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<a name="title">&nbsp;</a>
<h1 align=center>Valgrind, snapshot 20020516</h1>
<center>This manual was majorly updated on 20020501</center>
<center>This manual was minorly updated on 20020516</center>
<p>
<center>
<a href="mailto:jseward@acm.org">jseward@acm.org<br>
Copyright &copy; 2000-2002 Julian Seward
<p>
Valgrind is licensed under the GNU General Public License,
version 2<br>
An open-source tool for finding memory-management problems in
Linux-x86 executables.
</center>
<p>
<hr width="100%">
<a name="contents"></a>
<h2>Contents of this manual</h2>
<h4>1&nbsp; <a href="#intro">Introduction</a></h4>
1.1&nbsp; <a href="#whatfor">What Valgrind is for</a><br>
1.2&nbsp; <a href="#whatdoes">What it does with your program</a>
<h4>2&nbsp; <a href="#howtouse">How to use it, and how to make sense
of the results</a></h4>
2.1&nbsp; <a href="#starta">Getting started</a><br>
2.2&nbsp; <a href="#comment">The commentary</a><br>
2.3&nbsp; <a href="#report">Reporting of errors</a><br>
2.4&nbsp; <a href="#suppress">Suppressing errors</a><br>
2.5&nbsp; <a href="#flags">Command-line flags</a><br>
2.6&nbsp; <a href="#errormsgs">Explaination of error messages</a><br>
2.7&nbsp; <a href="#suppfiles">Writing suppressions files</a><br>
2.8&nbsp; <a href="#clientreq">The Client Request mechanism</a><br>
2.9&nbsp; <a href="#pthreads">Support for POSIX pthreads</a><br>
2.10&nbsp; <a href="#install">Building and installing</a><br>
2.11&nbsp; <a href="#problems">If you have problems</a><br>
<h4>3&nbsp; <a href="#machine">Details of the checking machinery</a></h4>
3.1&nbsp; <a href="#vvalue">Valid-value (V) bits</a><br>
3.2&nbsp; <a href="#vaddress">Valid-address (A)&nbsp;bits</a><br>
3.3&nbsp; <a href="#together">Putting it all together</a><br>
3.4&nbsp; <a href="#signals">Signals</a><br>
3.5&nbsp; <a href="#leaks">Memory leak detection</a><br>
<h4>4&nbsp; <a href="#limits">Limitations</a></h4>
<h4>5&nbsp; <a href="#howitworks">How it works -- a rough overview</a></h4>
5.1&nbsp; <a href="#startb">Getting started</a><br>
5.2&nbsp; <a href="#engine">The translation/instrumentation engine</a><br>
5.3&nbsp; <a href="#track">Tracking the status of memory</a><br>
5.4&nbsp; <a href="#sys_calls">System calls</a><br>
5.5&nbsp; <a href="#sys_signals">Signals</a><br>
<h4>6&nbsp; <a href="#example">An example</a></h4>
<h4>7&nbsp; <a href="#cache">Cache profiling</a></h4>
<h4>8&nbsp; <a href="techdocs.html">The design and implementation of Valgrind</a></h4>
<hr width="100%">
<a name="intro"></a>
<h2>1&nbsp; Introduction</h2>
<a name="whatfor"></a>
<h3>1.1&nbsp; What Valgrind is for</h3>
Valgrind is a tool to help you find memory-management problems in your
programs. When a program is run under Valgrind's supervision, all
reads and writes of memory are checked, and calls to
malloc/new/free/delete are intercepted. As a result, Valgrind can
detect problems such as:
<ul>
<li>Use of uninitialised memory</li>
<li>Reading/writing memory after it has been free'd</li>
<li>Reading/writing off the end of malloc'd blocks</li>
<li>Reading/writing inappropriate areas on the stack</li>
<li>Memory leaks -- where pointers to malloc'd blocks are lost
forever</li>
<li>Mismatched use of malloc/new/new [] vs free/delete/delete []</li>
</ul>
Problems like these can be difficult to find by other means, often
lying undetected for long periods, then causing occasional,
difficult-to-diagnose crashes.
<p>
Valgrind is closely tied to details of the CPU, operating system and
to a less extent, compiler and basic C libraries. This makes it
difficult to make it portable, so I have chosen at the outset to
concentrate on what I believe to be a widely used platform: Red Hat
Linux 7.2, on x86s. Valgrind uses the standard Unix
<code>./configure</code>, <code>make</code>, <code>make install</code>
mechanism, and I have attempted to ensure that it works on machines
with kernel 2.2 or 2.4 and glibc 2.1.X or 2.2.X. This should cover
the vast majority of modern Linux installations.
<p>
Valgrind is licensed under the GNU General Public License, version
2. Read the file LICENSE in the source distribution for details.
<a name="whatdoes">
<h3>1.2&nbsp; What it does with your program</h3>
Valgrind is designed to be as non-intrusive as possible. It works
directly with existing executables. You don't need to recompile,
relink, or otherwise modify, the program to be checked. Simply place
the word <code>valgrind</code> at the start of the command line
normally used to run the program. So, for example, if you want to run
the command <code>ls -l</code> on Valgrind, simply issue the
command: <code>valgrind ls -l</code>.
<p>Valgrind takes control of your program before it starts. Debugging
information is read from the executable and associated libraries, so
that error messages can be phrased in terms of source code
locations. Your program is then run on a synthetic x86 CPU which
checks every memory access. All detected errors are written to a
log. When the program finishes, Valgrind searches for and reports on
leaked memory.
<p>You can run pretty much any dynamically linked ELF x86 executable
using Valgrind. Programs run 25 to 50 times slower, and take a lot
more memory, than they usually would. It works well enough to run
large programs. For example, the Konqueror web browser from the KDE
Desktop Environment, version 3.0, runs slowly but usably on Valgrind.
<p>Valgrind simulates every single instruction your program executes.
Because of this, it finds errors not only in your application but also
in all supporting dynamically-linked (<code>.so</code>-format)
libraries, including the GNU C library, the X client libraries, Qt, if
you work with KDE, and so on. That often includes libraries, for
example the GNU C library, which contain memory access violations, but
which you cannot or do not want to fix.
<p>Rather than swamping you with errors in which you are not
interested, Valgrind allows you to selectively suppress errors, by
recording them in a suppressions file which is read when Valgrind
starts up. The build mechanism attempts to select suppressions which
give reasonable behaviour for the libc and XFree86 versions detected
on your machine.
<p><a href="#example">Section 6</a> shows an example of use.
<p>
<hr width="100%">
<a name="howtouse"></a>
<h2>2&nbsp; How to use it, and how to make sense of the results</h2>
<a name="starta"></a>
<h3>2.1&nbsp; Getting started</h3>
First off, consider whether it might be beneficial to recompile your
application and supporting libraries with optimisation disabled and
debugging info enabled (the <code>-g</code> flag). You don't have to
do this, but doing so helps Valgrind produce more accurate and less
confusing error reports. Chances are you're set up like this already,
if you intended to debug your program with GNU gdb, or some other
debugger.
<p>
A plausible compromise is to use <code>-g -O</code>.
Optimisation levels above <code>-O</code> have been observed, on very
rare occasions, to cause gcc to generate code which fools Valgrind's
error tracking machinery into wrongly reporting uninitialised value
errors. <code>-O</code> gets you the vast majority of the benefits of
higher optimisation levels anyway, so you don't lose much there.
<p>
Note that as of 1 May 2002 Valgrind does not understand the DWARF
debugging format, which is unfortunate since the upcoming gcc-3.1 uses
it by default. Valgrind only knows about the older "stabs" format.
If you use gcc-3.1 or above, you can still ask for stabs-format debug
info by passing <code>-gstabs</code> to gcc.
<p>
Then just run your application, but place the word
<code>valgrind</code> in front of your usual command-line invokation.
Note that you should run the real (machine-code) executable here. If
your application is started by, for example, a shell or perl script,
you'll need to modify it to invoke Valgrind on the real executables.
Running such scripts directly under Valgrind will result in you
getting error reports pertaining to <code>/bin/sh</code>,
<code>/usr/bin/perl</code>, or whatever interpreter you're using.
This almost certainly isn't what you want and can be confusing.
<a name="comment"></a>
<h3>2.2&nbsp; The commentary</h3>
Valgrind writes a commentary, detailing error reports and other
significant events. The commentary goes to standard output by
default. This may interfere with your program, so you can ask for it
to be directed elsewhere.
<p>All lines in the commentary are of the following form:<br>
<pre>
==12345== some-message-from-Valgrind
</pre>
<p>The <code>12345</code> is the process ID. This scheme makes it easy
to distinguish program output from Valgrind commentary, and also easy
to differentiate commentaries from different processes which have
become merged together, for whatever reason.
<p>By default, Valgrind writes only essential messages to the commentary,
so as to avoid flooding you with information of secondary importance.
If you want more information about what is happening, re-run, passing
the <code>-v</code> flag to Valgrind.
<a name="report"></a>
<h3>2.3&nbsp; Reporting of errors</h3>
When Valgrind detects something bad happening in the program, an error
message is written to the commentary. For example:<br>
<pre>
==25832== Invalid read of size 4
==25832== at 0x8048724: BandMatrix::ReSize(int, int, int) (bogon.cpp:45)
==25832== by 0x80487AF: main (bogon.cpp:66)
==25832== by 0x40371E5E: __libc_start_main (libc-start.c:129)
==25832== by 0x80485D1: (within /home/sewardj/newmat10/bogon)
==25832== Address 0xBFFFF74C is not stack'd, malloc'd or free'd
</pre>
<p>This message says that the program did an illegal 4-byte read of
address 0xBFFFF74C, which, as far as it can tell, is not a valid stack
address, nor corresponds to any currently malloc'd or free'd blocks.
The read is happening at line 45 of <code>bogon.cpp</code>, called
from line 66 of the same file, etc. For errors associated with an
identified malloc'd/free'd block, for example reading free'd memory,
Valgrind reports not only the location where the error happened, but
also where the associated block was malloc'd/free'd.
<p>Valgrind remembers all error reports. When an error is detected,
it is compared against old reports, to see if it is a duplicate. If
so, the error is noted, but no further commentary is emitted. This
avoids you being swamped with bazillions of duplicate error reports.
<p>If you want to know how many times each error occurred, run with
the <code>-v</code> option. When execution finishes, all the reports
are printed out, along with, and sorted by, their occurrence counts.
This makes it easy to see which errors have occurred most frequently.
<p>Errors are reported before the associated operation actually
happens. For example, if you program decides to read from address
zero, Valgrind will emit a message to this effect, and the program
will then duly die with a segmentation fault.
<p>In general, you should try and fix errors in the order that they
are reported. Not doing so can be confusing. For example, a program
which copies uninitialised values to several memory locations, and
later uses them, will generate several error messages. The first such
error message may well give the most direct clue to the root cause of
the problem.
<p>The process of detecting duplicate errors is quite an expensive
one and can become a significant performance overhead if your program
generates huge quantities of errors. To avoid serious problems here,
Valgrind will simply stop collecting errors after 300 different errors
have been seen, or 30000 errors in total have been seen. In this
situation you might as well stop your program and fix it, because
Valgrind won't tell you anything else useful after this. Note that
the 300/30000 limits apply after suppressed errors are removed. These
limits are defined in <code>vg_include.h</code> and can be increased
if necessary.
<a name="suppress"></a>
<h3>2.4&nbsp; Suppressing errors</h3>
Valgrind detects numerous problems in the base libraries, such as the
GNU C library, and the XFree86 client libraries, which come
pre-installed on your GNU/Linux system. You can't easily fix these,
but you don't want to see these errors (and yes, there are many!) So
Valgrind reads a list of errors to suppress at startup.
A default suppression file is cooked up by the
<code>./configure</code> script.
<p>You can modify and add to the suppressions file at your leisure,
or, better, write your own. Multiple suppression files are allowed.
This is useful if part of your project contains errors you can't or
don't want to fix, yet you don't want to continuously be reminded of
them.
<p>Each error to be suppressed is described very specifically, to
minimise the possibility that a suppression-directive inadvertantly
suppresses a bunch of similar errors which you did want to see. The
suppression mechanism is designed to allow precise yet flexible
specification of errors to suppress.
<p>If you use the <code>-v</code> flag, at the end of execution, Valgrind
prints out one line for each used suppression, giving its name and the
number of times it got used. Here's the suppressions used by a run of
<code>ls -l</code>:
<pre>
--27579-- supp: 1 socketcall.connect(serv_addr)/__libc_connect/__nscd_getgrgid_r
--27579-- supp: 1 socketcall.connect(serv_addr)/__libc_connect/__nscd_getpwuid_r
--27579-- supp: 6 strrchr/_dl_map_object_from_fd/_dl_map_object
</pre>
<a name="flags"></a>
<h3>2.5&nbsp; Command-line flags</h3>
You invoke Valgrind like this:
<pre>
valgrind [options-for-Valgrind] your-prog [options for your-prog]
</pre>
<p>Note that Valgrind also reads options from the environment variable
<code>$VALGRIND</code>, and processes them before the command-line
options.
