From: Daniel Barlow <d...@detached.demon.co.uk>
Subject: Linux ELF HOWTO (part 1/1)
Date: 1996/04/03
Message-ID: <ELF-HOWTO.1-11034.828576097@cc.gatech.edu>
X-Deja-AN: 145690910
sender: gr...@cc.gatech.edu
followup-to: poster
summary: How to install and migrate to the ELF binary file format
organization: Linux HOWTO Project
keywords: Linux, HOWTO, ELF
newsgroups: comp.os.linux.answers
nntp-posting-user: gregh
Archive-Name: linux/howto/elf
Last-modified: 11 Feb 96
-----BEGIN PGP SIGNED MESSAGE-----
*** The `Linux ELF HOWTO' is posted automatically by the
*** Linux HOWTO coordinator, Greg Hankins <gr...@sunsite.unc.edu>. Please
*** direct any comments or questions about this HOWTO to the author,
*** Daniel Barlow <d...@detached.demon.co.uk>.
- --- BEGIN Linux ELF HOWTO part 1/1 ---
The Linux ELF HOWTO
Daniel Barlow <daniel...@sjc.ox.ac.uk>
v1.11, 13 September 1995
This document describes how to migrate your Linux system to compile
and run programs in the ELF binary format. It falls into three con�
ceptual parts: (1) What ELF is, and why/whether you should upgrade,
(2) How to upgrade to ELF-capability, and (3) what you can do then.
1. What is ELF? An introduction
ELF (Executable and Linking Format) is a binary format originally
developed by USL (UNIX System Laboratories) and currently used in
Solaris and System V Release 4. Because of its increased flexibility
over the older a.out format that Linux currently uses, the GCC and C
library developers decided last year to move to using ELF as the Linux
standard binary format also.
This `increased flexibility' manifests as essentially two benefits to
the average applications progammer:
� It is much simpler to make shared libraries with ELF. Typically,
just compile all the object files with -fPIC, then link with a
command like
gcc -shared -Wl,-soname,libfoo.so.y -o libfoo.so.y.x *.o
If that looks complex, you obviously haven't ever read up on the
equivalent procedure for a.out shared libraries, which involves com�
piling the library twice, reserving space for all the data you think
that the library is likely to require in future, and registering that
address space with a third party (it's described in a document over 20
pages long --- look at
<ftp://tsx-11.mit.edu/pub/linux/packages/GCC/src/tools-2.16.tar.gz>
for details).
� It makes dynamic loading (ie programs which can load modules at
runtime) much simpler. This is used by Perl 5, Python, and the
ongoing port of Java to Linux, among other things. Other
suggestions for dynamic loading have included super-fast MUDs,
where extra code could be compiled and linked into the running
executable without having to stop and restart the program.
Against this it must be weighed that ELF is possibly a bit slower.
The figures that get bandied around are between 1% and 5%, though all
the actual tests that have been conducted so far indicate that the
difference is small enough to get lost in the noise of other events
happening at the same time. If you have TeX or a Postscript
viewer/printer, you can read speed.comp-1.0.tar.gz, which is available
from SunSite somewhere.
The slowdown comes from the fact that ELF library code must be
position independent (this is what the -fPIC above stands for) and so
a register must be devoted to holding offsets. That's one less for
holding variables in, and the 80x86 has a paucity of general-purpose
registers anyway.
1.1. What ELF isn't
There are a number of common misconceptions about what ELF will do for
your system:
It's not a way to run SVR4 or Solaris programs
Although it's the same binary `container' as SVR4 systems use,
that doesn't mean that SVR4 programs suddenly become runnable on
Linux. It's analogous to a disk format --- you can keep Linux
programs on MSDOS or Minix-format disks, and vice versa, but
that doesn't mean that these systems become able to run each
others' programs.
It is theoretically possible to run applications for other x86
Unices under Linux, but following the instructions in this HOWTO
will not have that effect. Start by looking at the iBCS kernel
module (somewhere on tsx-11.mit.edu) and see if it fits your
needs.
It's not intrinsically smaller or faster
You may well end up with smaller binaries anyway, though, as you
can more easily create shared libraries of common code between
many programs. In general, if you use the same compiler options
and your binaries come out smaller than they did with a.out,
it's more likely to be fluke or a different compiler version.
As for `faster', I'd be surprised. Speed increases could turn
up if your binaries are smaller, due to less swapping or larger
functional areas fitting in cache.
