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<?xml version="1.0"?> <!-- -*- sgml -*- -->
<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"
[ <!ENTITY % vg-entities SYSTEM "vg-entities.xml"> %vg-entities; ]>
<chapter id="manual-core" xreflabel="Valgrind's core">
<title>Using and understanding the Valgrind core</title>
<para>This chapter describes the Valgrind core services, command-line
options and behaviours. That means it is relevant regardless of what
particular tool you are using. The information should be sufficient for you
to make effective day-to-day use of Valgrind. Advanced topics related to
the Valgrind core are described in <xref linkend="manual-core-adv"/>.
</para>
<para>
A point of terminology: most references to "Valgrind" in this chapter
refer to the Valgrind core services. </para>
<sect1 id="manual-core.whatdoes"
xreflabel="What Valgrind does with your program">
<title>What Valgrind does with your program</title>
<para>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.</para>
<para>You invoke Valgrind like this:</para>
<programlisting><![CDATA[
valgrind [valgrind-options] your-prog [your-prog-options]]]></programlisting>
<para>The most important option is <option>--tool</option> which dictates
which Valgrind tool to run. For example, if want to run the command
<computeroutput>ls -l</computeroutput> using the memory-checking tool
Memcheck, issue this command:</para>
<programlisting><![CDATA[
valgrind --tool=memcheck ls -l]]></programlisting>
<para>However, Memcheck is the default, so if you want to use it you can
omit the <option>--tool</option> option.</para>
<para>Regardless of which tool is in use, Valgrind takes control of your
program before it starts. Debugging information is read from the
executable and associated libraries, so that error messages and other
outputs can be phrased in terms of source code locations, when
appropriate.</para>
<para>Your program is then run on a synthetic CPU provided by the
Valgrind core. As new code is executed for the first time, the core
hands the code to the selected tool. The tool adds its own
instrumentation code to this and hands the result back to the core,
which coordinates the continued execution of this instrumented
code.</para>
<para>The amount of instrumentation code added varies widely between
tools. At one end of the scale, Memcheck adds code to check every
memory access and every value computed,
making it run 10-50 times slower than natively.
At the other end of the spectrum, the minimal tool, called Nulgrind,
adds no instrumentation at all and causes in total "only" about a 4 times
slowdown.</para>
<para>Valgrind simulates every single instruction your program executes.
Because of this, the active tool checks, or profiles, not only the code
in your application but also in all supporting dynamically-linked libraries,
including the C library, graphical libraries, and so on.</para>
<para>If you're using an error-detection tool, Valgrind may
detect errors in system libraries, for example the GNU C or X11
libraries, which you have to use. You might not be interested in these
errors, since you probably have no control over that code. Therefore,
Valgrind allows you to selectively suppress errors, by recording them in
a suppressions file which is read when Valgrind starts up. The build
mechanism selects default suppressions which give reasonable
behaviour for the OS and libraries detected on your machine.
To make it easier to write suppressions, you can use the
<option>--gen-suppressions=yes</option> option. This tells Valgrind to
print out a suppression for each reported error, which you can then
copy into a suppressions file.</para>
<para>Different error-checking tools report different kinds of errors.
The suppression mechanism therefore allows you to say which tool or
tool(s) each suppression applies to.</para>
</sect1>
<sect1 id="manual-core.started" xreflabel="Getting started">
<title>Getting started</title>
<para>First off, consider whether it might be beneficial to recompile
your application and supporting libraries with debugging info enabled
(the <option>-g</option> option). Without debugging info, the best
Valgrind tools will be able to do is guess which function a particular
piece of code belongs to, which makes both error messages and profiling
output nearly useless. With <option>-g</option>, you'll get
messages which point directly to the relevant source code lines.</para>
<para>Another option you might like to consider, if you are working with
C++, is <option>-fno-inline</option>. That makes it easier to see the
function-call chain, which can help reduce confusion when navigating
around large C++ apps. For example, debugging
OpenOffice.org with Memcheck is a bit easier when using this option. 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. Alternatively, the Valgrind option
<option>--read-inline-info=yes</option> instructs Valgrind to read
the debug information describing inlining information. With this,
function call chain will be properly shown, even when your application
is compiled with inlining. </para>
<para>If you are planning to use Memcheck: On rare
occasions, compiler optimisations (at <option>-O2</option>
and above, and sometimes <option>-O1</option>) have been
observed to generate code which fools Memcheck into wrongly reporting
uninitialised value errors, or missing uninitialised value errors. We have
looked in detail into fixing this, and unfortunately the result is that
doing so would give a further significant slowdown in what is already a slow
tool. So the best solution is to turn off optimisation altogether. Since
this often makes things unmanageably slow, a reasonable compromise is to use
<option>-O</option>. This gets you the majority of the
benefits of higher optimisation levels whilst keeping relatively small the
chances of false positives or false negatives from Memcheck. Also, you
should compile your code with <option>-Wall</option> because
it can identify some or all of the problems that Valgrind can miss at the
higher optimisation levels. (Using <option>-Wall</option>
is also a good idea in general.) All other tools (as far as we know) are
unaffected by optimisation level, and for profiling tools like Cachegrind it
is better to compile your program at its normal optimisation level.</para>
<para>Valgrind understands the DWARF2/3/4 formats used by GCC 3.1 and
later. The reader for "stabs" debugging format (used by GCC versions
prior to 3.1) has been disabled in Valgrind 3.9.0.</para>
<para>When you're ready to roll, run Valgrind as described above.
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
<filename>/bin/sh</filename>,
<filename>/usr/bin/perl</filename>, or whatever interpreter
you're using. This may not be what you want and can be confusing. You
can force the issue by giving the option
<option>--trace-children=yes</option>, but confusion is still
likely.</para>
</sect1>
<!-- Referenced from both the manual and manpage -->
<sect1 id="&vg-comment-id;" xreflabel="&vg-comment-label;">
<title>The Commentary</title>
<para>Valgrind tools write a commentary, a stream of text, detailing
error reports and other significant events. All lines in the commentary
have following form:
<programlisting><![CDATA[
==12345== some-message-from-Valgrind]]></programlisting>
</para>
<para>The <computeroutput>12345</computeroutput> 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.</para>
<para>By default, Valgrind tools write 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 <option>-v</option> option to Valgrind. A second
<option>-v</option> gives yet more detail.
</para>
<para>You can direct the commentary to three different places:</para>
<orderedlist>
<listitem id="manual-core.out2fd" xreflabel="Directing output to fd">
<para>The default: send it to a file descriptor, which is by default
2 (stderr). So, if you give the core no options, it will write
commentary to the standard error stream. If you want to send it to
some other file descriptor, for example number 9, you can specify
<option>--log-fd=9</option>.</para>
<para>This is the simplest and most common arrangement, but can
cause problems when Valgrinding entire trees of processes which
expect specific file descriptors, particularly stdin/stdout/stderr,
to be available for their own use.</para>
</listitem>
<listitem id="manual-core.out2file"
xreflabel="Directing output to file"> <para>A less intrusive
option is to write the commentary to a file, which you specify by
<option>--log-file=filename</option>. There are special format
specifiers that can be used to use a process ID or an environment
variable name in the log file name. These are useful/necessary if your
program invokes multiple processes (especially for MPI programs).
See the <link linkend="manual-core.basicopts">basic options section</link>
for more details.</para>
</listitem>
<listitem id="manual-core.out2socket"
xreflabel="Directing output to network socket"> <para>The
least intrusive option is to send the commentary to a network
socket. The socket is specified as an IP address and port number
pair, like this: <option>--log-socket=192.168.0.1:12345</option> if
you want to send the output to host IP 192.168.0.1 port 12345
(note: we
have no idea if 12345 is a port of pre-existing significance). You
can also omit the port number:
<option>--log-socket=192.168.0.1</option>, in which case a default
port of 1500 is used. This default is defined by the constant
<computeroutput>VG_CLO_DEFAULT_LOGPORT</computeroutput> in the
sources.</para>
<para>Note, unfortunately, that you have to use an IP address here,
rather than a hostname.</para>
<para>Writing to a network socket is pointless if you don't
have something listening at the other end. We provide a simple
listener program,
<computeroutput>valgrind-listener</computeroutput>, which accepts
connections on the specified port and copies whatever it is sent to
stdout. Probably someone will tell us this is a horrible security
risk. It seems likely that people will write more sophisticated
listeners in the fullness of time.</para>
<para><computeroutput>valgrind-listener</computeroutput> can accept
simultaneous connections from up to 50 Valgrinded processes. In front
of each line of output it prints the current number of active
connections in round brackets.</para>
<para><computeroutput>valgrind-listener</computeroutput> accepts two
command-line options:</para>
<!-- start of xi:include in the manpage -->
<variablelist id="listener.opts.list">
<varlistentry>
<term><option>-e --exit-at-zero</option></term>
<listitem>
<para>When the number of connected processes falls back to zero,
exit. Without this, it will run forever, that is, until you
send it Control-C.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><option>portnumber</option></term>
<listitem>
<para>Changes the port it listens on from the default (1500).
