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22<div><div><h1 class="title">
23<a name="faq"></a>Valgrind Frequently Asked Questions</h1></div></div>
24<hr>
25</div>
26<div class="qandaset">
27<dl>
28<dt>1. <a href="faq.html#faq.background">Background</a>
29</dt>
30<dd>1.1. <a href="faq.html#faq.pronounce">How do you pronounce "Valgrind"?</a>
31</dd>
32<dd>1.2. <a href="faq.html#faq.whence">Where does the name "Valgrind" come from?</a>
33</dd>
34</dl>
35<dl>
36<dt>2. <a href="faq.html#faq.installing">Compiling, installing and configuring</a>
37</dt>
38<dd>2.1. <a href="faq.html#faq.make_dies">When building Valgrind, 'make' dies partway with
39 an assertion failure, something like this:</a>
40</dd>
41<dd>2.2. <a href="faq.html#faq.glibc_devel">When building Valgrind, 'make' fails with this:</a>
42</dd>
43</dl>
44<dl>
45<dt>3. <a href="faq.html#faq.abort">Valgrind aborts unexpectedly</a>
46</dt>
47<dd>3.1. <a href="faq.html#faq.exit_errors">Programs run OK on Valgrind, but at exit produce a bunch of
48 errors involving __libc_freeres and then die
49 with a segmentation fault.</a>
50</dd>
51<dd>3.2. <a href="faq.html#faq.bugdeath">My (buggy) program dies like this:</a>
52</dd>
53<dd>3.3. <a href="faq.html#faq.msgdeath">My program dies, printing a message like this along the
54 way:</a>
55</dd>
56<dd>3.4. <a href="faq.html#faq.java">I tried running a Java program (or another program that uses a
57 just-in-time compiler) under Valgrind but something went wrong.
58 Does Valgrind handle such programs?</a>
59</dd>
60</dl>
61<dl>
62<dt>4. <a href="faq.html#faq.unexpected">Valgrind behaves unexpectedly</a>
63</dt>
64<dd>4.1. <a href="faq.html#faq.reports">My program uses the C++ STL and string classes. Valgrind
65 reports 'still reachable' memory leaks involving these classes at
66 the exit of the program, but there should be none.</a>
67</dd>
68<dd>4.2. <a href="faq.html#faq.unhelpful">The stack traces given by Memcheck (or another tool) aren't
69 helpful. How can I improve them?</a>
70</dd>
71<dd>4.3. <a href="faq.html#faq.aliases">The stack traces given by Memcheck (or another tool) seem to
72 have the wrong function name in them. What's happening?</a>
73</dd>
74<dd>4.4. <a href="faq.html#faq.crashes">My program crashes normally, but doesn't under Valgrind, or vice
75 versa. What's happening?</a>
76</dd>
77<dd>4.5. <a href="faq.html#faq.hiddenbug"> Memcheck doesn't report any errors and I know my program has
78 errors.</a>
79</dd>
80<dd>4.6. <a href="faq.html#faq.overruns">Why doesn't Memcheck find the array overruns in this
81 program?</a>
82</dd>
83</dl>
84<dl>
85<dt>5. <a href="faq.html#faq.misc">Miscellaneous</a>
86</dt>
87<dd>5.1. <a href="faq.html#faq.writesupp">I tried writing a suppression but it didn't work. Can you
88 write my suppression for me?</a>
89</dd>
90<dd>5.2. <a href="faq.html#faq.deflost">With Memcheck's memory leak detector, what's the
91 difference between "definitely lost", "indirectly lost", "possibly
92 lost", "still reachable", and "suppressed"?</a>
93</dd>
94<dd>5.3. <a href="faq.html#faq.undeferrors">Memcheck's uninitialised value errors are hard to track down,
95 because they are often reported some time after they are caused. Could
96 Memcheck record a trail of operations to better link the cause to the
97 effect? Or maybe just eagerly report any copies of uninitialised
98 memory values?</a>
99</dd>
100<dd>5.4. <a href="faq.html#faq.attach">Is it possible to attach Valgrind to a program that is already
101 running?</a>
102</dd>
103</dl>
104<dl><dt>6. <a href="faq.html#faq.help">How To Get Further Assistance</a>
105</dt></dl>
106<br><table width="100%" summary="Q and A Div" cellpadding="2" cellspacing="2" border="0">
107<tr class="qandadiv"><td align="left" valign="top" colspan="2">
108<a name="faq.background"></a><h3 class="title">
109<a name="faq.background"></a>1. Background</h3>
110</td></tr>
111<tr class="toc" colspan="2"><td align="left" valign="top" colspan="2">1.1. <a href="faq.html#faq.pronounce">How do you pronounce "Valgrind"?</a><br>1.2. <a href="faq.html#faq.whence">Where does the name "Valgrind" come from?</a><br>
112</td></tr>
113<tr><td colspan="2"> </td></tr>
114<tr class="question">
115<td align="left" valign="top">
116<a name="faq.pronounce"></a><a name="q-pronounce"></a><b>1.1.</b>
117</td>
118<td align="left" valign="top"><b>How do you pronounce "Valgrind"?</b></td>
119</tr>
120<tr class="answer">
121<td align="left" valign="top"><a name="a-pronounce"></a></td>
122<td align="left" valign="top">
123<p>The "Val" as in the word "value". The "grind" is pronounced
124 with a short 'i' -- ie. "grinned" (rhymes with "tinned") rather than
