Re: libgcj missing symbol __data_start

----- Original Message ----- 
From: "Boehm, Hans" <[email protected]>
To: "Andreas Krebbel" <[email protected]>; "Andrew Haley" 
<[email protected]>
Cc: <[email protected]>; <[email protected]>; <[email protected]>
Sent: Wednesday, August 26, 2009 10:41 AM
Subject: RE: libgcj missing symbol __data_start


> [Adding the gc mailing list, since this still applies to current gc 
> versions.]

<snip>

<--

Unfortunately, I don't think this is the right solution.  If I understand 
correctly, DATASTART will end up as zero, which will cause the collector to 
start tracing at location zero, which seems unlikely to work, if the value 
is needed.  I suspect the value isn't actually used when dl_iterate_phdr is 
available and the application is dynamically linked, but still ...

Another popular option on other architectures is to define 
SEARCH_FOR_DATA_START instead of defining DATASTART to have a specific 
value.  This creates a weak reference to __data_start.  If that ends up 
being zero, the collector will walk the address space backwards starting at 
__end until an access faults.  It will assume that that's the beginning of 
the main data segment.  That might make debugging a bit harder because of 
the extra SIGSEGV at startup, but I still think it's a much better solution, 
assuming it actually works.  If it doesn't, this at least needs a 
conspicuous FIXME comment, since it's pretty clearly doing the wrong thing. 
In any case, I think this should also go in the upstream source.

Hans

-->


although not directly related, I will note that for my GC on Windows, I 
ended up switching to a strategy of using the ToolHelp library to iterate 
over the loaded modules and find their used address ranges...

it also makes the GC work more reliably as well...

I am not sure if something similar can be done on Linux (I have not looked 
into this).