<p>Valgrind's default settings succeed in giving reasonable behaviour
in most cases. Available options, in no particular order, are as
follows:
<ul>
<li><code>--help</code></li><br>
<li><code>--version</code><br>
<p>The usual deal.</li><br><p>
<li><code>-v --verbose</code><br>
<p>Be more verbose. Gives extra information on various aspects
of your program, such as: the shared objects loaded, the
suppressions used, the progress of the instrumentation engine,
and warnings about unusual behaviour.
</li><br><p>
<li><code>-q --quiet</code><br>
<p>Run silently, and only print error messages. Useful if you
are running regression tests or have some other automated test
machinery.
</li><br><p>
<li><code>--demangle=no</code><br>
<code>--demangle=yes</code> [the default]
<p>Disable/enable automatic demangling (decoding) of C++ names.
Enabled by default. When enabled, Valgrind will attempt to
translate encoded C++ procedure names back to something
approaching the original. The demangler handles symbols mangled
by g++ versions 2.X and 3.X.
<p>An important fact about demangling is that function
names mentioned in suppressions files should be in their mangled
form. Valgrind does not demangle function names when searching
for applicable suppressions, because to do otherwise would make
suppressions file contents dependent on the state of Valgrind's
demangling machinery, and would also be slow and pointless.
</li><br><p>
<li><code>--num-callers=&lt;number&gt;</code> [default=4]<br>
<p>By default, Valgrind shows four levels of function call names
to help you identify program locations. You can change that
number with this option. This can help in determining the
program's location in deeply-nested call chains. Note that errors
are commoned up using only the top three function locations (the
place in the current function, and that of its two immediate
callers). So this doesn't affect the total number of errors
reported.
<p>
The maximum value for this is 50. Note that higher settings
will make Valgrind run a bit more slowly and take a bit more
memory, but can be useful when working with programs with
deeply-nested call chains.
</li><br><p>
<li><code>--gdb-attach=no</code> [the default]<br>
<code>--gdb-attach=yes</code>
<p>When enabled, Valgrind will pause after every error shown,
and print the line
<br>
<code>---- Attach to GDB ? --- [Return/N/n/Y/y/C/c] ----</code>
<p>
Pressing <code>Ret</code>, or <code>N</code> <code>Ret</code>
or <code>n</code> <code>Ret</code>, causes Valgrind not to
start GDB for this error.
<p>
<code>Y</code> <code>Ret</code>
or <code>y</code> <code>Ret</code> causes Valgrind to
start GDB, for the program at this point. When you have
finished with GDB, quit from it, and the program will continue.
Trying to continue from inside GDB doesn't work.
<p>
<code>C</code> <code>Ret</code>
or <code>c</code> <code>Ret</code> causes Valgrind not to
start GDB, and not to ask again.
<p>
<code>--gdb-attach=yes</code> conflicts with
<code>--trace-children=yes</code>. You can't use them together.
Valgrind refuses to start up in this situation. 1 May 2002:
this is a historical relic which could be easily fixed if it
gets in your way. Mail me and complain if this is a problem for
you. </li><br><p>
<li><code>--partial-loads-ok=yes</code> [the default]<br>
<code>--partial-loads-ok=no</code>
<p>Controls how Valgrind handles word (4-byte) loads from
addresses for which some bytes are addressible and others
are not. When <code>yes</code> (the default), such loads
do not elicit an address error. Instead, the loaded V bytes
corresponding to the illegal addresses indicate undefined, and
those corresponding to legal addresses are loaded from shadow
memory, as usual.
<p>
When <code>no</code>, loads from partially
invalid addresses are treated the same as loads from completely
invalid addresses: an illegal-address error is issued,
and the resulting V bytes indicate valid data.
</li><br><p>
<li><code>--sloppy-malloc=no</code> [the default]<br>
<code>--sloppy-malloc=yes</code>
<p>When enabled, all requests for malloc/calloc are rounded up
to a whole number of machine words -- in other words, made
divisible by 4. For example, a request for 17 bytes of space
would result in a 20-byte area being made available. This works
around bugs in sloppy libraries which assume that they can
safely rely on malloc/calloc requests being rounded up in this
fashion. Without the workaround, these libraries tend to
generate large numbers of errors when they access the ends of
these areas.
<p>
Valgrind snapshots dated 17 Feb 2002 and later are
cleverer about this problem, and you should no longer need to
use this flag. To put it bluntly, if you do need to use this
flag, your program violates the ANSI C semantics defined for
<code>malloc</code> and <code>free</code>, even if it appears to
work correctly, and you should fix it, at least if you hope for
maximum portability.
</li><br><p>
<li><code>--trace-children=no</code> [the default]</br>
<code>--trace-children=yes</code>
<p>When enabled, Valgrind will trace into child processes. This
is confusing and usually not what you want, so is disabled by
default. As of 1 May 2002, tracing into a child process from a
parent which uses <code>libpthread.so</code> is probably broken
and is likely to cause breakage. Please report any such
problems to me. </li><br><p>
<li><code>--freelist-vol=&lt;number></code> [default: 1000000]
<p>When the client program releases memory using free (in C) or
delete (C++), that memory is not immediately made available for
re-allocation. Instead it is marked inaccessible and placed in
a queue of freed blocks. The purpose is to delay the point at
which freed-up memory comes back into circulation. This
increases the chance that Valgrind will be able to detect
invalid accesses to blocks for some significant period of time
after they have been freed.
<p>
This flag specifies the maximum total size, in bytes, of the
blocks in the queue. The default value is one million bytes.
Increasing this increases the total amount of memory used by
Valgrind but may detect invalid uses of freed blocks which would
otherwise go undetected.</li><br><p>
<li><code>--logfile-fd=&lt;number></code> [default: 2, stderr]
<p>Specifies the file descriptor on which Valgrind communicates
all of its messages. The default, 2, is the standard error
channel. This may interfere with the client's own use of
stderr. To dump Valgrind's commentary in a file without using
stderr, something like the following works well (sh/bash
syntax):<br>
<code>&nbsp;&nbsp;
valgrind --logfile-fd=9 my_prog 9> logfile</code><br>
That is: tell Valgrind to send all output to file descriptor 9,
and ask the shell to route file descriptor 9 to "logfile".
</li><br><p>
<li><code>--suppressions=&lt;filename></code>
[default: $PREFIX/lib/valgrind/default.supp]
<p>Specifies an extra
file from which to read descriptions of errors to suppress. You
may use as many extra suppressions files as you
like.</li><br><p>
<li><code>--leak-check=no</code> [default]<br>
<code>--leak-check=yes</code>
<p>When enabled, search for memory leaks when the client program
finishes. A memory leak means a malloc'd block, which has not
yet been free'd, but to which no pointer can be found. Such a
block can never be free'd by the program, since no pointer to it
exists. Leak checking is disabled by default because it tends
to generate dozens of error messages. </li><br><p>
<li><code>--show-reachable=no</code> [default]<br>
<code>--show-reachable=yes</code>
<p>When disabled, the memory leak detector only shows blocks for
which it cannot find a pointer to at all, or it can only find a
pointer to the middle of. These blocks are prime candidates for
memory leaks. When enabled, the leak detector also reports on
blocks which it could find a pointer to. Your program could, at
least in principle, have freed such blocks before exit.
Contrast this to blocks for which no pointer, or only an
interior pointer could be found: they are more likely to
indicate memory leaks, because you do not actually have a
pointer to the start of the block which you can hand to
<code>free</code>, even if you wanted to. </li><br><p>
<li><code>--leak-resolution=low</code> [default]<br>
<code>--leak-resolution=med</code> <br>
<code>--leak-resolution=high</code>
<p>When doing leak checking, determines how willing Valgrind is
to consider different backtraces to be the same. When set to
<code>low</code>, the default, only the first two entries need
match. When <code>med</code>, four entries have to match. When
<code>high</code>, all entries need to match.
<p>
For hardcore leak debugging, you probably want to use
<code>--leak-resolution=high</code> together with
<code>--num-callers=40</code> or some such large number. Note
however that this can give an overwhelming amount of
information, which is why the defaults are 4 callers and
low-resolution matching.
<p>
Note that the <code>--leak-resolution=</code> setting does not
affect Valgrind's ability to find leaks. It only changes how
the results are presented.
</li><br><p>
<li><code>--workaround-gcc296-bugs=no</code> [default]<br>
<code>--workaround-gcc296-bugs=yes</code> <p>When enabled,
assume that reads and writes some small distance below the stack
pointer <code>%esp</code> are due to bugs in gcc 2.96, and does
not report them. The "small distance" is 256 bytes by default.
Note that gcc 2.96 is the default compiler on some popular Linux
distributions (RedHat 7.X, Mandrake) and so you may well need to
use this flag. Do not use it if you do not have to, as it can
cause real errors to be overlooked. A better option is to use a
gcc/g++ which works properly; 2.95.3 seems to be a good choice.
<p>
Unfortunately (27 Feb 02) it looks like g++ 3.0.4 is similarly
buggy, so you may need to issue this flag if you use 3.0.4. A
while later (early Apr 02) this is confirmed as a scheduling bug
in g++-3.0.4.
</li><br><p>
<li><code>--cachesim=no</code> [default]<br>
<code>--cachesim=yes</code> <p>When enabled, turns off memory
checking, and turns on cache profiling. Cache profiling is
described in detail in <a href="#cache">Section 7</a>.
</li><br><p>
<li><code>--weird-hacks=hack1,hack2,...</code>
Pass miscellaneous hints to Valgrind which slightly modify the
simulated behaviour in nonstandard or dangerous ways, possibly
to help the simulation of strange features. By default no hacks
are enabled. Use with caution! Currently known hacks are:
<p>
<ul>
<li><code>ioctl-VTIME</code> Use this if you have a program
which sets readable file descriptors to have a timeout by
doing <code>ioctl</code> on them with a
<code>TCSETA</code>-style command <b>and</b> a non-zero
<code>VTIME</code> timeout value. This is considered
potentially dangerous and therefore is not engaged by
default, because it is (remotely) conceivable that it could
cause threads doing <code>read</code> to incorrectly block
the entire process.
<p>
You probably want to try this one if you have a program
which unexpectedly blocks in a <code>read</code> from a file
descriptor which you know to have been messed with by
<code>ioctl</code>. This could happen, for example, if the
descriptor is used to read input from some kind of screen
handling library.
<p>
To find out if your program is blocking unexpectedly in the
<code>read</code> system call, run with
<code>--trace-syscalls=yes</code> flag.
</ul>
</li><p>
</ul>
There are also some options for debugging Valgrind itself. You
shouldn't need to use them in the normal run of things. Nevertheless:
<ul>
<li><code>--single-step=no</code> [default]<br>
<code>--single-step=yes</code>
<p>When enabled, each x86 insn is translated seperately into
instrumented code. When disabled, translation is done on a
per-basic-block basis, giving much better translations.</li><br>
<p>
<li><code>--optimise=no</code><br>
<code>--optimise=yes</code> [default]
<p>When enabled, various improvements are applied to the
intermediate code, mainly aimed at allowing the simulated CPU's
registers to be cached in the real CPU's registers over several
simulated instructions.</li><br>
<p>
<li><code>--instrument=no</code><br>
<code>--instrument=yes</code> [default]
<p>When disabled, the translations don't actually contain any
instrumentation.</li><br>
<p>
<li><code>--cleanup=no</code><br>
<code>--cleanup=yes</code> [default]
<p>When enabled, various improvments are applied to the
post-instrumented intermediate code, aimed at removing redundant
value checks.</li><br>
<p>
<li><code>--trace-syscalls=no</code> [default]<br>
<code>--trace-syscalls=yes</code>
<p>Enable/disable tracing of system call intercepts.</li><br>
<p>
<li><code>--trace-signals=no</code> [default]<br>
<code>--trace-signals=yes</code>
<p>Enable/disable tracing of signal handling.</li><br>
<p>
<li><code>--trace-sched=no</code> [default]<br>
<code>--trace-sched=yes</code>
<p>Enable/disable tracing of thread scheduling events.</li><br>
<p>
<li><code>--trace-pthread=none</code> [default]<br>
<code>--trace-pthread=some</code> <br>
<code>--trace-pthread=all</code>
<p>Specifies amount of trace detail for pthread-related events.</li><br>
<p>
<li><code>--trace-symtab=no</code> [default]<br>
<code>--trace-symtab=yes</code>
<p>Enable/disable tracing of symbol table reading.</li><br>
<p>
<li><code>--trace-malloc=no</code> [default]<br>
<code>--trace-malloc=yes</code>
<p>Enable/disable tracing of malloc/free (et al) intercepts.
