What does rebaseall in Cygwin do?

Question

When bumping in dll/forking errors in cygwin, i'm calling the rebaseall script and everything magically works again. I know that it somehow modifies the dlls in the cygwin installation because i've run a diff between the problematic ones and the rebased ones.

What does it exactly modify in these binary files and makes them work again?

Answer 1

According to Error messages about module base addresses:

DLLs on Windows need to be loaded into memory at non-conflicting base addresses. rebaseall is a cygwin utility that scans all the libraries currently installed and sets each to request a different base address so that none will conflict.

A bit more technical explanation from Handling repeated failures of rebaseall to allow cygwin remaps:

Because of unix fork semantics (presumably), cygwin libraries must be mapped in the same location in both parent and child of a fork. All cygwin libraries have hints in them as to where they should be mapped in a processes address space; if those hints are followed, each library will be mapped in the same location in both address spaces. However, Windows is perfectly happy mapping a DLL anywhere in the address space; the hint is not considered controlling. The remapping error occurs when a cygwin process starts and one of its libraries cannot be mapped to the location specified by its hint.

/usr/bin/rebaseall changes the DLL hints for all of the cygwin libraries so that there are no inter-library conflicts; it does this by choosing a contiguous but not overlapping library layout starting at a base address and working down. This process makes sure there are no intra-cygwin conflicts, but cannot deal with conflicts with external DLLs that are in cygwin process address spaces (e.g. anti-virus DLLs).