125 "grined" (rhymes with "find").</p>
126<p>Don't feel bad: almost
127 everyone gets it wrong at first.</p>
128</td>
129</tr>
130<tr><td colspan="2"> </td></tr>
131<tr class="question">
132<td align="left" valign="top">
133<a name="faq.whence"></a><a name="q-whence"></a><b>1.2.</b>
134</td>
135<td align="left" valign="top"><b>Where does the name "Valgrind" come from?</b></td>
136</tr>
137<tr class="answer">
138<td align="left" valign="top"><a name="a-whence"></a></td>
139<td align="left" valign="top">
140<p>From Nordic mythology. Originally (before release) the project
141 was named Heimdall, after the watchman of the Nordic gods. He could
142 "see a hundred miles by day or night, hear the grass growing, see the
143 wool growing on a sheep's back", etc. This would have been a great
144 name, but it was already taken by a security package "Heimdal".</p>
145<p>Keeping with the Nordic theme, Valgrind was chosen. Valgrind is
146 the name of the main entrance to Valhalla (the Hall of the Chosen
147 Slain in Asgard). Over this entrance there resides a wolf and over it
148 there is the head of a boar and on it perches a huge eagle, whose eyes
149 can see to the far regions of the nine worlds. Only those judged
150 worthy by the guardians are allowed to pass through Valgrind. All
151 others are refused entrance.</p>
152<p>It's not short for "value grinder", although that's not a bad
153 guess.</p>
154</td>
155</tr>
156</table>
157<br><table width="100%" summary="Q and A Div" cellpadding="2" cellspacing="2" border="0">
158<tr class="qandadiv"><td align="left" valign="top" colspan="2">
159<a name="faq.installing"></a><h3 class="title">
160<a name="faq.installing"></a>2. Compiling, installing and configuring</h3>
161</td></tr>
162<tr class="toc" colspan="2"><td align="left" valign="top" colspan="2">2.1. <a href="faq.html#faq.make_dies">When building Valgrind, 'make' dies partway with
163 an assertion failure, something like this:</a><br>2.2. <a href="faq.html#faq.glibc_devel">When building Valgrind, 'make' fails with this:</a><br>
164</td></tr>
165<tr><td colspan="2"> </td></tr>
166<tr class="question">
167<td align="left" valign="top">
168<a name="faq.make_dies"></a><a name="q-make_dies"></a><b>2.1.</b>
169</td>
170<td align="left" valign="top">
171<b>When building Valgrind, 'make' dies partway with
172 an assertion failure, something like this:</b><pre class="screen">
173% make: expand.c:489: allocated_variable_append:
174 Assertion 'current_variable_set_list-&gt;next != 0' failed.
175</pre>
176</td>
177</tr>
178<tr class="answer">
179<td align="left" valign="top"><a name="a-make_dies"></a></td>
180<td align="left" valign="top"><p>It's probably a bug in 'make'. Some, but not all, instances of
181 version 3.79.1 have this bug, see
182 <a class="ulink" href="http://www.mail-archive.com/bug-make@gnu.org/msg01658.html" target="_top">this</a>.
183 Try upgrading to a more recent version of 'make'. Alternatively, we have
184 heard that unsetting the CFLAGS environment variable avoids the
185 problem.</p></td>
186</tr>
187<tr><td colspan="2"> </td></tr>
188<tr class="question">
189<td align="left" valign="top">
Elliott Hughesed398002017-06-21 14:41:24 -0700[diff] [blame^]190<a name="faq.glibc_devel"></a><a name="idm140394923880656"></a><b>2.2.</b>
Elliott Hughesa0664b92017-04-18 17:46:52 -0700[diff] [blame]191</td>
192<td align="left" valign="top">
193<b>When building Valgrind, 'make' fails with this:</b><pre class="screen">
194/usr/bin/ld: cannot find -lc
195collect2: ld returned 1 exit status
196</pre>
197</td>
198</tr>
199<tr class="answer">
200<td align="left" valign="top"></td>
201<td align="left" valign="top"><p>You need to install the glibc-static-devel package.</p></td>
202</tr>
203</table>
204<br><table width="100%" summary="Q and A Div" cellpadding="2" cellspacing="2" border="0">
205<tr class="qandadiv"><td align="left" valign="top" colspan="2">
206<a name="faq.abort"></a><h3 class="title">
207<a name="faq.abort"></a>3. Valgrind aborts unexpectedly</h3>
208</td></tr>
209<tr class="toc" colspan="2"><td align="left" valign="top" colspan="2">3.1. <a href="faq.html#faq.exit_errors">Programs run OK on Valgrind, but at exit produce a bunch of
210 errors involving __libc_freeres and then die
211 with a segmentation fault.</a><br>3.2. <a href="faq.html#faq.bugdeath">My (buggy) program dies like this:</a><br>3.3. <a href="faq.html#faq.msgdeath">My program dies, printing a message like this along the
212 way:</a><br>3.4. <a href="faq.html#faq.java">I tried running a Java program (or another program that uses a
213 just-in-time compiler) under Valgrind but something went wrong.