The specified port must be in the range 1024 to 65535.
The same restriction applies to port numbers specified by a
<option>--log-socket</option> to Valgrind itself.</para>
</listitem>
</varlistentry>
</variablelist>
<!-- end of xi:include in the manpage -->
<para>If a Valgrinded process fails to connect to a listener, for
whatever reason (the listener isn't running, invalid or unreachable
host or port, etc), Valgrind switches back to writing the commentary
to stderr. The same goes for any process which loses an established
connection to a listener. In other words, killing the listener
doesn't kill the processes sending data to it.</para>
</listitem>
</orderedlist>
<para>Here is an important point about the relationship between the
commentary and profiling output from tools. The commentary contains a
mix of messages from the Valgrind core and the selected tool. If the
tool reports errors, it will report them to the commentary. However, if
the tool does profiling, the profile data will be written to a file of
some kind, depending on the tool, and independent of what
<option>--log-*</option> options are in force. The commentary is
intended to be a low-bandwidth, human-readable channel. Profiling data,
on the other hand, is usually voluminous and not meaningful without
further processing, which is why we have chosen this arrangement.</para>
</sect1>
<sect1 id="manual-core.report" xreflabel="Reporting of errors">
<title>Reporting of errors</title>
<para>When an error-checking tool
detects something bad happening in the program, an error
message is written to the commentary. Here's an example from Memcheck:</para>
<programlisting><![CDATA[
==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== Address 0xBFFFF74C is not stack'd, malloc'd or free'd]]></programlisting>
<para>This message says that the program did an illegal 4-byte read of
address 0xBFFFF74C, which, as far as Memcheck can tell, is not a valid
stack address, nor corresponds to any current heap blocks or recently freed
heap blocks. The read is happening at line 45 of
<filename>bogon.cpp</filename>, called from line 66 of the same file,
etc. For errors associated with an identified (current or freed) heap block,
for example reading freed memory, Valgrind reports not only the
location where the error happened, but also where the associated heap block
was allocated/freed.</para>
<para>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.</para>
<para>If you want to know how many times each error occurred, run with
the <option>-v</option> 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.</para>
<para>Errors are reported before the associated operation actually
happens. For example, if you're using Memcheck and your program attempts to
read from address zero, Memcheck will emit a message to this effect, and
your program will then likely die with a segmentation fault.</para>
<para>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, when run on Memcheck.
The first such error message may well give the most direct clue to the
root cause of the problem.</para>
<para>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, Valgrind will simply stop collecting
errors after 1,000 different errors have been seen, or 10,000,000 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 1,000/10,000,000 limits
apply after suppressed errors are removed. These limits are
defined in <filename>m_errormgr.c</filename> and can be increased
if necessary.</para>
<para>To avoid this cutoff you can use the
<option>--error-limit=no</option> option. Then Valgrind will always show
errors, regardless of how many there are. Use this option carefully,
since it may have a bad effect on performance.</para>
</sect1>
<sect1 id="manual-core.suppress" xreflabel="Suppressing errors">
<title>Suppressing errors</title>
<para>The error-checking tools detect numerous problems in the system
libraries, such as the C library,
which come pre-installed with your OS. 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 created by the
<computeroutput>./configure</computeroutput> script when the system is
built.</para>
<para>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.</para>
<formalpara><title>Note:</title> <para>By far the easiest way to add
suppressions is to use the <option>--gen-suppressions=yes</option> option
described in <xref linkend="manual-core.options"/>. This generates
suppressions automatically. For best results,
though, you may want to edit the output
of <option>--gen-suppressions=yes</option> by hand, in which
case it would be advisable to read through this section.
</para>
</formalpara>
<para>Each error to be suppressed is described very specifically, to
minimise the possibility that a suppression-directive inadvertently
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.</para>
<para>If you use the <option>-v</option> option, at the end of execution,
Valgrind prints out one line for each used suppression, giving the number of times
it got used, its name and the filename and line number where the suppression is
defined. Depending on the suppression kind, the filename and line number are optionally
followed by additional information (such as the number of blocks and bytes suppressed
by a memcheck leak suppression). Here's the suppressions used by a
run of <computeroutput>valgrind -v --tool=memcheck ls -l</computeroutput>:</para>
<programlisting><![CDATA[
--1610-- used_suppression: 2 dl-hack3-cond-1 /usr/lib/valgrind/default.supp:1234
--1610-- used_suppression: 2 glibc-2.5.x-on-SUSE-10.2-(PPC)-2a /usr/lib/valgrind/default.supp:1234
]]></programlisting>
<para>Multiple suppressions files are allowed. Valgrind loads suppression
patterns from <filename>$PREFIX/lib/valgrind/default.supp</filename> unless
<option>--default-suppressions=no</option> has been specified. You can
ask to add suppressions from additional files by specifying
<option>--suppressions=/path/to/file.supp</option> one or more times.
</para>
<para>If you want to understand more about suppressions, look at an
existing suppressions file whilst reading the following documentation.
The file <filename>glibc-2.3.supp</filename>, in the source
distribution, provides some good examples.</para>
<para>Each suppression has the following components:</para>
<itemizedlist>
<listitem>
<para>First line: 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.</para>
</listitem>
<listitem>
<para>Second line: name of the tool(s) that the suppression is for
(if more than one, comma-separated), and the name of the suppression
itself, separated by a colon (n.b.: no spaces are allowed), eg:</para>
<programlisting><![CDATA[
tool_name1,tool_name2:suppression_name]]></programlisting>
<para>Recall that Valgrind is a modular system, in which
different instrumentation tools can observe your program whilst it
is running. Since different tools detect different kinds of errors,
it is necessary to say which tool(s) the suppression is meaningful
to.</para>
<para>Tools will complain, at startup, if a tool does not understand
any suppression directed to it. Tools ignore suppressions which are
not directed to them. As a result, it is quite practical to put
suppressions for all tools into the same suppression file.</para>
</listitem>
<listitem>
<para>Next line: a small number of suppression types have extra
information after the second line (eg. the <varname>Param</varname>
suppression for Memcheck)</para>
</listitem>
<listitem>
<para>Remaining lines: This is the calling context for the error --
the chain of function calls that led to it. There can be up to 24
of these lines.</para>
<para>Locations may be names of either shared objects or
functions. They begin
<computeroutput>obj:</computeroutput> and
<computeroutput>fun:</computeroutput> respectively. Function and
object names to match against may use the wildcard characters
<computeroutput>*</computeroutput> and
<computeroutput>?</computeroutput>.</para>
<para><command>Important note: </command> C++ function names must be
<command>mangled</command>. If you are writing suppressions by
hand, use the <option>--demangle=no</option> option to get the
mangled names in your error messages. An example of a mangled
C++ name is <computeroutput>_ZN9QListView4showEv</computeroutput>.
This is the form that the GNU C++ compiler uses internally, and
the form that must be used in suppression files. The equivalent
demangled name, <computeroutput>QListView::show()</computeroutput>,
is what you see at the C++ source code level.
