readByte
Tries to read a single byte from the stream or the buffer. Will block during this read.
On success, the byte read from the stream is returned. On end of stream -1 is returned but it's not an exceptional error and @error is not set.
If @cancellable is not null, then the operation can be cancelled by triggering the cancellable object from another thread. If the operation was cancelled, the error %G_IO_ERROR_CANCELLED will be returned. If an operation was partially finished when the operation was cancelled the partial result will be returned, without an error.
On error -1 is returned and @error is set accordingly.
Return
the byte read from the @stream, or -1 on end of stream or error.
Parameters
cancellable
optional #GCancellable object, null to ignore