#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>

#define readBits(start,size,data) ((*((uint64_t *) &data[(start) / 8]) >> ((start) & 7)) & ((1 << (size)) - 1))

uint64_t readData(const char* filename, unsigned int offset, unsigned int size) {
  uint64_t data = 0;
  int fd = open(filename, O_RDONLY);
  if(fd == -1) {
    return data;
  }
  if(lseek(fd, offset/8, SEEK_SET) == -1) {
    close(fd);
    return data;
  }
  ssize_t numBytes = read(fd, &data, sizeof(data));
  if(numBytes != sizeof(data)) {
    close(fd);
    return data;
  }
  uint64_t* dataPtr = &data;
  uint64_t aux = readBits(offset % 8, size, dataPtr);
  data = aux;
  close(fd);
  return data;
}

int main(int argc, char** argv) {
  char c;
  const char* filename = NULL;
  unsigned int offset = 0;
  unsigned int size = 0;
  while ((c = getopt (argc, argv, "f:o:s:")) != -1) {
    switch(c) {
      case 'f':
        filename = optarg;
        break;
      case 'o':
        offset = atoi(optarg);
        break;
      case 's':
        size = atoi(optarg);
        break;
      case '?':
        return 1;
    }
  }

  if(!filename || !offset || !size) {
    fprintf(stderr, "Need to specify filename (-f), offset (-o) and size (-s)\n");
    return 1;
  }
  uint64_t data = readData(filename, offset, size);
  printf("Read: 0x%016llX (%llu)\n", data, data);
  return 0;
}