How to list all externally-undefined symbols of a static library on Linux?

Question

I have a static library libfoo.a, which is just a compression of multiple .o files. I am looking for a way to list all symbols that

• appear in the static library as UND
• have no definition in this static library

So that I can find out all external symbol dependencies of this library.

Answer 1

You can use this method:

ld -r -o deleteme.o --whole-archive libfoo.a
nm -C --undefined-only deleteme.o       # `-C` if you might have C++ archive members
rm deleteme.o

Demo:

one.c

extern void two(void);

void one()
{
    two();
}

two.c

extern void three(void);

void two()
{
    three();
}

Make a static library libonetwo.a:

$ gcc -Wall -c one.c two.c
$ ar rcs libonetwo.a one.o two.o

nm parses the static library as if it were just a commandline list of its members:

$ nm --undefined-only libonetwo.a 

one.o:
                 U _GLOBAL_OFFSET_TABLE_
                 U two

two.o:
                 U _GLOBAL_OFFSET_TABLE_
                 U three

So incrementally link them all into a temporary object file:

$ ld -r -o deleteme.o --whole-archive libonetwo.a

Then see the residual undefined symbols of that object file and delete it:

$ nm --undefined-only deleteme.o && rm deleteme.o 
                 U _GLOBAL_OFFSET_TABLE_
                 U three