It doesn't require that you replace every binary on your system
At the end of this procedure you have a system capable of
compiling and running both ELF and a.out programs. New programs
will by default be compiled in ELF, though this can be
overridden with a command-line switch. There is admittedly a
memory penalty for running a mixed ELF/a.out system --- if you
have both breeds of program running at once you also have two
copies of the C library in core, and so on. I've had reports
that the speed difference from this is undetectable in normal
use on a 6Mb system though (I certainly haven't noticed much in
8Mb), so it's hardly pressing. You lose far more memory every
day by running bloated programs like Emacs and static
Mosaic/Netscape binaries :-)
It's nothing to do with Tolkien.
Or at least, not in this context.
1.2. Why you should(n't) convert to ELF
There are essentially two reasons to upgrade your system to compile
and run ELF programs: the first is the increased flexibility in
programming referred to above, and the second is that, due to the
first, everyone else will be too. Future releases of the C library
and GCC will only be compiled for ELF, and other developers are
expected to move ELFwards too.
Pleasingly for the purposes of symmetry, there are also two reasons
not to convert at this time. The first is that things are still
changing, some packages (including the `stable' 1.2 kernel series)
require patches to be made before they will compile in ELF, and there
may be residual bugs; one could make a strong case for waiting until
Linus himself has converted, for example.
The second is that although the installation described here is a
fairly small job in itself (it can be completed in well under an hour,
excepting the time taken to download the new software), an error at
almost any stage of it will probably leave you with an unbootable
system. If you are not comfortable with upgrading shared libraries
and the commands ldconfig and ldd mean nothing to you, you may want to
obtain or wait for a new Linux distribution in ELF, and backup,
reinstall and selectively restore your system using it. Then again
(and especially if the system is not mission-critical) you may want to
go through it anyway and look on it as a learning experience.
Still with us?
2. Installation
2.1. Background
The aim of this conversion is to leave you with a system which can
build and run both a.out and ELF programs, with each type of program
being able to find its appropriate breed of shared libraries. This
obviously requires a bit more intelligence in the library search
routines than the simple `look in /lib, /usr/lib and anywhere else
that the program was compiled to search' strategy that some other
systems can get away with.
The beastie responsible for searching out libraries in linux is
/lib/ld.so. The compiler and linker do not encode absolute library
pathnames into the programs they output; instead they put the library
name and the absolute path to ld.so in, and leave ld.so to match the
library name to the appropriate place at runtime. This has one very
important effect --- it means that the libraries that a program uses
can be moved to other directories without recompiling the program,
provided that ld.so is told to search the new directory. This is
essential functionality for the directory swapping operation that
follows.
The corollary of the above, of course, is that any attempt to delete
or move ld.so will cause every dynamically linked program on the
system to stop working. This is generally regarded as a Bad Thing.
For ELF binaries, an alternate dynamic loader is provided. This is
/lib/ld-linux.so.1, and does exactly the same thing as ld.so, but for
ELF programs. ld-linux.so.1 uses the same support files and programs
(ldd, ldconfig, and /etc/ld.so.conf) as the a.out loader ld.so does.
The basic plan, then, is that ELF development things (compilers,
include files and libraries) go into /usr/{bin,lib,include} where your
a.out ones currently are, and the a.out things will be moved into
/usr/i486-linuxaout/{bin, lib, include}. /etc/ld.so.conf lists all
the places on the system where libraries are expected to be found, and
ldconfig is intelligent enough to distinguish between ELF and a.out
variants.
There are a couple of exceptions to the library placement, though.
� Some old programs were built without the use of ld.so. These would
all cease working if their libraries were moved from under them.
Thus, libc.so* and libm.so* must stay where they are in /lib, and
the ELF versions have had their major numbers upgraded so that they
do not overwrite the a.out ones. Old X libraries (prior to version
6) are best left where they are also, although newer ones
(libX*so.6) must be moved. Moving the old ones will apparently
break xview programs, and not moving the new ones will cause them
to be overwritten when you install ELF X libraries.
If you have non-ld.so programs that require libraries other than
the above (if you know which programs they are, you can run ldd on
them to find out which libraries they need before breaking them)
you have essentially two options. One, you can extract the ELF
library tar files into a temporary directory, check whether your
precious library would be overwritten, and if so, move the ELF
version of the library into, say, /usr/i486-linux/lib instead of
/lib. Make sure your ld.so.conf has /usr/i486-linux/lib in it,
then run ldconfig and think no more on't. Two, you can recompile
or acquire a newer copy of the offending program. This might not
be a bad idea, if possible.
