[fpc-devel] Proposal: Supporting initial references for weakexternal

[Jeppe Johansen]

Den 12-01-2011 12:42, Jonas Maebe skrev:
>
> On 12 Jan 2011, at 06:29, Jeppe Johansen wrote:
>
>> While trying to find a nice solution for generating interrupt vector 
>> tables on ARM, I found that the usually employed solution is to 
>> generate weak references to symbols, that are initialized to a single 
>> handler. This way an application can override the weak reference, by 
>> declaring a new symbol with the same name. While it's possible to do 
>> that currently by using some external assembler code, I think a 
>> simpler solution could be made by using the weakexternal directive. 
>> The infrastructure is already there, but it's used in a slightly 
>> different way.
>
> How is it normally done in C? At first sight, using the weakexternal 
> directive to /define/ symbols seems like a wrong approach.
>
>
> Jonas
With C it's done using compiler dependent attributes. With gcc you can 
do it using __attribute__((weak,alias("SomeFunction")));. Though often 
it'll just be done using assembler using the .weak and .set/thumb_set 
directives

The weakexternal directive already "defines" a symbol, it will exist at 
runtime, but it might be nil. The only difference is that if you set the 
initial value it'll resolve to something at link time