file descriptors example.c

struct file_descriptor
{
  // implementation goes here
  int read(char * destination, int count);
  int write(char * source, int count);
};

struct disk_file : public file_descriptor
{
  // route requests to disk I/O functions
  int read(char * destination, int count);
  int write(char * source, int count);
}

struct socket_file : public file_descriptor
{
  // route requests to network I/O functions
  int read(char * destination, int count);
  int write(char * source, int count);
}

// kernel keeps record of the list of file descriptors you have open
int file_descriptor_count = /* some value meaning the maximum number of open files you are permitted to have */
file_descriptor * descriptors = malloc(file_descriptor, sizeof(file_descriptor) * file_descriptor_count;

// read() will read `count` bytes from the _file descriptor_ and put into `destination`.
// it will return the number of bytes which were successfully read (which may be different from the count you requested
// or it will set errno to an appropriate _error reason_ and return -1 if an error occurred.
int read(int fd_value, char * destination, int count)
{
    if ( (fd_value < 0) || (file_descriptor_count <= fd_value)|| (nullptr == destination) )
    {
         errno = /* something from #include <errno.h> which meands you gave bad function arguments */
         return -1;
     }

    // hand the request off to the file descriptor
    return descriptors[fd_value].read(destination, count);
}