how to copy a file

how to copy a file
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I use the code to read data from file and write the data to another place.

var v0=69632;//file size
var vvv= cc.FileUtils.getInstance().getFileData( “”, “r”,v0);
//jsb to write vvv to a file in writable path

but it reports

Cocos2d: Get data from file( failed!

the size of file ‘’ in writable path is only 12,288 bytes rather than 69,632 bytes