Segmentation faults and other memory bugs (reading uninitialized
memory, reading/writing beyond the bounds of an array, memory leaks,
etc.) can be hard to track down with a traditional debugger. Memory
errors can be elusive, and may not cause the program to crash
immediately. A program with memory errors may even appear to work
correctly on some datasets or on some machines.
We recommend using a special debugger to find memory errors, for
example Dr. Memory or
versions of these tools include Purify and
We'll discuss the Dr. Memory and Valgrind memory debugging tools
and the memory error reports these tools produce at the end of Lecture
7 and during Lab 5. You'll be expected to use one of these tools for
debugging starting with Homework 3. The homework submission server
and the TAs will use these tools for grading your homework.
Dr. Memory is available for
GNU/Linux, Microsoft Windows, and MacOSX operating systems. For
questions, bug reports, and discussion, use the Dr. Memory Users
Please report issues with Dr. Memory to the
Dr. Memory Users Group by email:
email@example.com. Be sure to include details
about your operating system and the Dr. Memory version number. Don't
send your full homework submission (it is a public mailing list).
Dr. Memory on GNU/Linux or Mac OSX
Obtain the Dr. Memory tar.gz file for your operating system from
Save and untar the package to a directory of your choice. Type something like:
tar -xvzf DrMemory-YourOperatingSystem-VersionXX.tar.gz
We'll assume DrMemory is in the directory
"~/DrMemory-YourOperatingSystem-VersionXX/" for the rest of these instructions.
Linux only: Ensure your Linux installation is able to
build 32-bit applications. On 64-bit Ubuntu you will want to install
these additional packages:
sudo apt-get install g++-multilib
Build your application as 32-bit by passing -m32 to
your C++ compiler (Dr. Memory does not yet support 64-bit
applications, but you can run 32-bit applications on a 64-bit
operating system). Be sure to include debug information by using
the -g option. For example:
g++ -g -m32 main.cpp foo_main.cpp foo_other.cpp -o foo.out
Run your program under Dr. Memory, replacing foo.out arg1 arg2
with your executable name and any command line arguments:
~/DrMemory-YourOperatingSystem-VersionXX/bin/drmemory -brief -- foo.out arg1 arg2
Dr. Memory will report errors to the screen as it runs. It will print a
summary at the end of what it found.
Installing Dr. Memory on Windows
Obtain Dr. Memory. To easily place it on the system path,
use the installer (the .msi file). Alternatively, you can
instead obtain the .zip file for a local install.
Double click on the .msi file to run the installer.
Check the box to accept the license and click Next.
The default location for Dr. Memory installation is fine
(it's probably C:\Program Files (x86)\Dr. Memory\ for 64-bit Windows).
Then click Install. You'll be asked to confirm that you want to make
administrative changes to the machine.
After a quick installation, press Finish.
Follow the instructions below to compile & run your program
using MinGW g++, the Visual Studio IDE, or the Visual Studio Command
Dr. Memory and MinGW
Dr. Memory does not support programs that use the Cygwin emulation
layer. Thus, we cannot use Dr. Memory with programs built using the
Cygwin version of g++. Instead, we will build our program with the
MinGW g++ compiler (Minimalist GNU for Windows):
If you haven't already done so, make sure to install the MinGW g++ compiler.
See Cygwin Installation Instructions.
Open a Cygwin terminal, navigate to the directory with your
files, and compile your program with the MinGW compiler by typing:
i686-pc-mingw32-g++.exe -static-libgcc -static-libstdc++ -ggdb -o foo.exe foo_main.cpp foo_other.cpp
If you've made the shortcut suggested
under Helpful edits to the Cygwin .bashrc
file, you can equivalently type:
memg++ -o foo.exe foo_main.cpp foo_other.cpp
You can run your program under Dr Memory by typing:
drmemory -brief -batch -- foo.exe arg1 arg2
Replace "foo.exe arg1 arg2" with your program name and any
command line arguments for your program.
Dr. Memory and Visual Studio
Alternatively, you can use Dr. Memory with the Microsoft Visual Studio compiler:
Build your application as 32-bit with Visual Studio (32-bit is
the default). Be sure to include debug information. You can verify
that you are including debug information by looking at the properties
of your build target:
Press Alt-F7 to bring up the configuration properties. Under "Configuration Properties | C/C++ | General", the "Debug Information Format" entry should either say "Program Database (/Zi)" or "Program Database for Edit and Continue (/ZI)". Additionally, under "Configuration Properties | Linker | Debugging", the "Generate Debug Info" entry should say "Yes (/DEBUG)". For Visual Studio 2015, under "Configuration Properties | Linker | Debugging", the "Generate Debug Info" entry should say "Optimize for debugging (/DEBUG)" -- it should not say "Optimize for faster linking (/DEBUG:FASTLINK)".