� If you have /usr and / on different partitions, you'll need to move
at least some of the libraries in /lib to somewhere on the root
disk, not on /usr. Either you can go through the programs that you
need to run at system startup or when in single-user mode, and
identify the libraries they use, or you can depend on your
system/distribution integrator to have done this for you and just
move all (er ... some. See above for the exceptions) of the
libraries in /lib to /lib-aout.
2.2. Before you start --- Notes and Caveats
� You will need to be running a post-1.1.52 kernel with ELF binary
format support.
� You are recommended to prepare or acquire a linux boot/root disk,
such as a Slackware rescue disk. You probably won't need it, but
if you do and you don't have one, you'll kick yourself. In a
similar `prevention is better than cure' vein, statically linked
copies of mv, ln, and maybe other file manipulation commands
(though in fact I think you can do everything else you actually
need to with shell builtins) may help you out of any awkward
situations you could end up in.
� If you have been following the ELF development, you may have ELF
libraries in /lib/elf (usually libc.so.4 and co). Applications
that you built using these should be rebuilt, then the directory
removed. There is no need for a /lib/elf directory!
� Most Linux installations these days have converged on the `FSSTND'
standard file system, but doubtless there are still installed
systems that haven't. If you see references to /sbin/something and
you don't have a /sbin directory, you'll probably find the program
referred to in /bin or /etc/.
2.3. You will need ...
The following packages are available from
<ftp://tsx-11.mit.edu/pub/linux/packages/GCC/> and
<ftp://sunsite.unc.edu/pub/Linux/GCC/>. Both sites are widely
mirrored; please take the time to look up your nearest mirror site and
use that instead of the master sites where possible. It's faster for
both you and everyone else.
These packages (either the listed version or a later one) are
required. Also download and read through the release notes for each
of them: these are the files named release.packagename. This applies
especially if you get newer versions than are listed here, as
procedures may have changed.
� ld.so-1.7.3.tar.gz --- the new dynamic linker
� libc-5.0.9.bin.tar.gz --- the ELF shared images for the C library
and its friends (m (maths), termcap, gdbm, and so on), plus the
corresponding static libraries and the include files needed to
compile programs with them. libc 5.2.something is expected to be
released during the lifetime of this HOWTO, and is considerably
different from 5.0.9; if you want to install it, you're on your
own, but I'd recommend installing 5.0.9 first and then installing
it over the top. There are several parts to libc 5.0.9 which are
not included in 5.2.x and for which the distribution channels are
not entirely set up yet.
� gcc-2.7.0.bin.tar.gz --- the ELF C compiler. Also includes an
a.out C compiler which understands the new directory layout.
� binutils-2.5.2l.17.bin.tar.gz --- the GNU binary utilities patched
for Linux. These are programs such as gas, ld, strings and so on,
most of which are required to make the C compiler go.
2.4. Rearranging your filesystem
Sooo... Note that in all that follows, when I say `remove' I
naturally mean `backup then remove' :-). Also, these instructions
directly apply only to people who haven't yet messed with ELF ---
those who have are expected to have the necessary nous to adapt as
appropriate. Let's go!
1. Make the new directories that you will move a.out things to
______________________________________________________________________
mkdir -p /usr/i486-linuxaout/bin
mkdir -p /usr/i486-linuxaout/include
mkdir -p /usr/i486-linuxaout/lib
mkdir /lib-aout
______________________________________________________________________
2. Untar the dynamic linker package ld.so-1.7.3 in the directory you
usually put source code, then read through the
ld.so-1.7.3/instldso.sh script just unpacked. If you have a really
standard system, run it by doing sh instldso.sh, but if you have
anything at all unusual then do the install by hand instead.
`Anything at all unusual' includes
� using zsh as a shell (some versions of zsh define $VERSION, which
seems to confuse instldso.sh)
� having symlinks from /lib/elf to /lib (which you shouldn't need,
but you may have valid reasons for if you have been following the
ELF development)
3. Edit /etc/ld.so.conf to add the new directory
/usr/i486-linuxaout/lib (and /lib-aout if you're going to need
one). Then rerun /sbin/ldconfig -v to check that it is picking up
the new directories.
