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symlinks and .so files on linux - what you need to know

The issue

I’ve seen this happen countless times.

First, a Linux user asks for help about an error looking like this:

Error when running `bar`: `libfoo.so.5: no such file or directory`

Then, someone who’s had the same issue suggests creating a symlink from libfoo.so.6 to libfoo.so.5.

And then I come across the dialog some time later and I start crying.

Why do I cry - it looks like the problem is solved, right?

  • the bar program seems to run just fine;
  • the change was made with just one command (sudo ln) and can be undone easily (just remove the symlink);
  • there was already a symlink from libfoo.so to libfoo.so.6 anyway!

Well, despite the appearances, the problem is not solved, and doing this is a terrible idea in general - like planting a ticking time bomb in the street you live or shooting yourself in the foot because you’ve got a plantar wart.

But to understand why we need to talk about the language C, shared libraries, soname bumps, Linux distributions, and package management.

And because I love telling stories, you, dear reader, will be the hero in this one.

Solving the Ultimate Question

Let’s assume a group of people you don’t really know (let’s called them The Experts), wrote a piece of C code than can get the answer to the Ultimate Question of Life, the Universe, and Everything.

For obvious reasons, they want to keep the source code private, so here’s what they did:

  • They wrote a header file containing the get_answer declaration named answer.h 1:
// in answer.h
#pragma once
#ifdef __cplusplus
extern "C" {
#endif

char* get_answer();

#ifdef __cplusplus
}
#endif
  • They created a shared library called libanswer.so from their source file (answer.c in this case):
$ gcc -shared libanswer.so answer.c

That way, everyone who needs to get the answer to the Ultimate Question of Life, the Universe, and Everything can buy the libanswer.so compiled library and the answer.h header and call the get_answer() function - let’s see how.

Using the library from The Experts

You’ve bought the libanswer.so and answer.h files from The Experts and have put them next to a file you’ve wrote named print-answer.c:

// in print-answer.c
#include <stdio.h>
#include <answer.h>

int main() {
  char* answer = get_answer();
  printf("The answer is %s\n", answer);
  return 0;
}

You were told that gcc can compile C code and link against shared libraries if you put them on the command line, so you try and run this:

$ gcc libanswer.so print-answer.c -o print-answer

But it does not work, and you get the following error message:

print-answer.c:2:10: fatal error: answer.h: No such file or directory

Wait a minute - the answer.h file is right there - what does that mean, “No such file or directory”?.

After a bit of research, you discover that gcc uses a list of paths called the “include path” where it looks for headers. You check on your machine, and sure enough, /usr/include/ is one of the elements of this list, and stdio.h is in /usr/include/stdio.h, which explains why gcc did not complain about the first include.

To fix the compilation error, you add the -I . option to the command line so that current directory is added to the list of include paths:

$ gcc -I . libanswer.so print-answer.c -o print-answer

And then it compiles.

Now you try and run the print-answer program, but you get a new error message:

$ ./print-answer
 error while loading shared libraries: libanswer.so
 cannot open shared object file: No such file or directory

It’s the same error message as in the introduction and the operating system is lying to us. The file libanswer.so is right there! What’s happening there?

After more investigation, you figure it out: when you compiled the print-answer executable, there was a small piece of binary inside it that recorded the name of the .so file it was linked against. You can check it by running the readelf command and display the dynamic section of your program:

$ readelf -d ./print-answer
Dynamic section at offset 0x2de8 contains 27 entries:
  Tag        Type                         Name/Value
 0x0000000000000001 (NEEDED)             Shared library: [libanswer.so]
 0x0000000000000001 (NEEDED)             Shared library: [libc.so.6]
 ...

In a way, the print-answer program “knows” that it needs libanswer.so to run. Crucially, it does not know nor cares about where libanswer.so really is. 2

Then, when you run ./print-answer, the operating system sees the name of the shared library in the dynamic section and tries to locate it. Like gcc, it finds libc.so.6 by itself (in /usr/lib/libc.so.6 for instance) - but it’s unable to find the libanswer.so shared library in the current directory.

Fortunately, you can fix that by using a special environment variable called LD_LIBRARY_PATH:

$ LD_LIBRARY_PATH=. ./print-answer
The answer is 42

This time, the operating system looks for libanswer.so in the current directory, finds it, and when required, invokes the code for the get_answer() function from the shared library.

That gets you thinking - you did not have to do any of this for print-answer to find the libc.so library and the stdio.h header.

  • What if it was possible to compile libanswer.so once and for all?
  • And what if there was a way to compile the print-answer.c source file without having to copy/paste the header and the shared library, and remember all the various gcc options?

Becoming a packager

Let’s assume The Experts realized that their business model was not going to work and decided to publish their source files for free instead. Yay open source!

Here’s the contents of their answer.c file:

#include <answer.h>
#include <stdio.h>
#include <string.h>

char * get_answer() {
  // Disappointing, I know
  int r = 6 * 7;
  char buf[3];
  snprintf(buf, 3, "%d", r);
  return strdup(buf);
}

Since you are an Arch Linux user, you decide to lookup documentation about the pacman package manager and how generate and publish Arch Linux packages 3.

