Thursday, April 26, 2007

Program Tools Serias: Part 1.Valgrind

Short Introduction And usage of Valgrind(From Valgrind official site)

Valgrind is an award-winning suite of tools for debugging and profiling Linux programs. With the tools that come with Valgrind, you can automatically detect many memory management and threading bugs, avoiding hours of frustrating bug-hunting, making your programs more stable. You can also perform detailed profiling, to speed up and reduce memory use of your programs.

The Valgrind distribution currently includes four tools: a memory error detector, a cache (time) profiler, a call-graph profiler, and a heap (space) profiler. It runs on the following platforms: X86/Linux, AMD64/Linux, PPC32/Linux, PPC64/Linux.

Valgrind is Open Source / Free Software, and is freely available under the GNU General Public License.

1. Introduction

The Valgrind distribution has multiple tools. The most popular is the memory checking tool (called Memcheck) which can detect many common memory errors such as:

  • touching memory you shouldn't (eg. overrunning heap block boundaries);

  • using values before they have been initialized;

  • incorrect freeing of memory, such as double-freeing heap blocks;

  • memory leaks.

What follows is the minimum information you need to start detecting memory errors in your program with Memcheck. Note that this guide applies to Valgrind version 2.4.0 and later; some of the information is not quite right for earlier versions.

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 Memchecking code compiled at -O1 works fairly well. 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 with a memory error and a memory leak.


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)
return 0;

Most error messages look like the following, which describes problem 1, the heap block overrun:

==19182== Invalid write of size 4
==19182== at 0x804838F: f (example.c:6)
==19182== by 0x80483AB: main (example.c:11)
==19182== Address 0x1BA45050 is 0 bytes after a block of size 40 alloc'd
==19182== at 0x1B8FF5CD: malloc (vg_replace_malloc.c:130)
==19182== by 0x8048385: f (example.c:5)
==19182== by 0x80483AB: main (example.c:11)

Things to notice:

  • There is a lot of information in each error message; read it carefully.

  • The 19182 is the process ID; it's usually unimportant.

  • The first line ("Invalid write...") tells you what kind of error it is. Here, the program wrote to some memory it should not have due to a heap block overrun.

  • Below the first line is a stack trace telling you where the problem occurred. Stack traces can get quite large, and be confusing, especially if you are using the C++ STL. Reading them from the bottom up can help. If the stack trace is not big enough, use the --num-callers option to make it bigger.

  • The code addresses (eg. 0x804838F) are usually unimportant, but occasionally crucial for tracking down weirder bugs.

  • Some error messages have a second component which describes the memory address involved. This one shows that the written memory is just past the end of a block allocated with malloc() on line 5 of example.c.

It's worth fixing errors in the order they are reported, as later errors can be caused by earlier errors.

Memory leak messages look like this:

==19182== 40 bytes in 1 blocks are definitely lost in loss record 1 of 1
==19182== at 0x1B8FF5CD: malloc (vg_replace_malloc.c:130)
==19182== by 0x8048385: f (a.c:5)
==19182== by 0x80483AB: main (a.c:11)

The stack trace tells you where the leaked memory was allocated. Memcheck cannot tell you why the memory leaked, unfortunately. (Ignore the "vg_replace_malloc.c", that's an implementation detail.)

There are several kinds of leaks; the two most important categories are:

  • "definitely lost": your program is leaking memory -- fix it!

  • "probably lost": your program is leaking memory, unless you're doing funny things with pointers (such as moving them to point to the middle of a heap block).

PS: Valgrind is my favourite memory leak check tool. I got a lot from it.
By the way, my most use option of valgrind is :
--leak-check=full --show-reachable=yes

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