a buffer to read data into (which should be at least count bytes long)
the [I/O priority][io-priority] of the request
optional gio.Cancellable object, NULL to ignore
callback to call when the request is satisfied
the data to pass to callback function
2.44
Request an asynchronous read of count bytes from the stream into the buffer starting at buffer.
This is the asynchronous equivalent of InputStream.readAll.
Call InputStream.readAllFinish to collect the result.
Any outstanding I/O request with higher priority (lower numerical value) will be executed before an outstanding request with lower priority. Default priority is G_PRIORITY_DEFAULT.