libstdc++/16612, empty basic_strings can't live in shared memory

[Mark Mitchell]

Nathan Myers wrote:

>On Thu, Aug 05, 2004 at 05:10:50PM -0700, Mark Mitchell wrote:
>  
>
>>Nathan Myers wrote:
>>    
>>
>>>I like the idea of using static space in libsupc++.  It's simpler,
>>>and should improve performance of the plain vanilla string, too, by 
>>>avoiding relocation indirections.  Can anybody say whether it could 
>>>be arranged for static storage in libsupc++ to appear at the same 
>>>address in all programs compiled for a given ABI version?
>>>      
>>>
>>It cannot be so arranged on some systems.  For example, some systems 
>>allocate memory to shared objects based on link order, so unless 
>>libsupc++ is always the first library, one can't be sure it will end up 
>>at the same place.  Also, if multiple libraries want to be loaded at the 
>>same address, the dynamic linker will have to pick one.
>>    
>>
>
>Perhaps I shouldn't have been so specific.  Might there already be 
>a block of 16 consecutive const (ideally, read-only) zero bytes in 
>crt1.o, or crti.o, or crtbegin.o at (a fixed offset from) an exported 
>symbol?  Might such a block be added to (e.g.) crtbegin.o?  There 
>might be plenty of other uses for such a thing besides std::string.
>
>I don't think that the need for a fixed address in all programs is 
>in any way universal.  I suspect that on most architectures, even 
>where it's not guaranteed, there would be a way to arrange to get a 
>common address, somehow, for (the few) programs that need it.  Other 
>programs would benefit just from the faster access to a non-relocated 
>symbol.
>  
>
I'm not enough of an expert to say for sure about these things.  Do you 
need the memory to be shared across cooperating processes, or just 
allocated at the same address in all cooperating processes?  If it's 
read-only, I'm guessing the latter.  In that case, you could probably 
also use mmap, requesting a particular address.  That will not work in 
the fully general case, but it might well be possible to make it work 
reliably on any given system by picking appropriate addresses.  You 
might also just be able to place an ELF section full of zeros in the 
executable with a fixed load address.

-- 
Mark Mitchell
CodeSourcery, LLC
(916) 791-8304
mark@codesourcery.com