4. Move all the a.out libraries in /usr/*/lib to
/usr/i486-linuxaout/lib. Note, I said `libraries' not
`everything'. That's files matching the specification lib*.so* ,
lib*.sa*, or lib*.a. Don't start moving /usr/lib/gcc-lib or
anything silly like that around.
5. Now look at /lib. Leave intact libc.so*, libm.so*, and libdl.so*.
If you have symlinks to X libraries (libX*.so.3*) leave them there
too --- XView and some other packages may require them. Leave
ld.so*, ld-linux.so* and any other files starting with ld. As for
the remaining libraries (if you have any left): if you have /usr on
the root partition, put them in /usr/i486-linuxaout/lib. If you
have /usr mounted separately, put them in /lib-aout. Now run
ldconfig -v
6. Remove the directory /usr/lib/ldscripts if it's there, in
preparation for installing the binutils (which will recreate it)
7. Remove any copies of ld and as (except for ld86 and as86) that you
can find in /usr/bin.
8. Some versions of GNU tar appear to have problems dealing with
symbolic links in the destination directory. You have two options
here:
a. (preferred) Use cpio instead of tar, it doesn't have this
problem. zcat /wherever/you/put/it/libc-5.0.9.tar.gz | cpio -iv
is the magic incantation here, to be executed from the root
directory.
b. (if you don't have cpio installed) Before installing the libc
images you might want to go through /usr/include and remove some
parts.
This is icky. Many packages (such as ncurses) are installed
into /usr/include by distribution maintainers and are not
supplied with the C library. Backup the /usr/include tree, use
tar tzf to see what's in the archive before untarring it, then
delete the bits of /usr/include that it actually fills. Then
untar the libc-5.0.9.bin.tar.gz package from root.
9. Install the binutils package. tar -xvzf
binutils-2.5.2.l17.bin.tar.gz -C / is one perfectly good way to do
this.
10.
You have now installed everything you need to run ELF executables.
Medical experts recommend that VDU workers take regular breaks away
from the screen; this would be an opportune moment. Don't forget
what you were doing, though; depending on the version of gcc you
were previously using, you may have left yourself unable to compile
programs in a.out until you unpack the new gcc.
11.
Backup and remove everything in /usr/lib/gcc-lib/{i486-linux,
i486-linuxelf, i486-linuxaout}/ If you use a non-standard gcc
driver (eg if you use Gnu ADA), copy that somewhere safe also.
Then install the gcc package, again by untarring from root.
12.
Some programs (notably various X programs) use /lib/cpp, which
under Linux is generally a link to /usr/lib/gcc-
lib/i486-linux/version/cpp. As the preceding step wiped out
whatever version of cpp it was pointing to, you'll need to recreate
the link:
$ cd /lib
$ ln -s /usr/lib/gcc-lib/i486-linux/2.7.0/cpp .
13.
The FSSTND people have once again justified their keep by moving
the utmp and wtmp files from /var/adm to /var/run and /var/log
respectively. You'll need to add some links dependent on where
they currently live, and you may need to make the /var/log and
/var/adm directories too. I reproduce below the ls -l output of
appropriate bits on my system:
$ ls -ld /var/adm /var/log /var/run /var/log/*tmp /var/run/*tmp
lrwxrwxrwx 1 root root 3 May 24 05:53 /var/adm -> log/
drwxr-xr-x 9 root root 1024 Aug 13 23:17 /var/log/
lrwxrwxrwx 1 root root 11 Aug 13 23:17 /var/log/utmp -> ../run/utmp
-rw-r--r-- 1 root root 451472 Aug 13 23:00 /var/log/wtmp
drwxr-xr-x 2 root root 1024 Aug 13 23:17 /var/run/
-rw-r--r-- 1 root root 448 Aug 13 23:00 /var/run/utmp
Check the FSSTND (from LDP archives such as
<ftp://sunsite.unc.edu/pub/Linux/docs/fsstnd/>) for the full story.
14.
This step is optional. If you're intending to continue compiling
programs in a.out, this is the appropriate time to install libc.so
4.7.x. Untar it from root, as you are now no doubt fully capable
of doing without further explanation.
