[LLVMdev] Some thought on handling ELF shared libraries in lld (original) (raw)

Rafael Espíndola rafael.espindola at gmail.com
Tue Jul 21 05:46:58 PDT 2015

Most ELF shared libraries can be sliced in two ways. One is following the information in the program headers (e_phoff). The other is following the information in the section headers (e_shoff).

Regular relocatable objects only have the section header.

At runtime, the dynamic linker only uses the program headers. In fact, the section headers is optional.

When given a shared library, how should the static linker handle it?

Note that, unlike the dynamic linker, the static one has to find all the defined symbol is a shared library. It is not enough to just look up the currently undefined symbols. To see that, consider

$ cat test.c void f(void) { } $ cat test2.c void f(void); void g(void) { f(); }

$ clang -c test.c test2.c -fPIC $ clang -shared test.o -o test.so $ rm -f test.a $ ar rc test.a test.o $ clang test.so test2.o test.a -o t.so -Wl,-t -shared $ clang test2.o test.a -o t.so -Wl,-t -shared

The second link will include the archive member, the first one will not.

It is tempting to use the program headers in the static linker. Doing so would let us support linking with shared libraries with no section headers, but there are a few issues:

Given that, it looks like we should use the sections. For what it is worth, it looks like that is what every other ELF linker does.

Cheers, Rafael

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