Disable Runtime Checks: The Visual Studio compiler's
/RTC1 flag can prevent Dr. Memory from reporting
uninitialized reads of local variables, and the /RTC1 checks
for uninitialized reads themselves may not catch everything that
Dr. Memory finds. However, /RTC1 does perform additional
stack checks that Dr. Memory does not, so for best results, your
application should be run under Dr. Memory without /RTC1, and
run natively (for development & testing without Dr. Memory) with
In the Visual Studio IDE, press Alt-F7 and then under
"Configuration Properties | C/C++ | Code Generation" ensure "Basic
Runtime Checks" says "Default".
The most recent Dr. Memory installer (for version 1.8 and later)
configures Dr. Memory as a Visual Studio "External Tool", which adds a
new menu item allowing you to run Dr. Memory within the IDE.
Now you can select the "Tools | Dr. Memory" menu item and Visual
Studio will run your application under Dr. Memory. You can add
arguments to your application in the box that pops up immediately
after selecting the men item by adding them at the end, after "$(TargetPath)".
The output of Dr. Memory (along with your program) will be
printed to the Visual Studio Output Window. Dr. Memory will report
errors to the screen as it runs. It will print a summary at the end
of what it found. You can double-click on a source file on any
error's callstack frame in order to automatically open up that file to the
line number indicated.
Using the Visual Studio compiler without the Visual Studio Integrated
Development Environment (IDE)
Launch the Visual Studio Command Prompt. From the Start menu,
under All Programs, find your Visual Studio version (e.g., 2010) and
expand it. Then expand Visual Studio Tools. Select the "Visual
Studio 2010 Command Prompt". (You don't want the x64 or Cross Tools
Note: this is not the Cygwin shell.
This Command Prompt is a cmd shell in which a batch file that
comes with Visual Studio has been executed. This batch file is called
vcvars.bat and it sets up the path and environment variables
needed to run the compiler from the command line.
Note: You can extract the environment variables from the batch file and set them
up in your .bashrc so you can build from a Cygwin shell.
At the command line, change to the directory containing your
Run the compiler, which is called "cl". This will build
hw.exe from all .cpp files in the current directory:
cl /Zi /MT /EHsc /Oy- /Ob0 /Fehw.exe *.cpp
If you installed Dr. Memory before you opened the Command Prompt, you
can run drmemory from the same prompt.
Run this command, replacing foo.exe arg1 arg2
with your executable name and any command line arguments:
drmemory -brief -batch -- foo.exe arg1 arg2
If you don't see any extra output from Dr. Memory as your program
runs, remove the -batch flag and the Dr. Memory output will be sent to
a file and notepad will launch automatically to display this file.
drmemory -brief -- foo.exe arg1 arg2
Dr. Memory will print a summary at the end of what errors it found.
only works on Unix-based systems (e.g., GNU/Linux, FreeBSD, and
MacOSX). Valgrind does not work on Cygwin because Cygwin emulates
UNIX at the library layer, but Valgrind operates at the system call
layer and the Windows system calls are significantly different than
UNIX system calls.
Note: Valgrind on the more recent Mac OSX versions
10.8 & 10.9 is still a work in progress -- you will likely see
false positive memory errors, but it may still be a helpful tool
To use Valgrind...
Valgrind is installed by default on most Linux distributions.
For MacOSX you'll need to install it yourself -- you may want to try
Homebrew, a package manager for Mac OSX.
Once Homebrew is setup, you can just type:
brew install valgrind
Your program should be compiled with debug information enabled
by specifying the -g flag. For example:
g++ -g main.cpp foo_main.cpp foo_other.cpp -o foo.out
Then run the program by adding Valgrind to the beginning of
your command line (replace foo.out arg1 arg2 with your
program name and any command line arguments for your program):
valgrind --leak-check=full --show-reachable=yes foo.out arg1 arg2
If that example run of your program contains any memory errors
Valgrind will output information to help you track down the error.
Note that using Valgrind can significantly slow down execution time as
it inspects every memory action. You may need to craft a smaller test
case that exhibits the same bug you would like to solve.
Note: Because some STL classes (including string) use their
own allocators (and do other optimization tricks), there may be a
warning about memory that is ``still reachable'' even though you've
deleted all your dynamically allocated memory. The newer versions of
Valgrind automatically suppresses some of these common false positive
errors, so you may see this listed as a ``suppressed leak''.
Suppression of False Positives in Valgrind
If you see false positive error messages in Valgrind (this is likely to
happen with Valgrind on the newest versions of Mac OSX), you will
probably want to create an error suppression file to allow you to focus on your actual errors.
Add the --gen-suppressions=all option to the valgrind command line:
valgrind --leak-check=full --gen-suppressions=all foo.out arg1 arg2
For each false positive (an error not obviously pointing at your
code), copy-paste the suppression text (a block of text in curly
braces) into a new file containing your custom suppressions, let's
call it my_suppressions.txt.
- Use that suppression file every time you run Valgrind:
valgrind --leak-check=full --suppressions=my_suppressions.txt foo.out arg1 arg2
You may need to add to that file in the future, when you use
additional library functions that cause different false positive errors.
Read more about Valgrind suppressions here: