#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/types.h>

void foo(char *buf)
{
  char a;

  a = buf[0];
  a = buf[4];
  a = buf[8];
  a = buf[10];
  buf[5]  = 't';
  buf[10] = 'e';
  buf[20] = 's';
  buf[30] = 't';
}

int main(int ac, char **av)
{
  int fd;
  char *buf;

  if (!(buf = malloc(256)))
    return -1;

  fd = open("./file.txt", O_RDONLY);
  read(fd, buf, 256), close(fd);
  foo(buf);
}