## 1. Introduction

## 1.1. An Overview of Valgrind

Valgrind is a suite of simulation-based debugging and profiling tools for programs running on Linux (x86, amd64 and ppc32). The system consists of a core, which provides a synthetic CPU in software, and a series of tools, each of which performs some kind of debugging, profiling, or similar task. The architecture is modular, so that new tools can be created easily and without disturbing the existing structure.

A number of useful tools are supplied as standard. In summary, these are:

1. Memcheck detects memory-management problems in your programs. All reads and writes of memory are checked, and calls to malloc/new/free/delete are intercepted. As a result, Memcheck can detect the following problems:

   - Use of uninitialised memory

   - Reading/writing memory after it has been free'd

   - Reading/writing off the end of malloc'd blocks

   - Reading/writing inappropriate areas on the stack

   - Memory leaks -- where pointers to malloc'd blocks are lost forever

   - Mismatched use of malloc/new/new [] vs free/delete/delete []

   - Overlapping `src` and `dst` pointers in `memcpy()` and related functions

   Problems like these can be difficult to find by other means, often lying undetected for long periods, then causing occasional, difficult-to-diagnose crashes.

2. Cachegrind is a cache profiler. It performs detailed simulation of the I1, D1 and L2 caches in your CPU and so can accurately pinpoint the sources of cache misses in your code. If you desire, it will show the number of cache misses, memory references and instructions accruing to each line of source code, with per-function, per-module and whole-program summaries. If you ask really nicely it will even show counts for each individual machine instruction.

   On x86 and AMD64, Cachegrind auto-detects your machine's cache configuration using the `CPUID` instruction, and so needs no further configuration info, in most cases.

   Cachegrind is nicely complemented by Josef Weidendorfer's amazing KCacheGrind visualisation tool (http://kcachegrind.sourceforge.net), a KDE application which presents these profiling results in a graphical and easier-to-understand form.

3. Helgrind finds data races in multithreaded programs. Helgrind looks for memory locations which are accessed by more than one (POSIX p-)thread, but for which no consistently used (pthread_mutex_)lock can be found. Such locations are indicative of missing synchronisation between threads, and could cause hard-to-find timing-dependent problems.

   Helgrind ("Hell's Gate", in Norse mythology) implements the so-called "Eraser" data-race-detection algorithm, along with various refinements (thread-segment lifetimes) which reduce the number of false errors it reports. It is as yet somewhat of an experimental tool, so your feedback is especially welcomed here.

   Helgrind has been hacked on extensively by Jeremy Fitzhardinge, and we have him to thank for getting it to a releasable state.

   NOTE: Helgrind is, unfortunately, not available in Valgrind 3.1.X, as a result of threading changes that happened in the 2.4.0 release. We hope to reinstate its functionality in a future 3.2.0 release.

A couple of minor tools (Lackey and Nulgrind) are also supplied. These aren't particularly useful -- they exist to illustrate how to create simple tools and to help the valgrind developers in various ways. Nulgrind is the null tool -- it adds no instrumentation. Lackey is a simple example tool which counts instructions, memory accesses, and the number of integer and floating point operations your program does.

Valgrind is closely tied to details of the CPU and operating system, and to a lesser extent, the compiler and basic C libraries. Nonetheless, as of version 3.1.0 it supports several platforms: x86/Linux (mature), AMD64/Linux (maturing), and PPC32/Linux (immature but works well). Valgrind uses the standard Unix `./configure`, `make`, `make install` mechanism, and we have attempted to ensure that it works on machines with kernel 2.4 or 2.6 and glibc 2.2.X--2.3.X.

Valgrind is licensed under the The GNU General Public License, version 2. The `valgrind/*.h` headers that you may wish to include in your code (eg. `valgrind.h`, `memcheck.h`) are distributed under a BSD-style license, so you may include them in your code without worrying about license conflicts. Some of the PThreads test cases, `pth_*.c`, are taken from "Pthreads Programming" by Bradford Nichols, Dick Buttlar & Jacqueline Proulx Farrell, ISBN 1-56592-115-1, published by O'Reilly & Associates, Inc.

## 1.2. How to navigate this manual

The Valgrind distribution consists of the Valgrind core, upon which are built Valgrind tools, which do different kinds of debugging and profiling. This manual is structured similarly.

First, we describe the Valgrind core, how to use it, and the flags it supports. Then, each tool has its own chapter in this manual. You only need to read the documentation for the core and for the tool(s) you actually use, although you may find it helpful to be at least a little bit familar with what all tools do. If you're new to all this, you probably want to run the Memcheck tool. The final chapter explains how to write a new tool.

Be aware that the core understands some command line flags, and the tools have their own flags which they know about. This means there is no central place describing all the flags that are accepted -- you have to read the flags documentation both for Valgrind's core and for the tool you want to use.