Returns the file descriptor associated with the specified channel.
- iOS 5.0+
- macOS 10.7+
- Mac Catalyst 13.0+
- tvOS 9.0+
- watchOS 2.0+
The channel whose file descriptor you want to retrieve.
The file descriptor associated with the channel or
-1 if the file descriptor is closed or not yet open.
If the path name associated with the channel has not yet been opened, calling this function does not normally open the corresponding file, with one exception. If you call the function from a barrier block scheduled on the channel, the function does open the file and return the resulting file descriptor.