Thanks Quincey, these things for sure will be a lot helpful, Im still pretty new to Objective-C, but I will look in all of those things that you said, thanks :) --- Wilker Lúcio http://about.me/wilkerlucio/bio Kajabi Consultant +55 81 82556600
On Wed, Jun 29, 2011 at 5:13 AM, Quincey Morris <quinceymor...@earthlink.net > wrote: > On Jun 29, 2011, at 00:27, Quincey Morris wrote: > > >> [fileData appendData:[file readDataOfLength:CHUNK_SIZE]]; > >> [file seekToFileOffset:MAX(0, fileSize - CHUNK_SIZE)]; > >> [fileData appendData:[file readDataOfLength:CHUNK_SIZE]]; > >> > >> [file closeFile]; > > > > This is not the worst way to read the entire contents of a file into a > NSData object, but it's pretty bad. :) > > > > It would be far easier and more efficient to use [NSData > dataWithContentsOfFile: path options: NSDataReadingMapped | > NSDataReadingUncached error: NULL]; > > Er, two things. > > I didn't notice until after I posted that you weren't reading the whole > file. So, sorry, "pretty bad" was too strong -- I'm upgrading my response to > "not the best". :) > > I think the way I suggested is still the best way, because (typically) > using the NSDataReadingMapped means the file contents are mapped into > virtual memory, so pages (4KB, or whatever the VM page size is) are not > actually read until you try to access them. > > If you use CC_MD5_Init/Update (first part)/Update (last part)/Final instead > of the convenience method that does it all, only the parts of the file you > need will actually be read. > > NSDataReadingMapped is in general the most efficient way to read a file > when you only need to read any part once, I think. > > The problem with NSFileHandle is that it pretty much sucks as a class. It > doesn't return any errors, so if something goes wrong it's either going to > throw an exception or just ignore it. IIRC it has no API contract regarding > error reporting. > > Second thing... > > I forgot to point out that your code has a glaring bug. With your > declarations, 'fileSize - CHUNK_SIZE' in 'MAX(0, fileSize - CHUNK_SIZE)' is > an unsigned expression. It can't go negative, and so that statement isn't > doing what it looks like it's doing. > > One more thing ... > > Since you have no release/retain calls, I'm assuming you're using garbage > collection. If that's the case, then this line is very, very dangerous: > > > return [SMEncodingUtilities md5HexDigestFromChar:[fileData bytes] > withLength:(unsigned int)[fileData length]]; > > You're passing an *interior* pointer to fileData's private storage, but the > (compiler-optimized) lifetime of 'fileData' ends *before* the md5 method is > entered, and so the object is subject to being collected *before* you've > used the pointer. This will crash. > > In your code, the window of danger is pretty small. But it will crash > eventually. > > The solution is to put a reference to the object after the call: > > > NSString* result = [SMEncodingUtilities > md5HexDigestFromChar:[fileData bytes] withLength:(unsigned int)[fileData > length]]; > > [fileData self]; > > return result; > > It's a PITA, but you gotta be careful with NSData. > > _______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