214 Does Valgrind handle such programs?</a><br>
215</td></tr>
216<tr><td colspan="2"> </td></tr>
217<tr class="question">
218<td align="left" valign="top">
219<a name="faq.exit_errors"></a><a name="q-exit_errors"></a><b>3.1.</b>
220</td>
221<td align="left" valign="top"><b>Programs run OK on Valgrind, but at exit produce a bunch of
222 errors involving <code class="literal">__libc_freeres</code> and then die
223 with a segmentation fault.</b></td>
224</tr>
225<tr class="answer">
226<td align="left" valign="top"><a name="a-exit_errors"></a></td>
227<td align="left" valign="top">
228<p>When the program exits, Valgrind runs the procedure
229 <code class="function">__libc_freeres</code> in glibc. This is a hook for
230 memory debuggers, so they can ask glibc to free up any memory it has
231 used. Doing that is needed to ensure that Valgrind doesn't
232 incorrectly report space leaks in glibc.</p>
233<p>The problem is that running <code class="literal">__libc_freeres</code> in
234 older glibc versions causes this crash.</p>
235<p>Workaround for 1.1.X and later versions of Valgrind: use the
236 <code class="option">--run-libc-freeres=no</code> option. You may then get space
237 leak reports for glibc allocations (please don't report these to
238 the glibc people, since they are not real leaks), but at least the
239 program runs.</p>
240</td>
241</tr>
242<tr><td colspan="2"> </td></tr>
243<tr class="question">
244<td align="left" valign="top">
245<a name="faq.bugdeath"></a><a name="q-bugdeath"></a><b>3.2.</b>
246</td>
247<td align="left" valign="top">
248<b>My (buggy) program dies like this:</b><pre class="screen">valgrind: m_mallocfree.c:248 (get_bszB_as_is): Assertion 'bszB_lo == bszB_hi' failed.</pre>
249<b>or like this:</b><pre class="screen">valgrind: m_mallocfree.c:442 (mk_inuse_bszB): Assertion 'bszB != 0' failed.</pre>
250<b>or otherwise aborts or crashes in m_mallocfree.c.</b>
251</td>
252</tr>
253<tr class="answer">
254<td align="left" valign="top"><a name="a-bugdeath"></a></td>
255<td align="left" valign="top"><p>If Memcheck (the memory checker) shows any invalid reads,
256 invalid writes or invalid frees in your program, the above may
257 happen. Reason is that your program may trash Valgrind's low-level
258 memory manager, which then dies with the above assertion, or
259 something similar. The cure is to fix your program so that it
260 doesn't do any illegal memory accesses. The above failure will
261 hopefully go away after that.</p></td>
262</tr>
263<tr><td colspan="2"> </td></tr>
264<tr class="question">
265<td align="left" valign="top">
266<a name="faq.msgdeath"></a><a name="q-msgdeath"></a><b>3.3.</b>
267</td>
268<td align="left" valign="top">
269<b>My program dies, printing a message like this along the
270 way:</b><pre class="screen">vex x86-&gt;IR: unhandled instruction bytes: 0x66 0xF 0x2E 0x5</pre>
271</td>
272</tr>
273<tr class="answer">
274<td align="left" valign="top"><a name="a-msgdeath"></a></td>
275<td align="left" valign="top">
276<p>One possibility is that your program has a bug and erroneously
277 jumps to a non-code address, in which case you'll get a SIGILL signal.
278 Memcheck may issue a warning just before this happens, but it might not
279 if the jump happens to land in addressable memory.</p>
280<p>Another possibility is that Valgrind does not handle the
281 instruction. If you are using an older Valgrind, a newer version might
282 handle the instruction. However, all instruction sets have some
283 obscure, rarely used instructions. Also, on amd64 there are an almost
284 limitless number of combinations of redundant instruction prefixes, many
285 of them undocumented but accepted by CPUs. So Valgrind will still have
286 decoding failures from time to time. If this happens, please file a bug
287 report.</p>
288</td>
289</tr>
290<tr><td colspan="2"> </td></tr>
291<tr class="question">
292<td align="left" valign="top">
293<a name="faq.java"></a><a name="q-java"></a><b>3.4.</b>
294</td>
295<td align="left" valign="top"><b>I tried running a Java program (or another program that uses a
296 just-in-time compiler) under Valgrind but something went wrong.
