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Sebastian Sumpf avatar

A brief history of symbol maps on Genode

When programming against Genode's shared libraries one might encounter things like this:

 Error: LD: jump slot relocation failed for symbol: 'wait_for_continue'

where wait_for_continue can be exchanged with any function that should be present within a shared library. The thing is, not all symbols within a shared library should be accessible from the outside.


In the early days of Genode we started to port things, like the dynamic linker and because it was ported we had to implement certain functions like read within the linker, while not using the libc - that implements read also. The dynamic linker version just read the program header of its own ELF file and was satisfied. Things changed when my colleague started to use Qt5 applications and ended up to see the ELF header within his applications while using the read call.

Symbol maps

Therefore, we started to use symbol maps for the linking of shared libraries. They allow to make certain symbols local to the library while others are global. In order to make sure your symbol/function is global call:

 objdump -TC <shared object>

and watch out for the g letter in the second row. If your function is marked l it is a local function which will not be accessible from the outside of the library.

ABI files

With Genode's packet management system there is the notion of ABI symbol files, within API archives, which are plain symbol files of a shared library and are linked against a binary. A symbol file contains all public function names as produced by

 nm <shared object>

for C++ methods, this means that they are mangled. So, if you encounter a function that is present within a shared library, but not visible from the outside, please add the symbol to the appropriate ABI file.

Symbol files reside under lib/symbols within a Genode repository. An example for libpng can be found here