Valgrind

Source Attribution

These notes are based on the tutorial Using Valgrind to Find Memory Leaks by Cprogramming.com.

1. Introduction & Installation

Valgrind runs your program in a monitored environment, tracking every call to memory allocation (malloc, new) and deallocation (free, delete).

Quick Installation (Source)

1. Decompress: bzip2 -d valgrind-XYZ.tar.bz2
2. Extract: tar -xf valgrind-XYZ.tar
3. Build & Install:

cd valgrind-XYZ
./configure
make
make install

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2. Detecting Memory Leaks

Memory leaks are silent bugs that occur when allocated memory is never freed. Over time, these can cause a program to crash once system memory is exhausted.

Basic Usage

To check for leaks, use the memcheck tool.

valgrind --tool=memcheck ./program_name

A balanced program will show equal numbers of allocs and frees. If they differ, you have a leak.

Detailed Analysis

To see exactly where the leak occurred, compile your code with the -g flag (to include debugging symbols) and run:

valgrind --tool=memcheck --leak-check=yes ./program_name

Pro Tip:

Use --show-reachable=yes to find absolutely every unpaired call to new or malloc.

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3. Finding Invalid Pointer Usage

Valgrind detects when your code accesses memory addresses that it does not “own,” such as writing past the end of an array.

• Invalid Write: Occurs when you try to change data outside allocated bounds.
• Invalid Read: Occurs when you try to access data outside allocated bounds.

Example Error:

If you malloc(10) but attempt to write to x[10], Valgrind will trigger an “Invalid write of size 1” and provide a stack trace to the exact line of code.

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4. Detecting Uninitialized Variables

Using a variable before assigning it a value can lead to unpredictable program behavior. Valgrind flags “Conditional jump or move depends on uninitialized value(s).”

• Logic Tracking: Valgrind is smart enough to track an uninitialized value even if it is passed through several functions (e.g., from main into a helper function).
• Requirement: You must test the specific branch of code containing the conditional statement for Valgrind to see the error.

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5. Other Memory Errors

• Double Frees: Calling free() or delete twice on the same pointer.
• Mismatched Deallocators: Using the wrong method to free memory.
  • malloc must be paired with free.
  • new must be paired with delete.
  • new[] must be paired with delete[].

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6. Limitations & Caveats

Feature	Supported?	Note
Heap Memory	Yes	Monitors malloc, new, etc.
Stack Memory	No	Does not check bounds for static arrays like char x[10].
Performance	Variable	Can slow down code significantly and use 2x more memory.
Edge Cases	Limited	Will not detect buffer overflows from long input strings unless that specific memory is touched.

TIP

If you suspect a bug in a static (stack) array, temporarily convert it to a dynamic (heap) array. Valgrind will then be able to perform bounds checking on it.

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Valgrind Cheat Sheet

Summary

Valgrind is an essential tool for C/C++ developers on Linux. By running your executable in Valgrind’s environment, you can proactively catch unpaired memory calls, invalid pointer math, and uninitialized variables before they become difficult-to-trace production crashes.