The Valgrind Quick Start Guide
The Valgrind Quick Start Guide
1. Introduction
The Valgrind tool suite provides a number of debugging and profiling tools that help you make your programs
faster and more correct. The most popular of these tools is called Memcheck. It can detect many memory-related
errors that are common in C and C++ programs and that can lead to crashes and unpredictable behaviour.
The rest of this guide gives the minimum information you need to start detecting memory errors in your program
with Memcheck. For full documentation of Memcheck and the other tools, please read the User Manual.
2. Preparing your program
Compile your program with -g to include debugging information so that Memcheck's error messages include
exact line numbers. Using -O0 is also a good idea, if you can tolerate the slowdown. With -O1 line numbers
in error messages can be inaccurate, although generally speaking running Memcheck on code compiled at -O1
works fairly well, and the speed improvement compared to running -O0 is quite significant. Use of -O2 and above
is not recommended as Memcheck occasionally reports uninitialised-value errors which don't really exist.
3. Running your program under Memcheck
If you normally run your program like this:
myprog arg1 arg2
Use this command line:
valgrind --leak-check=yes myprog arg1 arg2
Memcheck is the default tool. The --leak-check option turns on the detailed memory leak detector.
Your program will run much slower (eg. 20 to 30 times) than normal, and use a lot more memory. Memcheck will
issue messages about memory errors and leaks that it detects.
4. Interpreting Memcheck's output
Here's an example C program, in a file called a.c, with a memory error and a memory leak.
#include <stdlib.h>
void f(void)
{
int* x = malloc(10 * sizeof(int));
x[10] = 0; // problem 1: heap block overrun
} // problem 2: memory leak -- x not freed
int main(void)
{
f();
return 0;
}
Most error messages look like the following, which describes problem 1, the heap block overrun:
1
