(7,3), buf));
}

/*

% ./masks             
0000000000000000000000000000000000000000000000000000000000001100
1111111111111111111111111111111100000000000000000000000000000000
0000000000000000000000000000000000000000000000000000000001111000

*/