297 Does Valgrind handle such programs?</b></td>
298</tr>
299<tr class="answer">
300<td align="left" valign="top"><a name="a-java"></a></td>
301<td align="left" valign="top">
302<p>Valgrind can handle dynamically generated code, so long as
303 none of the generated code is later overwritten by other generated
304 code. If this happens, though, things will go wrong as Valgrind
305 will continue running its translations of the old code (this is true
306 on x86 and amd64, on PowerPC there are explicit cache flush
307 instructions which Valgrind detects and honours).
308 You should try running with
309 <code class="option">--smc-check=all</code> in this case. Valgrind will run
310 much more slowly, but should detect the use of the out-of-date
311 code.</p>
312<p>Alternatively, if you have the source code to the JIT compiler
313 you can insert calls to the
314 <code class="computeroutput">VALGRIND_DISCARD_TRANSLATIONS</code>
315 client request to mark out-of-date code, saving you from using
316 <code class="option">--smc-check=all</code>.</p>
317<p>Apart from this, in theory Valgrind can run any Java program
318 just fine, even those that use JNI and are partially implemented in
319 other languages like C and C++. In practice, Java implementations
320 tend to do nasty things that most programs do not, and Valgrind
321 sometimes falls over these corner cases.</p>
322<p>If your Java programs do not run under Valgrind, even with
323 <code class="option">--smc-check=all</code>, please file a bug report and
324 hopefully we'll be able to fix the problem.</p>
325</td>
326</tr>
327</table>
328<br><table width="100%" summary="Q and A Div" cellpadding="2" cellspacing="2" border="0">
329<tr class="qandadiv"><td align="left" valign="top" colspan="2">
330<a name="faq.unexpected"></a><h3 class="title">
331<a name="faq.unexpected"></a>4. Valgrind behaves unexpectedly</h3>
332</td></tr>
333<tr class="toc" colspan="2"><td align="left" valign="top" colspan="2">4.1. <a href="faq.html#faq.reports">My program uses the C++ STL and string classes. Valgrind
334 reports 'still reachable' memory leaks involving these classes at
335 the exit of the program, but there should be none.</a><br>4.2. <a href="faq.html#faq.unhelpful">The stack traces given by Memcheck (or another tool) aren't
336 helpful. How can I improve them?</a><br>4.3. <a href="faq.html#faq.aliases">The stack traces given by Memcheck (or another tool) seem to
337 have the wrong function name in them. What's happening?</a><br>4.4. <a href="faq.html#faq.crashes">My program crashes normally, but doesn't under Valgrind, or vice
338 versa. What's happening?</a><br>4.5. <a href="faq.html#faq.hiddenbug"> Memcheck doesn't report any errors and I know my program has
339 errors.</a><br>4.6. <a href="faq.html#faq.overruns">Why doesn't Memcheck find the array overruns in this
340 program?</a><br>
341</td></tr>
342<tr><td colspan="2"> </td></tr>
343<tr class="question">
344<td align="left" valign="top">
345<a name="faq.reports"></a><a name="q-reports"></a><b>4.1.</b>
346</td>
347<td align="left" valign="top"><b>My program uses the C++ STL and string classes. Valgrind
348 reports 'still reachable' memory leaks involving these classes at
349 the exit of the program, but there should be none.</b></td>
350</tr>
351<tr class="answer">
352<td align="left" valign="top"><a name="a-reports"></a></td>
353<td align="left" valign="top">
354<p>First of all: relax, it's probably not a bug, but a feature.
355 Many implementations of the C++ standard libraries use their own
356 memory pool allocators. Memory for quite a number of destructed
357 objects is not immediately freed and given back to the OS, but kept
358 in the pool(s) for later re-use. The fact that the pools are not
359 freed at the exit of the program cause Valgrind to report this
360 memory as still reachable. The behaviour not to free pools at the
361 exit could be called a bug of the library though.</p>
362<p>Using GCC, you can force the STL to use malloc and to free
363 memory as soon as possible by globally disabling memory caching.
