08 Jul 2020

What is this GLIBCXX error?

If you use enterprise or stable Linux distributions, sooner or later you will see an error like this:

app: /lib64/libc.so.6: version `GLIBC_3.1.45' not found (required by ./app)

Or like this:

app: /lib64/libstdc++.so.6: version `GLIBCXX_3.4.20' not found (required by ./app)

The application (or library) itself will vary. The path to the libc or libstdc++ might vary too. But the error will always mention that a GLIBCXX or GLIBC version was not found.

What can you do to fix it?

What are `glibc` and `libstdc++` ?

A little bit of background will help you understand the underlying issue better (and hopefully fix it).

glibc is the GNU C library . It’s an implementation of the standard C library. Any program written in C will use the standard library for things like accessing files and the network, displaying messages to the user, working with processes and so on. It’s a fundamental component of the operating system.

Hundreds of applications, libraries, and even other non-C programming languages installed on a typical Linux system will make use of the C library.

There are other C libraries on Linux too, such as musl , but glibc is the most common one.

libstdc++ is similar to glibc, but for C++: it’s an implementation of the standard library for C++. Any program written in C++ will use this to implement things in the C++ libraries. Things like threads, streams, files, Input/Output and so on.

There is a great reddit comment that goes into more detail about the relationship between these two libraries as well as how they relate with other core tools on the system such as the C compiler (gcc) and binutils.

An important aspect of a standard library is its Application Binary Interface (or ABI). A C program that was written and compiled against glibc in 2010 should continue to work against a new glibc version released in 2020. To make this happen, glibc provides an ABI, and promises to not change that particular ABI. glibc can add additional things, but not change or remove any part of the ABI. Removing anything in the ABI would break previously compiled applications.

How can glibc improve things if it can not ever change its ABI?

One way glibc preserves the ABI but still provides new features is through the use of symbol versioning. Each symbol, or function, provided by glibc is associated with a version. The linker (ld) links to the function-name-with-the-version. If your C program calls the function glob64, the linker will link it to not just the glob64 symbol in glibc, but to the fully versioned-symbol glob64@GLIBC_2.27. You can think of the text glob64@GLIBC_2.27 as the glob64 symbol with the version GLIBC_2.27.

This feature allows older program to use the older-but-compatible symbol glob64@GLIBC_2.2.5 but your new programs to make use of the newer symbol glob64@GLIBC_2.27.

The versions used by glibc for symbols generally look like GLIBC_<VERSION>. Some examples are GLIBC_2.27 and GLIBC_2.2.5.

You can find more information about how symbol versioning works in glibc here

libstdc++ has a similar concept and implementation ABI. In fact, some versions of libstdc++ even provide two different ABIs !

The versions exported by libstdc++ generally look like GLIBCXX_<VERSION>. Some examples: GLIBCXX_3.4 and CXXABI_1.3.

What does the error mean?

Now that you have a bit of a background on the C and C++ standard libraries and symbol versions, lets go back to the errors:

app: /lib64/libc.so.6: version `GLIBC_3.1.45' not found (required by ./app)

This error happens when the runtime linker tries to load the standard C library (libc) for app. The runtime linker sees that app has a dependency on a symbol and the version GLIBC_3.1.45 is not found in this C library.

app: /lib64/libstdc++.so.6: version `GLIBCXX_3.4.20' not found (required by ./app)

This error happens when the runtime linker tries to load the C++ library (libstdc++) for app. The runtime linker sees that app has a dependency on a symbol and the version GLIBCXX_3.4.20 of the symbol is not found in this C++ library.

In other words, the errors mean that app was linked against an newer version of the C/C++ library. The C/C++ library available on the system is too old and does not provide those symbols with those versions.

How can this happen?

This type of error is easy to run into.

You downloaded a pre-built binary compiled for a recent Linux distribution and try and run it on an older Linux distribution.

For example, an application compiled to run on RHEL 8 will show the error when run on RHEL 7. So will a a python package meant to run on a newer distribution.
You somehow installed a distribution package meant for a newer version of the distribution .
You downloaded an application that just isn’t supported on your distribution.

For example, you will run into this if you try and run .NET Core on RHEL 5.

The root cause is the same: there’s an incompatibility between your Operating System and the application or library you want to run.

You can check the GLIBC or GLIBCXX versions needed by an application or library using readelf:

$ readelf --dyn-syms /usr/bin/java | grep '@GLIBC'
     2: 0000000000000000     0 FUNC    GLOBAL DEFAULT  UND __libc_start_main@GLIBC_2.2.5 (3)
     6: 0000000000000000     0 FUNC    WEAK   DEFAULT  UND __cxa_finalize@GLIBC_2.2.5 (3)

Here, you can see that this application (java) uses two symbols that are versioned as GLIBC_2.2.5.

How to fix this error?

The correct way to fix this error is to make sure the Linux distribution you are using is compatible with the application (or library) that’s causing this error.

The application/library does not support your Linux distribution:

If you are using a Linux distribution which is too old and not supported by the application, upgrade to a newer one.

For example, if you need to run an application on RHEL 6 that reports this error, consider upgrading to RHEL 7 (or RHEL 8) to resolve this problem.

The application/library does support your Linux distribution:

If the application/library supports the version of your Linux distribution, then you got the wrong application or library binary. See if you can find a binary download that’s appropriate for your distribution. .NET Core, for example, supports RHEL 6, but has a separate binary download for RHEL 6. The regular .NET download supports Linux distributions that are newer than RHEL 6.

You might also want to file a bug/issue or report it to the application/library provider some other way. This will let them know that they need to change the downloads section and the documentation to help their users avoid this issue.

Here’s how not to fix the problem

There’s a some bad advice on how to fix this issue. Please do not follow it.

There’s some advice that says you can build your own version of glibc and/or libstdc++ and use them. I recommend not doing this. Once you build your own version, you have to take ownership of maintaining the build. That also means keeping up with all libc/libstdc++ security fixes and applying them and rebuilding libc/libstdc++. That might be okay for a hobby project, but it’s not an appropriate solution for a production environment.

If you really, really know what you are doing and understand all the maintenance, security and compatibility implications of it, there’s the last option: you can rebuild your own version of glibc or libstdc++. There’s steps on how to build your own version of glibc here . And there’s an even better way to run your just-built glibc that’s described here .

But then again, if you fully understand all the ramification, you wouldn’t be reading this post to help understand and fix the error. And you have probably spotted some ideas here that are not 100% correct.

There’s also some advice that it’s okay to simply grab a version of glibc from another version of your Linux distribution. That might work. It might also completely break your installation. It might also prevent any further updates to glibc/libstdc++ via the distribution’s package manager.

Conclusion

You should now know:

What symbol versioning is
How libc and libstdc++ use symbol versioning
What the version GLIBC_X.Y not found error messages mean
What are some good ways to fix the errors:
- Upgrading your Linux distribution
- Using a compatible version of the library
How not to fix this error!