After a while, you manage to write a working PGKBUILD for libanswer:

pkgname=libanswer
pkgver=1.0
pkgrel=1
pkgdesc="Answer the Ultimate Question"
arch=('x86_64')
...

build() {
  gcc -I . -shared answer.c -o libanswer.so
}

package ()
{
  mkdir -p $pkgdir/usr/{include,lib}
  install answer.h $pkgdir/usr/include/
  install libanswer.so $pkgdir/usr/lib
}

Then you build the package with makepkg

$ makepkg
==> Making package: libanswer 1.0-1
...
==> Finished making: libanswer 1.0-1

Everything goes well, and you now have a file named libanswer-1.0-1-x86_64.pkg.tar.xz next to your PGKBUILD.

You install the package with pacman:

$ sudo pacman -U libanswer-1.0-1-x86_64.pkg.tar.xz

Then you find out that you can compile and run print-answer.c from anywhere in the system using the following commands:

$ gcc print-answer.c -lanswer -o ./print-answer
$ ./print-answer
The answer is 42

This time you need only the name of the library (the part without the lib prefix and the .so suffix) after the -l option. You do not have to worry about include paths or use the LD_LIBRARY_PATH environment variable - neat!

What you’ve accomplished is called packaging the answer library, and it’s what package maintainers do - well done.

Impressed by your packaging skills4, the Arch Linux maintainers allow you to publish your package in official repositories, which means everyone using Arch Linux is now able to install the libanswer package in just one command!

A simple change

A few days later, The Experts release a new version of their library (1.1), containing a nice optimization:

char* get_answer() {
  return strdup("42");
}

Since you are the maintainer of the libanswer package, you quickly update the PKGBUILD and publish a new release.

-pkgver=1.0
+pkgver=1.1
 pkgrel=1
 pkgdesc="Answers the Ultimate Question"
 arch=('x86_64')
 source=(...)

You build and install the package on your machine and check it still works:

$ makepg
$ sudo pacman -U libanswer-1.1-1-x86_64.pkg.tar.xz

And then you publish the new version of the libanswer package.

That’s where Linux distributions really shine (and the whole reason we use shared libraries in the first place). Once the new package is published, any Arch Linux user who installs it will get the latest version of libanswer.so in their system, and all the programs that were linked against it will use the latest version - this is especially important if the new version contains a security bug fix for instance.

Another change

A week later, The Experts realize they don’t really need to return a string from the get_answer() function, and that a simple int would suffice.

So they modify both their header and source files:

- char* get_answer();
+ int get_answer();
#include <answer.h>

int get_answer() {
  return 42;
}

And they publish a release note:

# The answer project

## Version 2.0

* Change the return type of the `get_answer()` function from `char*` to `int`.

## Version 1.1

* This release implements performance optimizations.

## Version 1.0

* First public release!

Upon hearing the good news, you download the latest sources of the project, and you modify your print-answer.c source file to use the latest version of the library:

#include <stdio.h>
#include <answer.h>

int main() {
  int answer = get_answer();
  printf("The answer is %d\n", answer);
}

You compile everything and check the code still works:

$ gcc -I . -shared answer.c -o libanswer.so
$ gcc -I . libanswer.so print-answer.c -o ./print-answer
$ LD_LIBRARY_PATH=. ./print-answer
The answer is 42

Time to publish the v2!

-pkgver=1.0
+pkgver=2.0
 pkgrel=1
 pkgdesc="Answers the Ultimate Question"
 arch=('x86_64')
 source=(...)
$ makepg
$ sudo pacman -U libanswer-2.0-1-x86_64.pkg.tar.xz

Easy as pie - satisfied, you publish the new package and go to bed.

When shit hits the fan

The next morning you receive the following e-mail:

Subject : latest libanswer package update broke display-answer-pp program

Hello,

I'm using `display-answer-pp` version 0.4, and when I updated `libanswer` to
the version 2.0, I got the following error:

./display-answer-pp
zsh: segmentation fault (core dumped)  ./display-answer-pp

Downgrading the `libanswer` package fixes the problem. Please advise.

Signed: Bob

Welcome to the joys of packaging!

You’ve never heard of the display-answer-pp package - what is going on?

After a bit of research, you find out that someone wrote a display-answer-pp program using your libanswer package and published it on the official Arch Linux repositories a few days ago.

Here’s what the code for display-answer-pp looks like - it’s a single C++ file:

#include <answer.h>
#include <iostream>

int main() {
    auto answer = get_answer();
    std::cout << "The answer is: " << answer << std::endl;
    return 0;
}

So what happened?

Well, the problem is that you forgot to coordinate with your fellow packagers!

You see, when display-answer-pp was being packaged, the get_answer() function was returning a char*. When you published answer version 2.0, the code for the function get_answer() started returning an int instead.

So when Bob updated libanswer after having installed display-answer-pp, and tried to re-run the program, all hell break loose, because the compiled C++ code expected a pointer to a string and got an int instead.

