Request an asynchronous read of count bytes from the stream into the buffer starting at buffer. When the operation is finished callback will be called. You can then call InputStream.readFinish to get the result of the operation.
During an async request no other sync and async calls are allowed on stream, and will result in G_IO_ERROR_PENDING errors.
A value of count larger than G_MAXSSIZE will cause a G_IO_ERROR_INVALID_ARGUMENT error.
On success, the number of bytes read into the buffer will be passed to the callback. It is not an error if this is not the same as the requested size, as it can happen e.g. near the end of a file, but generally we try to read as many bytes as requested. Zero is returned on end of file (or if count is zero), but never otherwise.
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.
The asynchronous methods have a default fallback that uses threads to implement asynchronicity, so they are optional for inheriting classes. However, if you override one you must override all.
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