Done! Simple tests that you can try are
______________________________________________________________________
$ gcc -v
Reading specs from /usr/lib/gcc-lib/i486-linux/2.7.0/specs
gcc version 2.7.0
$ gcc -v -b i486-linuxaout
Reading specs from /usr/lib/gcc-lib/i486-linuxaout/2.7.0/specs
gcc version 2.7.0
$ ld -V
ld version cygnus/linux-2.5.2l.14 (with BFD cygnus/linux-2.5.2l.11)
Supported emulations:
elf_i386
i386linux
i386coff
______________________________________________________________________
followed of course by the traditional ``Hello, world'' program. Try
it with gcc and with gcc -b i486-linuxaout to check that both the
a.out and ELF compilers are set up corectly.
2.5. What it should look like (outline directory structure)
This is a deliberately vague guide to what the files you have just
installed are. It may be useful for troubleshooting or deciding what
to delete.
2.5.1.
/lib
� Dynamic linkers ld.so (a.out) and ld-linux.so.1 (ELF). Either of
these may be symlinks, but make sure that the files they point to
do exist.
� Basic shared libraries libc.so.4, libm.so.4 (a.out) These are
symlinks, but check that they point to real files.
� Basic shared libraries libc.so.5, libm.so.5,
libdl.so.1,libcurses.so.1,libtermcap.so.2, (ELF). Again, these are
symlinks.
� Lots of symlinks. For each library, there should be an actual file
(for example libc.so.5.0.9), a symlink to it with only the major
version number in its name (libc.so.5) and a symlink pointing to
that with no version number (libc.so). That's
______________________________________________________________________
lrwxrwxrwx 1 root root 9 May 24 05:52 libc.so -> libc.so.5
lrwxrwxrwx 1 root root 13 Aug 25 12:48 libc.so.5 -> libc.so.5.0.9
-rwxr-xr-x 1 bin bin 562683 May 19 04:47 libc.so.5.0.9
______________________________________________________________________
2.5.2. /usr/lib
� All the non-library files and directories that were there
previously.
� libbfd.so*,libdb.so*, libgdbm.so*, ELF shared libraries. All to
consist of three files as explained above in the /lib section.
� libbsd.a, libgmon.a, libldso.a, libmcheck.a, libieee.a, libmcheck.a
and one lib*.a file for every ELF shared library in /lib and
/usr/lib . ELF static libraries. The ones duplicating the shared
libraries may not be tremendously useful for most people --- when
using ELF, you can use the gcc -g switch with shared libraries, so
there's not much reason to compile static any longer.
� crt0.o, gcrt0.o. a.out `start of program' files; one of these is
linked as the first file in every a.out program you compile, unless
you take steps to avoid it.
� crt1.o, crtbegin.o, crtbeginS.o, crtend.o, crtendS.o, crti.o,
crtn.o, gcrt1.o. ELF startup files. These do similar things to
*crt0.o above for ELF programs.
2.5.3.
/usr/lib/ldscripts
� This is where the driver scripts for ld live, as the name suggests.
It should look like
______________________________________________________________________
$ ls /usr/lib/ldscripts/
elf_i386.x elf_i386.xs i386coff.xn i386linux.xbn
elf_i386.xbn elf_i386.xu i386coff.xr i386linux.xn
elf_i386.xn i386coff.x i386coff.xu i386linux.xr
elf_i386.xr i386coff.xbn i386linux.x i386linux.xu
______________________________________________________________________
2.5.4. /usr/i486-linux/bin
� ar, as, gasp, ld, nm, ranlib, strip. These are all actually
symlinks to the real binutils in /usr/bin
2.5.5. /usr/i486-linuxaout/bin
� as --- the a.out assembler, and gasp, its macro preprocessor
� ar, ld, nm, ranlib, strip --- symlinks to the real binutils in
/usr/bin
2.5.6. /usr/i486-linux/lib
� ldscripts is a symlink to /usr/lib/ldscripts.
2.5.7. /usr/i486-linuxaout/lib
� lib*.so*. a.out shared library images. Needed to run a.out
programs
� lib*.sa. a.out shared library stubs. Needed to compile a.out
programs that use shared libraries. If you don't intend to, you
can safely remove these.
� lib*.a. a.out static libraries. Needed to compile static a.out
programs (eg when compiling with -g). Again, you can delete them
if you don't intend to.
� ldscripts is a symbolic link to /usr/lib/ldscripts
2.5.8. /usr/lib/gcc-lib/i486-linux/2.7.0
� This directory contains a version of gcc 2.7.0 set up to compile
ELF programs.
