What is Valgrind? edit

Valgrind (downloadable here) is a utility for debugging programs for the x86 and x86-64 Linux platforms. It has recently become highly popular as it can be used to easily track down memory management and threading bugs that are hard to track down otherwise.

How to install it edit

There is a "generic" option, available on most systems, which involves compiling:

user> wget http://valgrind.org/downloads/valgrind-3.8.1.tar.bz2
user> bunzip2 valgrind-3.8.1.tar.bz2 
user> tar -xvf valgrind-3.8.1.tar
user> cd /valgrind-3.8.1
user> ./configure
user> make
user> su root
root> make install

On most Linux distributions, however, you can use the package management system. For example, in Debian GNU/Linux (and derivatives) simply run:

 apt-get install valgrind

How to use it edit

Valgrind can be run simply by prefixing the command line that you run with valgrind

 ./myprogram  -o option
 valgrind ./myprogram -o option

This simple test will check that the memory accesses with in the program are correct. Do not be surprised when you get messages about code that you know is not a problem. This program tests every access and some programs are forgiving about existing errors. It is always worth fixing these extra errors because they will be impossible to track down when they do cause a breakage in code.

You may prefer to run valgrind and log to a text file, with the following options:

valgrind --leak-check=full --freelist-vol=100000000 --log-file-exactly=log.txt -v ./myprogram

How it works edit

Valgrind is essentially an x86 machine-code interpreter. In fact it runs as a just-in-time compiler converting the machine code to an internal language, instrumenting that language and then code-generating from that language. Valgrind instruments the code to monitor memory allocation, deallocation, writes and reads, which lets it hold a bit-map of memory state. hence it can report attempts to read data from memory that has never yet been written, or using recently-freed memory. Under Valgrind a program will take something like 2-10 times longer than when run uninstrumented.

Callgrind is a related program that uses the same x86 interpreter technology to instrument the code to log routine calls and generate a file that can be analyzed to show time spent in various routines and the call paths involved.