</li><br>
<p>
<li><code>--stop-after=&lt;number></code>
[default: infinity, more or less]
<p>After &lt;number> basic blocks have been executed, shut down
Valgrind and switch back to running the client on the real CPU.
</li><br>
<p>
<li><code>--dump-error=&lt;number></code> [default: inactive]
<p>After the program has exited, show gory details of the
translation of the basic block containing the &lt;number>'th
error context. When used with <code>--single-step=yes</code>,
can show the exact x86 instruction causing an error. This is
all fairly dodgy and doesn't work at all if threads are
involved.</li><br>
<p>
</ul>
<a name="errormsgs">
<h3>2.6&nbsp; Explaination of error messages</h3>
Despite considerable sophistication under the hood, Valgrind can only
really detect two kinds of errors, use of illegal addresses, and use
of undefined values. Nevertheless, this is enough to help you
discover all sorts of memory-management nasties in your code. This
section presents a quick summary of what error messages mean. The
precise behaviour of the error-checking machinery is described in
<a href="#machine">Section 4</a>.
<h4>2.6.1&nbsp; Illegal read / Illegal write errors</h4>
For example:
<pre>
Invalid read of size 4
at 0x40F6BBCC: (within /usr/lib/libpng.so.2.1.0.9)
by 0x40F6B804: (within /usr/lib/libpng.so.2.1.0.9)
by 0x40B07FF4: read_png_image__FP8QImageIO (kernel/qpngio.cpp:326)
by 0x40AC751B: QImageIO::read() (kernel/qimage.cpp:3621)
Address 0xBFFFF0E0 is not stack'd, malloc'd or free'd
</pre>
<p>This happens when your program reads or writes memory at a place
which Valgrind reckons it shouldn't. In this example, the program did
a 4-byte read at address 0xBFFFF0E0, somewhere within the
system-supplied library libpng.so.2.1.0.9, which was called from
somewhere else in the same library, called from line 326 of
qpngio.cpp, and so on.
<p>Valgrind tries to establish what the illegal address might relate
to, since that's often useful. So, if it points into a block of
memory which has already been freed, you'll be informed of this, and
also where the block was free'd at. Likewise, if it should turn out
to be just off the end of a malloc'd block, a common result of
off-by-one-errors in array subscripting, you'll be informed of this
fact, and also where the block was malloc'd.
<p>In this example, Valgrind can't identify the address. Actually the
address is on the stack, but, for some reason, this is not a valid
stack address -- it is below the stack pointer, %esp, and that isn't
allowed. In this particular case it's probably caused by gcc
generating invalid code, a known bug in various flavours of gcc.
<p>Note that Valgrind only tells you that your program is about to
access memory at an illegal address. It can't stop the access from
happening. So, if your program makes an access which normally would
result in a segmentation fault, you program will still suffer the same
fate -- but you will get a message from Valgrind immediately prior to
this. In this particular example, reading junk on the stack is
non-fatal, and the program stays alive.
<h4>2.6.2&nbsp; Use of uninitialised values</h4>
For example:
<pre>
Conditional jump or move depends on uninitialised value(s)
at 0x402DFA94: _IO_vfprintf (_itoa.h:49)
by 0x402E8476: _IO_printf (printf.c:36)
by 0x8048472: main (tests/manuel1.c:8)
by 0x402A6E5E: __libc_start_main (libc-start.c:129)
</pre>
<p>An uninitialised-value use error is reported when your program uses
a value which hasn't been initialised -- in other words, is undefined.
Here, the undefined value is used somewhere inside the printf()
machinery of the C library. This error was reported when running the
following small program:
<pre>
int main()
{
int x;
printf ("x = %d\n", x);
}
</pre>
<p>It is important to understand that your program can copy around
junk (uninitialised) data to its heart's content. Valgrind observes
this and keeps track of the data, but does not complain. A complaint
is issued only when your program attempts to make use of uninitialised
data. In this example, x is uninitialised. Valgrind observes the
value being passed to _IO_printf and thence to _IO_vfprintf, but makes
no comment. However, _IO_vfprintf has to examine the value of x so it
can turn it into the corresponding ASCII string, and it is at this
point that Valgrind complains.
<p>Sources of uninitialised data tend to be:
<ul>
<li>Local variables in procedures which have not been initialised,
as in the example above.</li><br><p>
<li>The contents of malloc'd blocks, before you write something
there. In C++, the new operator is a wrapper round malloc, so
if you create an object with new, its fields will be
uninitialised until you fill them in, which is only Right and
Proper.</li>
</ul>
<h4>2.6.3&nbsp; Illegal frees</h4>
For example:
<pre>
Invalid free()
at 0x4004FFDF: free (ut_clientmalloc.c:577)
by 0x80484C7: main (tests/doublefree.c:10)
by 0x402A6E5E: __libc_start_main (libc-start.c:129)
by 0x80483B1: (within tests/doublefree)
Address 0x3807F7B4 is 0 bytes inside a block of size 177 free'd
at 0x4004FFDF: free (ut_clientmalloc.c:577)
by 0x80484C7: main (tests/doublefree.c:10)
by 0x402A6E5E: __libc_start_main (libc-start.c:129)
by 0x80483B1: (within tests/doublefree)
</pre>
<p>Valgrind keeps track of the blocks allocated by your program with
malloc/new, so it can know exactly whether or not the argument to
free/delete is legitimate or not. Here, this test program has
freed the same block twice. As with the illegal read/write errors,
Valgrind attempts to make sense of the address free'd. If, as
here, the address is one which has previously been freed, you wil
be told that -- making duplicate frees of the same block easy to spot.
<h4>2.6.4&nbsp; When a block is freed with an inappropriate
deallocation function</h4>
In the following example, a block allocated with <code>new []</code>
has wrongly been deallocated with <code>free</code>:
<pre>
Mismatched free() / delete / delete []
at 0x40043249: free (vg_clientfuncs.c:171)
by 0x4102BB4E: QGArray::~QGArray(void) (tools/qgarray.cpp:149)
by 0x4C261C41: PptDoc::~PptDoc(void) (include/qmemarray.h:60)
by 0x4C261F0E: PptXml::~PptXml(void) (pptxml.cc:44)
Address 0x4BB292A8 is 0 bytes inside a block of size 64 alloc'd
at 0x4004318C: __builtin_vec_new (vg_clientfuncs.c:152)
by 0x4C21BC15: KLaola::readSBStream(int) const (klaola.cc:314)
by 0x4C21C155: KLaola::stream(KLaola::OLENode const *) (klaola.cc:416)
by 0x4C21788F: OLEFilter::convert(QCString const &) (olefilter.cc:272)
</pre>
The following was told to me be the KDE 3 developers. I didn't know
any of it myself. They also implemented the check itself.
<p>
In C++ it's important to deallocate memory in a way compatible with
how it was allocated. The deal is:
<ul>
<li>If allocated with <code>malloc</code>, <code>calloc</code>,
<code>realloc</code>, <code>valloc</code> or
<code>memalign</code>, you must deallocate with <code>free</code>.
<li>If allocated with <code>new []</code>, you must deallocate with
<code>delete []</code>.
<li>If allocated with <code>new</code>, you must deallocate with
<code>delete</code>.
</ul>
The worst thing is that on Linux apparently it doesn't matter if you
do muddle these up, and it all seems to work ok, but the same program
may then crash on a different platform, Solaris for example. So it's
best to fix it properly. According to the KDE folks "it's amazing how
many C++ programmers don't know this".
<h4>2.6.5&nbsp; Passing system call parameters with inadequate
read/write permissions</h4>
Valgrind checks all parameters to system calls. If a system call
needs to read from a buffer provided by your program, Valgrind checks
that the entire buffer is addressible and has valid data, ie, it is
readable. And if the system call needs to write to a user-supplied
buffer, Valgrind checks that the buffer is addressible. After the
system call, Valgrind updates its administrative information to
precisely reflect any changes in memory permissions caused by the
system call.
<p>Here's an example of a system call with an invalid parameter:
<pre>
#include &lt;stdlib.h>
#include &lt;unistd.h>
int main( void )
{
char* arr = malloc(10);
(void) write( 1 /* stdout */, arr, 10 );
return 0;
}
</pre>
<p>You get this complaint ...
<pre>
Syscall param write(buf) contains uninitialised or unaddressable byte(s)
at 0x4035E072: __libc_write
by 0x402A6E5E: __libc_start_main (libc-start.c:129)
by 0x80483B1: (within tests/badwrite)
by &lt;bogus frame pointer> ???
Address 0x3807E6D0 is 0 bytes inside a block of size 10 alloc'd
at 0x4004FEE6: malloc (ut_clientmalloc.c:539)
by 0x80484A0: main (tests/badwrite.c:6)
by 0x402A6E5E: __libc_start_main (libc-start.c:129)
by 0x80483B1: (within tests/badwrite)
</pre>
<p>... because the program has tried to write uninitialised junk from
the malloc'd block to the standard output.
<h4>2.6.6&nbsp; Warning messages you might see</h4>
Most of these only appear if you run in verbose mode (enabled by
<code>-v</code>):
<ul>
<li> <code>More than 50 errors detected. Subsequent errors
will still be recorded, but in less detail than before.</code>
<br>
After 50 different errors have been shown, Valgrind becomes
more conservative about collecting them. It then requires only
the program counters in the top two stack frames to match when
deciding whether or not two errors are really the same one.
Prior to this point, the PCs in the top four frames are required
to match. This hack has the effect of slowing down the
appearance of new errors after the first 50. The 50 constant can
be changed by recompiling Valgrind.
<p>
<li> <code>More than 300 errors detected. I'm not reporting any more.
Final error counts may be inaccurate. Go fix your
program!</code>
<br>
After 300 different errors have been detected, Valgrind ignores
any more. It seems unlikely that collecting even more different
ones would be of practical help to anybody, and it avoids the
danger that Valgrind spends more and more of its time comparing
new errors against an ever-growing collection. As above, the 500
number is a compile-time constant.
<p>
<li> <code>Warning: client exiting by calling exit(&lt;number>).
Bye!</code>
<br>
Your program has called the <code>exit</code> system call, which
will immediately terminate the process. You'll get no exit-time
error summaries or leak checks. Note that this is not the same
as your program calling the ANSI C function <code>exit()</code>
-- that causes a normal, controlled shutdown of Valgrind.
<p>
<li> <code>Warning: client switching stacks?</code>
<br>
Valgrind spotted such a large change in the stack pointer, %esp,
that it guesses the client is switching to a different stack.
At this point it makes a kludgey guess where the base of the new
stack is, and sets memory permissions accordingly. You may get
many bogus error messages following this, if Valgrind guesses
wrong. At the moment "large change" is defined as a change of
more that 2000000 in the value of the %esp (stack pointer)
register.
<p>
<li> <code>Warning: client attempted to close Valgrind's logfile fd &lt;number>
</code>
<br>
Valgrind doesn't allow the client
to close the logfile, because you'd never see any diagnostic
information after that point. If you see this message,
you may want to use the <code>--logfile-fd=&lt;number></code>
option to specify a different logfile file-descriptor number.
<p>
<li> <code>Warning: noted but unhandled ioctl &lt;number></code>
<br>
Valgrind observed a call to one of the vast family of
<code>ioctl</code> system calls, but did not modify its
memory status info (because I have not yet got round to it).
The call will still have gone through, but you may get spurious
errors after this as a result of the non-update of the memory info.
<p>
<li> <code>Warning: unblocking signal &lt;number> due to
sigprocmask</code>
<br>
Really just a diagnostic from the signal simulation machinery.
This message will appear if your program handles a signal by
first <code>longjmp</code>ing out of the signal handler,
and then unblocking the signal with <code>sigprocmask</code>
-- a standard signal-handling idiom.
<p>
<li> <code>Warning: bad signal number &lt;number> in __NR_sigaction.</code>
<br>
Probably indicates a bug in the signal simulation machinery.
<p>
<li> <code>Warning: set address range perms: large range &lt;number></code>
<br>
Diagnostic message, mostly for my benefit, to do with memory
permissions.
</ul>
<a name="suppfiles"></a>
<h3>2.7&nbsp; Writing suppressions files</h3>
A suppression file describes a bunch of errors which, for one reason
or another, you don't want Valgrind to tell you about. Usually the
reason is that the system libraries are buggy but unfixable, at least
within the scope of the current debugging session. Multiple
suppresions files are allowed. By default, Valgrind uses
<code>$PREFIX/lib/valgrind/default.supp</code>.
<p>
You can ask to add suppressions from another file, by specifying
<code>--suppressions=/path/to/file.supp</code>.