2.5.9. /usr/lib/gcc-lib/i486-linuxaout/2.7.0
� This directory contains a version of gcc 2.7.0 set up to compile
a.out programs, which knows about the new directory structure. If
you're not going to compile anything in a.out, deleting this may
free up around 4Mb.
2.6. Common errors (Don't Panic!)
You moved the wrong thing and now nothing runs
You still have a shell running, though, and with a little
ingenuity you can do an awful lot with shell builtins. Remember
that echo * is an acceptable substitute for ls, and echo
>>filename can be used to add lines to a file. Also, don't
forget that ldconfig is linked static. If you moved, say,
libc.so.4 to /lib-aout mistakenly, you can do echo "lib-aout"
>>/etc/ld.so.conf ; ldconfig -v/ and be back up again. If you
moved /lib/ld.so you may be able to do sln /silly/place/ld.so
/lib/ld.so, if you have a statically linked ln, and probably be
back up again.
no such file or directory: /usr/bin/gcc
that the ELF dynamic loader /lib/ld-linux.so.1 is not installed,
or is unreadable for some reason. You should have installed it
at around step 2 previously.
not a ZMAGIC file, skipping
from ldconfig. You have an old version of the ld.so package, so
get a recent one. Again, see step 2 of the installation.
bad address
on attempting to run anything ELF. You're using kernel 1.3.x,
where x<3. Upgrade to 1.3.3 or downgrade to 1.2.something
_setutent: Can't open utmp file
This message is often seen in multiples of three when you start
an xterm. Go and read the FSSTND tirade near the end of the
installation procedure.
gcc: installation problem, cannot exec something: No such file or
directory
when attempting to do a.out compilations (something is usually
one of cpp or cc1). Either it's right, or alternatively you
typed
$ gcc -b -i486-linuxaout
when you should have typed
$ gcc -b i486-linuxaout
Note that the `i486' does not start with a dash.
3. Building programs in ELF
3.1. Ordinary programs
To build a program in ELF, use gcc as always. To build in a.out, use
gcc -b i486-linuxaout .
______________________________________________________________________
$ cat >hello.c
main() { printf("hello, world\n"); }
^D
$ gcc -o hello hello.c
$ file hello
hello: ELF 32-bit LSB executable i386 (386 and up) Version 1
$ ./hello
hello, world
______________________________________________________________________
This is perhaps an appropriate time to answer the question ``if a.out
compilers default to producing a program called a.out, what name does
an ELF compiler give its output?''. Still a.out, is the answer.
Boring boring boring ... :-)
3.2. Building libraries
To build libfoo.so as a shared library, the basic steps look like
this:
______________________________________________________________________
$ gcc -fPIC -c *.c
$ gcc -shared -Wl,-soname,libfoo.so.1 -o libfoo.so.1.0 *.o
$ ln -s libfoo.so.1.0 libfoo.so.1
$ ln -s libfoo.so.1 libfoo.so
$ export LD_LIBRARY_PATH=`pwd`:$LD_LIBRARY_PATH
______________________________________________________________________
This will generate a shared library called libfoo.so.1.0, and the
appropriate links for ld (libfoo.so) and the dynamic linker
(libfoo.so.1) to find it. To test, we add the current directory to
LD_LIBRARY_PATH.
When you're happpy that the library works, you'll have to move it to,
say, /usr/local/lib, and recreate the appropriate links. Note that
the libfoo.so link should point to libfoo.so.1, so it doesn't need
updating on every minor version number change. The link from
libfoo.so.1 to libfoo.so.1.0 is kept up to date by ldconfig, which on
most systems is run as part of the boot process.
______________________________________________________________________
$ su
# cp libfoo.so.1.0 /usr/local/lib
# /sbin/ldconfig
# ( cd /usr/local/lib ; ln -s libfoo.so.1 libfoo.so )
______________________________________________________________________
3.3. Programs with dynamic loading
These are covered extensively in H J Lu's ELF programming document,
and the dlopen(3) manual page, which can be found in the ld.so
package. Here's a nice simple example though: link it with -ldl
______________________________________________________________________
#include <dlfcn.h>
#include <stdio.h>
main()
{
void *libc;
void (*printf_call)();
if(libc=dlopen("/lib/libc.so.5",RTLD_LAZY))
{
printf_call=dlsym(libc,"printf");
(*printf_call)("hello, world\n");
}
}
______________________________________________________________________
3.4. Debugging
Your existing copy of gdb will most likely work unchanged with ELF
programs. The new version in the GCC directory on tsx-11 is reported
to be better at debugging programs that use shared libraries and
dynamic loading, and to understand ELF core dumps.
