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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">
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<HTML>
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<HEAD>
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<link rel="stylesheet" href="designstyle.css">
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<title>Google Heap Leak Checker</title>
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</HEAD>
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2007-03-22 03:28:56 +00:00
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2007-04-16 20:49:32 +00:00
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<BODY>
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2007-03-22 03:28:56 +00:00
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2007-04-16 20:49:32 +00:00
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<p align=right>
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<i>Last modified
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<script type=text/javascript>
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var lm = new Date(document.lastModified);
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document.write(lm.toDateString());
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</script></i>
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2007-03-22 03:28:56 +00:00
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</p>
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2007-04-16 20:49:32 +00:00
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<p>This is the heap checker we use at Google to detect memory leaks in
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C++ programs. There are three parts to using it: linking the library
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into an application, running the code, and analyzing the output.</p>
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2007-04-16 20:49:32 +00:00
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<H1>Linking in the Library</H1>
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<p>The heap-checker is part of tcmalloc, so to install the heap
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checker into your executable, add <code>-ltcmalloc</code> to the
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link-time step for your executable. Also, while we don't necessarily
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recommend this form of usage, it's possible to add in the profiler at
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run-time using <code>LD_PRELOAD</code>:
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<pre>% env LD_PRELOAD="/usr/lib/libtcmalloc.so" <binary></pre>
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2007-04-16 20:49:32 +00:00
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<p>This does <i>not</i> turn on heap checking; it just inserts the
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code. For that reason, it's practical to just always link
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<code>-ltcmalloc</code> into a binary while developing; that's what we
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do at Google. (However, since any user can turn on the profiler by
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setting an environment variable, it's not necessarily recommended to
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install heapchecker-linked binaries into a production, running
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system.) Note that if you wish to use the heap checker, you must
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also use the tcmalloc memory-allocation library. There is no way
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currently to use the heap checker separate from tcmalloc.</p>
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2007-03-22 03:28:56 +00:00
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2007-04-16 20:49:32 +00:00
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<h1>Running the Code</h1>
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<p>Note: For security reasons, heap profiling will not write to a file
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-- and is thus not usable -- for setuid programs.</p>
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<h2><a name="whole_program">Whole-program Heap Leak Checking</a></h2>
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<p>The recommended way to use the heap checker is in "whole program"
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mode. In this case, the heap-checker starts tracking memory
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allocations before the start of <code>main()</code>, and checks again
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at program-exit. If it finds any memory leaks -- that is, any memory
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not pointed to by objects that are still "live" at program-exit -- it
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aborts the program (via <code>exit(1)</code>) and prints a message
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describing how to track down the memory leak (using <A
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HREF="heapprofile.html#pprof">pprof</A>).
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<p>Here's how to run a program with whole-program heap checking:</p>
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<ol>
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<li> <p>Define the environment variable HEAPCHECK to the <A
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HREF="#types">type of heap-checking</A> to do. For instance,
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to heap-check
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<code>/usr/local/bin/my_binary_compiled_with_tcmalloc</code>:</p>
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<pre>% env HEAPCHECK=normal /usr/local/bin/my_binary_compiled_with_tcmalloc</pre>
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</ol>
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<p>No other action is required.</p>
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<p>Note that since the heap-checker uses the heap-profiling framework
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internally, it is not possible to run both the heap-checker and <A
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HREF="heapprofile.html">heap profiler</A> at the same time.</p>
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<h3><a name="types">Flavors of Heap Checking</a></h3>
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<p>These are the legal values when running a whole-program heap
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check:</p>
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<ol>
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<li> <code>minimal</code>
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<li> <code>normal</code>
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<li> <code>strict</code>
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<li> <code>draconian</code>
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</ol>
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<p>"Minimal" heap-checking starts as late as possible ina
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initialization, meaning you can leak some memory in your
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initialization routines (that run before <code>main()</code>, say),
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and not trigger a leak message. If you frequently (and purposefully)