<p>Each suppression has the following components:<br>
<ul>
<li>Its name. This merely gives a handy name to the suppression, by
which it is referred to in the summary of used suppressions
printed out when a program finishes. It's not important what
the name is; any identifying string will do.
<p>
<li>The nature of the error to suppress. Either:
<code>Value1</code>,
<code>Value2</code>,
<code>Value4</code> or
<code>Value8</code>,
meaning an uninitialised-value error when
using a value of 1, 2, 4 or 8 bytes.
Or
<code>Cond</code> (or its old name, <code>Value0</code>),
meaning use of an uninitialised CPU condition code. Or:
<code>Addr1</code>,
<code>Addr2</code>,
<code>Addr4</code> or
<code>Addr8</code>, meaning an invalid address during a
memory access of 1, 2, 4 or 8 bytes respectively. Or
<code>Param</code>,
meaning an invalid system call parameter error. Or
<code>Free</code>, meaning an invalid or mismatching free.</li><br>
<p>
<li>The "immediate location" specification. For Value and Addr
errors, is either the name of the function in which the error
occurred, or, failing that, the full path the the .so file
containing the error location. For Param errors, is the name of
the offending system call parameter. For Free errors, is the
name of the function doing the freeing (eg, <code>free</code>,
<code>__builtin_vec_delete</code>, etc)</li><br>
<p>
<li>The caller of the above "immediate location". Again, either a
function or shared-object name.</li><br>
<p>
<li>Optionally, one or two extra calling-function or object names,
for greater precision.</li>
</ul>
<p>
Locations may be either names of shared objects or wildcards matching
function names. They begin <code>obj:</code> and <code>fun:</code>
respectively. Function and object names to match against may use the
wildcard characters <code>*</code> and <code>?</code>.
A suppression only suppresses an error when the error matches all the
details in the suppression. Here's an example:
<pre>
{
__gconv_transform_ascii_internal/__mbrtowc/mbtowc
Value4
fun:__gconv_transform_ascii_internal
fun:__mbr*toc
fun:mbtowc
}
</pre>
<p>What is means is: suppress a use-of-uninitialised-value error, when
the data size is 4, when it occurs in the function
<code>__gconv_transform_ascii_internal</code>, when that is called
from any function of name matching <code>__mbr*toc</code>,
when that is called from
<code>mbtowc</code>. It doesn't apply under any other circumstances.
The string by which this suppression is identified to the user is
__gconv_transform_ascii_internal/__mbrtowc/mbtowc.
<p>Another example:
<pre>
{
libX11.so.6.2/libX11.so.6.2/libXaw.so.7.0
Value4
obj:/usr/X11R6/lib/libX11.so.6.2
obj:/usr/X11R6/lib/libX11.so.6.2
obj:/usr/X11R6/lib/libXaw.so.7.0
}
</pre>
<p>Suppress any size 4 uninitialised-value error which occurs anywhere
in <code>libX11.so.6.2</code>, when called from anywhere in the same
library, when called from anywhere in <code>libXaw.so.7.0</code>. The
inexact specification of locations is regrettable, but is about all
you can hope for, given that the X11 libraries shipped with Red Hat
7.2 have had their symbol tables removed.
<p>Note -- since the above two examples did not make it clear -- that
you can freely mix the <code>obj:</code> and <code>fun:</code>
styles of description within a single suppression record.
<a name="clientreq"></a>
<h3>2.8&nbsp; The Client Request mechanism</h3>
Valgrind has a trapdoor mechanism via which the client program can
pass all manner of requests and queries to Valgrind. Internally, this
is used extensively to make malloc, free, signals, threads, etc, work,
although you don't see that.
<p>
For your convenience, a subset of these so-called client requests is
provided to allow you to tell Valgrind facts about the behaviour of
your program, and conversely to make queries. In particular, your
program can tell Valgrind about changes in memory range permissions
that Valgrind would not otherwise know about, and so allows clients to
get Valgrind to do arbitrary custom checks.
<p>
Clients need to include the header file <code>valgrind.h</code> to
make this work. The macros therein have the magical property that
they generate code in-line which Valgrind can spot. However, the code
does nothing when not run on Valgrind, so you are not forced to run
your program on Valgrind just because you use the macros in this file.
Also, you are not required to link your program with any extra
supporting libraries.
<p>
A brief description of the available macros:
<ul>
<li><code>VALGRIND_MAKE_NOACCESS</code>,
<code>VALGRIND_MAKE_WRITABLE</code> and
<code>VALGRIND_MAKE_READABLE</code>. These mark address
ranges as completely inaccessible, accessible but containing
undefined data, and accessible and containing defined data,
respectively. Subsequent errors may have their faulting
addresses described in terms of these blocks. Returns a
"block handle". Returns zero when not run on Valgrind.
<p>
<li><code>VALGRIND_DISCARD</code>: At some point you may want
Valgrind to stop reporting errors in terms of the blocks
defined by the previous three macros. To do this, the above
macros return a small-integer "block handle". You can pass
this block handle to <code>VALGRIND_DISCARD</code>. After
doing so, Valgrind will no longer be able to relate
addressing errors to the user-defined block associated with
the handle. The permissions settings associated with the
handle remain in place; this just affects how errors are
reported, not whether they are reported. Returns 1 for an
invalid handle and 0 for a valid handle (although passing
invalid handles is harmless). Always returns 0 when not run
on Valgrind.
<p>
<li><code>VALGRIND_CHECK_NOACCESS</code>,
<code>VALGRIND_CHECK_WRITABLE</code> and
<code>VALGRIND_CHECK_READABLE</code>: check immediately
whether or not the given address range has the relevant
property, and if not, print an error message. Also, for the
convenience of the client, returns zero if the relevant
property holds; otherwise, the returned value is the address
of the first byte for which the property is not true.
Always returns 0 when not run on Valgrind.
<p>
<li><code>VALGRIND_CHECK_NOACCESS</code>: a quick and easy way
to find out whether Valgrind thinks a particular variable
(lvalue, to be precise) is addressible and defined. Prints
an error message if not. Returns no value.
<p>
<li><code>VALGRIND_MAKE_NOACCESS_STACK</code>: a highly
experimental feature. Similarly to
<code>VALGRIND_MAKE_NOACCESS</code>, this marks an address
range as inaccessible, so that subsequent accesses to an
address in the range gives an error. However, this macro
does not return a block handle. Instead, all annotations
created like this are reviewed at each client
<code>ret</code> (subroutine return) instruction, and those
which now define an address range block the client's stack
pointer register (<code>%esp</code>) are automatically
deleted.
<p>
In other words, this macro allows the client to tell
Valgrind about red-zones on its own stack. Valgrind
automatically discards this information when the stack
retreats past such blocks. Beware: hacky and flaky, and
probably interacts badly with the new pthread support.
<p>
<li><code>RUNNING_ON_VALGRIND</code>: returns 1 if running on
Valgrind, 0 if running on the real CPU.
<p>
<li><code>VALGRIND_DO_LEAK_CHECK</code>: run the memory leak detector
right now. Returns no value. I guess this could be used to
incrementally check for leaks between arbitrary places in the
program's execution. Warning: not properly tested!
<p>
<li><code>VALGRIND_DISCARD_TRANSLATIONS</code>: discard translations
of code in the specified address range. Useful if you are
debugging a JITter or some other dynamic code generation system.
After this call, attempts to execute code in the invalidated
address range will cause valgrind to make new translations of that
code, which is probably the semantics you want. Note that this is
implemented naively, and involves checking all 200191 entries in
the translation table to see if any of them overlap the specified
address range. So try not to call it often, or performance will
nosedive. Note that you can be clever about this: you only need
to call it when an area which previously contained code is
overwritten with new code. You can choose to write code into
fresh memory, and just call this occasionally to discard large
chunks of old code all at once.
<p>
Warning: minimally tested. Also, doesn't interact well with the
cache simulator.
</ul>
<p>
<a name="pthreads"></a>
<h3>2.9&nbsp; Support for POSIX Pthreads</h3>
As of late April 02, Valgrind supports programs which use POSIX
pthreads. Doing this has proved technically challenging and is still
in progress, but it works well enough, as of 1 May 02, for significant
threaded applications to work.
<p>
It works as follows: threaded apps are (dynamically) linked against
<code>libpthread.so</code>. Usually this is the one installed with
your Linux distribution. Valgrind, however, supplies its own
<code>libpthread.so</code> and automatically connects your program to
it instead.
<p>
The fake <code>libpthread.so</code> and Valgrind cooperate to
implement a user-space pthreads package. This approach avoids the
horrible implementation problems of implementing a truly
multiprocessor version of Valgrind, but it does mean that threaded
apps run only on one CPU, even if you have a multiprocessor machine.
<p>
Valgrind schedules your threads in a round-robin fashion, with all
threads having equal priority. It switches threads every 20000 basic
blocks (typically around 120000 x86 instructions), which means you'll
get a much finer interleaving of thread executions than when run
natively. This in itself may cause your program to behave differently
if you have some kind of concurrency, critical race, locking, or
similar, bugs.
<p>
The current (1 May 02) state of pthread support is as follows. Please
note that things are advancing rapidly, so the situation may have
improved by the time you read this -- check the web site for further
updates.
<ul>
<li>Mutexes, condition variables, thread-specific data and
<code>pthread_once</code> currently work.
<p>
<li>Various attribute-like calls are handled but ignored.
You get a warning message.
<p>
<li>The main big omission is proper cleanup support for cancellation.
<code>pthread_cancel</code> works, but instantly nukes the target
thread without giving it any chance to clean up. Also, when a
thread exits, it does not run any cleanup handlers.
<p>
<li>Currently the following syscalls are thread-safe (nonblocking):
<code>write</code> <code>read</code> <code>nanosleep</code>
<code>sleep</code> <code>select</code> and <code>poll</code>.
<p>
<li>The POSIX requirement that each thread have its own
signal-blocking mask is not done; the signal handling mechanism is
thread-unaware and all signals are delivered to the main thread,
antidisirregardless.
</ul>
As of 1 May 02, the following programs now work fine on my RedHat 7.2
box: Opera 6.0Beta2, KNode in KDE 3.0, Mozilla-0.9.2.1 and
Galeon-0.11.3, both as supplied with RedHat 7.2.
<p>
Mozilla 1.0RC1 works fine too, provided that you patch it as described
here: <a href="http://bugzilla.mozilla.org/show_bug.cgi?id=124335">
http://bugzilla.mozilla.org/show_bug.cgi?id=124335</a>. This fixes a
bug in Mozilla which assumes that memory returned from
<code>malloc</code> is 8-aligned. Valgrind's allocator only
guarantees 4-alignment, so without the patch Mozilla makes an illegal
memory access, which Valgrind of course spots, and then bombs.
Mozilla 1.0RC2 works fine out-of-the-box.
<a name="install"></a>
<h3>2.10&nbsp; Building and installing</h3>
We now use the standard Unix <code>./configure</code>,
<code>make</code>, <code>make install</code> mechanism, and I have
attempted to ensure that it works on machines with kernel 2.2 or 2.4
and glibc 2.1.X or 2.2.X. I don't think there is much else to say.
There are no options apart from the usual <code>--prefix</code> that
you should give to <code>./configure</code>.
<p>
Let me know if you have build problems.
<a name="problems"></a>
<h3>2.11&nbsp; If you have problems</h3>
Mail me (<a href="mailto:jseward@acm.org">jseward@acm.org</a>).
<p>See <a href="#limits">Section 4</a> for the known limitations of
Valgrind, and for a list of programs which are known not to work on
it.
<p>The translator/instrumentor has a lot of assertions in it. They
are permanently enabled, and I have no plans to disable them. If one
of these breaks, please mail me!
<p>If you get an assertion failure on the expression
<code>chunkSane(ch)</code> in <code>vg_free()</code> in
<code>vg_malloc.c</code>, this may have happened because your program
wrote off the end of a malloc'd block, or before its beginning.
Valgrind should have emitted a proper message to that effect before
dying in this way. This is a known problem which I should fix.
<p>
<hr width="100%">
<a name="machine"></a>
<h2>3&nbsp; Details of the checking machinery</h2>
Read this section if you want to know, in detail, exactly what and how
Valgrind is checking.
<a name="vvalue"></a>
<h3>3.1&nbsp; Valid-value (V) bits</h3>
It is simplest to think of Valgrind implementing a synthetic Intel x86
CPU which is identical to a real CPU, except for one crucial detail.
Every bit (literally) of data processed, stored and handled by the
real CPU has, in the synthetic CPU, an associated "valid-value" bit,
which says whether or not the accompanying bit has a legitimate value.
In the discussions which follow, this bit is referred to as the V
(valid-value) bit.
<p>Each byte in the system therefore has a 8 V bits which follow
it wherever it goes. For example, when the CPU loads a word-size item
(4 bytes) from memory, it also loads the corresponding 32 V bits from
a bitmap which stores the V bits for the process' entire address
space. If the CPU should later write the whole or some part of that
value to memory at a different address, the relevant V bits will be
stored back in the V-bit bitmap.