Note that 1.2 series kernels cannot generate core dumps from ELF
programs anyway. 1.3 can.
4. Patches and binaries
At this point in the proceedings, you can, if you like, stop. You
have installed everything necessary to compile and run ELF programs.
You may wish to rebuild some programs in ELF, either for purposes of
`neatness' or to minimise memory usage. For most end-user
applications, this is pretty simple; some packages however do assume
too much about the systems they run on, and may fail due to one or
more of:
� Different underscore conventions in the assembler: in an a.out
executable, external labels get _ prefixed to them; in an ELF
executable, they don't. This makes no difference until you start
integrating hand-written assembler: all the labels of the form _foo
must be translated to foo, or (if you want to be portable about it)
to EXTERNAL(foo) where EXTERNAL is some macro which returns either
its argument (if __ELF__ is defined) or _ concatenated with its
argument if not.
� Differences in libc 5 from libc 4. The interface to the locale
support has changed, for one.
� The application or build process depending on knowledge of the
binary format used --- emacs, for example, dumps its memory image
to disk in executable format, so obviously needs to know what
format your executables are in.
� The application consists of or includes shared libraries (X11 is
the obvious example). These will obviously need changes to
accomodate the different method of shared library creation in ELF.
Anyway, here are two lists: the first is of programs that needed
changing for ELF where the changes have been made (ie that you will
need new versions of to compile as ELF), and the second is of programs
that still need third-party patches of some kind.
4.1. Upgrade:
� Dosemu. Modulo the three or four cuurrent dosemu development trees
(don't ask, just join the linux-msdos mailing list), dosemu runs
with ELF. You'll need to monkey with the Makefile. Current
versions of dosemu are available from
<ftp://tsx-11.mit.edu/pub/linux/ALPHA/dosemu/>
� Emacs. Emacs has a rather odd build procedure that involves
running a minimal version of itself, loading in all the useful bits
as lisp, then dumping its memory image back to disk as an
executable file. (FSF) Emacs 19.29 and XEmacs 19.12 (formerly
Lucid Emacs) can both detect whether you are compiling as ELF and
Do The Right Thing automatically.
� MAKEDEV. In some incarnations, this utility removes existing
entries for devices before recreating them. This is Bad News if it
happens to touch /dev/zero, as said device is necessary to the
operation of all ELF programs. See the util-linux package(q.v.)
for a fixed version.
� perl 5.001. Perl 5.001 plus the ``official unofficial'' patches a-
e will compile unchanged on an ELF system, complete with dynamic
loading. The patches are available from ftp.metronet.com or
ftp.wpi.edu
� The cal program in util-linux 2.2 doesn't work. Upgrade to version
2.4 <ftp://tsx-11.mit.edu/pub/linux/packages/utils> or later.
� XFree86. XFree86 3.1.2 comes in both a.out and ELF formats. ftp
to ftp.xfree86.org, read the `too many users' message that you are
almost guaranteed to get, and pick the closest mirror site network-
wise to you. Once you have the contents of the common and elf
directories, you must edit /usr/X11R6/lib/X11/config/linux.cf to
change the lines saying
#define LinuxElfDefault NO
#define UseElfFormat NO
to say YES instead. Otherwise an xpm build will attempt to do odd
stuff with jumpas and its associated relics of the past.
� Mosaic. I don't have the facilities to build this myself, but the
Mosaic 2.7b1 binary available from NCSA comes in ELF. It has been
linked against an odd X setup though, with the result that on
normal systems it will complain about not finding libXpm.so.4.5.
The simple fix is to edit it carefully with emacs or another editor
that copes with binary files. Find the occurence of the string
libXpm.so.4.5^@ (where ^@ is a NUL --- ASCII zero --- character),
delete the .5 and add two more characters after the NUL to aviod
changing the file length.