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leak data in one-time global initializers, "minimal" mode is useful
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for you. Otherwise, you should avoid it for stricter modes.</p>
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<p>"Normal" heap-checking tracks <A HREF="#live">live objects</A> and
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reports a leak for any data that is not reachable via a live object
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when the program exits.</p>
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<p>"Strict" heap-checking is much like "normal" but has a few extra
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checks that memory isn't lost in global destructors. In particular,
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if you have a global variable that allocates memory during program
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execution, and then "forgets" about the memory in the global
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destructor (say, by setting the pointer to it to NULL) without freeing
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it, that will prompt a leak message in "strict" mode, though not in
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"normal" mode.</p>
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<p>"Draconian" heap-checking is appropriate for those who like to be
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very precise about their memory management, and want the heap-checker
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to help them enforce it. In "draconian" mode, the heap-checker does
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not do "live object" checking at all, so it reports a leak unless
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<i>all</i> allocated memory is freed before program exit. (However,
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you can use <A HREF="#disable">IgnoreObject()</A> to re-enable
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liveness-checking on an object-by-object basis.)</p>
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<p>"Normal" mode, as the name implies, is the one used most often at
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Google. It's appropriate for everyday heap-checking use.</p>
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<p>In addition, there are two other possible modes:</p>
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<ul>
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<li> <code>as-is</code>
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<li> <code>local</code>
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</ul>
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<p><code>as-is</code> is the most flexible mode; it allows you to
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specify the various <A HREF="#options">knobs</A> of the heap checker
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explicitly. <code>local</code> activates the <A
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HREF="#explicit">explicit heap-check instrumentation</A>, but does not
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turn on any whole-program leak checking.</p>
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<h3><A NAME="tweaking">Tweaking whole-program checking</A></h3>
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<p>In some cases you want to check the whole program for memory leaks,
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but waiting for after <code>main()</code> exits to do the first
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whole-program leak check is waiting too long: e.g. in a long-running
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server one might wish to simply periodically check for leaks while the
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server is running. In this case, you can call the static method
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<code>NoGlobalLeaks()</code>, to verify no global leaks have happened
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as of that point in the program.</p>
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<p>Alternately, doing the check after <code>main()</code> exits might
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be too late. Perhaps you have some objects that are known not to
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clean up properly at exit. You'd like to do the "at exit" check
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before those objects are destroyed (since while they're live, any
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memory they point to will not be considered a leak). In that case,
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you can call <code>NoGlobalLeaks()</code> manually, near the end of
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<code>main()</code>, and then call <code>CancelGlobalCheck()</code> to
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turn off the automatic post-<code>main()</code> check.</p>
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<p>Finally, there's a helper macro for "strict" and "draconian" modes,
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which require all global memory to be freed before program exit. This
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freeing can be time-consuming and is often unnecessary, since libc
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cleans up all memory at program-exit for you. If you want the
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benefits of "strict"/"draconian" modes without the cost of all that
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freeing, look at <code>REGISTER_HEAPCHECK_CLEANUP</code> (in
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<code>heap-checker.h</code>). This macro allows you to mark specific
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cleanup code as active only when the heap-checker is turned on.</p>
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<h2><a name="explicit">Explicit (Partial-program) Heap Leak Checking</h2>
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<p>Instead of whole-program checking, you can check certain parts of
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your code to verify they do not have memory leaks. There are two
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types of checks you can do. The "no leak" check verifies that between
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two parts of a program, no memory is allocated without being freed; it
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checks that memory does not grow. The stricter "same heap" check
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verifies that two parts of a program share the same heap profile; that
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is, that the memory does not grow <i>or shrink</i>, or change in any
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way.</p>
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<p>To use this kind of checking code, bracket the code you want
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checked by creating a <code>HeapLeakChecker</code> object at the
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beginning of the code segment, and calling <code>*SameHeap()</code> or
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<code>*NoLeaks()</code> at the end. These functions, and all others
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referred to in this file, are declared in
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<code><google/heap-checker.h></code>.
