How to debug C++ memory leaks using Valgrind?

How to use Valgrind to debug C++ memory leaks

Valgrind is a powerful memory debugger that can be used to detect memory leaks in C++ programs , illegal use and distribution issues. Here's how to use Valgrind to debug C++ memory leaks:

1. Install Valgrind

Use the following command to install Valgrind:

sudo apt install valgrind

2. Compiling and Debugging

When compiling the program, add the -g flag to generate debugging information:

g++ -g my_program.cpp -o my_program

Then, use Valgrind to run the program and use --leak-check=full Flag to check for memory leaks:

valgrind --leak-check=full ./my_program

3. Analyze Valgrind output

The output of Valgrind will contain information about the detected memory leaked information.

Practical case

The following is a simple C++ program that simulates a memory leak:

#include <iostream>

int* leak() {
  int* ptr = new int;
  return ptr;
}

int main() {
  int* ptr = leak();
  return 0;
}

Compile and use Valgrind to run this program:

g++ -g leak.cpp -o leak
valgrind --leak-check=full ./leak

The output of Valgrind will contain the following information:

==27244== Memcheck, a memory error detector
==27244== Copyright (C) 2002-2017, and GNU GPL'd by, Julian Seward et al.
==27244== Using Valgrind-3.15.0.
==27244== Command: ./leak
==27244==
==27244== HEAP SUMMARY:
==27244==     in use at exit: 4 bytes in 1 blocks
==27244==   total heap usage: 1 allocs, 0 frees, 4 bytes allocated
==27244==
==27244== LEAK SUMMARY:
==27244==    definitely lost: 4 bytes in 1 blocks
==27244==    indirectly lost: 0 bytes in 0 blocks
==27244==      possibly lost: 0 bytes in 0 blocks
==27244==    still reachable: 0 bytes in 0 blocks
==27244==         suppressed: 0 bytes in 0 blocks
==27244== Rerun with --leak-check=full to see what's still reachable
==27244==
==27244== For counts of detected and suppressed errors, rerun with: -v
==27244== Use --track-origins=yes to see where unfreed memory was allocated
==27244== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
==27244==
==27244== 1 errors in context 0 of 1:
==27244== Invalid read of size 8
==27244==    at 0x4842E10: leak (leak.cpp:5)
==27244==    by 0x483D8E7: main (leak.cpp:12)
==27244==  Address 0x555555555600 is not stack'd, malloc'd or (recently) free'd
==27244==
==27244== LEAK SUMMARY:
==27244==    definitely lost: 0 bytes in 0 blocks
==27244==    indirectly lost: 0 bytes in 0 blocks
==27244==      possibly lost: 4 bytes in 1 blocks
==27244==    still reachable: 0 bytes in 0 blocks
==27244==         suppressed: 0 bytes in 0 blocks
==27244== Rerun with --leak-check=full to see what's still reachable
==27244==
==27244== For counts of detected and suppressed errors, rerun with: -v
==27244== Use --track-origins=yes to see where unfreed memory was allocated

This output indicates that there is a 4-byte memory leak in the program, which comes from an unreleased ## in the function leak() #int Pointer.

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