</para>
<para>A location line may also be
simply "<computeroutput>...</computeroutput>" (three dots). This is
a frame-level wildcard, which matches zero or more frames. Frame
level wildcards are useful because they make it easy to ignore
varying numbers of uninteresting frames in between frames of
interest. That is often important when writing suppressions which
are intended to be robust against variations in the amount of
function inlining done by compilers.</para>
</listitem>
<listitem>
<para>Finally, the entire suppression must be between curly
braces. Each brace must be the first character on its own
line.</para>
</listitem>
</itemizedlist>
<para>A suppression only suppresses an error when the error matches all
the details in the suppression. Here's an example:</para>
<programlisting><![CDATA[
{
__gconv_transform_ascii_internal/__mbrtowc/mbtowc
Memcheck:Value4
fun:__gconv_transform_ascii_internal
fun:__mbr*toc
fun:mbtowc
}]]></programlisting>
<para>What it means is: for Memcheck only, suppress a
use-of-uninitialised-value error, when the data size is 4, when it
occurs in the function
<computeroutput>__gconv_transform_ascii_internal</computeroutput>, when
that is called from any function of name matching
<computeroutput>__mbr*toc</computeroutput>, when that is called from
<computeroutput>mbtowc</computeroutput>. It doesn't apply under any
other circumstances. The string by which this suppression is identified
to the user is
<computeroutput>__gconv_transform_ascii_internal/__mbrtowc/mbtowc</computeroutput>.</para>
<para>(See <xref linkend="mc-manual.suppfiles"/> for more details
on the specifics of Memcheck's suppression kinds.)</para>
<para>Another example, again for the Memcheck tool:</para>
<programlisting><![CDATA[
{
libX11.so.6.2/libX11.so.6.2/libXaw.so.7.0
Memcheck: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
}]]></programlisting>
<para>This suppresses any size 4 uninitialised-value error which occurs
anywhere in <filename>libX11.so.6.2</filename>, when called from
anywhere in the same library, when called from anywhere in
<filename>libXaw.so.7.0</filename>. The inexact specification of
locations is regrettable, but is about all you can hope for, given that
the X11 libraries shipped on the Linux distro on which this example
was made have had their symbol tables removed.</para>
<para>Although the above two examples do not make this clear, you can
freely mix <computeroutput>obj:</computeroutput> and
<computeroutput>fun:</computeroutput> lines in a suppression.</para>
<para>Finally, here's an example using three frame-level wildcards:</para>
<programlisting><![CDATA[
{
a-contrived-example
Memcheck:Leak
fun:malloc
...
fun:ddd
...
fun:ccc
...
fun:main
}
]]></programlisting>
This suppresses Memcheck memory-leak errors, in the case where
the allocation was done by <computeroutput>main</computeroutput>
calling (though any number of intermediaries, including zero)
<computeroutput>ccc</computeroutput>,
calling onwards via
<computeroutput>ddd</computeroutput> and eventually
to <computeroutput>malloc.</computeroutput>.
</sect1>
<sect1 id="manual-core.options"
xreflabel="Core Command-line Options">
<title>Core Command-line Options</title>
<para>As mentioned above, Valgrind's core accepts a common set of options.
The tools also accept tool-specific options, which are documented
separately for each tool.</para>
<para>Valgrind's default settings succeed in giving reasonable behaviour
in most cases. We group the available options by rough categories.</para>
<sect2 id="manual-core.toolopts" xreflabel="Tool-selection Option">
<title>Tool-selection Option</title>
<para id="tool.opts.para">The single most important option.</para>
<variablelist id="tool.opts.list">
<varlistentry id="tool_name" xreflabel="--tool">
<term>
<option><![CDATA[--tool=<toolname> [default: memcheck] ]]></option>
</term>
<listitem>
<para>Run the Valgrind tool called <varname>toolname</varname>,
e.g. memcheck, cachegrind, callgrind, helgrind, drd, massif,
lackey, none, exp-sgcheck, exp-bbv, exp-dhat, etc.</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2 id="manual-core.basicopts" xreflabel="Basic Options">
<title>Basic Options</title>
<!-- start of xi:include in the manpage -->
<para id="basic.opts.para">These options work with all tools.</para>
<variablelist id="basic.opts.list">
<varlistentry id="opt.help" xreflabel="--help">
<term><option>-h --help</option></term>
<listitem>
<para>Show help for all options, both for the core and for the
selected tool. If the option is repeated it is equivalent to giving
<option>--help-debug</option>.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.help-debug" xreflabel="--help-debug">
<term><option>--help-debug</option></term>
<listitem>
<para>Same as <option>--help</option>, but also lists debugging
options which usually are only of use to Valgrind's
developers.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.version" xreflabel="--version">
<term><option>--version</option></term>
<listitem>
<para>Show the version number of the Valgrind core. Tools can have
their own version numbers. There is a scheme in place to ensure
that tools only execute when the core version is one they are
known to work with. This was done to minimise the chances of
strange problems arising from tool-vs-core version
incompatibilities.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.quiet" xreflabel="--quiet">
<term><option>-q</option>, <option>--quiet</option></term>
<listitem>
<para>Run silently, and only print error messages. Useful if you
are running regression tests or have some other automated test
machinery.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.verbose" xreflabel="--verbose">
<term><option>-v</option>, <option>--verbose</option></term>
<listitem>
<para>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 and
execution engines, and warnings about unusual behaviour. Repeating
the option increases the verbosity level.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.trace-children" xreflabel="--trace-children">
<term>
<option><![CDATA[--trace-children=<yes|no> [default: no] ]]></option>
</term>
<listitem>
<para>When enabled, Valgrind will trace into sub-processes
initiated via the <varname>exec</varname> system call. This is
necessary for multi-process programs.
</para>
<para>Note that Valgrind does trace into the child of a
<varname>fork</varname> (it would be difficult not to, since
<varname>fork</varname> makes an identical copy of a process), so this
option is arguably badly named. However, most children of
<varname>fork</varname> calls immediately call <varname>exec</varname>
anyway.
</para>
</listitem>
</varlistentry>
<varlistentry id="opt.trace-children-skip" xreflabel="--trace-children-skip">
<term>
<option><![CDATA[--trace-children-skip=patt1,patt2,... ]]></option>
</term>
<listitem>
<para>This option only has an effect when
<option>--trace-children=yes</option> is specified. It allows
for some children to be skipped. The option takes a comma
separated list of patterns for the names of child executables
that Valgrind should not trace into. Patterns may include the
metacharacters <computeroutput>?</computeroutput>
and <computeroutput>*</computeroutput>, which have the usual
meaning.</para>
<para>
This can be useful for pruning uninteresting branches from a
tree of processes being run on Valgrind. But you should be
careful when using it. When Valgrind skips tracing into an
executable, it doesn't just skip tracing that executable, it
also skips tracing any of that executable's child processes.
In other words, the flag doesn't merely cause tracing to stop
at the specified executables -- it skips tracing of entire
process subtrees rooted at any of the specified
executables.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.trace-children-skip-by-arg"
xreflabel="--trace-children-skip-by-arg">
<term>
<option><![CDATA[--trace-children-skip-by-arg=patt1,patt2,... ]]></option>
</term>
<listitem>
<para>This is the same as
<option>--trace-children-skip</option>, with one difference:
the decision as to whether to trace into a child process is
made by examining the arguments to the child process, rather
than the name of its executable.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.child-silent-after-fork"
xreflabel="--child-silent-after-fork">
<term>
<option><![CDATA[--child-silent-after-fork=<yes|no> [default: no] ]]></option>
</term>
<listitem>
<para>When enabled, Valgrind will not show any debugging or
logging output for the child process resulting from
a <varname>fork</varname> call. This can make the output less
confusing (although more misleading) when dealing with processes
that create children. It is particularly useful in conjunction
with <varname>--trace-children=</varname>. Use of this option is also
strongly recommended if you are requesting XML output
(<varname>--xml=yes</varname>), since otherwise the XML from child and
parent may become mixed up, which usually makes it useless.
</para>
</listitem>
</varlistentry>
<varlistentry id="opt.vgdb" xreflabel="--vgdb">
<term>
<option><![CDATA[--vgdb=<no|yes|full> [default: yes] ]]></option>
</term>
<listitem>
<para>Valgrind will provide "gdbserver" functionality when
<option>--vgdb=yes</option> or <option>--vgdb=full</option> is
specified. This allows an external GNU GDB debugger to control
and debug your program when it runs on Valgrind.
<option>--vgdb=full</option> incurs significant performance
overheads, but provides more precise breakpoints and
watchpoints. See <xref linkend="manual-core-adv.gdbserver"/> for
a detailed description.
</para>
<para> If the embedded gdbserver is enabled but no gdb is
currently being used, the <xref linkend="manual-core-adv.vgdb"/>
command line utility can send "monitor commands" to Valgrind
from a shell. The Valgrind core provides a set of
<xref linkend="manual-core-adv.valgrind-monitor-commands"/>. A tool
can optionally provide tool specific monitor commands, which are
documented in the tool specific chapter.
