As a veteran Linux administrator, I have compiled my fair share of Apache servers over the years. And if there is one seemingly innocuous error that can drive even experienced admins mad, it is the infamous “redefinition of ap_hack_apr_allocator_create” failing Apache compilation with cryptic logs.
In this guide, I want to provide both an in-depth technical explanation of precisely why this error occurs as well as a foolproof walkthrough to resolve it quickly. Whether you are compiling your first Apache server or your five hundredth, let’s banish this annoyance once and for all!
Why Apache Compile Errors Still Catch Us Off Guard
Before we dig into the details, I think it is important to level set on what makes Apache dependency errors like this one so pernicious…
The fact is Apache compilation is complex, often involving tracking multiple underground dependencies. Making things more confusing, you can frequently compile Apache itself completely fine only to have things fall apart when you attempt to start it.
This delayed onset failure can make issues like apr-util version mismatches extremely difficult to diagnose.
And that does not even cover intermittent build server errors that pass locally but then fail CI!
While on paper, the solution to misaligned dependency versions is simple – in practice, it remains an all too common frustration for even senior engineers. Identifying the root cause without clear error messages is half the battle.
Fortunately, the technical cause behind these frustrating “redefinition errors” and their ideal resolution is quite straightforward once we demystify exactly what is failing behind the scenes…
Peering Behind the Compiler Error
When we see ramblings in our compilation logs about the compiler not knowing whether ap_hack_apr_allocator_create is coming or going, what exactly does this mean?
The Linking Step Gone Awry
To understand redefinition errors, we have to quickly discuss how compilers like gcc work. When you kick off Apache compilation, the process looks something like this:
- The individual .c files are compiled into .o object files
- Those object files are linked together into the final Apache executable
It is this linking phase where our error emerges.
You see, the linker‘s job is to stitch all those compiled objects into a cohesive program by connecting referenced functions and libraries together.
Colliding Symbols
The “redefinition” error occurs when two different object files try to link different versions of the same function – like our friend ap_hack_apr_allocator_create.
With multiple conflicting definitions, the compiler does not know which version of this “symbol” to include in the executable.
These collisions cripple the build process since the final program would be left with duplicates of the same function!
And what is causing this symbol confusion? You guessed it…inconsistent dependency versions!
Tracking Down Mismatched Versions
So just how do these duplicate symbol definitions manage to sneak into our code base? Let’s explore the prime culprits:
Apache-Apr-Util Version Mismatch
The #1 offender stems from Apache being compiled against out-of-date apr-util sources.
As a refresher, apr-util builds on top of APR to provide Apache higher level constructs for tasks like string handling. Much like Python or Perl, Apache leverages these modular libraries so it does not have to reimplement common utilities itself.
Over time, the apr-util interface evolves, updating function names and definitions. However, if we compile the latest Apache point release against older apr-util code, those renamed symbols turn into collisions.
Hence the golden rule: Always use the apr-util version specifically designed for your Apache version!
Pinpointing the Proper Software Stack
By this point, the core cause of these avoidable errors should be clear – version mismatches in our Apache to apr-util dependency chain.
Thankfully, the fix is just as straightforward…
Scanning for Version Skew
First, we need to scan our existing compilation environment to reveal any skewed versions:
$ apxs -q CFLAGS
$ apr-1-config --version
$ apu-1-config --version This outputs the compilation flags as well as both APR and apr-util releases Apache got built against.
Getting Back in Sync
If any versions fail to match expectation for our Apache target, we simply need to:
- Download and compile the desired apr-util release
- Recompile Apache pointing properly at upgraded dependencies
Following this simple synchronization, order is restored and we avoid any further collisions!
Automating the Dependency Dance
The best approach for sustainable Apache compilation is having a CI/CD pipeline codifying the build…
Version Controlled Infrastructure
With a GitOps model, our entire runtime environment from OS distro to individual dependency versions are tracked as code. This prevents skew from creeping in as manual tribal knowledge fades.
Having the expected state version controlled gives us a single source of truth. Our automation simply needs to align the current state with the desired one.
Checking Dependencies in CI
We can then create pipeline steps to automatically:
- Validate expected vs actual versions
- Download/compile missing dependencies
- Trigger Apache rebuild against updated languages
This dependency verification contained within the pipeline acts as a safety net for changes that might introduce skew. The faster CI alerts us to mismatches, the faster we can rectify and avoid those menacing redefinition errors!
Cutting-Edge Innovations
Finally, the Apache community itself continues working on enhancements to the venerable httpd server designed specifically with sustainable compilation in mind…
One exciting area is retooling the build process for compiler independence. Features like improved modularization and renaming ambiguous symbols will help protect against inadvertent collisions across platforms and toolchains.
Investments like these will hopefully relegate tedious errors stemming from version mismatches to a thing of the past!
Key Takeaways: Conquering Apr-Util Redefinition
We covered quite a bit of ground explaining both the technical mechanics and comprehensive resolution for the age old “ap_hack_apr_allocator errors”. Let’s recap the key takeaways:
- Redefinition errors surface during Apache linking when duplicate symbols detected
- Mismatched Apache vs apr-util versions is the prime culprit
- Check versions with apxs and configs to catch mismatches
- Recompile against properly synced dependency chain
- Codify infrastructure for automatic version enforcement
Hopefully with this expanded context, you feel empowered to diagnose and overcome these sneaky compiler warnings once and for all on your systems! Let me know if any other questions come up.
