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Re: Debugger thread dump confusion


On Aug 28, 2006, at 4:49 PM, Scott Ribe wrote:

Any ideas how to find the offending code (even though I know it's a
duff reference when the UTF8String message is sent to the null ref
user)?  Shouldn't it show:

0 - objc_msgSend
1 - main
2 - start


Well, actually no, or maybe ;-)

- In Objective-C it's legal to send messages to nil. You might or might not
get a useful return value (IIRC the rule is that if the method returns an
object you get back nil, but if the method returns a C type the return is
undefined. Even worse, there are cases where on PPC the 0'ish value will be
returned, but not on Intel.)

"In Objective-C, it is valid to send a message to a nil object. The Objective-C runtime assumes that the return value of a message sent to a nil object is nil, as long as the message returns an object or any integer scalar of size less than or equal to sizeof(void*).
On Intel-based Macintosh computers, messages to a nil object always return 0.0 for methods whose return type is float, double, long double, or long long. Methods whose return value is a struct, as defined by the Mac OS X ABI Function Call Guide to be returned in registers, will return 0.0 for every field in the data structure. Other struct data types will not be filled with zeros. This is also true under Rosetta. On PowerPC Macintosh computers, the behavior is undefined."

<http://developer.apple.com/documentation/MacOSX/Conceptual/ universal_binary/universal_binary_tips/chapter_5_section_22.html>



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