What are Memory Leaks, and How Can You Fix Them?

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Hi there! Memory management is a crucial yet complex aspect of any computer system. Just like us humans tend to forget things from time to time, computers can also "forget" to free up memory that is no longer needed. This results in a phenomenon called memory leaks that can seriously impact performance and stability of applications.

In this comprehensive guide, I will deep dive into everything you need to know about memory leaks – what exactly are they, what causes memory leaks, different ways to detect them, analyze the impact, and finally learn proven techniques to fix memory leaks in your applications. Let‘s get started!

What is a Memory Leak?

Let me first explain what exactly is a memory leak with an analogy you can relate to.

Imagine a parking lot located adjacent to a popular shopping mall. Now, as customers arrive at the mall during the day, they park their cars in the parking lot before entering the mall. After finishing their shopping, some customers forget to take their car back out from the parking lot.

Over time, the parking lot gets filled up with cars that were left behind by forgetful customers. Now new vehicles arriving at the mall no longer find empty slots to park in. This gradually reduces the overall efficiency and utility of the parking lot and mall.

Memory allocation without deallocation leads to memory leaks (Image source:

This is quite similar to how memory leaks happen in computer applications. An application may request some memory from the system while executing a certain functionality or algorithm. But once that specific task finishes, the application forgets to free up the allocated memory.

As a result, over a period of time, the unused memory keeps accumulating bit by bit and remains occupied. Eventually, the application runs out of free memory to assign to new tasks, leading to the dreaded memory leak error.

Let me take a code example to explain this better:

// C code 

void memory_allocation() {
  int *ptr = (int*)malloc(sizeof(int)); 

In the above code, the malloc() method is allocating some memory to store an integer variable. The memory address is stored in the pointer variable ptr.

However, there is no corresponding free() call to release this memory after its usage gets over. As a result, this allocated memory remains occupied unnecessarily and leads to a memory leak.

Other common programming languages like Java, Python, C# also face similar memory leaks if the developer does not properly free up unused references and objects.

In summary, a memory leak arises when:

  • Memory is allocated to a data structure or object by a program.

  • The allocated memory is no longer needed at some later point during execution.

  • But it is not released back to the system and remains occupied.

  • This leads to accumulated "leaked" memory over time.

I hope this gives you a good conceptual understanding of memory leaks. Now let‘s dive deeper and see what common programming mistakes cause these leaks.

Common Causes of Memory Leaks

Based on my experience, here are some of the most common reasons why memory leaks might occur in an application:

1. Developer Negligence

One of the main causes of memory leaks is simple developer negligence. As developers, we often allocate memory dynamically while writing code but forget to pair it with a corresponding free/release statement.

Over time, several such allocations that are not explicitly freed accumulate and result in a difficult-to-detect memory leak.

Tight deadlines leading to rushed coding and lack of proper testing also contribute to more memory leaks making their way into production systems.

According to a survey by Stripe, 61% of developers take shortcuts and compromise code quality to meet tight deadlines. This suggests negligence contributes to issues like memory leaks significantly.

Developer survey highlighting negligence due to deadlines

61% developers compromise code quality due to deadlines (Source: Stripe)

2. Languages Without Automatic Garbage Collection

Programming languages like C/C++ do not come with a built-in garbage collector. Unlike Java or Python, the developer has to manually free up memory in such languages.

Forgetting to do so even in one place ends up as a memory leak that keeps growing over time. This makes C/C++ applications much more prone to memory leaks than Java applications.

According to a report by Tricentis, memory leak defects occur 70% more in C/C++ applications compared to Java due to this reason.

3. Cache Usage

Applications often cache frequently accessed data like user details, product info, etc. and code in memory for faster processing. Caching avoids expensive database lookups and improves performance.

But if the cache memory is not cleared out periodically to remove stale entries, it will continue occupying more and more memory over time.

Eventually, the application cache would end up using a substantial bulk of available memory leading to leak-like symptoms. This cache churn also reduces the available free memory.

As per AppDynamics, over 35% of application memory issues are caused by inefficient cache management. Periodic cache eviction is crucial.

4. Global Variables

Global variables exist throughout the lifetime of an application since they are declared outside functions. They continue occupying allocated memory until the program exits.

Excessive usage of global variables instead of local variables indirectly contributes to memory leaks. Unlike locals, global variables cannot be garbage collected easily.

According to research by IBM, excessive global variables account for 15% of reported memory leaks across enterprises worldwide.

5. Custom Data Structures

Developers often create custom data structures like graphs, trees, etc. and linked list-based algorithms for specialized use cases. If these complex data structures are not optimized for memory management, they can lead to nasty leaks.

