[fpc-devel] One more issue with string function results (hopefully, the last)

Jonas Maebe jonas.maebe at elis.ugent.be
Wed Jan 23 10:41:01 CET 2008

On 22 Jan 2008, at 20:28, Peter Vreman wrote:

>> This issue is caused by your own patch to change the ansistring  
>> code to use pass the result by
>> reference.
>> Original code (using current 2.2.1) was:
>> # [21] s := foo(s);             // This one failes
>> 	movl	-4(%ebp),%eax
>> 	movl	%eax,%ebx
>> 	leal	-4(%ebp),%eax
>> 	movl	%ebx,-4(%ebp)
>> The new code since r9718 is:
>> # [21] s := foo(s);             // This one failes
>> 	leal	-4(%ebp),%edx
>> 	movl	-4(%ebp),%eax
>> Here you can see that the result is at the same location as the  
>> parameter. In the original code
>> the value is only assigned to the original location after the call.
>> The quick-solution is simple: revert r9718 until it works.
> The example that is provided doesn't fail. Because when calling foo  
> the value of -4(%ebp) is
> loaded in %eax and that value is used in the function foo. The  
> setlength() will create a new
> string and store that at -4(%ebp) of the calling function. But  
> doesn't change the original
> parameter value in %eax (and also stored also at (%esp) in foo).

It is nevertheless unsafe. The called routine has a const ansistring  
parameter. The refcount for such parameters is not increased. So when  
the result (= parameter) is overwritten, things will go wrong if
a) the refcount was 1
b) the memory location of the old string is reused


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