calc.c

/*
 * Byte calculator. Sums bytes of a given file and displays to screen.
 *
 * Compile:
 *  gcc calc.c -o calc -O2
 *
 */
#include <stdlib.h>
#include <stdio.h>
#include <stdint.h>
#define BUFFER (1 << 20)

unsigned long long do_calc(FILE *f) {
    uint8_t buf[BUFFER];
    unsigned long long sum = 0;
    size_t nbytes, i;

    while ((nbytes = fread(buf, 1, BUFFER, f)) > 0)
        for (i = 0; i < nbytes; i++)
            sum += buf[i];

    return sum;
}

int main(int argc, const char* argv[]) {
    FILE *f;
    unsigned long long res;

    if (argc != 2)
        return 1;
   
    if ((f = fopen(argv[1], "rb")) == NULL) {
        perror("fopen");
        return 1;
    };

    res = do_calc(f);
    fclose(f);

    printf("%llu\n", res);
    return 0;
}

Best I can do for now: https://gist.github.com/yfyf/6639162