invert-bits.c

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

void printBits(char *bits, size_t n) {
    for (int i = 0; i < n; ++i) {
        printf("%c", (bits[i/8] & (1 << (7 - i%8))) == 0 ? '0' : '1');
    }
    printf("\n");
}

void invertBits(char *bits, size_t n) {
    int i = 0, j = n-1;
    char ib, jb;
    while (i < j) {
        ib = (bits[i/8] & (1 << (7 - i%8))) > 0 ? 1 : 0;
        jb = (bits[j/8] & (1 << (7 - j%8))) > 0 ? 1 : 0;

        bits[i/8] = (bits[i/8] & ~(1 << (7 - i%8))) | (jb << (7 - i%8));
        bits[j/8] = (bits[j/8] & ~(1 << (7 - j%8))) | (ib << (7 - j%8));

        ++i;
        --j;
    }
}

int main(int argc, char *argv[]) {
    size_t n = strlen(argv[1]);
    char c[2] = { 0x8f, 0x4f };
    printBits(c, 13);
    invertBits(c, 13);
    printBits(c, 13);
}