In other terms, the application binary interface (or ABI for short) of the libanswer library broke.

You break it, you fix it

Unfortunately, there’s only one way to fix an ABI breakage: you need to recompile everything that was linked against the old version of the library.

That’s one of the main issues package maintainers have to solve. They need two very different features when it comes to libraries updates:

  • If the new version of the library is ABI-compatible with the previous one, end-users should be able to get it by simply updating the one package that contains it.

  • If not, they need to make sure none of the programs that depend on the library break when the update is made.

The Arch Way

Here’s how Arch maintainers solve this problem. In our example, they would have published libanswer v2 in a special repository named “staging”. Then, they would have rebuild every package that depends on libanswer (so both print-answer and display-answer-pp) and pushed those to the staging repository.

Finally, after a period of testing, they would have moved libanswer, print-answer and display-answer-pp in the official repositories in one swift update. They would have used a to do list like this one to coordinate the packaging tasks.

This means that if you try and update libanswer without upgrading every other package that depends on it, you risk breaking your installation - and that’s why partial updates are not supported on Arch Linux.

The Debian Way

Debian maintainers use another strategy. When they package a library, they include its version number in the name of the package. What’s more, they have a separate development package that contains the files required for compiling programs that use the library. They also use a compilation trick called the soname option.

Let’s see how this works.

First, they tell gcc to use the correct soname option when linking the library 5:

$ gcc -I . answer.c -shared -Wl,-soname=libanswer.so.1 -o libanswer.so.1

They use the outcome of the build to generate two packages:

  • libanswer-dev, that contains a symlink libanswer.so -> libanswer.so.1 and the answer.h header
  • and libanswer1, that contains only the libanswer.so.1 file

Then, they build display-answer-pp for the first time, using the libanswer-dev package:

$ g++ -lanswer display-answer.cpp -o ./display-answer-pp

Because libanswer.so was built with the -soname=libanswer.so.1 option, the display-answer-pp binary now knows that it needs libanswer.so.1 at runtime:

$ readelf -d display-answer-pp
Tag                  Type               Name/value
0x0000000000000001 (NEEDED)             Shared library: [libanswer.so.1]
...

When the v2 version of libanswer is published by The Experts, they rebuild the shared library with an appropriate soname:

$ gcc -I . answer.c -shared -Wl,-soname=libanswer.so.2 -o libanswer.so.2

They use the outcome of the build to update the libanswer-dev package and to create a brand new libanswer2 package.

At this point:

  • libanswer-dev contains a symlink libanswer.so -> libanswer.so.2 and the updated answer.h header
  • libanswer2 contains only the libanswer.so.2 file

Then, they build display-answer-pp for the second time, using the updated libanswer-dev package:

$ g++ -lanswer display-answer.cpp -o ./display-answer

This time, display-answer-pp knows that it needs libanswer.so.2 at runtime:

$ readelf -d display-answer-pp
Tag                  Type               Name/value
0x0000000000000001 (NEEDED)             Shared library: [libanswer.so.2]
...

Let’s sum up:

  • Both libanswer1 and libanswer2 packages can co-exist in the same system, since they contain different filenames
  • The libanswer-dev package does not need to be installed for display-answer-pp to run since the program contain the versioned soname in its dynamic section
  • If you want to rebuild a program when a new version of the library is out, you just install the latest libanswer-dev package - the command used for compilation does not have to change at all!

Clever, right?

Because of this technique, the update from libanswer v1 to libanswer v2 is often called a soname bump - the update from v1 to v1.1 is just a regular update.

Quite often, a backward incompatible update correspond to the first digit of the soname to be updated. 6

Conclusion

Now you know - different versions of a library have various sonames, and the symlinks are carefully crafted by distribution maintainers.

So, when you create a symlink yourself, you are taking a huge risk, especially when creating links between libraries that have a different leading digit in their soname!

As we saw, using the incorrect library at runtime can cause crashes, and if you break an essential binary (like bash for instance), you may no longer be able to log in, which means your only choice may be to re-install your whole system from scratch. This is not theoretical by the way: it happened to me years ago, and I still remember it to this day.

So, what to do if you get this error?

Error when running `bar`: `libfoo.so.5: no such file or directory`
  • If bar comes from a package on your distribution, update all packages and then open a bug report if the problem persists
  • If not, try and re-compile bar - possibly after updating the libfoo-dev package.

But please, please do not create a symlink from libfoo.so.6 to libfoo.so.5 or suggest this solution to someone else. Together, let’s put an end to this bad, bad, advice. Thank you!


  1. If you’re puzzled by the __cplusplus stuff don’t worry - it’s just so that the answer library is usable from C++ too. ↩︎

  2. By the way, it needs libc.so because the compiled code for printf lives there. ↩︎

  3. You’re using Arch Linux because you’re cool and you want to learn - good for you! ↩︎

  4. Reminder: it’s a story I made up! ↩︎

  5. Actually, this is often done automatically by whatever build system the library is using ↩︎

  6. Sadly it’s not always the case: lua and libpng are notable exceptions. ↩︎


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