<p>In short, each bit in the system has an associated V bit, which
follows it around everywhere, even inside the CPU. Yes, the CPU's
(integer and <code>%eflags</code>) registers have their own V bit
vectors.
<p>Copying values around does not cause Valgrind to check for, or
report on, errors. However, when a value is used in a way which might
conceivably affect the outcome of your program's computation, the
associated V bits are immediately checked. If any of these indicate
that the value is undefined, an error is reported.
<p>Here's an (admittedly nonsensical) example:
<pre>
int i, j;
int a[10], b[10];
for (i = 0; i &lt; 10; i++) {
j = a[i];
b[i] = j;
}
</pre>
<p>Valgrind emits no complaints about this, since it merely copies
uninitialised values from <code>a[]</code> into <code>b[]</code>, and
doesn't use them in any way. However, if the loop is changed to
<pre>
for (i = 0; i &lt; 10; i++) {
j += a[i];
}
if (j == 77)
printf("hello there\n");
</pre>
then Valgrind will complain, at the <code>if</code>, that the
condition depends on uninitialised values.
<p>Most low level operations, such as adds, cause Valgrind to
use the V bits for the operands to calculate the V bits for the
result. Even if the result is partially or wholly undefined,
it does not complain.
<p>Checks on definedness only occur in two places: when a value is
used to generate a memory address, and where control flow decision
needs to be made. Also, when a system call is detected, valgrind
checks definedness of parameters as required.
<p>If a check should detect undefinedness, an error message is
issued. The resulting value is subsequently regarded as well-defined.
To do otherwise would give long chains of error messages. In effect,
we say that undefined values are non-infectious.
<p>This sounds overcomplicated. Why not just check all reads from
memory, and complain if an undefined value is loaded into a CPU register?
Well, that doesn't work well, because perfectly legitimate C programs routinely
copy uninitialised values around in memory, and we don't want endless complaints
about that. Here's the canonical example. Consider a struct
like this:
<pre>
struct S { int x; char c; };
struct S s1, s2;
s1.x = 42;
s1.c = 'z';
s2 = s1;
</pre>
<p>The question to ask is: how large is <code>struct S</code>, in
bytes? An int is 4 bytes and a char one byte, so perhaps a struct S
occupies 5 bytes? Wrong. All (non-toy) compilers I know of will
round the size of <code>struct S</code> up to a whole number of words,
in this case 8 bytes. Not doing this forces compilers to generate
truly appalling code for subscripting arrays of <code>struct
S</code>'s.
<p>So s1 occupies 8 bytes, yet only 5 of them will be initialised.
For the assignment <code>s2 = s1</code>, gcc generates code to copy
all 8 bytes wholesale into <code>s2</code> without regard for their
meaning. If Valgrind simply checked values as they came out of
memory, it would yelp every time a structure assignment like this
happened. So the more complicated semantics described above is
necessary. This allows gcc to copy <code>s1</code> into
<code>s2</code> any way it likes, and a warning will only be emitted
if the uninitialised values are later used.
<p>One final twist to this story. The above scheme allows garbage to
pass through the CPU's integer registers without complaint. It does
this by giving the integer registers V tags, passing these around in
the expected way. This complicated and computationally expensive to
do, but is necessary. Valgrind is more simplistic about
floating-point loads and stores. In particular, V bits for data read
as a result of floating-point loads are checked at the load
instruction. So if your program uses the floating-point registers to
do memory-to-memory copies, you will get complaints about
uninitialised values. Fortunately, I have not yet encountered a
program which (ab)uses the floating-point registers in this way.
<a name="vaddress"></a>
<h3>3.2&nbsp; Valid-address (A) bits</h3>
Notice that the previous section describes how the validity of values
is established and maintained without having to say whether the
program does or does not have the right to access any particular
memory location. We now consider the latter issue.
<p>As described above, every bit in memory or in the CPU has an
associated valid-value (V) bit. In addition, all bytes in memory, but
not in the CPU, have an associated valid-address (A) bit. This
indicates whether or not the program can legitimately read or write
that location. It does not give any indication of the validity or the
data at that location -- that's the job of the V bits -- only whether
or not the location may be accessed.
<p>Every time your program reads or writes memory, Valgrind checks the
A bits associated with the address. If any of them indicate an
invalid address, an error is emitted. Note that the reads and writes
themselves do not change the A bits, only consult them.
<p>So how do the A bits get set/cleared? Like this:
<ul>
<li>When the program starts, all the global data areas are marked as
accessible.</li><br>
<p>
<li>When the program does malloc/new, the A bits for the exactly the
area allocated, and not a byte more, are marked as accessible.
Upon freeing the area the A bits are changed to indicate
inaccessibility.</li><br>
<p>
<li>When the stack pointer register (%esp) moves up or down, A bits
are set. The rule is that the area from %esp up to the base of
the stack is marked as accessible, and below %esp is
inaccessible. (If that sounds illogical, bear in mind that the
stack grows down, not up, on almost all Unix systems, including
GNU/Linux.) Tracking %esp like this has the useful side-effect
that the section of stack used by a function for local variables
etc is automatically marked accessible on function entry and
inaccessible on exit.</li><br>
<p>
<li>When doing system calls, A bits are changed appropriately. For
example, mmap() magically makes files appear in the process's
address space, so the A bits must be updated if mmap()
succeeds.</li><br>
<p>
<li>Optionally, your program can tell Valgrind about such changes
explicitly, using the client request mechanism described above.
</ul>
<a name="together"></a>
<h3>3.3&nbsp; Putting it all together</h3>
Valgrind's checking machinery can be summarised as follows:
<ul>
<li>Each byte in memory has 8 associated V (valid-value) bits,
saying whether or not the byte has a defined value, and a single
A (valid-address) bit, saying whether or not the program
currently has the right to read/write that address.</li><br>
<p>
<li>When memory is read or written, the relevant A bits are
consulted. If they indicate an invalid address, Valgrind emits
an Invalid read or Invalid write error.</li><br>
<p>
<li>When memory is read into the CPU's integer registers, the
relevant V bits are fetched from memory and stored in the
simulated CPU. They are not consulted.</li><br>
<p>
<li>When an integer register is written out to memory, the V bits
for that register are written back to memory too.</li><br>
<p>
<li>When memory is read into the CPU's floating point registers, the
relevant V bits are read from memory and they are immediately
checked. If any are invalid, an uninitialised value error is
emitted. This precludes using the floating-point registers to
copy possibly-uninitialised memory, but simplifies Valgrind in
that it does not have to track the validity status of the
floating-point registers.</li><br>
<p>
<li>As a result, when a floating-point register is written to
memory, the associated V bits are set to indicate a valid
value.</li><br>
<p>
<li>When values in integer CPU registers are used to generate a
memory address, or to determine the outcome of a conditional
branch, the V bits for those values are checked, and an error
emitted if any of them are undefined.</li><br>
<p>
<li>When values in integer CPU registers are used for any other
purpose, Valgrind computes the V bits for the result, but does
not check them.</li><br>
<p>
<li>One the V bits for a value in the CPU have been checked, they
are then set to indicate validity. This avoids long chains of
errors.</li><br>
<p>
<li>When values are loaded from memory, valgrind checks the A bits
for that location and issues an illegal-address warning if
needed. In that case, the V bits loaded are forced to indicate
Valid, despite the location being invalid.
<p>
This apparently strange choice reduces the amount of confusing
information presented to the user. It avoids the
unpleasant phenomenon in which memory is read from a place which
is both unaddressible and contains invalid values, and, as a
result, you get not only an invalid-address (read/write) error,
but also a potentially large set of uninitialised-value errors,
one for every time the value is used.
<p>
There is a hazy boundary case to do with multi-byte loads from
addresses which are partially valid and partially invalid. See
details of the flag <code>--partial-loads-ok</code> for details.
</li><br>
</ul>
Valgrind intercepts calls to malloc, calloc, realloc, valloc,
memalign, free, new and delete. The behaviour you get is:
<ul>
<li>malloc/new: the returned memory is marked as addressible but not
having valid values. This means you have to write on it before
you can read it.</li><br>
<p>
<li>calloc: returned memory is marked both addressible and valid,
since calloc() clears the area to zero.</li><br>
<p>
<li>realloc: if the new size is larger than the old, the new section
is addressible but invalid, as with malloc.</li><br>
<p>
<li>If the new size is smaller, the dropped-off section is marked as
unaddressible. You may only pass to realloc a pointer
previously issued to you by malloc/calloc/new/realloc.</li><br>
<p>
<li>free/delete: you may only pass to free a pointer previously
issued to you by malloc/calloc/new/realloc, or the value
NULL. Otherwise, Valgrind complains. If the pointer is indeed
valid, Valgrind marks the entire area it points at as
unaddressible, and places the block in the freed-blocks-queue.
The aim is to defer as long as possible reallocation of this
block. Until that happens, all attempts to access it will
elicit an invalid-address error, as you would hope.</li><br>
</ul>
<a name="signals"></a>
<h3>3.4&nbsp; Signals</h3>
Valgrind provides suitable handling of signals, so, provided you stick
to POSIX stuff, you should be ok. Basic sigaction() and sigprocmask()
are handled. Signal handlers may return in the normal way or do
longjmp(); both should work ok. As specified by POSIX, a signal is
blocked in its own handler. Default actions for signals should work
as before. Etc, etc.
<p>Under the hood, dealing with signals is a real pain, and Valgrind's
simulation leaves much to be desired. If your program does
way-strange stuff with signals, bad things may happen. If so, let me
know. I don't promise to fix it, but I'd at least like to be aware of
it.
<a name="leaks"><a/>
<h3>3.5&nbsp; Memory leak detection</h3>
Valgrind keeps track of all memory blocks issued in response to calls
to malloc/calloc/realloc/new. So when the program exits, it knows
which blocks are still outstanding -- have not been returned, in other
words. Ideally, you want your program to have no blocks still in use
at exit. But many programs do.
<p>For each such block, Valgrind scans the entire address space of the
process, looking for pointers to the block. One of three situations
may result:
<ul>
<li>A pointer to the start of the block is found. This usually
indicates programming sloppiness; since the block is still
pointed at, the programmer could, at least in principle, free'd
it before program exit.</li><br>
<p>
<li>A pointer to the interior of the block is found. The pointer
might originally have pointed to the start and have been moved
along, or it might be entirely unrelated. Valgrind deems such a
block as "dubious", that is, possibly leaked,
because it's unclear whether or
not a pointer to it still exists.</li><br>
<p>
<li>The worst outcome is that no pointer to the block can be found.
The block is classified as "leaked", because the
programmer could not possibly have free'd it at program exit,
since no pointer to it exists. This might be a symptom of
having lost the pointer at some earlier point in the
program.</li>
</ul>
Valgrind reports summaries about leaked and dubious blocks.
For each such block, it will also tell you where the block was
allocated. This should help you figure out why the pointer to it has
been lost. In general, you should attempt to ensure your programs do
not have any leaked or dubious blocks at exit.
<p>The precise area of memory in which Valgrind searches for pointers
is: all naturally-aligned 4-byte words for which all A bits indicate
addressibility and all V bits indicated that the stored value is
actually valid.
<p><hr width="100%">
<a name="limits"></a>
<h2>4&nbsp; Limitations</h2>
The following list of limitations seems depressingly long. However,
most programs actually work fine.
<p>Valgrind will run x86-GNU/Linux ELF dynamically linked binaries, on
a kernel 2.2.X or 2.4.X system, subject to the following constraints:
<ul>
<li>No MMX, SSE, SSE2, 3DNow instructions. If the translator
encounters these, Valgrind will simply give up. It may be
possible to add support for them at a later time. Intel added a
few instructions such as "cmov" to the integer instruction set
on Pentium and later processors, and these are supported.
Nevertheless it's safest to think of Valgrind as implementing
the 486 instruction set.</li><br>
<p>
<li>Pthreads support is improving, but there are still significant
limitations in that department. See the section above on
Pthreads. Note that your program must be dynamically linked
against <code>libpthread.so</code>, so that Valgrind can
substitute its own implementation at program startup time. If
you're statically linked against it, things will fail
badly.</li><br>
<p>
<li>Valgrind assumes that the floating point registers are not used
as intermediaries in memory-to-memory copies, so it immediately
checks V bits in floating-point loads/stores. If you want to
write code which copies around possibly-uninitialised values,
you must ensure these travel through the integer registers, not
the FPU.</li><br>
<p>
<li>If your program does its own memory management, rather than
using malloc/new/free/delete, it should still work, but
Valgrind's error checking won't be so effective.</li><br>
<p>
<li>Valgrind's signal simulation is not as robust as it could be.
