Read bytes

Given an API ReadFromDisk() which returns a block of 256 bytes from disk. Implement the API Read(SizeInBytes) which reads SizeInBytes bytes from the disk by calling only ReadFromDisk. Notice that every call to ReadFromDisk() will read the next 256 bytes from disk.

Update 2015 - 03 - 30
I realize that I understood the ReadN problem wrong, the char[] buff is the output array. Here is the updated version. I updated the ReadN too.

public int read2(byte[] buff, int sizeInBytes){
  int index = 0, next = 0;
  //assume return -1 if reach the end of the file
  byte[] tmp = new byte[256];
  while(index < sizeInBytes && (next = readFromDisk(tmp))!= -1)
   for(int i = 0; i< next && index < sizeInBytes; buff[index++] = tmp[i++]);
  return index;
 }
 public int readFromDisk(byte[] buff){
  return 256;
 }

This is similar to the LeetCode's ReadN question. I hijacked the code from java src (see here if you want the original).

public void read(int sizeInBytes) throws IOException{
  //readFromDisk() will read 256 bytes and put into 
  //the byte array, so we only need the size / 256
  int size = sizeInBytes / 256;
  byte[] b = new byte[size];
  int c = readFromDisk();
  //assume readFromDisk() will return -1 if the 
  //length is less than 256
  if(c == -1)
   return;
  b[0] = (byte)c;
  int i = 1;
  for(; i < size; i++){
   c = readFromDisk();
   //reach the end of the file
   if(c == -1)
    break;
   b[i] = (byte)c;
  }
 }
 public int readFromDisk(){
  return 255;
 }