Question of the necessity of rebaseall

Lenik lenik@bodz.net
Thu May 14 01:50:00 GMT 2009


On 2009-5-14 9:34, Larry Hall (Cygwin) wrote:
> Lenik wrote:
>> On 2009-5-14 8:55, Larry Hall (Cygwin) wrote:
>>> Lenik wrote:
>>>> Imagebase in PE header is something a bit of optional, that means if
>>>> two dlls with same imagebase, the OS kernel will automaticly relocate
>>>> one of them, to put them in different virtual spaces. So, rebase
>>>> should be a utility for optimizing the overall start-up speed, to
>>>> reduce avoidable relocations, rather then fix start-up errors.
>>>>
>>>> Any idea?
>>>
>>> cygwin1.dll cannot be relocated if the fork semantics are to work. So
>>> while what you say above is true for apps in general, it's not true for
>>> Cygwin apps.
>>>
>>
>> I can't figure out how fork will break the relocation, for libraries
>> have been loaded, they're already relocated before invoke into fork,
>> and for libraries haven't been loaded, they don't need to share memory
>> after new processes created by fork.
>
> You have it backwards. Forking doesn't break the relocation. Relocation
> breaks forking. cygwin1.dll needs to have a very special memory layout to
> implement the fork semantics in Win32. If this memory layout is disrupted,
> fork breaks. Relocating cygwin1.dll disrupts the required memory layout.
> 'rebaseall' does its best to locate all Cygwin DLLs that it knows of
> into a layout that avoids collisions. This maintains the required
> memory layout so fork can do its job.
>
Could you explain in more detail? I can't find any document about this 
special memory layout.

Thanks


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