Rewinding an NSInputStream based on an NSData

Question

I have an NSData that I would like to read as an NSInputStream. This way I can have a consistent API for processing both files and in-memory data. As part of the processing, I would like to make sure that the stream begins with some set of bytes (if it doesn't, I need to process it differently). I'd like to avoid reading a whole file into memory if it's of the wrong type.

So I'm looking for either a way to rewind the stream, or a way to "peek" at the upcoming bytes without moving the read pointer. If this is an NSInputStream created with URL, I can use setProperty:forKey: on NSStreamFileCurrentOffsetKey, but bizarrely this does not work on an NSInputStream created from an NSData (even though you would presume this would have been even easier to implement than the file version). I can't close and reopen the steam to reset the input pointer either (this is explicitly not allowed by NSStream).

I can rework this problem using an NSData-only interface and -initWithContentsOfMappedFile, but I'd rather stay with the NSStream approach if I can.

Answer 1

I think I don't understand something here. An NSInputStream can take data from three places: a socket, an NSData object, or a file. You haven't said that you want to use a socket, which leaves the other two as your data sources. Also, docs for NSStream say that only file-based streams are seekable. (NSStream, overview, 3rd paragraph)

Given that, I'd think that an NSData object would be a better choice. An NSData object will handle both files and bytes (which I think is what you mean by data in memory).

But you consider that and say that you'd prefer to stick with streams. Is there some other consideration here?

(Edit) Sorry, I should have made this a real answer. My answer for the issue you've described is that using NSData really is the right thing to do.

If you prefer a different answer, then please give more details.