What does the hasSpaceAvailable property on NSOutputStream mean?

Question

I'm trying to wrap my head around the logic behind hasSpaceAvailable on NSOutputStream.

In my app, I'm sending large amounts of data (100MB) broken up into 4080byte chunks (hard limit) over a CFSocket managed by NSInput/output streams.

When I start writing the data, about a quarter way through hasSpaceAvailable suddenly becomes NO, and so I add the data to a queue. However, if I ignore that and try to write the data anyways, the write seems to work as the return value of write:maxLength: matches the maxLength parameter (4080).

What does the output stream have space for? As far as I can tell, when using UNIX/Berkley sockets there is no logic available to determine if the socket can be written to, you just write and determine if all of the data was written.

The documentation for the property states:

A boolean value that indicates whether the receiver can be written to. (read-only)
YES if the receiver can be written to or if a write must be attempted in order to determine if space is available, NO otherwise.

In my example where I'm seeing a NO, what factor is causing this result when I can still write to that socket.

Answer 1

I think the hasSpaceAvailable property just returns YES if the stream has sent a "space available" stream event since the last time you called the write method. You shouldn't poll that property, and it arguably shouldn't even exist. Instead, you should wait for a stream event on the output stream that says that there's space available for writing instead.

When that stream event occurs, it means that the outgoing packet queue has at least one byte fewer than the maximum number of bytes that the socket is configured to allow you to queue up. In other words, a send() or write() system call on the socket is guaranteed to write at least one byte without blocking, and the socket is guaranteed to be in a nonblocking mode.

Note that after you write data, the stream will send another space available event immediately if the stream's buffer can take more data (or after it has sent some data if the buffer is full).