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</p>
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<p>Here's an example:</p>
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<pre>
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HeapLeakChecker heap_checker("test_foo");
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{
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code that exercises some foo functionality;
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this code should preserve memory allocation state;
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}
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2008-02-13 00:55:09 +00:00
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if (!heap_checker.SameHeap()) assert(NULL == "heap memory leak");
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</pre>
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2007-04-16 20:49:32 +00:00
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<p>The various flavors of these functions -- <code>SameHeap()</code>,
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<code>QuickSameHeap()</code>, <code>BriefSameHeap()</code> -- trade
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off running time for accuracy: the faster routines might miss some
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legitimate leaks. For instance, the briefest tests might be confused
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by code like this:</p>
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<pre>
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void LeakTwentyBytes() {
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char* a = malloc(20);
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HeapLeakChecker heap_checker("test_malloc");
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char* b = malloc(20);
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free(a);
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// This will pass: it totes up 20 bytes allocated and 20 bytes freed
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assert(heap_checker.BriefNoLeaks()); // doesn't detect that b is leaked
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}
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</pre>
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2007-04-16 20:49:32 +00:00
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<p>(This is because <code>BriefSameHeap()</code> does not use <A
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HREF="#pprof">pprof</A>, which is slower but is better able to track
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allocations in tricky situations like the above.)</p>
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2007-04-16 20:49:32 +00:00
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<p>Note that adding in the <code>HeapLeakChecker</code> object merely
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instruments the code for leak-checking. To actually turn on this
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leak-checking on a particular run of the executable, you must still
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run with the heap-checker turned on:</p>
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<pre>% env HEAPCHECK=local /usr/local/bin/my_binary_compiled_with_tcmalloc</pre>
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<p>If you want to do whole-program leak checking in addition to this
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manual leak checking, you can run in <code>normal</code> or some other
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mode instead: they'll run the "local" checks in addition to the
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whole-program check.</p>
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2007-04-16 20:49:32 +00:00
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<h2><a name="disable">Disabling Heap-checking of Known Leaks</a></h2>
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<p>Sometimes your code has leaks that you know about and are willing
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to accept. You would like the heap checker to ignore them when
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checking your program. You can do this by bracketing the code in
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question with an appropriate heap-checking construct:</p>
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<pre>
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...
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void *mark = HeapLeakChecker::GetDisableChecksStart();
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<leaky code>
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HeapLeakChecker::DisableChecksToHereFrom(mark);
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...
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</pre>
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2007-04-16 20:49:32 +00:00
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<p>Alternately, you can use <code>IgnoreObject()</code>, which takes a
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pointer to an object to ignore. That memory, and everything reachable
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from it (by following pointers), is ignored for the purposes of leak
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checking. You can call <code>UnignoreObject()</code> to undo the
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effects of <code>IgnoreObject()</code>.</p>
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<h2><a name="options">Tuning the Heap Checker</h2>
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<p>The heap leak checker has many options, some that trade off running
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time and accuracy, and others that increase the sensitivity at the
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risk of returning false positives. For most uses, the range covered
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by the <A HREF="#types">heap-check flavors</A> is enough, but in
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specialized cases more control can be helpful.</p>
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<p>
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These options are specified via environment varaiables.
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</p>
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2007-04-16 20:49:32 +00:00
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<p>This first set of options controls sensitivity and accuracy. These
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options are ignored unless you run the heap checker in <A
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HREF="#types">as-is</A> mode.
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<table frame=box rules=sides cellpadding=5 width=100%>
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<tr valign=top>
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<td><code>HEAP_CHECK_AFTER_DESTRUCTORS</code></td>
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<td>Default: false</td>
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<td>
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When true, do the final leak check after all other global
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destructors have run. When false, do it after all
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<code>REGISTER_HEAPCHECK_CLEANUP</code>, typically much earlier in
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the global-destructor process.
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</td>
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</tr>
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<tr valign=top>
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<td><code>HEAP_CHECK_IGNORE_THREAD_LIVE</code></td>
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<td>Default: true</td>
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<td>
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If true, ignore objects reachable from thread stacks and registers
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(that is, do not report them as leaks).
