Attempts to read up to
buffer.length bytes from this into
buffer, as with
Note that since this method never blocks, you cannot actually use
cancellable to cancel it. However, it will return an error if
cancellable has already been cancelled when you call, which may happen if you call this method after a source triggers due to
having been cancelled.
a buffer to read data into (which should be at least
a Cancellable, or null
the number of bytes you want to read
the number of bytes read, or -1 on error (including g_io_error_would_block).