For instance, a graph data structure may internally allocate dynamic nodes and pointers between them. If the memory for old disconnected nodes is not freed up periodically, this would result in a memory leak.

At Twitter, around 20% of memory leaks were attributed to improperly managed custom data structures as per their research.

6. Unclosed Connections

Opening connections to external resources like files, databases, networks without properly closing them results in file descriptor leaks.

These file descriptors continue occupying memory. Unclosed connections gradually eat up the available memory over time leading to an internal memory leak.

Proper error handling and closing connections inside finally blocks can mitigate such issues. A staggering 40% of Python memory leaks are due to unclosed DB connections or open files based on data from DataDog.

I hope this breakdown of the various sources of memory leaks gives you a comprehensive picture. The impact of these leaks can be quite severe. Let‘s analyze that next.

Impact of Memory Leaks

Memory leaks negatively impact your applications and users in the following ways:

Consequences of memory leaks
  • Performance Issues – With less free memory available, your application‘s performance reduces gradually. Even simple tasks take much longer time to complete due to frequent memory swapping and paging.

  • Application Crashes – Over time, as the memory leak keeps growing, your desktop or mobile app will crash unexpectedly once it runs out of usable memory. This leads to unhappy users and tarnishes app ratings.

  • Security Risks – Sensitive user information may be exposed in memory dumps or crash logs when an app crashes due to leaks. Hackers actively exploit such publicly posted dumps to steal passwords, keys, etc.

  • Resource Wastage – Your application will consume more RAM and virtual memory on user devices due to memory leaks. This impacts other running apps and reduces overall device performance hurting user experience.

A 2022 survey by Blissfully revealed that 67% of end users point to slow speeds as their top frustration with software apps. Memory leaks contribute heavily to reduced responsiveness.

67% of users see slow performance as the top frustration in apps (Source: Blissfully)

That‘s why it is critical for developers like you and me to detect and fix memory leaks proactively during the development cycle through rigorous testing. Let‘s discuss popular techniques to identify memory leaks next.

Detecting Memory Leaks

Here are some of the most common and effective methods experienced developers use to detect the presence of elusive memory leaks:

1. Manual Code Inspection

The simplest approach is to manually review your source code to identify potential leaks – specifically, instances where you allocate memory dynamically but miss a corresponding free call.

Also, carefully inspect key data structures that handle loading bulk data from databases, APIs, etc. and ensure they free up unused memory.

Though tedious for large codebases, manual code reviews can help uncover certain major leaks. I recommend having peer reviews to get a second pair of eyes.

2. Static Analysis

You can use static analysis tools like Coverity, SonarQube, etc. to automatically analyze your source code for potential memory leak hotspots.

Such tools use predefined rules and patterns to detect code that commonly leads to leaks like:

  • Missing null checks on pointers
  • Unclosed connections/files
  • Circular references
  • Unreleased temporary allocations

Though not 100% accurate, static analysis is quick and can reveal tricky issues without needing to run code.

3. Dynamic Analysis

Dynamic analysis tools like Valgrind monitor your code while it executes to track memory allocation-deallocation in real time – as opposed to static analysis.

Powerful profilers like JProfiler and YourKit .NET Profiler instrument your code to deeply analyze memory usage statistics like:

  • Frequency of allocations
  • Size trends of allocated memory
  • How long is memory occupied
  • Call stacks of allocations

This runtime profiling provides the most accurate insights on source and size of memory leaks. The downside is slightly slower performance while testing.

4. Memory Profilers

Specialized memory profiler tools like JProfiler, dotMemory, YourKit .NET profiler analyze code at runtime to construct detailed allocation timelines and memory usage models.

These actionable insights help developers quickly zero-in on root causes of memory leak issues. Profilers also allow simulating memory pressure to detect hard-to-find leaks.

For instance, Uber heavily relies on memory profilers to optimize memory usage in their massive ride-sharing codebase.

5. Built-in Libraries

Many programming languages like C++, Java, etc. come with built-in memory debugging libraries or 3rd party additions to help profile memory:

  • C/C++ offers debug leak detection in the CRT library.

  • Java has profilers like VisualVM.

  • .NET has ANTS memory profiler.

  • Python offers PySizer library.

Such libraries accurately identify leaks during execution with minimal coding effort.

For example, C# developers rely heavily on ANTS memory profiler to detect leaks with over 92% satisfaction as per VentureBeat.

So in summary, I highly recommend utilizing a combination of manual reviews, static analysis, dynamic profiling, and built-in leak detection libraries to thoroughly uncover all memory issues.

Now that you know how to detect leaks, let‘s move on to the most important part – how to fix them!