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</td>
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</tr>
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<tr valign=top>
|
|
|
|
|
<td><code>HEAP_CHECK_IGNORE_GLOBAL_LIVE</code></td>
|
|
|
|
|
<td>Default: true</td>
|
|
|
|
|
<td>
|
|
|
|
|
If true, ignore objects reachable from global variables and data
|
|
|
|
|
(that is, do not report them as leaks).
|
|
|
|
|
</td>
|
|
|
|
|
</tr>
|
|
|
|
|
|
|
|
|
|
</table>
|
|
|
|
|
|
|
|
|
|
<p>These options modify the behavior of whole-program leak
|
|
|
|
|
checking.</p>
|
|
|
|
|
|
|
|
|
|
<table frame=box rules=sides cellpadding=5 width=100%>
|
|
|
|
|
|
|
|
|
|
<tr valign=top>
|
|
|
|
|
<td><code>HEAP_CHECK_REPORT</code></td>
|
|
|
|
|
<td>Default: true</td>
|
|
|
|
|
<td>
|
|
|
|
|
If true, use <code>pprof</code> to report more info about found leaks.
|
|
|
|
|
</td>
|
|
|
|
|
</tr>
|
|
|
|
|
|
|
|
|
|
<tr valign=top>
|
|
|
|
|
<td><code>HEAP_CHECK_STRICT_CHECK</code></td>
|
|
|
|
|
<td>Default: true</td>
|
|
|
|
|
<td>
|
|
|
|
|
If true, do the program-end check via <code>SameHeap()</code>;
|
|
|
|
|
if false, use <code>NoLeaks()</code>.
|
|
|
|
|
</td>
|
|
|
|
|
</tr>
|
|
|
|
|
|
|
|
|
|
</table>
|
|
|
|
|
|
|
|
|
|
<p>These options apply to all types of leak checking.</p>
|
|
|
|
|
|
|
|
|
|
<table frame=box rules=sides cellpadding=5 width=100%>
|
|
|
|
|
|
|
|
|
|
<tr valign=top>
|
|
|
|
|
<td><code>HEAP_CHECK_IDENTIFY_LEAKS</code></td>
|
|
|
|
|
<td>Default: false</td>
|
|
|
|
|
<td>
|
|
|
|
|
If true, generate the addresses of the leaked objects in the
|
|
|
|
|
generated memory leak profile files.
|
|
|
|
|
</td>
|
|
|
|
|
</tr>
|
|
|
|
|
|
|
|
|
|
<tr valign=top>
|
|
|
|
|
<td><code>HEAP_CHECK_TEST_POINTER_ALIGNMENT</code></td>
|
|
|
|
|
<td>Default: false</td>
|
|
|
|
|
<td>
|
|
|
|
|
If true, check all leaks to see if they might be due to the use
|
|
|
|
|
of unaligned pointers.
|
|
|
|
|
</td>
|
|
|
|
|
</tr>
|
|
|
|
|
|
|
|
|
|
<tr valign=top>
|
|
|
|
|
<td><code>PPROF_PATH</code></td>
|
|
|
|
|
<td>Default: pprof</td>
|
|
|
|
|
<td>
|
|
|
|
|
The location of the <code>pprof</code> executable.
|
|
|
|
|
</td>
|
|
|
|
|
</tr>
|
|
|
|
|
|
|
|
|
|
<tr valign=top>
|
|
|
|
|
<td><code>HEAP_CHECK_DUMP_DIRECTORY</code></td>
|
|
|
|
|
<td>Default: /tmp</td>
|
|
|
|
|
<td>
|
|
|
|
|
Where the heap-profile files are kept while the program is running.
|
|
|
|
|
</td>
|
|
|
|
|
</tr>
|
|
|
|
|
|
|
|
|
|
</table>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<h2>Tips for Handling Detected Leaks</h2>
|
|
|
|
|
|
|
|
|
|
<p>What do you do when the heap leak checker detects a memory leak?
