Linking static libraries with clang independent of order

Question

In GCC, I can use the linker flags -Wl,--start-group and -Wl,--end-group to resolve linking problems with libraries that have circular dependencies. I'd like to do the same with clang, but it seems like this feature was dropped in lld version 3.2. How do I do it?

Ogbomosho

en.wikipedia.org/wiki/Technical_debt – Anthropophagy

list all the libs (in any order) twice: -la -lb ...-lx -la -lb ... -lx (best done with a script) – Authority

@Authority This may or may not be enough. – Ambagious

@Ambagious should be -- this guarantees that for every symbol referenced in the object code, the ~definition will appear at least once after the reference / declaration – Authority

@Authority GNU ld will only include objects from an archive that it actually needs at that point. Knowing this one can easily construct a counterexample: pastebin.com/YSrNqT4h – Ambagious

@Ambagious Well, this was my point exactly (on including only the already referenced objects), so your example fails? – Authority

The example fails to link even though the libraries liba.a and libb.a are linked twice as you suggested. For this specific example you would need to add another -la in order to make it link. – Ambagious

According to this change the switches should now be ignored and not needed since circular dependencies should always be resolved automatically. Maybe they are adopting same behavior as MSVC (which searches for symbols all libraries at any time)? Can someone confirm? – Tousle

Answer 1

The release notes of LLVM 3.2 state that

llvm-ld and llvm-stub have been removed, llvm-ld functionality can be partially replaced by llvm-link | opt | {llc | as, llc -filetype=obj} | ld, or fully replaced by Clang.

By default clang seems to use the system linker. That is on Linux for example it uses the GNU ld:

$ clang --version
clang version 3.2 (branches/release_32 170558)
...
$ clang -Wl,--verbose
GNU ld (GNU Binutils; devel:gcc / openSUSE_12.3) 2.24.0.20140403-196
...

This suggests that you can use -Wl,--start-group and -Wl,--end-group as with GCC.

Ambagious

This is the case. Apple's ld tool doesn't support the --start-group flags, but it doesn't care about circular dependencies either. – Ogbomosho

Answer 2

I'm not a fan of circular dependencies :) but here some say they handle this cases by linking some libraries twice. I haven't tried it but this might increase your code's footprint.

$(CC) -o myApp -lfoo -lbar -lfoo

I don't know if this works with clang but it might worth a shot.

Best solution would be removing the circular dependencies as it will generate you more problems in the future.

Butter

Answer 3

I had a similar problem linking with a custom clang compiler using a QT c++ project.

The problem turned out to be that QT was using clang (c compiler) rather than clang++ (c++ compiler). By defining the compiler as clang++ instead of clang in the QT project (setting QMAKE_CXX=clang++) the project linked successfully.

As I understand it, the same holds true for gcc and g++.

Phagy