364 Beware! Doing so will probably slow down your program, sometimes
365 drastically.</p>
366<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
367<li class="listitem"><p>With GCC 2.91, 2.95, 3.0 and 3.1, compile all source using
368 the STL with <code class="literal">-D__USE_MALLOC</code>. Beware! This was
369 removed from GCC starting with version 3.3.</p></li>
370<li class="listitem"><p>With GCC 3.2.2 and later, you should export the
371 environment variable <code class="literal">GLIBCPP_FORCE_NEW</code> before
372 running your program.</p></li>
373<li class="listitem"><p>With GCC 3.4 and later, that variable has changed name to
374 <code class="literal">GLIBCXX_FORCE_NEW</code>.</p></li>
375</ul></div>
376<p>There are other ways to disable memory pooling: using the
377 <code class="literal">malloc_alloc</code> template with your objects (not
378 portable, but should work for GCC) or even writing your own memory
379 allocators. But all this goes beyond the scope of this FAQ. Start
380 by reading
381 <a class="ulink" href="http://gcc.gnu.org/onlinedocs/libstdc++/faq/index.html#4_4_leak" target="_top">
382 http://gcc.gnu.org/onlinedocs/libstdc++/faq/index.html#4_4_leak</a>
383 if you absolutely want to do that. But beware:
384 allocators belong to the more messy parts of the STL and
385 people went to great lengths to make the STL portable across
386 platforms. Chances are good that your solution will work on your
387 platform, but not on others.</p>
388</td>
389</tr>
390<tr><td colspan="2"> </td></tr>
391<tr class="question">
392<td align="left" valign="top">
393<a name="faq.unhelpful"></a><a name="q-unhelpful"></a><b>4.2.</b>
394</td>
395<td align="left" valign="top"><b>The stack traces given by Memcheck (or another tool) aren't
396 helpful. How can I improve them?</b></td>
397</tr>
398<tr class="answer">
399<td align="left" valign="top"><a name="a-unhelpful"></a></td>
400<td align="left" valign="top">
401<p>If they're not long enough, use <code class="option">--num-callers</code>
402 to make them longer.</p>
403<p>If they're not detailed enough, make sure you are compiling
404 with <code class="option">-g</code> to add debug information. And don't strip
405 symbol tables (programs should be unstripped unless you run 'strip'
406 on them; some libraries ship stripped).</p>
407<p>Also, for leak reports involving shared objects, if the shared
408 object is unloaded before the program terminates, Valgrind will
409 discard the debug information and the error message will be full of
410 <code class="literal">???</code> entries. The workaround here is to avoid
411 calling <code class="function">dlclose</code> on these shared objects.</p>
412<p>Also, <code class="option">-fomit-frame-pointer</code> and
413 <code class="option">-fstack-check</code> can make stack traces worse.</p>
414<p>Some example sub-traces:</p>
415<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
416<li class="listitem">
417<p>With debug information and unstripped (best):</p>
418<pre class="programlisting">
419Invalid write of size 1
420 at 0x80483BF: really (malloc1.c:20)
421 by 0x8048370: main (malloc1.c:9)
422</pre>
423</li>
424<li class="listitem">
425<p>With no debug information, unstripped:</p>
426<pre class="programlisting">
427Invalid write of size 1
428 at 0x80483BF: really (in /auto/homes/njn25/grind/head5/a.out)
429 by 0x8048370: main (in /auto/homes/njn25/grind/head5/a.out)
430</pre>
431</li>
432<li class="listitem">
433<p>With no debug information, stripped:</p>
434<pre class="programlisting">
435Invalid write of size 1
436 at 0x80483BF: (within /auto/homes/njn25/grind/head5/a.out)
437 by 0x8048370: (within /auto/homes/njn25/grind/head5/a.out)
438 by 0x42015703: __libc_start_main (in /lib/tls/libc-2.3.2.so)
439 by 0x80482CC: (within /auto/homes/njn25/grind/head5/a.out)
440</pre>
441</li>
442<li class="listitem">
443<p>With debug information and -fomit-frame-pointer:</p>
444<pre class="programlisting">
445Invalid write of size 1
446 at 0x80483C4: really (malloc1.c:20)
447 by 0x42015703: __libc_start_main (in /lib/tls/libc-2.3.2.so)
448 by 0x80482CC: ??? (start.S:81)
449</pre>
450</li>
451<li class="listitem">
452<p>A leak error message involving an unloaded shared object:</p>
453<pre class="programlisting">
45484 bytes in 1 blocks are possibly lost in loss record 488 of 713
455 at 0x1B9036DA: operator new(unsigned) (vg_replace_malloc.c:132)
456 by 0x1DB63EEB: ???
457 by 0x1DB4B800: ???
458 by 0x1D65E007: ???
