symbol lookup in shared libraries

Question

I have tested such a simple program below

/* a shared library */
dispatch_write_hello(void)
{
        fprintf(stderr, "hello\n");
}

extern void
print_hello(void)
{
        dispatch_write_hello();
}

My main program is like this:

extern void
dispatch_write_hello(void)
{
        fprintf(stderr, "overridden\n");
}

int
main(int argc, char **argv)
{
        print_hello();
        return 0;
}

The result of the program is "overriden".
To figure this out why this happens, I used gdb. The call chain is like this:
_dl_runtime_resolve -> _dl_fixup ->_dl_lookup_symbol_x
I found the definition of _dl_lookup_symbol_x in glibc is

Search loaded objects' symbol tables for a definition of the symbol UNDEF_NAME, perhaps with a requested version for the symbol

So I think when trying to find the symbol dispatch_write_hello, it first of all looks up in the main object file, and then in the shared library. This is the cause of this problem. Is my understanding right? Many thanks for your time.

yugr yugr · Accepted Answer · 2017-03-12T15:33:09

Given that you mention _dl_runtime_resolve, I assume that you're on Linux system (thanks @Olaf for clarifying this).

A short answer to your question - yes, during symbols interposition dynamic linker will first look inside the executable and only then scan shared libraries. So definition of dispatch_write_hello will prevail.

EDIT

In case you wonder why runtime linker needs to resolve the call to dispatch_write_hello in print_hello to anything besides dispatch_write_hello in the same translation unit - this is caused by so called semantic interposition support in GCC. By default compiler treats any call inside library code (i.e. code compiled with -fPIC) as potentially interposable at runtime unless you specifically tell it not to, via -fvisibility-hidden, -Wl,-Bsymbolic, -fno-semantic-interposition or __attribute__((visibility("hidden"))). This has been discussed on the net many times e.g. in the infamous Sorry state of dynamic libraries on Linux.

As a side note, this feature incures significant performance penalty compared to other compilers (Clang, Visual Studio).

symbol lookup in shared libraries

1 Answers