</para>
</listitem>
</varlistentry>
<varlistentry id="opt.vgdb-error" xreflabel="--vgdb-error">
<term>
<option><![CDATA[--vgdb-error=<number> [default: 999999999] ]]></option>
</term>
<listitem>
<para> Use this option when the Valgrind gdbserver is enabled with
<option>--vgdb=yes</option> or <option>--vgdb=full</option>.
Tools that report errors will wait
for "<computeroutput>number</computeroutput>" errors to be
reported before freezing the program and waiting for you to
connect with GDB. It follows that a value of zero will cause
the gdbserver to be started before your program is executed.
This is typically used to insert GDB breakpoints before
execution, and also works with tools that do not report
errors, such as Massif.
</para>
</listitem>
</varlistentry>
<varlistentry id="opt.vgdb-stop-at" xreflabel="--vgdb-stop-at">
<term>
<option><![CDATA[--vgdb-stop-at=<set> [default: none] ]]></option>
</term>
<listitem>
<para> Use this option when the Valgrind gdbserver is enabled with
<option>--vgdb=yes</option> or <option>--vgdb=full</option>.
The Valgrind gdbserver will be invoked for each error after
<option>--vgdb-error</option> have been reported.
You can additionally ask the Valgrind gdbserver to be invoked
for other events, specified in one of the following ways: </para>
<itemizedlist>
<listitem><para>a comma separated list of one or more of
<option>startup exit valgrindabexit</option>.</para>
<para>The values <option>startup</option> <option>exit</option>
<option>valgrindabexit</option> respectively indicate to
invoke gdbserver before your program is executed, after the
last instruction of your program, on Valgrind abnormal exit
(e.g. internal error, out of memory, ...).</para>
<para>Note: <option>startup</option> and
<option>--vgdb-error=0</option> will both cause Valgrind
gdbserver to be invoked before your program is executed. The
<option>--vgdb-error=0</option> will in addition cause your
program to stop on all subsequent errors.</para>
</listitem>
<listitem><para><option>all</option> to specify the complete set.
It is equivalent to
<option>--vgdb-stop-at=startup,exit,valgrindabexit</option>.</para>
</listitem>
<listitem><para><option>none</option> for the empty set.</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry id="opt.track-fds" xreflabel="--track-fds">
<term>
<option><![CDATA[--track-fds=<yes|no> [default: no] ]]></option>
</term>
<listitem>
<para>When enabled, Valgrind will print out a list of open file
descriptors on exit or on request, via the gdbserver monitor
command <varname>v.info open_fds</varname>. Along with each
file descriptor is printed a stack backtrace of where the file
was opened and any details relating to the file descriptor such
as the file name or socket details.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.time-stamp" xreflabel="--time-stamp">
<term>
<option><![CDATA[--time-stamp=<yes|no> [default: no] ]]></option>
</term>
<listitem>
<para>When enabled, each message is preceded with an indication of
the elapsed wallclock time since startup, expressed as days,
hours, minutes, seconds and milliseconds.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.log-fd" xreflabel="--log-fd">
<term>
<option><![CDATA[--log-fd=<number> [default: 2, stderr] ]]></option>
</term>
<listitem>
<para>Specifies that Valgrind should send all of its messages to
the specified file descriptor. The default, 2, is the standard
error channel (stderr). Note that this may interfere with the
client's own use of stderr, as Valgrind's output will be
interleaved with any output that the client sends to
stderr.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.log-file" xreflabel="--log-file">
<term>
<option><![CDATA[--log-file=<filename> ]]></option>
</term>
<listitem>
<para>Specifies that Valgrind should send all of its messages to
the specified file. If the file name is empty, it causes an abort.
There are three special format specifiers that can be used in the file
name.</para>
<para><option>%p</option> is replaced with the current process ID.
This is very useful for program that invoke multiple processes.
WARNING: If you use <option>--trace-children=yes</option> and your
program invokes multiple processes OR your program forks without
calling exec afterwards, and you don't use this specifier
(or the <option>%q</option> specifier below), the Valgrind output from
all those processes will go into one file, possibly jumbled up, and
possibly incomplete.</para>
<para><option>%q{FOO}</option> is replaced with the contents of the
environment variable <varname>FOO</varname>. If the
<option>{FOO}</option> part is malformed, it causes an abort. This
specifier is rarely needed, but very useful in certain circumstances
(eg. when running MPI programs). The idea is that you specify a
variable which will be set differently for each process in the job,
for example <computeroutput>BPROC_RANK</computeroutput> or whatever is
applicable in your MPI setup. If the named environment variable is not
set, it causes an abort. Note that in some shells, the
<option>{</option> and <option>}</option> characters may need to be
escaped with a backslash.</para>
<para><option>%%</option> is replaced with <option>%</option>.</para>
<para>If an <option>%</option> is followed by any other character, it
causes an abort.</para>
<para>If the file name specifies a relative file name, it is put
in the program's initial working directory : this is the current
directory when the program started its execution after the fork
or after the exec. If it specifies an absolute file name (ie.
starts with '/') then it is put there.
</para>
</listitem>
</varlistentry>
<varlistentry id="opt.log-socket" xreflabel="--log-socket">
<term>
<option><![CDATA[--log-socket=<ip-address:port-number> ]]></option>
</term>
<listitem>
<para>Specifies that Valgrind should send all of its messages to
the specified port at the specified IP address. The port may be
omitted, in which case port 1500 is used. If a connection cannot
be made to the specified socket, Valgrind falls back to writing
output to the standard error (stderr). This option is intended to
be used in conjunction with the
<computeroutput>valgrind-listener</computeroutput> program. For
further details, see
<link linkend="&vg-comment-id;">the commentary</link>
in the manual.</para>
</listitem>
</varlistentry>
</variablelist>
<!-- end of xi:include in the manpage -->
</sect2>
<sect2 id="manual-core.erropts" xreflabel="Error-related Options">
<title>Error-related Options</title>
<!-- start of xi:include in the manpage -->
<para id="error-related.opts.para">These options are used by all tools
that can report errors, e.g. Memcheck, but not Cachegrind.</para>
<variablelist id="error-related.opts.list">
<varlistentry id="opt.xml" xreflabel="--xml">
<term>
<option><![CDATA[--xml=<yes|no> [default: no] ]]></option>
</term>
<listitem>
<para>When enabled, the important parts of the output (e.g. tool error
messages) will be in XML format rather than plain text. Furthermore,
the XML output will be sent to a different output channel than the
plain text output. Therefore, you also must use one of
<option>--xml-fd</option>, <option>--xml-file</option> or
<option>--xml-socket</option> to specify where the XML is to be sent.
</para>
<para>Less important messages will still be printed in plain text, but
because the XML output and plain text output are sent to different
output channels (the destination of the plain text output is still
controlled by <option>--log-fd</option>, <option>--log-file</option>
and <option>--log-socket</option>) this should not cause problems.
</para>
<para>This option is aimed at making life easier for tools that consume
Valgrind's output as input, such as GUI front ends. Currently this
option works with Memcheck, Helgrind, DRD and SGcheck. The output
format is specified in the file
<computeroutput>docs/internals/xml-output-protocol4.txt</computeroutput>
in the source tree for Valgrind 3.5.0 or later.</para>
<para>The recommended options for a GUI to pass, when requesting
XML output, are: <option>--xml=yes</option> to enable XML output,
<option>--xml-file</option> to send the XML output to a (presumably
GUI-selected) file, <option>--log-file</option> to send the plain
text output to a second GUI-selected file,
<option>--child-silent-after-fork=yes</option>, and
<option>-q</option> to restrict the plain text output to critical
error messages created by Valgrind itself. For example, failure to
read a specified suppressions file counts as a critical error message.
In this way, for a successful run the text output file will be empty.