|
|
|
|
|
First, you should run the reported <code>pprof</code> command;
|
|
|
|
|
hopefully, that is enough to track down the location where the leak
|
|
|
|
|
occurs.</p>
|
|
|
|
|
|
|
|
|
|
<p>If the leak is a real leak, you should fix it!</p>
|
|
|
|
|
|
|
|
|
|
<p>If you are sure that the reported leaks are not dangerous and there
|
|
|
|
|
is no good way to fix them, then you can use
|
|
|
|
|
<code>HeapLeakChecker::GetDisableChecksStart()</code> and/or
|
|
|
|
|
<code>HeapLeakChecker::IgnoreObject()</code> to disable heap-checking
|
|
|
|
|
for certain parts of the codebase.</p>
|
|
|
|
|
|
|
|
|
|
<p>In "strict" or "draconian" mode, leaks may be due to incomplete
|
|
|
|
|
cleanup in the destructors of global variables. If you don't wish to
|
|
|
|
|
augment the cleanup routines, but still want to run in "strict" or
|
|
|
|
|
"draconian" mode, consider using <A
|
|
|
|
|
HREF="#tweaking"><code>REGISTER_HEAPCHECK_CLEANUP</code></A>.</p>
|
|
|
|
|
|
2007-07-18 18:30:50 +00:00
|
|
|
<h2>Hints for Debugging Detected Leaks</h2>
|
|
|
|
|
|
|
|
|
|
<p>Sometimes it can be useful to not only know the exact code that
|
|
|
|
|
allocates the leaked objects, but also the addresses of the leaked objects.
|
|
|
|
|
Combining this e.g. with additional logging in the program
|
|
|
|
|
one can then track which subset of the allocations
|
|
|
|
|
made at a certain spot in the code are leaked.
|
|
|
|
|
<br/>
|
|
|
|
|
To get the addresses of all leaked objects
|
|
|
|
|
define the environment variable <code>HEAP_CHECK_IDENTIFY_LEAKS</code>
|
|
|
|
|
to be <code>1</code>.
|
|
|
|
|
The object addresses will be reported in the form of addresses
|
|
|
|
|
of fake immediate callers of the memory allocation routines.
|
|
|
|
|
Note that the performance of doing leak-checking in this mode
|
|
|
|
|
can be noticeably worse than the default mode.
|
|
|
|
|
</p>
|
|
|
|
|
|
|
|
|
|
<p>One relatively common class of leaks that don't look real
|
|
|
|
|
is the case of multiple initialization.
|
|
|
|
|
In such cases the reported leaks are typically things that are
|
|
|
|
|
linked from some global objects,
|
|
|
|
|
which are initialized and say never modified again.
|
|
|
|
|
The non-obvious cause of the leak is frequently the fact that
|
|
|
|
|
the initialization code for these objects executes more than once.
|
|
|
|
|
<br/>
|
|
|
|
|
E.g. if the code of some <code>.cc</code> file is made to be included twice
|
|
|
|
|
into the binary, then the constructors for global objects defined in that file
|
|
|
|
|
will execute twice thus leaking the things allocated on the first run.
|
|
|
|
|
<br/>
|
|
|
|
|
Similar problems can occur if object initialization is done more explicitly
|
|
|
|
|
e.g. on demand by a slightly buggy code
|
|
|
|
|
that does not always ensure only-once initialization.
|
|
|
|
|
</p>
|
|
|
|
|
|
|
|
|
|
<p>
|
|
|
|
|
A more rare but even more puzzling problem can be use of not properly
|
|
|
|
|
aligned pointers (maybe inside of not properly aligned objects).
|
|
|
|
|
Normally such pointers are not followed by the leak checker,
|
|
|
|
|
hence the objects reachable only via such pointers are reported as leaks.
|
|
|
|
|
If you suspect this case
|
|
|
|
|
define the environment variable <code>HEAP_CHECK_TEST_POINTER_ALIGNMENT</code>
|
|
|
|
|
to be <code>1</code>
|
|
|
|
|
and then look closely at the generated leak report messages.
|
|
|
|
|
</p>
|
2007-04-16 20:49:32 +00:00
|
|
|
|
|
|
|
|
<h1>How It Works</h1>
|
|
|
|
|
|
|
|
|
|
<p>When a <code>HeapLeakChecker</code> object is constructed, it dumps
|
|
|
|
|
a memory-usage profile named
|
|
|
|
|
<code><prefix>.<name>-beg.heap</code> to a temporary
|
|
|
|
|
directory. When <code>*NoLeaks()</code> or <code>*SameHeap()</code>
|
|
|
|
|
is called (for whole-program checking, this happens automatically at
|
|
|
|
|
program-exit), it dumps another profile, named
|
|
|
|
|
<code><prefix>.<name>-end.heap</code>.
