## 6 Helgrind: a data-race detector

To use this tool, you must specify `--tool=helgrind` on the Valgrind command line.

Helgrind is a Valgrind tool for detecting data races in C and C++ programs that use the Pthreads library.

It uses the Eraser algorithm described in

> Eraser: A Dynamic Data Race Detector for Multithreaded Programs
> Stefan Savage, Michael Burrows, Greg Nelson, Patrick Sobalvarro and Thomas Anderson
> ACM Transactions on Computer Systems, 15(4):391-411
> November 1997.

We also incorporate significant improvements from this paper:

> Runtime Checking of Multithreaded Applications with Visual Threads Jerry J. Harrow, Jr.
> Proceedings of the 7th International SPIN Workshop on Model Checking of Software
> Stanford, California, USA
> August 2000
> LNCS 1885, pp331--342
> K. Havelund, J. Penix, and W. Visser, editors.

Basically what Helgrind does is to look for memory locations which are accessed by more than one thread. For each such location, Helgrind records which of the program's (pthread_mutex_)locks were held by the accessing thread at the time of the access. The hope is to discover that there is indeed at least one lock which is used by all threads to protect that location. If no such lock can be found, then there is (apparently) no consistent locking strategy being applied for that location, and so a possible data race might result.

Helgrind also allows for "thread segment lifetimes". If the execution of two threads cannot overlap -- for example, if your main thread waits on another thread with a `pthread_join()` operation -- they can both access the same variable without holding a lock.

There's a lot of other sophistication in Helgrind, aimed at reducing the number of false reports, and at producing useful error reports. We hope to have more documentation one day...