Combining static libraries

Question

Suppose I have three C static libraries say libColor.a which depends on *libRGB.*a which in turn depends on libPixel.a . The library libColor.a is said to depend on library libRGB.a since there are some references in libColor.a to some of symbols defined in libRGB.a. How do I combine all the above libraries to a new libNewColor.a which is independent?

Independent means the new library should have all symbols defined. So while linking I just need to give -lNewColor. The size of the new library should be minimal i.e it should not contain any symbols in libRGB.a which is not used by libColor.a etc. I tried my luck using various options in ar command (used to create and update static libraries/archives).

Answer 1

A little used feature of the GNU archiver is the archive script, it is a simple but powerful interface, and it can do exactly what you want, for example if the following script is called script.ar:

CREATE libNewColor.a
ADDLIB libColor.a
ADDLIB libRGB.a
ADDLIB libPixel.a
SAVE
END

Then you could invoke ar as follows:

ar -M < script.ar

and you would get libNewColor.a that contains all of the .o files from libColor.a libRGB.a and libPixel.a.

Additionally you can also add regular .o files as well with the ADDMOD command:

CREATE libNewColor.a
ADDLIB libColor.a
ADDLIB libRGB.a
ADDLIB libPixel.a
ADDMOD someRandomCompiledFile.o
SAVE
END

Furthermore it is super easy to generate these scripts in Makefiles, so I typically create a somewhat generic makefile rule for creating archives which actually generates the script and invokes ar on the script. Something like this:

$(OUTARC): $(OBJECTS)
    $(SILENT)echo "CREATE $@" > $(ODIR)/$(ARSCRIPT)
    $(SILENT)for a in $(ARCHIVES); do (echo "ADDLIB $$a" >> $(ODIR)/$(ARSCRIPT)); done
    $(SILENT)echo "ADDMOD $(OBJECTS)" >> $(ODIR)/$(ARSCRIPT)
    $(SILENT)echo "SAVE" >> $(ODIR)/$(ARSCRIPT)
    $(SILENT)echo "END" >> $(ODIR)/$(ARSCRIPT)
    $(SILENT)$(AR) -M < $(ODIR)/$(ARSCRIPT)

Though now that I look at it I guess it doesn't work if $(OBJECTS) is empty (i.e. if you just want to combine archives without adding extra object files) but I will leave it as an exercise for the reader to fix that issue if needed... :D

Here are the docs for this feature:

https://sourceware.org/binutils/docs/binutils/ar-scripts.html#ar-scripts

Answer 2

1/ Extract ALL of the object files from each library (using ar) and try to compile your code without the libraries or any of the object files. You'll probably get an absolute bucket-load of undefined symbols. If you get no undefined symbols, go to step 5.

2/ Grab the first one and find out which object file satisfies that symbol (using nm).

3/ Write down that object file then compile your code, including the new object file. You'll get a new list of undefined symbols or, if there's none, go to step 5.

4/ Go to step 2.

5/ Combine all the object files in your list (if any) into a single library (again with ar).

Bang! There you have it. Try to link your code without any of the objects but with the new library.

This whole thing could be relatively easily automated with a shell script.