|
|
|
|
|
(<code><prefix></code> is typically determined automatically,
|
|
|
|
|
and <code><name></code> is typically <code>argv[0]</code>.) It
|
|
|
|
|
then compares the two profiles. If the second profile shows more
|
|
|
|
|
memory use than the first (or, for <code>*SameHeap()</code> calls,
|
|
|
|
|
any different pattern of memory use than the first), the
|
|
|
|
|
<code>*NoLeaks()</code> or <code>*SameHeap()</code> function will
|
|
|
|
|
return false. For "whole program" profiling, this will cause the
|
|
|
|
|
executable to abort (via <code>exit(1)</code>). In all cases, it will
|
|
|
|
|
print a message on how to process the dumped profiles to locate
|
|
|
|
|
leaks.</p>
|
|
|
|
|
|
|
|
|
|
<h3><A name=live>Detecting Live Objects</A></h3>
|
|
|
|
|
|
|
|
|
|
<p>At any point during a program's execution, all memory that is
|
|
|
|
|
accessible at that time is considered "live." This includes global
|
|
|
|
|
variables, and also any memory that is reachable by following pointers
|
|
|
|
|
from a global variable. It also includes all memory reachable from
|
|
|
|
|
the current stack frame and from current CPU registers (this captures
|
|
|
|
|
local variables). Finally, it includes the thread equivalents of
|
|
|
|
|
these: thread-local storage and thread heaps, memory reachable from
|
|
|
|
|
thread-local storage and thread heaps, and memory reachable from
|
|
|
|
|
thread CPU registers.</p>
|
|
|
|
|
|
|
|
|
|
<p>In all modes except "draconian," live memory is not
|
|
|
|
|
considered to be a leak. We detect this by doing a liveness flood,
|
|
|
|
|
traversing pointers to heap objects starting from some initial memory
|
|
|
|
|
regions we know to potentially contain live pointer data. Note that
|
|
|
|
|
this flood might potentially not find some (global) live data region
|
|
|
|
|
to start the flood from. If you find such, please file a bug.</p>
|
|
|
|
|
|
|
|
|
|
<p>The liveness flood attempts to treat any properly aligned byte
|
|
|
|
|
sequences as pointers to heap objects and thinks that it found a good
|
|
|
|
|
pointer whenever the current heap memory map contains an object with
|
|
|
|
|
the address whose byte representation we found. Some pointers into
|
|
|
|
|
not-at-start of object will also work here.</p>
|
|
|
|
|
|
|
|
|
|
<p>As a result of this simple approach, it's possible (though
|
|
|
|
|
unlikely) for the flood to be inexact and occasionally result in
|
|
|
|
|
leaked objects being erroneously determined to be live. For instance,
|
|
|
|
|
random bit patterns can happen to look like pointers to leaked heap
|
|
|
|
|
objects. More likely, stale pointer data not corresponding to any
|
|
|
|
|
live program variables can be still present in memory regions,
|
|
|
|
|
especially in thread stacks. For instance, depending on how the local
|
|
|
|
|
<code>malloc</code> is implemented, it may reuse a heap object
|
|
|
|
|
address:</p>
|
|
|
|
|
<pre>
|
|
|
|
|
char* p = new char[1]; // new might return 0x80000000, say.