But if it isn't empty, then it will contain important information
which the GUI user should be made aware
of.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.xml-fd" xreflabel="--xml-fd">
<term>
<option><![CDATA[--xml-fd=<number> [default: -1, disabled] ]]></option>
</term>
<listitem>
<para>Specifies that Valgrind should send its XML output to the
specified file descriptor. It must be used in conjunction with
<option>--xml=yes</option>.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.xml-file" xreflabel="--xml-file">
<term>
<option><![CDATA[--xml-file=<filename> ]]></option>
</term>
<listitem>
<para>Specifies that Valgrind should send its XML output
to the specified file. It must be used in conjunction with
<option>--xml=yes</option>. Any <option>%p</option> or
<option>%q</option> sequences appearing in the filename are expanded
in exactly the same way as they are for <option>--log-file</option>.
See the description of <option>--log-file</option> for details.
</para>
</listitem>
</varlistentry>
<varlistentry id="opt.xml-socket" xreflabel="--xml-socket">
<term>
<option><![CDATA[--xml-socket=<ip-address:port-number> ]]></option>
</term>
<listitem>
<para>Specifies that Valgrind should send its XML output the
specified port at the specified IP address. It must be used in
conjunction with <option>--xml=yes</option>. The form of the argument
is the same as that used by <option>--log-socket</option>.
See the description of <option>--log-socket</option>
for further details.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.xml-user-comment" xreflabel="--xml-user-comment">
<term>
<option><![CDATA[--xml-user-comment=<string> ]]></option>
</term>
<listitem>
<para>Embeds an extra user comment string at the start of the XML
output. Only works when <option>--xml=yes</option> is specified;
ignored otherwise.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.demangle" xreflabel="--demangle">
<term>
<option><![CDATA[--demangle=<yes|no> [default: yes] ]]></option>
</term>
<listitem>
<para>Enable/disable automatic demangling (decoding) of C++ names.
Enabled by default. When enabled, Valgrind will attempt to
translate encoded C++ names back to something approaching the
original. The demangler handles symbols mangled by g++ versions
2.X, 3.X and 4.X.</para>
<para>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
suppression file contents dependent on the state of Valgrind's
demangling machinery, and also slow down suppression matching.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.num-callers" xreflabel="--num-callers">
<term>
<option><![CDATA[--num-callers=<number> [default: 12] ]]></option>
</term>
<listitem>
<para>Specifies the maximum number of entries shown in stack traces
that identify program locations. Note that errors are commoned up
using only the top four function locations (the place in the current
function, and that of its three immediate callers). So this doesn't
affect the total number of errors reported.</para>
<para>The maximum value for this is 500. 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.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.unw-stack-scan-thresh"
xreflabel="--unw-stack-scan-thresh">
<term>
<option><![CDATA[--unw-stack-scan-thresh=<number> [default: 0] ]]></option>
</term>
<term>
<option><![CDATA[--unw-stack-scan-frames=<number> [default: 5] ]]></option>
</term>
<listitem>
<para>Stack-scanning support is available only on ARM
targets.</para>
<para>These flags enable and control stack unwinding by stack
scanning. When the normal stack unwinding mechanisms -- usage
of Dwarf CFI records, and frame-pointer following -- fail, stack
scanning may be able to recover a stack trace.</para>
<para>Note that stack scanning is an imprecise, heuristic
mechanism that may give very misleading results, or none at all.
It should be used only in emergencies, when normal unwinding
fails, and it is important to nevertheless have stack
traces.</para>
<para>Stack scanning is a simple technique: the unwinder reads
words from the stack, and tries to guess which of them might be
return addresses, by checking to see if they point just after
ARM or Thumb call instructions. If so, the word is added to the
backtrace.</para>
<para>The main danger occurs when a function call returns,
leaving its return address exposed, and a new function is
called, but the new function does not overwrite the old address.
The result of this is that the backtrace may contain entries for
functions which have already returned, and so be very
confusing.</para>
<para>A second limitation of this implementation is that it will
scan only the page (4KB, normally) containing the starting stack
pointer. If the stack frames are large, this may result in only
a few (or not even any) being present in the trace. Also, if
you are unlucky and have an initial stack pointer near the end
of its containing page, the scan may miss all interesting
frames.</para>
<para>By default stack scanning is disabled. The normal use
case is to ask for it when a stack trace would otherwise be very
short. So, to enable it,
use <computeroutput>--unw-stack-scan-thresh=number</computeroutput>.
This requests Valgrind to try using stack scanning to "extend"
stack traces which contain fewer
than <computeroutput>number</computeroutput> frames.</para>
<para>If stack scanning does take place, it will only generate
at most the number of frames specified
by <computeroutput>--unw-stack-scan-frames</computeroutput>.
Typically, stack scanning generates so many garbage entries that
this value is set to a low value (5) by default. In no case
will a stack trace larger than the value specified
by <computeroutput>--num-callers</computeroutput> be
created.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.error-limit" xreflabel="--error-limit">
<term>
<option><![CDATA[--error-limit=<yes|no> [default: yes] ]]></option>
</term>
<listitem>
<para>When enabled, Valgrind stops reporting errors after 10,000,000
in total, or 1,000 different ones, have been seen. This is to
stop the error tracking machinery from becoming a huge performance
overhead in programs with many errors.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.error-exitcode" xreflabel="--error-exitcode">
<term>
<option><![CDATA[--error-exitcode=<number> [default: 0] ]]></option>
</term>
<listitem>
<para>Specifies an alternative exit code to return if Valgrind
reported any errors in the run. When set to the default value
(zero), the return value from Valgrind will always be the return
value of the process being simulated. When set to a nonzero value,
that value is returned instead, if Valgrind detects any errors.
This is useful for using Valgrind as part of an automated test
suite, since it makes it easy to detect test cases for which
Valgrind has reported errors, just by inspecting return codes.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.sigill-diagnostics" xreflabel="--sigill-diagnostics">
<term>
<option><![CDATA[--sigill-diagnostics=<yes|no> [default: yes] ]]></option>
</term>
<listitem>
<para>Enable/disable printing of illegal instruction diagnostics.
Enabled by default, but defaults to disabled when
<option>--quiet</option> is given. The default can always be explicitly
overridden by giving this option.</para>
<para>When enabled, a warning message will be printed, along with some
diagnostics, whenever an instruction is encountered that Valgrind
cannot decode or translate, before the program is given a SIGILL signal.
Often an illegal instruction indicates a bug in the program or missing
support for the particular instruction in Valgrind. But some programs
do deliberately try to execute an instruction that might be missing
and trap the SIGILL signal to detect processor features. Using
this flag makes it possible to avoid the diagnostic output
that you would otherwise get in such cases.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.show-below-main" xreflabel="--show-below-main">
<term>
<option><![CDATA[--show-below-main=<yes|no> [default: no] ]]></option>
</term>
<listitem>
<para>By default, stack traces for errors do not show any
functions that appear beneath <function>main</function> because
most of the time it's uninteresting C library stuff and/or
gobbledygook. Alternatively, if <function>main</function> is not
present in the stack trace, stack traces will not show any functions
below <function>main</function>-like functions such as glibc's
<function>__libc_start_main</function>. Furthermore, if
<function>main</function>-like functions are present in the trace,
they are normalised as <function>(below main)</function>, in order to
make the output more deterministic.</para>
<para>If this option is enabled, all stack trace entries will be
shown and <function>main</function>-like functions will not be
normalised.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.fullpath-after" xreflabel="--fullpath-after">
<term>
<option><![CDATA[--fullpath-after=<string>
[default: don't show source paths] ]]></option>
</term>
<listitem>
<para>By default Valgrind only shows the filenames in stack
traces, but not full paths to source files. When using Valgrind
in large projects where the sources reside in multiple different
directories, this can be inconvenient.
<option>--fullpath-after</option> provides a flexible solution
to this problem. When this option is present, the path to each
source file is shown, with the following all-important caveat:
if <option>string</option> is found in the path, then the path
up to and including <option>string</option> is omitted, else the
path is shown unmodified. Note that <option>string</option> is
not required to be a prefix of the path.</para>
<para>For example, consider a file named
<computeroutput>/home/janedoe/blah/src/foo/bar/xyzzy.c</computeroutput>.
Specifying <option>--fullpath-after=/home/janedoe/blah/src/</option>
will cause Valgrind to show the name
as <computeroutput>foo/bar/xyzzy.c</computeroutput>.</para>
<para>Because the string is not required to be a prefix,
<option>--fullpath-after=src/</option> will produce the same
output. This is useful when the path contains arbitrary
machine-generated characters. For example, the
path
<computeroutput>/my/build/dir/C32A1B47/blah/src/foo/xyzzy</computeroutput>
can be pruned to <computeroutput>foo/xyzzy</computeroutput>
using
<option>--fullpath-after=/blah/src/</option>.</para>
<para>If you simply want to see the full path, just specify an
empty string: <option>--fullpath-after=</option>. This isn't a
special case, merely a logical consequence of the above rules.</para>
<para>Finally, you can use <option>--fullpath-after</option>
multiple times. Any appearance of it causes Valgrind to switch
to producing full paths and applying the above filtering rule.
