Introduction to Memory Leaks and how to fix them

Goals

• Understand why memory leaks occur
• Learn how tofind and fix memory leaks
• use memory leak detection tools

Introduction

• A memory leak is a heap-allocated unreachable memory location
• Consequences: resource depletion and memory leaks
• In large programs, hard to detect
• For large enough executions and notable leaks, after a long time the operating system can kill an application that would return out of memory error
• Note: This topic does not cover buffer overflow issues

Memory leak example

void leak() {
    int *ptr = new int[10]; // C++
}

Valgrind

• Open source tool for memory leaks detection
• Broadly, an instrumentation framework for building dynamic analysis tools
• Other usages: profiling or concurrency issues …
• Supported in platforms such as: AMD64, AARCH64, …
• More information available at https://valgrind.org>

Using Valgrind

1. Compile with -g and 0 or 1 optimization level, -O[01]
2. Higher optimization levels may generate inexisting uninitialised-value errors
3. Run your application within valgrind environment:

   valgrind --leak-check=yes app <list of args>

Valgrind's output

• The output can be large, so you could preserve it on a file; e.g,

  valgrind --leak-check=yes app <list of args> 2>1 | tee output.txt | less

• Memory leaks looks as:

  ==19182== 40 bytes in 1 blocks are definitely lost in loss record 1 of 1
  ==19182==    at 0x1B8FF5CD: malloc (vg_replace_malloc.c:130)
  ==19182==    by 0x8048385: f (a.c:5)
  ==19182==    by 0x80483AB: main (a.c:11)

Suggested workflow

• With the least of memory leaks from valgrind try to remove them all
• When working with external libraries, being leakage free may be impossible

Quiz

• Can Valgrind detect this error?

Address Sanitizers

• Address Sanitizers (ASan) detects memory issues at runtime
• Contrary to valgrind, ASan requires linking with its library
• To use, you have to add the following compilation flags:

  -fsanitize=address -static-libasan

Summary

• Remeber to use smart pointers
• Test your code with memory-issues tools
• Explore tools such linters as well