NetBSD ELF FAQ

ELF Issues

• What is ELF?
• A dynamically loaded module at run-time couldn't find symbols from my program image.
• No need for ldconfig or for ld.so.conf!
• My program can't find its shared library
• Elf Shared Library Examples
• But I Want ldconfig! I Want ld.so.conf! I want it! I want it! I want it!
• Updating a system from a.out to ELF
• How do I tell if my system is ELF?
• Comments on the NetBSD ELF FAQ?

ELF Issues

What is ELF? (top)

ELF is a binary format designed to support dynamic objects and shared libraries. On older COFF and ECOFF systems, dynamic support and shared libraries were not naturally supported by the underlying format, leading to dynamic implementations that were at times complex, quirky, and slow.

A dynamically loaded module at run-time couldn't find symbols from my program image. (top)

You probably left off the --export-dynamic option when linking the application. This is required only if arbitrary symbols in the program might be needed by the dynamically loaded module, for example, if a program intends to make run-time decisions to dynamically load modules it was never linked with. Note, when running cc(1) instead of ld(1) this will be specified as -Wl,--export-dynamic.

No need for ldconfig or for ld.so.conf! (top)

Ideally, there is no need for ldconfig or for /etc/ld.so.conf, since ELF provides good and predictable (read portable) mechanism to locate shared libraries. Unfortunately there are still a few corner cases (like wanting to run setuid binaries that you don't have the source for, that want shared libraries to be installed somewhere you don't like). For those corner cases, you'll find that creating an /etc/ld.so.conf file, will still work. Read on though about other ways of doing this and why it is not a good idea. The next section discusses the ELF mechanisms for locating shared libraries.

My program can't find its shared library (top)

An ELF program needs to know the directory and the filename required to mmap(2) its shared libraries. Encoded within the file name is version information. There are one set of mechanisms for the directories and a different mechanism for the file names.

Elf Shared Library Examples (top)

To compile f.c and make an installable shared library out of it:

cc -O  -Werror  -c -fpic -DPIC f.c -o f.so
ar cq libf_pic.a `NM=nm lorder f.so | tsort -q`
ld -x -shared -R/my/directory/lib -soname libf.so.4 -o \
   libf.so.4.9  /usr/lib/crtbeginS.o  --whole-archive \
   libf_pic.a /usr/lib/crtendS.o

There is another way:

% cat Makefile
LIB=f
SRCS=f.c
.include <bsd.lib.mk>
% cat shlib_version
major=4
minor=9
% make

You can disable some of the Makefile targets with NOPROFILE=1 and NOSTATICLIB=1.

And there is another way:

But I Want ldconfig!    I Want ld.so.conf! I want it! I want it! I want it! (top)

At first glance, it might seem reasonable, why shouldn't people be able to move things around to anywhere they want and correct the consequent lossage in a /etc file?

In fact, some developers of ELF systems have apparently added such a file, but with mixed results. The ELF mechanism was designed to correct some of the previous problems, and introducing the old mechanism would bring many of those old problems back.

Currently we are even supporting the /etc/ld.so.conf functionality in our ELF linker, but it is not at all clear that a hybrid mechanism is the right solution. For that reason we do not advertise its existence, advocate its use, or even provide a default installation template. It is there for those who think that they really need it, and cannot live without it.

Here are some of the problems.

ELF tools are standardized packages maintained by third parties; these tools are used consistently on different operating systems and platforms. In the long run, the standardization provided by ELF will increase the quality of both systems and applications.

Updating a system from a.out to ELF (top)

The recommended method is to install a ELF snapshot. If you try upgrading via source and something goes wrong it is very easy to hose your system such that it will not even boot to single-user mode.

However for those who wish to to upgrade from source: (this should work on i386, sparc and any platform that uses ld.new and gas.new)

If you find you need to make any changes to the above script, please let us know on <www@NetBSD.org>.

How do I tell if my system is ELF? (top)

If you are running an ELF system your compiler will define the constant __ELF__. You can use this in your C programs of course but you can also use the following shell script to determine it as well.

if echo __ELF__ | ${CC:-cc} -E - | grep -q __ELF__
then echo "Not ELF"
else echo "It is an ELF system"
fi

Comments on the NetBSD ELF FAQ? (top)

The ELF faq is now maintained by Christos Zoulas <christos@NetBSD.org>

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