fs.read have a different behaviour - node.js

Question

Any idea why fs.read cannot behave as fs.readSync?
My code is very simple, just read out the songs file chunk by chunk. And I find out with fs.readSync function that the song file can read out 512 bytes everytime while with fs.read function, there is no log info printed out and if i delete the while(readPosition < fileSize), it executes only one time.
var chunkSize = 512; //the chunk size that will be read every time
var readPostion = 0; //the first byte which will be read from the file.
var fileSize =0;
var fs=require('fs');
//var Buffer = require("buffer");
//var songsBuf = Buffer.alloc(512);
var songsBuf = new Buffer(chunkSize);
fs.open('/media/sdcard/song.mp3','r',function(err,fd){
if(err)
throw err;
console.log("The file had been opened");
var fileSize = fs.fstatSync(fd).size;
console.log("The total size of this file is:%d Bytes",fileSize);
console.log("Start to read the file chunk by chunk");
//read the file in sync mode
while(readPostion<fileSize)
{
fs.readSync(fd,songsBuf,0,chunkSize,readPostion);
if(readPostion+chunkSize>fileSize)
chunkSize = fileSize-readPostion;
readPostion+=chunkSize;
console.log("the read position is %d",readPostion);
console.log("The chunk size is %d",chunkSize);
console.log(songsBuf);
}
//the code above can readout the file chunk by chunk but the below one cannot
//read the file in Async mode.
while(readPostion<fileSize)
{
// console.log("ff");
fs.read(fd,songsBuf,0,chunkSize,1,function(err,byteNum,buffer){
if(err)
throw err;
console.log("Start to read from %d byte",readPostion);
console.log("Total bytes are %d",byteNum);
console.log(buffer);
if(readPostion+chunkSize>fileSize)
chunkSize = fileSize-readPostion; //if the files to read is smaller than one chunk
readPostion+=chunkSize;
});
}
fs.close(fd);
});

Answer 1

You can do this with the async library.
async.whilst(
function () { return readPostion < fileSize },
function (callback) {
fs.read(fd, songsBuf, 0, chunkSize, 1, function (err, byteNum, buffer) {
if (err) return callback(err)
console.log("Start to read from %d byte",readPostion);
console.log("Total bytes are %d",byteNum);
console.log(buffer);
if(readPostion + chunkSize > fileSize)
chunkSize = fileSize - readPostion; //if the files to read is smaller than one chunk
readPostion += chunkSize
callback(null, songBuffer)
})
},
function (err, n) {
if (err) console.error(err)
fs.close(fd)
// Do something with songBuffer here
}
)