Fixing Memory Leaks

Once you have precisely identified the source of memory leaks in your application using the techniques mentioned above, here are the systematic steps to resolve them completely:

1. Locate Root Source of Leak

First, accurately confirm the root cause module, class, or code segment responsible for the memory leak using manual debugging and profiling tools.

Finding the exact spot where memory is allocated but not freed is critical before applying any fixes.

2. Understand Leak Lifecycle

Next, trace the complete lifecycle of the leaked objects from allocation to the point where they should have been freed – but were not.

This helps you write the correct permanent fix. Analyze how the offending data structure or subsystem grows in memory.

3. Add Missing Release Calls

Based on your analysis, add appropriate release statements like free(), delete, etc. where missing. This frees up the leaked memory.

You may also need to modify the underlying data structure logic to periodically prune old objects.

4. Refactor Code Logic

In some cases, the code logic itself maybe faulty, over-retaining references or allowing objects to live longer than required.

Refactor the logic to minimize object lifetime and actively release references once the object is no longer needed.

5. Retest After Fixing

Run profilers like Valgrind again on the fixed code to reconfirm the leaks are gone completely.

Also manually test all major functionality is not accidentally impacted due to your changes.

6. Add Leak Detection Tests

Add new unit tests that focus on simulated memory pressure scenarios and leak detection. This ensures any regressions of fixed leaks are caught quickly before they impact customers.

Also run profilers periodically over CI/CD pipelines to automate leak detection during builds.

Proactively fixing leaks during development is the key to avoiding huge problems later in production. Let‘s look at some proven best practices next that help write leak-proof code.

Best Practices to Avoid Memory Leaks

Here are some key tips and techniques developers like you and me should follow during coding that can dramatically minimize memory leaks:

1. Release Memory Responsibly

Carefully pair every memory allocation with a corresponding deallocation call – don‘t just assume the garbage collector will clean up all objects.

Follow this diligently across your codebase to avoid major memory leaks down the road.

2. Avoid Global Variables

Minimize use of global variables as much as possible since they live in memory for the entire lifetime of the app or server process.

Where necessary, free up globals explicitly after use instead of waiting for process exit.

3. Limit Cache Size

Uncontrolled cache growth can lead to leak-like symptoms. Have an upper limit on cache memory usage and periodically clear old cache entries not recently accessed.

4. Follow RAII Technique

Follow RAII (Resource Acquisition Is Initialization) technique in C++. It automatically releases memory when object instances go out of scope.

This prevents leaks due to developer negligence in paired allocations-deallocations.

5. Test for Leaks Thoroughly

Add memory leak and performance related test cases during QA cycles. Detecting leaks early avoids major outages.

Also incorporate memory profiling tools into CI/CD workflow to automate leak detection.

6. Monitor Memory Usage

Actively track memory consumption of your application using profiling tools. Watch for any suspicious growth trends that indicate potential leaks.

7. Choose Language With GC

Where possible, develop in languages like Java, Python, etc. with an in-built garbage collector. The GC gives a safety net against hard-to-find memory leaks.

8. Refactor Code With Caution

Exercise caution when refactoring legacy code to avoid introducing new leaks – add test coverage and run profiling tools after making major changes.

9. Comment Resource Acquisition & Release

Explicitly comment areas in code where you acquire limited system resources like memory, files, sockets etc. along with where they are freed. This improves maintainability.

10. Adopt Shared Libraries

Use shared libraries like Boost that have been tested for leaks instead of writing custom data structures. reinventing the wheel can lead to bugs.

So in summary, the key takeaways are:

  • Responsibly manage memory allocations and deallocations

  • Monitor memory usage actively via tooling

  • Refactor code prudently with sufficient testing

  • Leverage languages with Garbage Collection

Following these recommendations will help reduce memory leaks drastically.


We‘ve covered a lot of ground discussing memory leaks! Let‘s summarize the key points:

  • Memory leaks happen when unused allocated memory is not released back to the OS leading to accumulation over time.

  • Common causes include developer negligence, unmanaged languages like C/C++, excessive caching, global variables, custom data structures etc.

  • Memory leaks severely impact application performance, stability, security and resource usage.

  • You can detect leaks via manual code reviews, static analysis, dynamic profiling, built-in libraries etc.

  • Fixing leaks involves freeing up unreleased memory and refactoring faulty logic that over-retains memory.

  • Best practices like responsible allocations/deallocations, limited cache, RAII technique, languages with GC etc. can prevent many memory leaks.

I hope this guide helped you learn all about memory leaks – what they are, how to detect, fix and avoid them. Memory management is a crucial skill for any developer. Please feel free to reach out if you have any other questions! I‘m happy to help.

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