[xml] encoding parameter usage for xmlReaderForXXX

["Prashant R" <ramapra gmail com>]

xmlReaderForIO (xmlInputReadCallback ioread,

xmlInputCloseCallback ioclose,

void *ioctx,

const char *URL,

const char *encoding,

int options);

I have been typically passing NULL for the const char * encoding parameter for this API . Are there any limitations to doing this ? 

How is this parameter used ? In other words what other values could be supplied to this ? 

Thanks in advance