Basic POSIX-compliant sigaction and sigprocmask functionality is
supplied, but it's conceivable that things could go badly awry
if you do wierd things with signals. Workaround: don't.
Programs that do non-POSIX signal tricks are in any case
inherently unportable, so should be avoided if
possible.</li><br>
<p>
<li>Programs which try to handle signals on
an alternate stack (sigaltstack) are not supported, although
they could be, with a bit of effort.</li><br>
<p>
<li>Programs which switch stacks are not well handled. Valgrind
does have support for this, but I don't have great faith in it.
It's difficult -- there's no cast-iron way to decide whether a
large change in %esp is as a result of the program switching
stacks, or merely allocating a large object temporarily on the
current stack -- yet Valgrind needs to handle the two situations
differently. 1 May 02: this probably interacts badly with the
new pthread support. I haven't checked properly.</li><br>
<p>
<li>x86 instructions, and system calls, have been implemented on
demand. So it's possible, although unlikely, that a program
will fall over with a message to that effect. If this happens,
please mail me ALL the details printed out, so I can try and
implement the missing feature.</li><br>
<p>
<li>x86 floating point works correctly, but floating-point code may
run even more slowly than integer code, due to my simplistic
approach to FPU emulation.</li><br>
<p>
<li>You can't Valgrind-ize statically linked binaries. Valgrind
relies on the dynamic-link mechanism to gain control at
startup.</li><br>
<p>
<li>Memory consumption of your program is majorly increased whilst
running under Valgrind. This is due to the large amount of
adminstrative information maintained behind the scenes. Another
cause is that Valgrind dynamically translates the original
executable. Translated, instrumented code is 14-16 times larger
than the original (!) so you can easily end up with 30+ MB of
translations when running (eg) a web browser.
</li>
</ul>
Programs which are known not to work are:
<ul>
<li>emacs starts up but immediately concludes it is out of memory
and aborts. Emacs has it's own memory-management scheme, but I
don't understand why this should interact so badly with
Valgrind. Emacs works fine if you build it to use the standard
malloc/free routines.</li><br>
<p>
</ul>
<p><hr width="100%">
<a name="howitworks"></a>
<h2>5&nbsp; How it works -- a rough overview</h2>
Some gory details, for those with a passion for gory details. You
don't need to read this section if all you want to do is use Valgrind.
<a name="startb"></a>
<h3>5.1&nbsp; Getting started</h3>
Valgrind is compiled into a shared object, valgrind.so. The shell
script valgrind sets the LD_PRELOAD environment variable to point to
valgrind.so. This causes the .so to be loaded as an extra library to
any subsequently executed dynamically-linked ELF binary, viz, the
program you want to debug.
<p>The dynamic linker allows each .so in the process image to have an
initialisation function which is run before main(). It also allows
each .so to have a finalisation function run after main() exits.
<p>When valgrind.so's initialisation function is called by the dynamic
linker, the synthetic CPU to starts up. The real CPU remains locked
in valgrind.so for the entire rest of the program, but the synthetic
CPU returns from the initialisation function. Startup of the program
now continues as usual -- the dynamic linker calls all the other .so's
initialisation routines, and eventually runs main(). This all runs on
the synthetic CPU, not the real one, but the client program cannot
tell the difference.
<p>Eventually main() exits, so the synthetic CPU calls valgrind.so's
finalisation function. Valgrind detects this, and uses it as its cue
to exit. It prints summaries of all errors detected, possibly checks
for memory leaks, and then exits the finalisation routine, but now on
the real CPU. The synthetic CPU has now lost control -- permanently
-- so the program exits back to the OS on the real CPU, just as it
would have done anyway.
<p>On entry, Valgrind switches stacks, so it runs on its own stack.
On exit, it switches back. This means that the client program
continues to run on its own stack, so we can switch back and forth
between running it on the simulated and real CPUs without difficulty.
This was an important design decision, because it makes it easy (well,
significantly less difficult) to debug the synthetic CPU.
<a name="engine"></a>
<h3>5.2&nbsp; The translation/instrumentation engine</h3>
Valgrind does not directly run any of the original program's code. Only
instrumented translations are run. Valgrind maintains a translation
table, which allows it to find the translation quickly for any branch
target (code address). If no translation has yet been made, the
translator - a just-in-time translator - is summoned. This makes an
instrumented translation, which is added to the collection of
translations. Subsequent jumps to that address will use this
translation.
<p>Valgrind no longer directly supports detection of self-modifying
code. Such checking is expensive, and in practice (fortunately)
almost no applications need it. However, to help people who are
debugging dynamic code generation systems, there is a Client Request
(basically a macro you can put in your program) which directs Valgrind
to discard translations in a given address range. So Valgrind can
still work in this situation provided the client tells it when
code has become out-of-date and needs to be retranslated.
<p>The JITter translates basic blocks -- blocks of straight-line-code
-- as single entities. To minimise the considerable difficulties of
dealing with the x86 instruction set, x86 instructions are first
translated to a RISC-like intermediate code, similar to sparc code,
but with an infinite number of virtual integer registers. Initially
each insn is translated seperately, and there is no attempt at
instrumentation.
<p>The intermediate code is improved, mostly so as to try and cache
the simulated machine's registers in the real machine's registers over
several simulated instructions. This is often very effective. Also,
we try to remove redundant updates of the simulated machines's
condition-code register.
<p>The intermediate code is then instrumented, giving more
intermediate code. There are a few extra intermediate-code operations
to support instrumentation; it is all refreshingly simple. After
instrumentation there is a cleanup pass to remove redundant value
checks.
<p>This gives instrumented intermediate code which mentions arbitrary
numbers of virtual registers. A linear-scan register allocator is
used to assign real registers and possibly generate spill code. All
of this is still phrased in terms of the intermediate code. This
machinery is inspired by the work of Reuben Thomas (MITE).
<p>Then, and only then, is the final x86 code emitted. The
intermediate code is carefully designed so that x86 code can be
generated from it without need for spare registers or other
inconveniences.
<p>The translations are managed using a traditional LRU-based caching
scheme. The translation cache has a default size of about 14MB.
<a name="track"></a>
<h3>5.3&nbsp; Tracking the status of memory</h3> Each byte in the
process' address space has nine bits associated with it: one A bit and
eight V bits. The A and V bits for each byte are stored using a
sparse array, which flexibly and efficiently covers arbitrary parts of
the 32-bit address space without imposing significant space or
performance overheads for the parts of the address space never
visited. The scheme used, and speedup hacks, are described in detail
at the top of the source file vg_memory.c, so you should read that for
the gory details.
<a name="sys_calls"></a>
<h3>5.4 System calls</h3>
All system calls are intercepted. The memory status map is consulted
before and updated after each call. It's all rather tiresome. See
vg_syscall_mem.c for details.
<a name="sys_signals"></a>
<h3>5.5&nbsp; Signals</h3>
All system calls to sigaction() and sigprocmask() are intercepted. If
the client program is trying to set a signal handler, Valgrind makes a
note of the handler address and which signal it is for. Valgrind then
arranges for the same signal to be delivered to its own handler.
<p>When such a signal arrives, Valgrind's own handler catches it, and
notes the fact. At a convenient safe point in execution, Valgrind
builds a signal delivery frame on the client's stack and runs its
handler. If the handler longjmp()s, there is nothing more to be said.
If the handler returns, Valgrind notices this, zaps the delivery
frame, and carries on where it left off before delivering the signal.
<p>The purpose of this nonsense is that setting signal handlers
essentially amounts to giving callback addresses to the Linux kernel.
We can't allow this to happen, because if it did, signal handlers
would run on the real CPU, not the simulated one. This means the
checking machinery would not operate during the handler run, and,
worse, memory permissions maps would not be updated, which could cause
spurious error reports once the handler had returned.
<p>An even worse thing would happen if the signal handler longjmp'd
rather than returned: Valgrind would completely lose control of the
client program.
<p>Upshot: we can't allow the client to install signal handlers
directly. Instead, Valgrind must catch, on behalf of the client, any
signal the client asks to catch, and must delivery it to the client on
the simulated CPU, not the real one. This involves considerable
gruesome fakery; see vg_signals.c for details.
<p>
<hr width="100%">
<a name="example"></a>
<h2>6&nbsp; Example</h2>
This is the log for a run of a small program. The program is in fact
correct, and the reported error is as the result of a potentially serious
code generation bug in GNU g++ (snapshot 20010527).
<pre>
sewardj@phoenix:~/newmat10$
~/Valgrind-6/valgrind -v ./bogon
==25832== Valgrind 0.10, a memory error detector for x86 RedHat 7.1.
==25832== Copyright (C) 2000-2001, and GNU GPL'd, by Julian Seward.
==25832== Startup, with flags:
==25832== --suppressions=/home/sewardj/Valgrind/redhat71.supp
==25832== reading syms from /lib/ld-linux.so.2
==25832== reading syms from /lib/libc.so.6
==25832== reading syms from /mnt/pima/jrs/Inst/lib/libgcc_s.so.0
==25832== reading syms from /lib/libm.so.6
==25832== reading syms from /mnt/pima/jrs/Inst/lib/libstdc++.so.3
==25832== reading syms from /home/sewardj/Valgrind/valgrind.so
==25832== reading syms from /proc/self/exe
==25832== loaded 5950 symbols, 142333 line number locations
==25832==
==25832== Invalid read of size 4
==25832== at 0x8048724: _ZN10BandMatrix6ReSizeEiii (bogon.cpp:45)
==25832== by 0x80487AF: main (bogon.cpp:66)
==25832== by 0x40371E5E: __libc_start_main (libc-start.c:129)
==25832== by 0x80485D1: (within /home/sewardj/newmat10/bogon)
==25832== Address 0xBFFFF74C is not stack'd, malloc'd or free'd
==25832==
==25832== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
==25832== malloc/free: in use at exit: 0 bytes in 0 blocks.
==25832== malloc/free: 0 allocs, 0 frees, 0 bytes allocated.
==25832== For a detailed leak analysis, rerun with: --leak-check=yes
==25832==
==25832== exiting, did 1881 basic blocks, 0 misses.
==25832== 223 translations, 3626 bytes in, 56801 bytes out.
</pre>
<p>The GCC folks fixed this about a week before gcc-3.0 shipped.
<hr width="100%">
<p>
<a name="cache"></a>
<h2>7&nbsp; Cache profiling</h2>
As well as memory debugging, Valgrind also allows you to do cache simulations
and annotate your source line-by-line with the number of cache misses. In
particular, it records:
<ul>
<li>L1 instruction cache reads and misses;
<li>L1 data cache reads and read misses, writes and write misses;
<li>L2 unified cache reads and read misses, writes and writes misses.
</ul>
On a modern x86 machine, an L1 miss will typically cost around 10 cycles,
and an L2 miss can cost as much as 200 cycles. Detailed cache profiling can be
very useful for improving the performance of your program.<p>
Also, since one instruction cache read is performed per instruction executed,
you can find out how many instructions are executed per line, which can be
useful for optimisation and test coverage.<p>
Please note that this is an experimental feature. Any feedback, bug-fixes,
suggestions, etc, welcome.
<h3>7.1&nbsp; Overview</h3>
First off, as for normal Valgrind use, you probably want to turn on debugging
info (the <code>-g</code> flag). But by contrast with normal Valgrind use, you
probably <b>do</b> want to turn optimisation on, since you should profile your
program as it will be normally run.
The three steps are:
<ol>
<li>Generate a cache simulator for your machine's cache
configuration with the supplied <code>vg_cachegen</code>
program, and recompile Valgrind with <code>make install</code>.
<p>
The default settings are for an AMD Athlon, and you will get
useful information with the defaults, so you can skip this step
if you want. Nevertheless, for accurate cache profiles you will
need use <code>vg_cachegen</code> to customise
<code>cachegrind</code> for your system.
<p>
This step only needs to be done once, unless you are interested
in simulating different cache configurations (eg. first
concentrating on instruction cache misses, then on data cache
misses).
</li>
<p>
<li>Run your program with <code>cachegrind</code> in front of the
normal command line invocation. When the program finishes,
Valgrind will print summary cache statistics. It also collects
line-by-line information in a file <code>cachegrind.out</code>.
<p>
This step should be done every time you want to collect
information about a new program, a changed program, or about the
same program with different input.
</li>
<p>
<li>Generate a function-by-function summary, and possibly annotate
source files with 'vg_annotate'. Source files to annotate can be
specified manually, or manually on the command line, or
"interesting" source files can be annotated automatically with
the <code>--auto=yes</code> option. You can annotate C/C++
files or assembly language files equally easily.</li>
<p>
This step can be performed as many times as you like for each
Step 2. You may want to do multiple annotations showing
different information each time.<p>
</ol>
The steps are described in detail in the following sections.<p>
<a name="generate"></a>
<h3>7.3&nbsp; Generating a cache simulator</h3>
Although Valgrind comes with a pre-generated cache simulator, it most
likely won't match the cache configuration of your machine, so you
should generate a new simulator.<p>
You need to generate three files, one for each of the I1, D1 and L2
caches. For each cache, you need to know the:
<ul>
<li>Cache size (bytes);
<li>Line size (bytes);
<li>Associativity.