459 by 0x8049EE6: main (main.cpp:24)
460</pre>
461</li>
462</ul></div>
463</td>
464</tr>
465<tr><td colspan="2"> </td></tr>
466<tr class="question">
467<td align="left" valign="top">
468<a name="faq.aliases"></a><a name="q-aliases"></a><b>4.3.</b>
469</td>
470<td align="left" valign="top"><b>The stack traces given by Memcheck (or another tool) seem to
471 have the wrong function name in them. What's happening?</b></td>
472</tr>
473<tr class="answer">
474<td align="left" valign="top"><a name="a-aliases"></a></td>
475<td align="left" valign="top"><p>Occasionally Valgrind stack traces get the wrong function
476 names. This is caused by glibc using aliases to effectively give
477 one function two names. Most of the time Valgrind chooses a
478 suitable name, but very occasionally it gets it wrong. Examples we know
479 of are printing <code class="function">bcmp</code> instead of
480 <code class="function">memcmp</code>, <code class="function">index</code> instead of
481 <code class="function">strchr</code>, and <code class="function">rindex</code> instead of
482 <code class="function">strrchr</code>.</p></td>
483</tr>
484<tr><td colspan="2"> </td></tr>
485<tr class="question">
486<td align="left" valign="top">
487<a name="faq.crashes"></a><a name="q-crashes"></a><b>4.4.</b>
488</td>
489<td align="left" valign="top"><b>My program crashes normally, but doesn't under Valgrind, or vice
490 versa. What's happening?</b></td>
491</tr>
492<tr class="answer">
493<td align="left" valign="top"><a name="a-crashes"></a></td>
494<td align="left" valign="top">
495<p>When a program runs under Valgrind, its environment is slightly
496 different to when it runs natively. For example, the memory layout is
497 different, and the way that threads are scheduled is different.</p>
498<p>Most of the time this doesn't make any difference, but it can,
499 particularly if your program is buggy. For example, if your program
500 crashes because it erroneously accesses memory that is unaddressable,
501 it's possible that this memory will not be unaddressable when run under
502 Valgrind. Alternatively, if your program has data races, these may not
503 manifest under Valgrind.</p>
504<p>There isn't anything you can do to change this, it's just the
505 nature of the way Valgrind works that it cannot exactly replicate a
506 native execution environment. In the case where your program crashes
507 due to a memory error when run natively but not when run under Valgrind,
508 in most cases Memcheck should identify the bad memory operation.</p>
509</td>
510</tr>
511<tr><td colspan="2"> </td></tr>
512<tr class="question">
513<td align="left" valign="top">
514<a name="faq.hiddenbug"></a><a name="q-hiddenbug"></a><b>4.5.</b>
515</td>
516<td align="left" valign="top"><b> Memcheck doesn't report any errors and I know my program has
517 errors.</b></td>
518</tr>
519<tr class="answer">
520<td align="left" valign="top"><a name="a-hiddenbug"></a></td>
521<td align="left" valign="top">
522<p>There are two possible causes of this.</p>
523<p>First, by default, Valgrind only traces the top-level process.
524 So if your program spawns children, they won't be traced by Valgrind
525 by default. Also, if your program is started by a shell script,
526 Perl script, or something similar, Valgrind will trace the shell, or
527 the Perl interpreter, or equivalent.</p>
528<p>To trace child processes, use the
529 <code class="option">--trace-children=yes</code> option.</p>
530<p>If you are tracing large trees of processes, it can be less
531 disruptive to have the output sent over the network. Give Valgrind
532 the option <code class="option">--log-socket=127.0.0.1:12345</code> (if you want
533 logging output sent to port <code class="literal">12345</code> on
534 <code class="literal">localhost</code>). You can use the valgrind-listener
535 program to listen on that port:</p>
536<pre class="programlisting">
537valgrind-listener 12345
538</pre>
539<p>Obviously you have to start the listener process first. See
540 the manual for more details.</p>
541<p>Second, if your program is statically linked, most Valgrind
542 tools will only work well if they are able to replace certain
543 functions, such as <code class="function">malloc</code>, with their own
544 versions. By default, statically linked <code class="function">malloc
545 functions</code> are not replaced. A key indicator of this is
546 if Memcheck says:
547</p>
548<pre class="programlisting">
549All heap blocks were freed -- no leaks are possible
550</pre>
551<p>
552 when you know your program calls <code class="function">malloc</code>. The
553 workaround is to use the option
554 <code class="option">--soname-synonyms=somalloc=NONE</code>
555 or to avoid statically linking your program.</p>
556<p>There will also be no replacement if you use an alternative
557 <code class="function">malloc library</code> such as tcmalloc, jemalloc,
558 ... In such a case, the
559 option <code class="option">--soname-synonyms=somalloc=zzzz</code> (where
560 zzzz is the soname of the alternative malloc library) will allow