Each produced path is compared against all
the <option>--fullpath-after</option>-specified strings, in the
order specified. The first string to match causes the path to
be truncated as described above. If none match, the full path
is shown. This facilitates chopping off prefixes when the
sources are drawn from a number of unrelated directories.
</para>
</listitem>
</varlistentry>
<varlistentry id="opt.extra-debuginfo-path" xreflabel="--extra-debuginfo-path">
<term>
<option><![CDATA[--extra-debuginfo-path=<path> [default: undefined and unused] ]]></option>
</term>
<listitem>
<para>By default Valgrind searches in several well-known paths
for debug objects, such
as <computeroutput>/usr/lib/debug/</computeroutput>.</para>
<para>However, there may be scenarios where you may wish to put
debug objects at an arbitrary location, such as external storage
when running Valgrind on a mobile device with limited local
storage. Another example might be a situation where you do not
have permission to install debug object packages on the system
where you are running Valgrind.</para>
<para>In these scenarios, you may provide an absolute path as an extra,
final place for Valgrind to search for debug objects by specifying
<option>--extra-debuginfo-path=/path/to/debug/objects</option>.
The given path will be prepended to the absolute path name of
the searched-for object. For example, if Valgrind is looking
for the debuginfo
for <computeroutput>/w/x/y/zz.so</computeroutput>
and <option>--extra-debuginfo-path=/a/b/c</option> is specified,
it will look for a debug object at
<computeroutput>/a/b/c/w/x/y/zz.so</computeroutput>.</para>
<para>This flag should only be specified once. If it is
specified multiple times, only the last instance is
honoured.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.debuginfo-server" xreflabel="--debuginfo-server">
<term>
<option><![CDATA[--debuginfo-server=ipaddr:port [default: undefined and unused]]]></option>
</term>
<listitem>
<para>This is a new, experimental, feature introduced in version
3.9.0.</para>
<para>In some scenarios it may be convenient to read debuginfo
from objects stored on a different machine. With this flag,
Valgrind will query a debuginfo server running
on <computeroutput>ipaddr</computeroutput> and listening on
port <computeroutput>port</computeroutput>, if it cannot find
the debuginfo object in the local filesystem.</para>
<para>The debuginfo server must accept TCP connections on
port <computeroutput>port</computeroutput>. The debuginfo
server is contained in the source
file <computeroutput>auxprogs/valgrind-di-server.c</computeroutput>.
It will only serve from the directory it is started
in. <computeroutput>port</computeroutput> defaults to 1500 in
both client and server if not specified.</para>
<para>If Valgrind looks for the debuginfo for
<computeroutput>/w/x/y/zz.so</computeroutput> by using the
debuginfo server, it will strip the pathname components and
merely request <computeroutput>zz.so</computeroutput> on the
server. That in turn will look only in its current working
directory for a matching debuginfo object.</para>
<para>The debuginfo data is transmitted in small fragments (8
KB) as requested by Valgrind. Each block is compressed using
LZO to reduce transmission time. The implementation has been
tuned for best performance over a single-stage 802.11g (WiFi)
network link.</para>
<para>Note that checks for matching primary vs debug objects,
using GNU debuglink CRC scheme, are performed even when using
the debuginfo server. To disable such checking, you need to
also specify
<computeroutput>--allow-mismatched-debuginfo=yes</computeroutput>.
</para>
<para>By default the Valgrind build system will
build <computeroutput>valgrind-di-server</computeroutput> for
the target platform, which is almost certainly not what you
want. So far we have been unable to find out how to get
automake/autoconf to build it for the build platform. If
you want to use it, you will have to recompile it by hand using
the command shown at the top
of <computeroutput>auxprogs/valgrind-di-server.c</computeroutput>.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.allow-mismatched-debuginfo"
xreflabel="--allow-mismatched-debuginfo">
<term>
<option><![CDATA[--allow-mismatched-debuginfo=no|yes [no] ]]></option>
</term>
<listitem>
<para>When reading debuginfo from separate debuginfo objects,
Valgrind will by default check that the main and debuginfo
objects match, using the GNU debuglink mechanism. This
guarantees that it does not read debuginfo from out of date
debuginfo objects, and also ensures that Valgrind can't crash as
a result of mismatches.</para>
<para>This check can be overridden using
<computeroutput>--allow-mismatched-debuginfo=yes</computeroutput>.
This may be useful when the debuginfo and main objects have not
been split in the proper way. Be careful when using this,
though: it disables all consistency checking, and Valgrind has
been observed to crash when the main and debuginfo objects don't
match.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.suppressions" xreflabel="--suppressions">
<term>
<option><![CDATA[--suppressions=<filename> [default: $PREFIX/lib/valgrind/default.supp] ]]></option>
</term>
<listitem>
<para>Specifies an extra file from which to read descriptions of
errors to suppress. You may use up to 100 extra suppression
files.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.gen-suppressions" xreflabel="--gen-suppressions">
<term>
<option><![CDATA[--gen-suppressions=<yes|no|all> [default: no] ]]></option>
</term>
<listitem>
<para>When set to <varname>yes</varname>, Valgrind will pause
after every error shown and print the line:
<literallayout><computeroutput> ---- Print suppression ? --- [Return/N/n/Y/y/C/c] ----</computeroutput></literallayout>
The prompt's behaviour is the same as for the
<option>--db-attach</option> option (see below).</para>
<para>If you choose to, Valgrind will print out a suppression for
this error. You can then cut and paste it into a suppression file
if you don't want to hear about the error in the future.</para>
<para>When set to <varname>all</varname>, Valgrind will print a
suppression for every reported error, without querying the
user.</para>
<para>This option is particularly useful with C++ programs, as it
prints out the suppressions with mangled names, as
required.</para>
<para>Note that the suppressions printed are as specific as
possible. You may want to common up similar ones, by adding
wildcards to function names, and by using frame-level wildcards.
The wildcarding facilities are powerful yet flexible, and with a
bit of careful editing, you may be able to suppress a whole
family of related errors with only a few suppressions.
<!-- commented out because it causes broken links in the man page
For details on how to do this, see
<xref linkend="manual-core.suppress"/>.
-->
</para>
<para>Sometimes two different errors
are suppressed by the same suppression, in which case Valgrind
will output the suppression more than once, but you only need to
have one copy in your suppression file (but having more than one
won't cause problems). Also, the suppression name is given as
<computeroutput>&lt;insert a suppression name
here&gt;</computeroutput>; the name doesn't really matter, it's
only used with the <option>-v</option> option which prints out all
used suppression records.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.db-attach" xreflabel="--db-attach">
<term>
<option><![CDATA[--db-attach=<yes|no> [default: no] ]]></option>
</term>
<listitem>
<para>When enabled, Valgrind will pause after every error shown
and print the line:
<literallayout><computeroutput> ---- Attach to debugger ? --- [Return/N/n/Y/y/C/c] ----</computeroutput></literallayout>
Pressing <varname>Ret</varname>, or <varname>N Ret</varname> or
<varname>n Ret</varname>, causes Valgrind not to start a debugger
for this error.</para>
<para>Pressing <varname>Y Ret</varname> or
<varname>y Ret</varname> causes Valgrind to start a debugger for
the program at this point. When you have finished with the
debugger, quit from it, and the program will continue. Trying to
continue from inside the debugger doesn't work.</para>
<para>
Note: if you use GDB, more powerful debugging support is
provided by the <option>--vgdb=</option> <varname>yes</varname>
or <varname>full</varname> value. This activates Valgrind's
internal gdbserver, which provides more-or-less full GDB-style
control of the application: insertion of breakpoints, continuing
from inside GDB, inferior function calls, and much more.
