An utility for making quines, basically..

pack.c

// $ gcc pack.c -o pack
// $ ./pack e "Hello World!" | ./pack d
// Hello World!

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

typedef uint64_t box;

int help(int argc) {
    int exit = 0;
    if (argc != 1) {
        printf("Bad command\n");
        exit = 1;
    }

    printf("Concept: pack a string into a sequence of 64 bits integers.\nAn utility for making quines, basically..\n");
    printf("Usage:\n");
    printf("pack e \"Hello World\" > out.txt\n");
    printf("pack d < out.txt\n");

    printf("\nNo-op test:\n");
    printf("pack e \"Hello World\" | pack d\n");
    return exit;
}

void encode(const char str[]) {
    size_t len = strlen(str);
    size_t i = 0;
    printf("%i\n", (len / 8) + (len % 8 != 0));
    while (i < len) {
        box enc = 0;
        for (size_t j = 0; j < 8; j++) {
            // char c = i + j < len ? 1 : '\0'; // debug 2**(k*8) + 2**((k-1)*8) + .. + 1
            char c = i + j < len ? str[i + j] : '\0';
            enc = enc | (box)c << j * 8;
        }
        i += 8;
        printf("%" PRIx64 "\n", enc);
    }
}

void decode() {
    size_t count;
    scanf("%i", &count);

    while (count--) {
        uint64_t n;
        scanf("%" PRIx64 "", &n);
        for (size_t i = 0; i < 8 * sizeof(box); i += 8) {
            char c = 0xff & (n >> i);
            if (c == '\0')
                break;
            else
                putchar(c);
        }
    }
}

int main(int argc, const char* argv[]) {
    if (argc == 2 && *argv[1] == 'd') {
        decode();
        return 0;
    } else if (argc == 3 && *argv[1] == 'e') {
        encode(argv[2]);
        return 0;
    }
    return help(argc);
}