[Gc] Re: libgcj missing symbol __data_start

BGB cr88192 at hotmail.com
Wed Aug 26 11:25:06 PDT 2009

----- Original Message ----- 
From: "Boehm, Hans" <hans.boehm at hp.com>
To: "Andreas Krebbel" <krebbel at linux.vnet.ibm.com>; "Andrew Haley" 
<aph at redhat.com>
Cc: <gcc-patches at gcc.gnu.org>; <java at gcc.gnu.org>; <gc at linux.hpl.hp.com>
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.]



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.



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).

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