</para>
<para><varname>C Ret</varname> or <varname>c Ret</varname> causes
Valgrind not to start a debugger, and not to ask again.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.db-command" xreflabel="--db-command">
<term>
<option><![CDATA[--db-command=<command> [default: gdb -nw %f %p] ]]></option>
</term>
<listitem>
<para>Specify the debugger to use with the
<option>--db-attach</option> command. The default debugger is
GDB. This option is a template that is expanded by Valgrind at
runtime. <literal>%f</literal> is replaced with the executable's
file name and <literal>%p</literal> is replaced by the process ID
of the executable.</para>
<para>This specifies how Valgrind will invoke the debugger. By
default it will use whatever GDB is detected at build time, which
is usually <computeroutput>/usr/bin/gdb</computeroutput>. Using
this command, you can specify some alternative command to invoke
the debugger you want to use.</para>
<para>The command string given can include one or instances of the
<literal>%p</literal> and <literal>%f</literal> expansions. Each
instance of <literal>%p</literal> expands to the PID of the
process to be debugged and each instance of <literal>%f</literal>
expands to the path to the executable for the process to be
debugged.</para>
<para>Since <computeroutput>&lt;command&gt;</computeroutput> is likely
to contain spaces, you will need to put this entire option in
quotes to ensure it is correctly handled by the shell.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.input-fd" xreflabel="--input-fd">
<term>
<option><![CDATA[--input-fd=<number> [default: 0, stdin] ]]></option>
</term>
<listitem>
<para>When using <option>--db-attach=yes</option> or
<option>--gen-suppressions=yes</option>, Valgrind will stop so as
to read keyboard input from you when each error occurs. By
default it reads from the standard input (stdin), which is
problematic for programs which close stdin. This option allows
you to specify an alternative file descriptor from which to read
input.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.dsymutil" xreflabel="--dsymutil">
<term>
<option><![CDATA[--dsymutil=no|yes [no] ]]></option>
</term>
<listitem>
<para>This option is only relevant when running Valgrind on
Mac OS X.</para>
<para>Mac OS X uses a deferred debug information (debuginfo)
linking scheme. When object files containing debuginfo are
linked into a <computeroutput>.dylib</computeroutput> or an
executable, the debuginfo is not copied into the final file.
Instead, the debuginfo must be linked manually by
running <computeroutput>dsymutil</computeroutput>, a
system-provided utility, on the executable
or <computeroutput>.dylib</computeroutput>. The resulting
combined debuginfo is placed in a directory alongside the
executable or <computeroutput>.dylib</computeroutput>, but with
the extension <computeroutput>.dSYM</computeroutput>.</para>
<para>With <option>--dsymutil=no</option>, Valgrind
will detect cases where the
<computeroutput>.dSYM</computeroutput> directory is either
missing, or is present but does not appear to match the
associated executable or <computeroutput>.dylib</computeroutput>,
most likely because it is out of date. In these cases, Valgrind
will print a warning message but take no further action.</para>
<para>With <option>--dsymutil=yes</option>, Valgrind
will, in such cases, automatically
run <computeroutput>dsymutil</computeroutput> as necessary to
bring the debuginfo up to date. For all practical purposes, if
you always use <option>--dsymutil=yes</option>, then
there is never any need to
run <computeroutput>dsymutil</computeroutput> manually or as part
of your applications's build system, since Valgrind will run it
as necessary.</para>
<para>Valgrind will not attempt to
run <computeroutput>dsymutil</computeroutput> on any
executable or library in
<computeroutput>/usr/</computeroutput>,
<computeroutput>/bin/</computeroutput>,
<computeroutput>/sbin/</computeroutput>,
<computeroutput>/opt/</computeroutput>,
<computeroutput>/sw/</computeroutput>,
<computeroutput>/System/</computeroutput>,
<computeroutput>/Library/</computeroutput> or
<computeroutput>/Applications/</computeroutput>
since <computeroutput>dsymutil</computeroutput> will always fail
in such situations. It fails both because the debuginfo for
such pre-installed system components is not available anywhere,
and also because it would require write privileges in those
directories.</para>
<para>Be careful when
using <option>--dsymutil=yes</option>, since it will
cause pre-existing <computeroutput>.dSYM</computeroutput>
directories to be silently deleted and re-created. Also note that
<computeroutput>dsymutil</computeroutput> is quite slow, sometimes
excessively so.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.max-stackframe" xreflabel="--max-stackframe">
<term>
<option><![CDATA[--max-stackframe=<number> [default: 2000000] ]]></option>
</term>
<listitem>
<para>The maximum size of a stack frame. If the stack pointer moves by
more than this amount then Valgrind will assume that
the program is switching to a different stack.</para>
<para>You may need to use this option if your program has large
stack-allocated arrays. Valgrind keeps track of your program's
stack pointer. If it changes by more than the threshold amount,
Valgrind assumes your program is switching to a different stack,
and Memcheck behaves differently than it would for a stack pointer
change smaller than the threshold. Usually this heuristic works
well. However, if your program allocates large structures on the
stack, this heuristic will be fooled, and Memcheck will
subsequently report large numbers of invalid stack accesses. This
option allows you to change the threshold to a different
value.</para>
<para>You should only consider use of this option if Valgrind's
debug output directs you to do so. In that case it will tell you
the new threshold you should specify.</para>
<para>In general, allocating large structures on the stack is a
bad idea, because you can easily run out of stack space,
especially on systems with limited memory or which expect to
support large numbers of threads each with a small stack, and also
because the error checking performed by Memcheck is more effective
for heap-allocated data than for stack-allocated data. If you
have to use this option, you may wish to consider rewriting your
code to allocate on the heap rather than on the stack.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.main-stacksize" xreflabel="--main-stacksize">
<term>
<option><![CDATA[--main-stacksize=<number>
[default: use current 'ulimit' value] ]]></option>
</term>
<listitem>
<para>Specifies the size of the main thread's stack.</para>
<para>To simplify its memory management, Valgrind reserves all
required space for the main thread's stack at startup. That
means it needs to know the required stack size at
startup.</para>
<para>By default, Valgrind uses the current "ulimit" value for
the stack size, or 16 MB, whichever is lower. In many cases
this gives a stack size in the range 8 to 16 MB, which almost
never overflows for most applications.</para>
<para>If you need a larger total stack size,
use <option>--main-stacksize</option> to specify it. Only set
it as high as you need, since reserving far more space than you
need (that is, hundreds of megabytes more than you need)
constrains Valgrind's memory allocators and may reduce the total
amount of memory that Valgrind can use. This is only really of
significance on 32-bit machines.</para>
<para>On Linux, you may request a stack of size up to 2GB.
Valgrind will stop with a diagnostic message if the stack cannot
be allocated.</para>
<para><option>--main-stacksize</option> only affects the stack
size for the program's initial thread. It has no bearing on the
size of thread stacks, as Valgrind does not allocate
those.</para>
<para>You may need to use both <option>--main-stacksize</option>
and <option>--max-stackframe</option> together. It is important
to understand that <option>--main-stacksize</option> sets the
maximum total stack size,
whilst <option>--max-stackframe</option> specifies the largest
size of any one stack frame. You will have to work out
the <option>--main-stacksize</option> value for yourself
(usually, if your applications segfaults). But Valgrind will
tell you the needed <option>--max-stackframe</option> size, if
necessary.</para>
<para>As discussed further in the description
of <option>--max-stackframe</option>, a requirement for a large
stack is a sign of potential portability problems. You are best
advised to place all large data in heap-allocated memory.</para>
</listitem>
</varlistentry>
</variablelist>
<!-- end of xi:include in the manpage -->
</sect2>
<sect2 id="manual-core.mallocopts" xreflabel="malloc-related Options">
<title>malloc-related Options</title>
<!-- start of xi:include in the manpage -->
<para id="malloc-related.opts.para">For tools that use their own version of
<computeroutput>malloc</computeroutput> (e.g. Memcheck,
Massif, Helgrind, DRD), the following options apply.</para>
<variablelist id="malloc-related.opts.list">
<varlistentry id="opt.alignment" xreflabel="--alignment">
<term>
<option><![CDATA[--alignment=<number> [default: 8 or 16, depending on the platform] ]]></option>
</term>
<listitem>
<para>By default Valgrind's <function>malloc</function>,
<function>realloc</function>, etc, return a block whose starting
address is 8-byte aligned or 16-byte aligned (the value depends on the
platform and matches the platform default). This option allows you to
specify a different alignment. The supplied value must be greater
than or equal to the default, less than or equal to 4096, and must be
a power of two.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.redzone-size" xreflabel="--redzone-size">
<term>
<option><![CDATA[--redzone-size=<number> [default: depends on the tool] ]]></option>
</term>
<listitem>
<para> Valgrind's <function>malloc, realloc,</function> etc, add
padding blocks before and after each heap block allocated by the
program being run. Such padding blocks are called redzones. The
default value for the redzone size depends on the tool. For
example, Memcheck adds and protects a minimum of 16 bytes before
and after each block allocated by the client. This allows it to
detect block underruns or overruns of up to 16 bytes.