561 Valgrind to replace the functions.</p>
562</td>
563</tr>
564<tr><td colspan="2"> </td></tr>
565<tr class="question">
566<td align="left" valign="top">
567<a name="faq.overruns"></a><a name="q-overruns"></a><b>4.6.</b>
568</td>
569<td align="left" valign="top">
570<b>Why doesn't Memcheck find the array overruns in this
571 program?</b><pre class="programlisting">
572int static[5];
573
574int main(void)
575{
576 int stack[5];
577
578 static[5] = 0;
579 stack [5] = 0;
580
581 return 0;
582}
583</pre>
584</td>
585</tr>
586<tr class="answer">
587<td align="left" valign="top"><a name="a-overruns"></a></td>
588<td align="left" valign="top">
589<p>Unfortunately, Memcheck doesn't do bounds checking on global
590 or stack arrays. We'd like to, but it's just not possible to do in
591 a reasonable way that fits with how Memcheck works. Sorry.</p>
592<p>However, the experimental tool SGcheck can detect errors like
593 this. Run Valgrind with the <code class="option">--tool=exp-sgcheck</code> option
594 to try it, but be aware that it is not as robust as Memcheck.</p>
595</td>
596</tr>
597</table>
598<br><table width="100%" summary="Q and A Div" cellpadding="2" cellspacing="2" border="0">
599<tr class="qandadiv"><td align="left" valign="top" colspan="2">
600<a name="faq.misc"></a><h3 class="title">
601<a name="faq.misc"></a>5. Miscellaneous</h3>
602</td></tr>
603<tr class="toc" colspan="2"><td align="left" valign="top" colspan="2">5.1. <a href="faq.html#faq.writesupp">I tried writing a suppression but it didn't work. Can you
604 write my suppression for me?</a><br>5.2. <a href="faq.html#faq.deflost">With Memcheck's memory leak detector, what's the
605 difference between "definitely lost", "indirectly lost", "possibly
606 lost", "still reachable", and "suppressed"?</a><br>5.3. <a href="faq.html#faq.undeferrors">Memcheck's uninitialised value errors are hard to track down,
607 because they are often reported some time after they are caused. Could
608 Memcheck record a trail of operations to better link the cause to the
609 effect? Or maybe just eagerly report any copies of uninitialised
610 memory values?</a><br>5.4. <a href="faq.html#faq.attach">Is it possible to attach Valgrind to a program that is already
611 running?</a><br>
612</td></tr>
613<tr><td colspan="2"> </td></tr>
614<tr class="question">
615<td align="left" valign="top">
616<a name="faq.writesupp"></a><a name="q-writesupp"></a><b>5.1.</b>
617</td>
618<td align="left" valign="top"><b>I tried writing a suppression but it didn't work. Can you
619 write my suppression for me?</b></td>
620</tr>
621<tr class="answer">
622<td align="left" valign="top"><a name="a-writesupp"></a></td>
623<td align="left" valign="top">
624<p>Yes! Use the <code class="option">--gen-suppressions=yes</code> feature
625 to spit out suppressions automatically for you. You can then edit
626 them if you like, eg. combining similar automatically generated
627 suppressions using wildcards like <code class="literal">'*'</code>.</p>
628<p>If you really want to write suppressions by hand, read the
629 manual carefully. Note particularly that C++ function names must be
630 mangled (that is, not demangled).</p>
631</td>
632</tr>
633<tr><td colspan="2"> </td></tr>
634<tr class="question">
635<td align="left" valign="top">
636<a name="faq.deflost"></a><a name="q-deflost"></a><b>5.2.</b>
637</td>
638<td align="left" valign="top"><b>With Memcheck's memory leak detector, what's the
639 difference between "definitely lost", "indirectly lost", "possibly
640 lost", "still reachable", and "suppressed"?</b></td>
641</tr>
642<tr class="answer">
643<td align="left" valign="top"><a name="a-deflost"></a></td>
644<td align="left" valign="top">
645<p>The details are in the Memcheck section of the user manual.</p>
646<p>In short:</p>
647<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
648<li class="listitem"><p>"definitely lost" means your program is leaking memory --
649 fix those leaks!</p></li>
650<li class="listitem"><p>"indirectly lost" means your program is leaking memory in
651 a pointer-based structure. (E.g. if the root node of a binary tree
652 is "definitely lost", all the children will be "indirectly lost".)
653 If you fix the "definitely lost" leaks, the "indirectly lost" leaks
654 should go away.
655 </p></li>
656<li class="listitem"><p>"possibly lost" means your program is leaking
657 memory, unless you're doing unusual things with pointers that could
658 cause them to point into the middle of an allocated block; see the
659 user manual for some possible causes. Use
660 <code class="option">--show-possibly-lost=no</code> if you don't want to see
661 these reports.</p></li>
662<li class="listitem"><p>"still reachable" means your program is probably ok -- it
663 didn't free some memory it could have. This is quite common and
664 often reasonable. Don't use
665 <code class="option">--show-reachable=yes</code> if you don't want to see
666 these reports.</p></li>
667<li class="listitem"><p>"suppressed" means that a leak error has been suppressed.
