What’s the difference between `-rpath-link` and `-L`?

Here is a demo, for GNU ld, of the difference between -L and -rpath-link –
and for good measure, the difference between -rpath-link and -rpath.

foo.c

#include <stdio.h>

void foo(void)
{
    puts(__func__);
}

bar.c

#include <stdio.h>

void bar(void)
{
    puts(__func__);
}

foobar.c

extern void foo(void);
extern void bar(void);

void foobar(void)
{
    foo();
    bar();
}

main.c

extern void foobar(void);

int main(void)
{
    foobar();
    return 0;
}

Make two shared libraries, libfoo.so and libbar.so:

$ gcc -c -Wall -fPIC foo.c bar.c
$ gcc -shared -o libfoo.so foo.o
$ gcc -shared -o libbar.so bar.o

Make a third shared library, libfoobar.so that depends on the first two;

$ gcc -c -Wall -fPIC foobar.c
$ gcc -shared -o libfoobar.so foobar.o -lfoo -lbar
/usr/bin/ld: cannot find -lfoo
/usr/bin/ld: cannot find -lbar
collect2: error: ld returned 1 exit status

Oops. The linker doesn’t know where to look to resolve -lfoo or -lbar.

The -L option fixes that.

$ gcc -shared -o libfoobar.so foobar.o -L. -lfoo -lbar

The -Ldir option tells the linker that dir is one of the directories to
search for libraries that resolve the -lname options it is given. It searches
the -L directories first, in their commandline order; then it searches its
configured default directories, in their configured order.

Now make a program that depends on libfoobar.so:

$ gcc -c -Wall main.c
$ gcc -o prog main.o -L. -lfoobar
/usr/bin/ld: warning: libfoo.so, needed by ./libfoobar.so, not found (try using -rpath or -rpath-link)
/usr/bin/ld: warning: libbar.so, needed by ./libfoobar.so, not found (try using -rpath or -rpath-link)
./libfoobar.so: undefined reference to `bar'
./libfoobar.so: undefined reference to `foo'
collect2: error: ld returned 1 exit status

Oops again. The linker detects the dynamic dependencies requested by libfoobar.so
but can’t satisfy them. Let’s resist its advice – try using -rpath or -rpath-link –
for a bit and see what we can do with -L and -l:

$ gcc -o prog main.o -L. -lfoobar -lfoo -lbar

So far so good. But:

$ ./prog
./prog: error while loading shared libraries: libfoobar.so: cannot open shared object file: No such file or directory

at runtime, the loader can’t find libfoobar.so.

What about the linker’s advice then? With -rpath-link, we can do:

$ gcc -o prog main.o -L. -lfoobar -Wl,-rpath-link=$(pwd)

and that linkage also succeeds. ($(pwd) means “Print Working Directory” and just “copies” the current path.)

The -rpath-link=dir option tells the linker that when it encounters an input file that
requests dynamic dependencies – like libfoobar.so – it should search directory dir to
resolve them. So we don’t need to specify those dependencies with -lfoo -lbar and don’t
even need to know what they are. What they are is information already written in the
dynamic section of libfoobar.so:-

$ readelf -d libfoobar.so

Dynamic section at offset 0xdf8 contains 26 entries:
  Tag        Type                         Name/Value
 0x0000000000000001 (NEEDED)             Shared library: [libfoo.so]
 0x0000000000000001 (NEEDED)             Shared library: [libbar.so]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
 ...
 ...

We just need to know a directory where they can be found, whatever they are.

But does that give us a runnable prog?

$ ./prog
./prog: error while loading shared libraries: libfoobar.so: cannot open shared object file: No such file or directory

No. Same as story as before. That’s because -rpath-link=dir gives the linker the information
that the loader would need to resolve some of the dynamic dependencies of prog
at runtime – assuming it remained true at runtime – but it doesn’t write that information into the dynamic section of prog.
It just lets the linkage succeed, without our needing to spell out all the recursive dynamic
dependencies of the linkage with -l options.

At runtime, libfoo.so, libbar.so – and indeed libfoobar.so –
might well not be where they are now – $(pwd) – but the loader might be able to locate them
by other means: through the ldconfig cache or a setting
of the LD_LIBRARY_PATH environment variable, e.g:

$ export LD_LIBRARY_PATH=.; ./prog
foo
bar

rpath=dir provides the linker with the same information as rpath-link=dir
and instructs the linker to bake that information into the dynamic section of
the output file. Let’s try that:

$ export LD_LIBRARY_PATH=
$ gcc -o prog main.o -L. -lfoobar -Wl,-rpath=$(pwd)
$ ./prog
foo
bar

All good. Because now, prog contains the information that $(pwd) is a runtime search
path for shared libraries that it depends on, as we can see:

$ readelf -d prog

Dynamic section at offset 0xe08 contains 26 entries:
  Tag        Type                         Name/Value
 0x0000000000000001 (NEEDED)             Shared library: [libfoobar.so]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
 0x000000000000000f (RPATH)              Library rpath: [/home/imk/develop/so/scrap]
 ...                                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 ...

That search path will be tried after the directories listed in LD_LIBRARY_PATH, if any are set, and before the system defaults – the ldconfig-ed directories, plus /lib and /usr/lib.