On Jan 13, 2010, at 5:54 PM, Graham Cox wrote:
> @implementation NSData (SHA1Hash)
>
> - (NSData*) sha1Hash
> {
> // calculates the 160 bit SHA-1 digest of the given data
>
> unsigned char* digest = (unsigned char*) malloc(20);
> SHA1([self bytes], [self length], digest);
>
> return [[self class] dataWithBytesNoCopy:digest length:20];
> }
>
> @end
>
> The static analyzer reports this:
>
> Potential leak of an object allocated on line 622
>
> Method returns an Objective-C object with a +1 retain count
> (owning reference)
> Object returned to caller as an owning reference (single retain
> count transferred to caller)
> Object allocated on line 622 is returned from a method whose
> name ('sha1Hash') does not contain 'copy' or otherwise starts with 'new' or
> 'alloc'. This violates the naming convention rules given in
> the Memory Management Guide for Cocoa (object leaked)
>
> But I think this is wrong - [NSData dataWithBytesNoCopy] returns an object I
> don't own, and it in turn takes ownership of <digest>. So where's the leak?
> Is clang simply mistaking the fact that +dataWithBytesNoCopy CONTAINS 'copy'
> and not STARTS WITH copy?
Yes, the static analyzer is wrong here.
The Official Rule is '... contains "copy" ...'. This is a case where a
mechanical interpretation of the Official Rule isn't quite right.
The static analyzer does have a special case for +[NSData
dataWithBytesNoCopy:length:].
http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20090511/017258.html
But your code sends the message to [self class] instead of NSData, so perhaps
the special case is not recognized. Alternatively, perhaps your Xcode isn't the
newest available.
Try this to suppress the complaint:
return [(NSData *)[self class] dataWithBytesNoCopy:digest length:20];
--
Greg Parker gpar...@apple.com Runtime Wrangler
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