|
|
|
|
|
delete p;
|
|
|
|
|
new char[1]; // new might return 0x80000000 again
|
|
|
|
|
// This last new is a leak, but doesn't seem it: p looks like it points to it
|
|
|
|
|
</pre>
|
|
|
|
|
|
|
|
|
|
<p>In other words, imprecisions in the liveness flood mean that for
|
|
|
|
|
any heap leak check we might miss some memory leaks. This means that
|
|
|
|
|
for local leak checks, we might report a memory leak in the local
|
|
|
|
|
area, even though the leak actually happened before the
|
|
|
|
|
<code>HeapLeakChecker</code> object was constructed. Note that for
|
|
|
|
|
whole-program checks, a leak report <i>does</i> always correspond to a
|
|
|
|
|
real leak (since there's no "before" to have created a false-live
|
|
|
|
|
object).</p>
|
|
|
|
|
|
|
|
|
|
<p>While this liveness flood approach is not very portable and not
|
|
|
|
|
100% accurate, it works in most cases and saves us from writing a lot
|
|
|
|
|
of explicit clean up code and other hassles when dealing with thread
|
|
|
|
|
data.</p>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<h3><A NAME="pprof">More Exact Checking via pprof</A></h3>
|
|
|
|
|
|
|
|
|
|
<p>The perftools profiling tool, <code>pprof</code>, is primarily
|
|
|
|
|
intended for users to use interactively in order to explore heap and
|
|
|
|
|
CPU usage. However, the heap-checker can -- and, by default, does -
|
|
|
|
|
call <code>pprof</code> internally, in order to improve its leak
|
|
|
|
|
checking.</p>
|
|
|
|
|
|
|
|
|
|
<p>In particular, the heap-checker calls <code>pprof</code> to utilize
|
|
|
|
|
the full call-path for all allocations. <code>pprof</code> uses this
|
|
|
|
|
data to disambiguate allocations. When the time comes to do a
|
|
|
|
|
<code>SameHeap</code> or <code>NoLeaks</code> check, the heap-checker
|
|
|
|
|
asks <code>pprof</code> to do this check on an
|
|
|
|
|
allocation-by-allocation basis, rather than just by comparing global
|
|
|
|
|
counts.</p>
|
|
|
|
|
|
|
|
|
|
<p>Here's an example. Consider the following function:</p>
|
|
|
|
|
<pre>
|
|
|
|
|
void LeakTwentyBytes() {
|
|
|
|
|
char* a = malloc(20);
|
|
|
|
|
HeapLeakChecker heap_checker("test_malloc");
|
|
|
|
|
char* b = malloc(20);
|
|
|
|
|
free(a);
|
|
|
|
|
heap_checker.NoLeaks();
|
|
|
|
|
}
|
|
|
|
|
</pre>
|
|
|
|
|
|
|
|
|
|
<p>Without using pprof, the only thing we will do is count up the
|
|
|
|
|
number of allocations and frees inside the leak-checked interval.
|
|
|
|
|
Twenty bytes allocated, twenty bytes freed, and the code looks ok.</p>
|
|
|
|
|
|
|
|
|
|
<p>With pprof, however, we can track the call-path for each
|
|
|
|
|
allocation, and account for them separately. In the example function
|
|
|
|
|
above, there are two call-paths that end in an allocation, one that
|
|
|
|
|
ends in "LeakTwentyBytes:line1" and one that ends in
|
|
|
|
|
"LeakTwentyBytes:line3".</p>
|
|
|
|
|
|
|
|
|
|
<p>Here's how the heap-checker works when it can use pprof in this
|
|
|
|
|
way:</p>
|
|
|
|
|
<ol>
|
|
|
|
|
<li> <b>Line 1:</b> Allocate 20 bytes, mark <code>a</code> as having
|
|
|
|
|
call-path "LeakTwentyBytes:line1", and update the count-map
|
|
|
|
|
<pre>count["LeakTwentyByte:line1"] += 20;</pre>
|
|
|
|
|
<li> <b>Line 2:</b> Dump the current <code>count</code> map to a file.
|
|
|
|
|
<li> <b>Line 3:</b> Allocate 20 bytes, mark <code>b</code> as having
|
|
|
|
|
call-path "LeakTwentyBytes:line3", and update the count-map:
|
|
|
|
|
<pre>count["LeakTwentyByte:line3"] += 20;</pre>
|
|
|
|
|
<li> <b>Line 4:</b> Look up <code>a</code> to find its call-path
|
|
|
|
|
(stored in line 1), and use that to update the count-map:
|
|
|
|
|
<pre>count["LeakTwentyByte:line1"] -= 20;</pre>
|
|
|
|
|
<li> <b>Line 5:</b> Look at each bucket in the current count-map,
|
|
|
|
|
minus what was dumped in line 2. Here's the diffs we'll have
|
|
|
|
|
in each bucket:
|
|
|
|
|
<pre>
|
|
|
|
|
count["LeakTwentyByte:line1"] == -20;
|
|
|
|
|
count["LeakTwentyByte:line3"] == 20;
|
|
|
|
|
</pre>
|
|
|
|
|
Since <i>at least one</i> bucket has a positive number, we
|
|
|
|
|
complain of a leak. (Note if line 5 had been
|
|
|
|
|
<code>SameHeap</code> instead of <code>NoLeaks</code>, we would
|
|
|
|
|
have complained if any bucket had had a <i>non-zero</i>
|
|
|
|
|
number.)