</ul>
vg_cachegen takes three options:
<ul>
<li><code>--I1=size,line_size,associativity</code>
<li><code>--D1=size,line_size,associativity</code>
<li><code>--L2=size,line_size,associativity</code>
</ul>
You can specify one, two or all three caches per invocation of
vg_cachegen. It checks that the configuration is sensible before
generating the simulators; to see the allowed values, run
<code>vg_cachegen -h</code>.<p>
An example invocation would be:
<blockquote><code>
vg_cachegen --I1=65536,64,2 --D1=65536,64,2 --L2=262144,64,8
</code></blockquote>
This simulates a machine with a 128KB split L1 2-way associative
cache, and a 256KB unified 8-way associative L2 cache. Both caches
have 64B lines.<p>
If you don't know your cache configuration, you'll have to find it
out. (Ideally <code>vg_cachegen</code> could auto-identify your cache
configuration using the CPUID instruction, which could be done
automatically during installation, and this whole step could be
skipped.)<p>
<h3>7.4&nbsp; Cache simulation specifics</h3>
<code>vg_cachegen</code> only generates simulations for a machine with
a split L1 cache and a unified L2 cache. This configuration is used
for all (modern) x86-based machines we are aware of. Old Cyrix CPUs
had a unified I and D L1 cache, but they are ancient history now.<p>
The more specific characteristics of the simulation are as follows.
<ul>
<li>Write-allocate: when a write miss occurs, the block written to
is brought into the D1 cache. Most modern caches have this
property.</li><p>
<li>Bit-selection hash function: the line(s) in the cache to which a
memory block maps is chosen by the middle bits M--(M+N-1) of the
byte address, where:
<ul>
<li>&nbsp;line size = 2^M bytes&nbsp;</li>
<li>(cache size / line size) = 2^N bytes</li>
</ul> </li><p>
<li>Inclusive L2 cache: the L2 cache replicates all the entries of
the L1 cache. This is standard on Pentium chips, but AMD
Athlons use an exclusive L2 cache that only holds blocks evicted
from L1. Ditto AMD Durons and most modern VIAs.</li><p>
</ul>
Other noteworthy behaviour:
<ul>
<li>References that straddle two cache lines are treated as follows:</li>
<ul>
<li>If both blocks hit --&gt; counted as one hit</li>
<li>If one block hits, the other misses --&gt; counted as one miss</li>
<li>If both blocks miss --&gt; counted as one miss (not two)</li>
</ul><p>
<li>Instructions that modify a memory location (eg. <code>inc</code> and
<code>dec</code>) are counted as doing just a read, ie. a single data
reference. This may seem strange, but since the write can never cause a
miss (the read guarantees the block is in the cache) it's not very
interesting.<p>
Thus it measures not the number of times the data cache is accessed, but
the number of times a data cache miss could occur.<p>
</li>
</ul>
If you are interested in simulating a cache with different properties, it is
not particularly hard to write your own cache simulator, or to modify existing
ones in <code>vg_cachesim_I1.c</code>, <code>vg_cachesim_I1.c</code> and
<code>vg_cachesim_I1.c</code>. We'd be interested to hear from anyone who
does.
<a name="profile"></a>
<h3>7.5&nbsp; Profiling programs</h3>
Cache profiling is enabled by using the <code>--cachesim=yes</code>
option to the <code>valgrind</code> shell script. Alternatively, it
is probably more convenient to use the <code>cachegrind</code> script.
This automatically turns off Valgrind's memory checking functions,
since the cache simulation is slow enough already, and you probably
don't want to do both at once.
<p>
To gather cache profiling information about the program <code>ls
-l<code, type:
<blockquote><code>cachegrind ls -l</code></blockquote>
The program will execute (slowly). Upon completion, summary statistics
that look like this will be printed:
<pre>
==31751== I refs: 27,742,716
==31751== I1 misses: 276
==31751== L2 misses: 275
==31751== I1 miss rate: 0.0%
==31751== L2i miss rate: 0.0%
==31751==
==31751== D refs: 15,430,290 (10,955,517 rd + 4,474,773 wr)
==31751== D1 misses: 41,185 ( 21,905 rd + 19,280 wr)
==31751== L2 misses: 23,085 ( 3,987 rd + 19,098 wr)
==31751== D1 miss rate: 0.2% ( 0.1% + 0.4%)
==31751== L2d miss rate: 0.1% ( 0.0% + 0.4%)
==31751==
==31751== L2 misses: 23,360 ( 4,262 rd + 19,098 wr)
==31751== L2 miss rate: 0.0% ( 0.0% + 0.4%)
</pre>
Cache accesses for instruction fetches are summarised first, giving the
number of fetches made (this is the number of instructions executed, which
can be useful to know in its own right), the number of I1 misses, and the
number of L2 instruction (<code>L2i</code>) misses.<p>
Cache accesses for data follow. The information is similar to that of the
instruction fetches, except that the values are also shown split between reads
and writes (note each row's <code>rd</code> and <code>wr</code> values add up
to the row's total).<p>
Combined instruction and data figures for the L2 cache follow that.<p>
<h3>7.6&nbsp; Output file</h3>
As well as printing summary information, Cachegrind also writes
line-by-line cache profiling information to a file named
<code>cachegrind.out</code>. This file is human-readable, but is best
interpreted by the accompanying program <code>vg_annotate</code>,
described in the next section.
<p>
Things to note about the <code>cachegrind.out</code> file:
<ul>
<li>It is written every time <code>valgrind --cachesim=yes</code> or
<code>cachegrind</code> is run, and will overwrite any existing
<code>cachegrind.out</code> in the current directory.</li>
<p>
<li>It can be huge: <code>ls -l</code> generates a file of about
350KB. Browsing a few files and web pages with a Konqueror
built with full debugging information generates a file
of around 15 MB.</li>
</ul>
<a name="annotate"></a>
<h3>7.7&nbsp; Annotating C/C++ programs</h3>
Before using <code>vg_annotate</code>, it is worth widening your
window to be at least 120-characters wide if possible, as the output
lines can be quite long.
<p>
To get a function-by-function summary, run <code>vg_annotate</code> in
directory containing a <code>cachegrind.out</code> file. The output
looks like this:
<pre>
--------------------------------------------------------------------------------
I1 cache: 65536 B, 64 B, 2-way associative
D1 cache: 65536 B, 64 B, 2-way associative
L2 cache: 262144 B, 64 B, 8-way associative
Command: concord vg_to_ucode.c
Events recorded: Ir I1mr I2mr Dr D1mr D2mr Dw D1mw D2mw
Events shown: Ir I1mr I2mr Dr D1mr D2mr Dw D1mw D2mw
Event sort order: Ir I1mr I2mr Dr D1mr D2mr Dw D1mw D2mw
Threshold: 99%
Chosen for annotation:
Auto-annotation: on
--------------------------------------------------------------------------------
Ir I1mr I2mr Dr D1mr D2mr Dw D1mw D2mw
--------------------------------------------------------------------------------
27,742,716 276 275 10,955,517 21,905 3,987 4,474,773 19,280 19,098 PROGRAM TOTALS
--------------------------------------------------------------------------------
Ir I1mr I2mr Dr D1mr D2mr Dw D1mw D2mw file:function
--------------------------------------------------------------------------------
8,821,482 5 5 2,242,702 1,621 73 1,794,230 0 0 getc.c:_IO_getc
5,222,023 4 4 2,276,334 16 12 875,959 1 1 concord.c:get_word
2,649,248 2 2 1,344,810 7,326 1,385 . . . vg_main.c:strcmp
2,521,927 2 2 591,215 0 0 179,398 0 0 concord.c:hash
2,242,740 2 2 1,046,612 568 22 448,548 0 0 ctype.c:tolower
1,496,937 4 4 630,874 9,000 1,400 279,388 0 0 concord.c:insert
897,991 51 51 897,831 95 30 62 1 1 ???:???
598,068 1 1 299,034 0 0 149,517 0 0 ../sysdeps/generic/lockfile.c:__flockfile
598,068 0 0 299,034 0 0 149,517 0 0 ../sysdeps/generic/lockfile.c:__funlockfile
598,024 4 4 213,580 35 16 149,506 0 0 vg_clientmalloc.c:malloc
446,587 1 1 215,973 2,167 430 129,948 14,057 13,957 concord.c:add_existing
341,760 2 2 128,160 0 0 128,160 0 0 vg_clientmalloc.c:vg_trap_here_WRAPPER
320,782 4 4 150,711 276 0 56,027 53 53 concord.c:init_hash_table
298,998 1 1 106,785 0 0 64,071 1 1 concord.c:create
149,518 0 0 149,516 0 0 1 0 0 ???:tolower@@GLIBC_2.0
149,518 0 0 149,516 0 0 1 0 0 ???:fgetc@@GLIBC_2.0
95,983 4 4 38,031 0 0 34,409 3,152 3,150 concord.c:new_word_node
85,440 0 0 42,720 0 0 21,360 0 0 vg_clientmalloc.c:vg_bogus_epilogue
</pre>
First up is a summary of the annotation options:
<ul>
<li>I1 cache, D1 cache, L2 cache: cache configuration. So you know the
configuration with which these results were obtained.</li><p>
<li>Command: the command line invocation of the program under
examination.</li><p>
<li>Events recorded: event abbreviations are:<p>
<ul>
<li><code>Ir </code>: I cache reads (ie. instructions executed)</li>
<li><code>I1mr</code>: I1 cache read misses</li>
<li><code>I2mr</code>: L2 cache instruction read misses</li>
<li><code>Dr </code>: D cache reads (ie. memory reads)</li>
<li><code>D1mr</code>: D1 cache read misses</li>
<li><code>D2mr</code>: L2 cache data read misses</li>
<li><code>Dw </code>: D cache writes (ie. memory writes)</li>
<li><code>D1mw</code>: D1 cache write misses</li>
<li><code>D2mw</code>: L2 cache data write misses</li>
</ul><p>
Note that D1 total accesses is given by <code>D1mr</code> +
<code>D1mw</code>, and that L2 total accesses is given by
<code>I2mr</code> + <code>D2mr</code> + <code>D2mw</code>.</li><p>
<li>Events shown: the events shown (a subset of events gathered). This can
be adjusted with the <code>--show</code> option.</li><p>
<li>Event sort order: the sort order in which functions are shown. For
example, in this case the functions are sorted from highest
<code>Ir</code> counts to lowest. If two functions have identical
<code>Ir</code> counts, they will then be sorted by <code>I1mr</code>
counts, and so on. This order can be adjusted with the
<code>--sort</code> option.<p>
Note that this dictates the order the functions appear. It is <b>not</b>
the order in which the columns appear; that is dictated by the "events
shown" line (and can be changed with the <code>--sort</code> option).
</li><p>
<li>Threshold: <code>vg_annotate</code> by default omits functions
that cause very low numbers of misses to avoid drowning you in
information. In this case, vg_annotate shows summaries the
functions that account for 99% of the <code>Ir</code> counts;
<code>Ir</code> is chosen as the threshold event since it is the
primary sort event. The threshold can be adjusted with the
<code>--threshold</code> option.</li><p>
<li>Chosen for annotation: names of files specified manually for annotation;
in this case none.</li><p>
<li>Auto-annotation: whether auto-annotation was requested via the
<code>--auto=yes</code> option. In this case no.</li><p>
</ul>
Then follows summary statistics for the whole program. These are similar
to the summary provided when running <code>valgrind --cachesim=yes</code>.<p>
Then follows function-by-function statistics. Each function is
identified by a <code>file_name:function_name</code> pair. If a column
contains only a dot it means the function never performs
that event (eg. the third row shows that <code>strcmp()</code>
contains no instructions that write to memory). The name
<code>???</code> is used if the the file name and/or function name
could not be determined from debugging information. If most of the
entries have the form <code>???:???</code> the program probably wasn't
compiled with <code>-g</code>. <p>
It is worth noting that functions will come from three types of source files:
<ol>
<li> From the profiled program (<code>concord.c</code> in this example).</li>
<li>From libraries (eg. <code>getc.c</code>)</li>
<li>From Valgrind's implementation of some libc functions (eg.
<code>vg_clientmalloc.c:malloc</code>). These are recognisable because
the filename begins with <code>vg_</code>, and is probably one of
<code>vg_main.c</code>, <code>vg_clientmalloc.c</code> or
<code>vg_mylibc.c</code>.