4.2. Patch
� e2fsutils. The Utilities for the Second Extended File System need
a patch from
<ftp://ftp.ibp.fr/pub/linux/ELF/patches/e2fsprogs-0.5b.elf.diff.gz>
to compile correctly as a shared library. Remy Card says ``This is
the ELF patch which will probably be included in the next release
of e2fsck and co''
� file. This works anyway, but can be improved:
<http://sable.ox.ac.uk/~jo95004/patches/file.diff> adds support for
identifying stripped and mixed-endian ELF binaries.
� The Kernel. As from at least 1.3.8, the development 1.3 series
have a make config option to build using ELF tools. If you are
using the 1.2 series, you have two options:
1. Patch the Makefile slightly to use the a.out compiler. Just
change the CC and LD definitions to be
___________________________________________________________________
LD =ld -m i386linux
CC =gcc -b i486-linuxaout -D__KERNEL__ -I$(TOPDIR)/include
___________________________________________________________________
Alternatively,
2. Apply H J Lu's patch which allows compiling the kernel in ELF
(and also adds the ability to do ELF core dumps).
Let me reiterate that neither of these is necessary for the 1.3
series.
� ps (procps-0.97) The psupdate program needs a patch to work if you
have compiled the kernel as ELF. It's available in
<linux.nrao.edu:/pub/people/juphoff/procps>, both as a patch to
vanilla 0.97 and as an entire tar-file. A new version of procps is
expected to be released soon with this patch in place, so if you
can find procps 0.98 by the time you read this, this patch will
probably be obsolete.
5. Further information
� The linux-gcc mailing list is really the best place to see what's
happening, usually without even posting to it. Remember, it's not
Usenet, so keep the questions down unless you're actually
developing. For instructions on joining the mailing list, mail a
message containing the word help to majo...@vger.rutgers.edu.
Archives of the list are at <http://homer.ncm.com/>.
� There's a certain amount of information about what the linux-gcc
list is doing at my ELF web page
<http://sable.ox.ac.uk/~jo95004/elf.html>, when I remember to
update it. This also has a link to the latest version of this
HOWTO, and the patches it refers to. For US people and others with
poor links to UK academic sites (that's nearly everyone outside of
UK academia), this is all mirrored at
<http://www.blackdown.org/elf/elf.html>
� See also Bobby Shmit's ELF upgrade experience
<http://www.intac.com/~cully/elf.html> web page.
� The GCC-FAQ <ftp://sunsite.unc.edu/pub/Linux/docs/faqs/GCC-
FAQ.html> contains much general development information and some
more technical ELF details.
� There's also documentation for the file format on tsx-11
<ftp://tsx-11.mit.edu/pub/linux/packages/GCC/ELF.doc.tar.gz>. This
is probably of most use to people who want to understand, debug or
rewrite programs that deal directly with binary objects.
� H J Lu's document ELF: From The Programmer's Perspective
<ftp://tsx-11.mit.edu/pub/linux/packages/GCC/elf.latex.tar.gz>
contains much useful and more detailed information on programming
with ELF. If you aren't LaTeX-capable, it is also available as
PostScript.
� There is a manual page for dlopen(3) supplied with the ld.so
package.
6. Legalese
All trademarks used in this document are acknowledged as being owned
by their respective owners. (Spot the teeth-gritting irony there...)
The right of Daniel Barlow to be identified as the author of this work
has been asserted in accordance with sections 77 and 78 of the
Copyright Designs and Patents Act 1988. (Proof by assertion
This document is copyright (C) 1995 Daniel Barlow
<daniel...@sjc.ox.ac.uk> It may be reproduced and distributed in
whole or in part, in any medium physical or electronic, as long as
this copyright notice is retained on all copies. Commercial
redistribution is allowed and encouraged; however, the author would
like to be notified of any such distributions.
All translations, derivative works, or aggregate works incorporating
any Linux HOWTO documents must be covered under this copyright notice.
That is, you may not produce a derivative work from a HOWTO and impose
additional restrictions on its distribution. Exceptions to these rules
may be granted under certain conditions; please contact the Linux
HOWTO coordinator at the address given below.
In short, we wish to promote dissemination of this information through
as many channels as possible. However, we do wish to retain copyright
on the HOWTO documents, and would like to be notified of any plans to
redistribute the HOWTOs.
If you have questions, please contact Greg Hankins, the Linux HOWTO
coordinator, at gr...@sunsite.unc.edu via email, or at +1 404 853
9989.
- --- END Linux ELF HOWTO part 1/1 ---
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