668 There are some suppressions in the default suppression files.
669 You can ignore suppressed errors.</p></li>
670</ul></div>
671</td>
672</tr>
673<tr><td colspan="2"> </td></tr>
674<tr class="question">
675<td align="left" valign="top">
676<a name="faq.undeferrors"></a><a name="q-undeferrors"></a><b>5.3.</b>
677</td>
678<td align="left" valign="top"><b>Memcheck's uninitialised value errors are hard to track down,
679 because they are often reported some time after they are caused. Could
680 Memcheck record a trail of operations to better link the cause to the
681 effect? Or maybe just eagerly report any copies of uninitialised
682 memory values?</b></td>
683</tr>
684<tr class="answer">
685<td align="left" valign="top"><a name="a-undeferrors"></a></td>
686<td align="left" valign="top">
687<p>Prior to version 3.4.0, the answer was "we don't know how to do it
688 without huge performance penalties". As of 3.4.0, try using the
689 <code class="option">--track-origins=yes</code> option. It will run slower than
690 usual, but will give you extra information about the origin of
691 uninitialised values.</p>
692<p>Or if you want to do it the old fashioned way, you can use the
693 client request
694 <code class="computeroutput">VALGRIND_CHECK_VALUE_IS_DEFINED</code> to help
695 track these errors down -- work backwards from the point where the
696 uninitialised error occurs, checking suspect values until you find the
697 cause. This requires editing, compiling and re-running your program
698 multiple times, which is a pain, but still easier than debugging the
699 problem without Memcheck's help.</p>
700<p>As for eager reporting of copies of uninitialised memory values,
701 this has been suggested multiple times. Unfortunately, almost all
702 programs legitimately copy uninitialised memory values around (because
703 compilers pad structs to preserve alignment) and eager checking leads to
704 hundreds of false positives. Therefore Memcheck does not support eager
705 checking at this time.</p>
706</td>
707</tr>
708<tr><td colspan="2"> </td></tr>
709<tr class="question">
710<td align="left" valign="top">
711<a name="faq.attach"></a><a name="q-attach"></a><b>5.4.</b>
712</td>
713<td align="left" valign="top"><b>Is it possible to attach Valgrind to a program that is already
714 running?</b></td>
715</tr>
716<tr class="answer">
717<td align="left" valign="top"><a name="a-attach"></a></td>
718<td align="left" valign="top">
719<p>No. The environment that Valgrind provides for running programs
720 is significantly different to that for normal programs, e.g. due to
721 different layout of memory. Therefore Valgrind has to have full control
722 from the very start.</p>
723<p>It is possible to achieve something like this by running your
724 program without any instrumentation (which involves a slow-down of about
725 5x, less than that of most tools), and then adding instrumentation once
726 you get to a point of interest. Support for this must be provided by
727 the tool, however, and Callgrind is the only tool that currently has
728 such support. See the instructions on the
729 <code class="computeroutput">callgrind_control</code> program for details.
730 </p>
731</td>
732</tr>
733</table>
734<br><table width="100%" summary="Q and A Div" cellpadding="2" cellspacing="2" border="0">
735<tr class="qandadiv"><td align="left" valign="top" colspan="2">
736<a name="faq.help"></a><h3 class="title">
737<a name="faq.help"></a>6. How To Get Further Assistance</h3>
738</td></tr>
739<tr class="toc" colspan="2"><td align="left" valign="top" colspan="2"></td></tr>
740<tr class="answer">
741<td align="left" valign="top"><a name="a-help"></a></td>
742<td align="left" valign="top">
743<p>Read the appropriate section(s) of the
744 <a class="ulink" href="http://www.valgrind.org/docs/manual/index.html" target="_top">Valgrind Documentation</a>.</p>
745<p><a class="ulink" href="http://search.gmane.org" target="_top">Search</a> the
746 <a class="ulink" href="http://news.gmane.org/gmane.comp.debugging.valgrind" target="_top">valgrind-users</a> mailing list archives, using the group name
747 <code class="computeroutput">gmane.comp.debugging.valgrind</code>.</p>
748<p>If you think an answer in this FAQ is incomplete or inaccurate, please
749 e-mail <a class="ulink" href="mailto:valgrind@valgrind.org" target="_top">valgrind@valgrind.org</a>.</p>
750<p>If you have tried all of these things and are still
751 stuck, you can try mailing the
752 <a class="ulink" href="http://www.valgrind.org/support/mailing_lists.html" target="_top">valgrind-users mailing list</a>.
753 Note that an email has a better change of being answered usefully if it is
754 clearly written. Also remember that, despite the fact that most of the
755 community are very helpful and responsive to emailed questions, you are
756 probably requesting help from unpaid volunteers, so you have no guarantee
757 of receiving an answer.</p>
758</td>
759</tr>
760</table>
761</div>
762</div>
763<div>
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