</li>
</ol>
There are two ways to annotate source files -- by choosing them
manually, or with the <code>--auto=yes</code> option. To do it
manually, just specify the filenames as arguments to
<code>vg_annotate</code>. For example, the output from running
<code>vg_annotate concord.c</code> for our example produces the same
output as above followed by an annotated version of
<code>concord.c</code>, a section of which looks like:
<pre>
--------------------------------------------------------------------------------
-- User-annotated source: concord.c
--------------------------------------------------------------------------------
Ir I1mr I2mr Dr D1mr D2mr Dw D1mw D2mw
[snip]
. . . . . . . . . void init_hash_table(char *file_name, Word_Node *table[])
3 1 1 . . . 1 0 0 {
. . . . . . . . . FILE *file_ptr;
. . . . . . . . . Word_Info *data;
1 0 0 . . . 1 1 1 int line = 1, i;
. . . . . . . . .
5 0 0 . . . 3 0 0 data = (Word_Info *) create(sizeof(Word_Info));
. . . . . . . . .
4,991 0 0 1,995 0 0 998 0 0 for (i = 0; i < TABLE_SIZE; i++)
3,988 1 1 1,994 0 0 997 53 52 table[i] = NULL;
. . . . . . . . .
. . . . . . . . . /* Open file, check it. */
6 0 0 1 0 0 4 0 0 file_ptr = fopen(file_name, "r");
2 0 0 1 0 0 . . . if (!(file_ptr)) {
. . . . . . . . . fprintf(stderr, "Couldn't open '%s'.\n", file_name);
1 1 1 . . . . . . exit(EXIT_FAILURE);
. . . . . . . . . }
. . . . . . . . .
165,062 1 1 73,360 0 0 91,700 0 0 while ((line = get_word(data, line, file_ptr)) != EOF)
146,712 0 0 73,356 0 0 73,356 0 0 insert(data->;word, data->line, table);
. . . . . . . . .
4 0 0 1 0 0 2 0 0 free(data);
4 0 0 1 0 0 2 0 0 fclose(file_ptr);
3 0 0 2 0 0 . . . }
</pre>
(Although column widths are automatically minimised, a wide terminal is clearly
useful.)<p>
Each source file is clearly marked (<code>User-annotated source</code>) as
having been chosen manually for annotation. If the file was found in one of
the directories specified with the <code>-I</code>/<code>--include</code>
option, the directory and file are both given.<p>
Each line is annotated with its event counts. Events not applicable for a line
are represented by a `.'; this is useful for distinguishing between an event
which cannot happen, and one which can but did not.<p>
Sometimes only a small section of a source file is executed. To minimise
uninteresting output, Valgrind only shows annotated lines and lines within a
small distance of annotated lines. Gaps are marked with the line numbers so
you know which part of a file the shown code comes from, eg:
<pre>
(figures and code for line 704)
-- line 704 ----------------------------------------
-- line 878 ----------------------------------------
(figures and code for line 878)
</pre>
The amount of context to show around annotated lines is controlled by the
<code>--context</code> option.<p>
To get automatic annotation, run <code>vg_annotate --auto=yes</code>.
vg_annotate will automatically annotate every source file it can find that is
mentioned in the function-by-function summary. Therefore, the files chosen for
auto-annotation are affected by the <code>--sort</code> and
<code>--threshold</code> options. Each source file is clearly marked
(<code>Auto-annotated source</code>) as being chosen automatically. Any files
that could not be found are mentioned at the end of the output, eg:
<pre>
--------------------------------------------------------------------------------
The following files chosen for auto-annotation could not be found:
--------------------------------------------------------------------------------
getc.c
ctype.c
../sysdeps/generic/lockfile.c
</pre>
This is quite common for library files, since libraries are usually compiled
with debugging information, but the source files are often not present on a
system. If a file is chosen for annotation <b>both</b> manually and
automatically, it is marked as <code>User-annotated source</code>.
Use the <code>-I/--include</code> option to tell Valgrind where to look for
source files if the filenames found from the debugging information aren't
specific enough.
Beware that vg_annotate can take some time to digest large
<code>cachegrind.out</code> files, eg. 30 seconds or more. Also beware that
auto-annotation can produce a lot of output if your program is large!
<h3>7.8&nbsp; Annotating assembler programs</h3>
Valgrind can annotate assembler programs too, or annotate the
assembler generated for your C program. Sometimes this is useful for
understanding what is really happening when an interesting line of C
code is translated into multiple instructions.<p>
To do this, you just need to assemble your <code>.s</code> files with
assembler-level debug information. gcc doesn't do this, but you can
use the GNU assembler with the <code>--gstabs</code> option to
generate object files with this information, eg:
<blockquote><code>as --gstabs foo.s</code></blockquote>
You can then profile and annotate source files in the same way as for C/C++
programs.
<h3>7.9&nbsp; <code>vg_annotate</code> options</h3>
<ul>
<li><code>-h, --help</code></li><p>
<li><code>-v, --version</code><p>
Help and version, as usual.</li>
<li><code>--sort=A,B,C</code> [default: order in
<code>cachegrind.out</code>]<p>
Specifies the events upon which the sorting of the function-by-function
entries will be based. Useful if you want to concentrate on eg. I cache
misses (<code>--sort=I1mr,I2mr</code>), or D cache misses
(<code>--sort=D1mr,D2mr</code>), or L2 misses
(<code>--sort=D2mr,I2mr</code>).</li><p>
<li><code>--show=A,B,C</code> [default: all, using order in
<code>cachegrind.out</code>]<p>
Specifies which events to show (and the column order). Default is to use
all present in the <code>cachegrind.out</code> file (and use the order in
the file).</li><p>
<li><code>--threshold=X</code> [default: 99%] <p>
Sets the threshold for the function-by-function summary. Functions are
shown that account for more than X% of the primary sort event. If
auto-annotating, also affects which files are annotated.
Note: thresholds can be set for more than one of the events by appending
any events for the <code>--sort</code> option with a colon and a number
(no spaces, though). E.g. if you want to see the functions that cover
99% of L2 read misses and 99% of L2 write misses, use this option:
<blockquote><code>--sort=D2mr:99,D2mw:99</code></blockquote>
</li><p>
<li><code>--auto=no</code> [default]<br>
<code>--auto=yes</code> <p>
When enabled, automatically annotates every file that is mentioned in the
function-by-function summary that can be found. Also gives a list of
those that couldn't be found.
<li><code>--context=N</code> [default: 8]<p>
Print N lines of context before and after each annotated line. Avoids
printing large sections of source files that were not executed. Use a
large number (eg. 10,000) to show all source lines.
</li><p>
<li><code>-I=&lt;dir&gt;, --include=&lt;dir&gt;</code>
[default: empty string]<p>
Adds a directory to the list in which to search for files. Multiple
-I/--include options can be given to add multiple directories.
</ul>
<h3>7.10&nbsp; Warnings</h3>
There are a couple of situations in which vg_annotate issues warnings.
<ul>
<li>If a source file is more recent than the <code>cachegrind.out</code>
file. This is because the information in <code>cachegrind.out</code> is
only recorded with line numbers, so if the line numbers change at all in
the source (eg. lines added, deleted, swapped), any annotations will be
incorrect.<p>
<li>If information is recorded about line numbers past the end of a file.
This can be caused by the above problem, ie. shortening the source file
while using an old <code>cachegrind.out</code> file. If this happens,
the figures for the bogus lines are printed anyway (clearly marked as
bogus) in case they are important.</li><p>
</ul>
<h3>7.10&nbsp; Things to watch out for</h3>
Some odd things that can occur during annotation:
<ul>
<li>If annotating at the assembler level, you might see something like this:
<pre>
1 0 0 . . . . . . leal -12(%ebp),%eax
1 0 0 . . . 1 0 0 movl %eax,84(%ebx)
2 0 0 0 0 0 1 0 0 movl $1,-20(%ebp)
. . . . . . . . . .align 4,0x90
1 0 0 . . . . . . movl $.LnrB,%eax
1 0 0 . . . 1 0 0 movl %eax,-16(%ebp)
</pre>
How can the third instruction be executed twice when the others are
executed only once? As it turns out, it isn't. Here's a dump of the
executable, from objdump:
<pre>
8048f25: 8d 45 f4 lea 0xfffffff4(%ebp),%eax
8048f28: 89 43 54 mov %eax,0x54(%ebx)
8048f2b: c7 45 ec 01 00 00 00 movl $0x1,0xffffffec(%ebp)
8048f32: 89 f6 mov %esi,%esi
8048f34: b8 08 8b 07 08 mov $0x8078b08,%eax
8048f39: 89 45 f0 mov %eax,0xfffffff0(%ebp)
</pre>
Notice the extra <code>mov %esi,%esi</code> instruction. Where did this
come from? The GNU assembler inserted it to serve as the two bytes of
padding needed to align the <code>movl $.LnrB,%eax</code> instruction on
a four-byte boundary, but pretended it didn't exist when adding debug
information. Thus when Valgrind reads the debug info it thinks that the
<code>movl $0x1,0xffffffec(%ebp)</code> instruction covers the address
range 0x8048f2b--0x804833 by itself, and attributes the counts for the
<code>mov %esi,%esi</code> to it.<p>
</li>
<li>Inlined functions can cause strange results in the function-by-function
summary. If a function <code>inline_me()</code> is defined in
<code>foo.h</code> and inlined in the functions <code>f1()</code>,
<code>f2()</code> and <code>f3()</code> in <code>bar.c</code>, there will
not be a <code>foo.h:inline_me()</code> function entry. Instead, there
will be separate function entries for each inlining site, ie.
<code>foo.h:f1()</code>, <code>foo.h:f2()</code> and
<code>foo.h:f3()</code>. To find the total counts for
<code>foo.h:inline_me()</code>, add up the counts from each entry.<p>
The reason for this is that although the debug info output by gcc
indicates the switch from <code>bar.c</code> to <code>foo.h</code>, it
doesn't indicate the name of the function in <code>foo.h</code>, so
Valgrind keeps using the old one.<p>
<li>Sometimes, the same filename might be represented with a relative name
and with an absolute name in different parts of the debug info, eg:
<code>/home/user/proj/proj.h</code> and <code>../proj.h</code>. In this
case, if you use auto-annotation, the file will be annotated twice with
the counts split between the two.<p>
</li>
<li>Files with more than 65,535 lines cause difficulties for the stabs debug
info reader. This is because the line number in the <code>struct
nlist</code> defined in <code>a.out.h</code> under Linux is only a 16-bit
number. Valgrind can handle some files with more than 65,535 lines
correctly by making some guesses to identify line number overflows. But
some cases are beyond it, in which case you'll get a warning message
explaining that annotations for the file might be incorrect.<p>
</li>
<li>If you compile some files with <code>-g</code> and some without, some
events that take place in a file without debug info could be attributed
to the last line of a file with debug info (whichever one gets placed
before the non-debug-info file in the executable).<p>
</li>
</ul>
This list looks long, but these cases should be fairly rare.<p>
Note: stabs is not an easy format to read. If you come across bizarre
annotations that look like might be caused by a bug in the stabs reader,
please let us know.<p>
<h3>7.11&nbsp; Accuracy</h3>
Valgrind's cache profiling has a number of shortcomings:
<ul>
<li>It doesn't account for kernel activity -- the effect of system calls on
the cache contents is ignored.</li><p>
<li>It doesn't account for other process activity (although this is probably
desirable when considering a single program).</li><p>
<li>It doesn't account for virtual-to-physical address mappings; hence the
entire simulation is not a true representation of what's happening in the
cache.</li><p>
<li>It doesn't account for cache misses not visible at the instruction level,
eg. those arising from TLB misses, or speculative execution.</li><p>
<li>Valgrind's custom <code>malloc()</code> will allocate memory in different
ways to the standard <code>malloc()</code>, which could warp the results.
</li><p>
<li>The instructions <code>bts</code>, <code>btr</code> and <code>btc</code>
will incorrectly be counted as doing a data read if both the arguments
are registers, eg:
<blockquote><code>btsl %eax, %edx</code></blockquote>
This should only happen rarely.
</ul>
Another thing worth nothing is that results are very sensitive. Changing the
size of the <code>valgrind.so</code> file, the size of the program being
profiled, or even the length of its name can perturb the results. Variations
will be small, but don't expect perfectly repeatable results if your program
changes at all.<p>
While these factors mean you shouldn't trust the results to be super-accurate,
hopefully they should be close enough to be useful.<p>
<h3>7.12&nbsp; Todo</h3>
<ul>
<li>Use CPUID instruction to auto-identify cache configuration during
installation. This would save the user from having to know their cache
configuration and using vg_cachegen.</li>
<p>
<li>Program start-up/shut-down calls a lot of functions that aren't
interesting and just complicate the output. Would be nice to exclude
these somehow.</li>
<p>
</ul>
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