|
|
|
|
|
</ol>
|
|
|
|
|
|
|
|
|
|
<p>Note that one way to visualize the non-<code>pprof</code> mode is
|
|
|
|
|
that we do the same thing as above, but always use "unknown" as the
|
|
|
|
|
call-path. That is, our count-map always only has one entry in it:
|
|
|
|
|
<code>count["unknown"]</code>. Looking at the example above shows how
|
|
|
|
|
having only one entry in the map can lead to incorrect results.</p>
|
|
|
|
|
|
|
|
|
|
<p>Here is when <code>pprof</code> is used by the heap-checker:</p>
|
|
|
|
|
<ul>
|
|
|
|
|
<li> <code>NoLeaks()</code> and <code>SameHeap()</code> both use
|
|
|
|
|
<code>pprof</code>.
|
|
|
|
|
<li> <code>BriefNoLeaks()</code> and <code>BriefSameHeap()</code> do
|
|
|
|
|
not use <code>pprof</code>.
|
|
|
|
|
<li> <code>QuickNoLeaks</code> and <code>QuickSameHeap()</code> are
|
|
|
|
|
a kind of compromise: they do <i>not</i> use pprof for their
|
|
|
|
|
leak check, but if that check happens to find a leak anyway,
|
|
|
|
|
then they re-do the leak calculation using <code>pprof</code>.
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This means they do not always find leaks, but when they do,
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they will be as accurate as possible in their leak report.
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</ul>
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<h3>Leak-checking and Threads</h3>
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<p>At the time of HeapLeakChecker's construction and during
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<code>*NoLeaks()</code>/<code>*SameHeap()</code> calls, we grab a lock
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and then pause all other threads so other threads do not interfere
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with recording or analyzing the state of the heap.</p>
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<p>In general, leak checking works correctly in the presence of
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threads. However, thread stack data liveness determination (via
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<code>base/thread_lister.h</code>) does not work when the program is
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running under GDB, because the ptrace functionality needed for finding
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threads is already hooked to by GDB. Conversely, leak checker's
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ptrace attempts might also interfere with GDB. As a result, GDB can
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result in potentially false leak reports. For this reason, the
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heap-checker turns itself off when running under GDB.</p>
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<p>Also, <code>thread_lister</code> only works for Linux pthreads;
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leak checking is unlikely to handle other thread implementations
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correctly.</p>
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<p>As mentioned in the discussion of liveness flooding, thread-stack
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liveness determination might mis-classify as reachable objects that
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very recently became unreachable (leaked). This can happen when the
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pointers to now-logically-unreachable objects are present in the
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active thread stack frame. In other words, trivial code like the
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following might not produce the expected leak checking outcome
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depending on how the compiled code works with the stack:</p>
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<pre>
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int* foo = new int [20];
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HeapLeakChecker check("a_check");
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foo = NULL;
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CHECK(check.NoLeaks()); // this might succeed
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</pre>
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2007-03-22 03:28:56 +00:00
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<hr>
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2007-04-16 20:49:32 +00:00
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<address>Maxim Lifantsev<br>
|
2007-03-22 03:28:56 +00:00
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|
<!-- Created: Tue Dec 19 10:43:14 PST 2000 -->
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<!-- hhmts start -->
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2007-07-18 18:30:50 +00:00
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Last modified: Fri Jul 13 13:14:33 PDT 2007
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2007-03-22 03:28:56 +00:00
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<!-- hhmts end -->
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2007-04-16 20:49:32 +00:00
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</address>
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2007-03-22 03:28:56 +00:00
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</body>
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</html>
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