</para>
<para>Increasing the redzone size makes it possible to detect
overruns of larger distances, but increases the amount of memory
used by Valgrind. Decreasing the redzone size will reduce the
memory needed by Valgrind but also reduces the chances of
detecting over/underruns, so is not recommended.</para>
</listitem>
</varlistentry>
</variablelist>
<!-- end of xi:include in the manpage -->
</sect2>
<sect2 id="manual-core.rareopts" xreflabel="Uncommon Options">
<title>Uncommon Options</title>
<!-- start of xi:include in the manpage -->
<para id="uncommon.opts.para">These options apply to all tools, as they
affect certain obscure workings of the Valgrind core. Most people won't
need to use them.</para>
<variablelist id="uncommon.opts.list">
<varlistentry id="opt.smc-check" xreflabel="--smc-check">
<term>
<option><![CDATA[--smc-check=<none|stack|all|all-non-file> [default: stack] ]]></option>
</term>
<listitem>
<para>This option controls Valgrind's detection of self-modifying
code. If no checking is done, if a program executes some code, then
overwrites it with new code, and executes the new code, Valgrind will
continue to execute the translations it made for the old code. This
will likely lead to incorrect behaviour and/or crashes.</para>
<para>Valgrind has four levels of self-modifying code detection:
no detection, detect self-modifying code on the stack (which is used by
GCC to implement nested functions), detect self-modifying code
everywhere, and detect self-modifying code everywhere except in
file-backed mappings.
Note that the default option will catch the vast majority
of cases. The main case it will not catch is programs such as JIT
compilers that dynamically generate code <emphasis>and</emphasis>
subsequently overwrite part or all of it. Running with
<varname>all</varname> will slow Valgrind down noticeably.
Running with
<varname>none</varname> will rarely speed things up, since very little
code gets put on the stack for most programs. The
<function>VALGRIND_DISCARD_TRANSLATIONS</function> client
request is an alternative to <option>--smc-check=all</option>
that requires more programmer effort but allows Valgrind to run
your program faster, by telling it precisely when translations
need to be re-made.
<!-- commented out because it causes broken links in the man page
; see <xref
linkend="manual-core-adv.clientreq"/> for more details.
-->
</para>
<para><option>--smc-check=all-non-file</option> provides a
cheaper but more limited version
of <option>--smc-check=all</option>. It adds checks to any
translations that do not originate from file-backed memory
mappings. Typical applications that generate code, for example
JITs in web browsers, generate code into anonymous mmaped areas,
whereas the "fixed" code of the browser always lives in
file-backed mappings. <option>--smc-check=all-non-file</option>
takes advantage of this observation, limiting the overhead of
checking to code which is likely to be JIT generated.</para>
<para>Some architectures (including ppc32, ppc64, ARM and MIPS)
require programs which create code at runtime to flush the
instruction cache in between code generation and first use.
Valgrind observes and honours such instructions. Hence, on
ppc32/Linux, ppc64/Linux and ARM/Linux, Valgrind always provides
complete, transparent support for self-modifying code. It is
only on platforms such as x86/Linux, AMD64/Linux, x86/Darwin and
AMD64/Darwin that you need to use this option.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.read-inline-info" xreflabel="--read-inline-info">
<term>
<option><![CDATA[--read-inline-info=<yes|no> [default: no] ]]></option>
</term>
<listitem>
<para>When enabled, Valgrind will read information about
inlined function calls from DWARF3 debug info.
This slows Valgrind startup and makes it use more memory (typically
for each inlined piece of code, 6 words + the function name), but
it results in more descriptive stacktraces:</para>
<programlisting><![CDATA[
==15380== Conditional jump or move depends on uninitialised value(s)
==15380== at 0x80484EA: main (inlinfo.c:6)
==15380==
==15380== Conditional jump or move depends on uninitialised value(s)
==15380== at 0x8048550: fun_noninline (inlinfo.c:6)
==15380== by 0x804850E: main (inlinfo.c:34)
==15380==
==15380== Conditional jump or move depends on uninitialised value(s)
==15380== at 0x8048520: main (inlinfo.c:6)]]></programlisting>
<para>And here are the same errors with
<option>--read-inline-info=yes</option>:</para>
<programlisting><![CDATA[
==15377== Conditional jump or move depends on uninitialised value(s)
==15377== at 0x80484EA: fun_d (inlinfo.c:6)
==15377== by 0x80484EA: fun_c (inlinfo.c:14)
==15377== by 0x80484EA: fun_b (inlinfo.c:20)
==15377== by 0x80484EA: fun_a (inlinfo.c:26)
==15377== by 0x80484EA: main (inlinfo.c:33)
==15377==
==15377== Conditional jump or move depends on uninitialised value(s)
==15377== at 0x8048550: fun_d (inlinfo.c:6)
==15377== by 0x8048550: fun_noninline (inlinfo.c:41)
==15377== by 0x804850E: main (inlinfo.c:34)
==15377==
==15377== Conditional jump or move depends on uninitialised value(s)
==15377== at 0x8048520: fun_d (inlinfo.c:6)
==15377== by 0x8048520: main (inlinfo.c:35)
]]></programlisting>
</listitem>
</varlistentry>
<varlistentry id="opt.read-var-info" xreflabel="--read-var-info">
<term>
<option><![CDATA[--read-var-info=<yes|no> [default: no] ]]></option>
</term>
<listitem>
<para>When enabled, Valgrind will read information about
variable types and locations from DWARF3 debug info.
This slows Valgrind startup significantly and makes it use significantly
more memory, but for the tools that can take advantage of it (Memcheck,
Helgrind, DRD) it can result in more precise error messages. For example,
here are some standard errors issued by Memcheck:</para>
<programlisting><![CDATA[
==15363== Uninitialised byte(s) found during client check request
==15363== at 0x80484A9: croak (varinfo1.c:28)
==15363== by 0x8048544: main (varinfo1.c:55)
==15363== Address 0x80497f7 is 7 bytes inside data symbol "global_i2"
==15363==
==15363== Uninitialised byte(s) found during client check request
==15363== at 0x80484A9: croak (varinfo1.c:28)
==15363== by 0x8048550: main (varinfo1.c:56)
==15363== Address 0xbea0d0cc is on thread 1's stack
==15363== in frame #1, created by main (varinfo1.c:45)
></programlisting>
<para>And here are the same errors with
<option>--read-var-info=yes</option>:</para>
<programlisting><![CDATA[
==15370== Uninitialised byte(s) found during client check request
==15370== at 0x80484A9: croak (varinfo1.c:28)
==15370== by 0x8048544: main (varinfo1.c:55)
==15370== Location 0x80497f7 is 0 bytes inside global_i2[7],
==15370== a global variable declared at varinfo1.c:41
==15370==
==15370== Uninitialised byte(s) found during client check request
==15370== at 0x80484A9: croak (varinfo1.c:28)
==15370== by 0x8048550: main (varinfo1.c:56)
==15370== Location 0xbeb4a0cc is 0 bytes inside local var "local"
==15370== declared at varinfo1.c:46, in frame #1 of thread 1
]]></programlisting>
</listitem>
</varlistentry>
<varlistentry id="opt.vgdb-poll" xreflabel="--vgdb-poll">
<term>
<option><![CDATA[--vgdb-poll=<number> [default: 5000] ]]></option>
</term>
<listitem>
<para> As part of its main loop, the Valgrind scheduler will
poll to check if some activity (such as an external command or
some input from a gdb) has to be handled by gdbserver. This
activity poll will be done after having run the given number of
basic blocks (or slightly more than the given number of basic
blocks). This poll is quite cheap so the default value is set
relatively low. You might further decrease this value if vgdb
cannot use ptrace system call to interrupt Valgrind if all
threads are (most of the time) blocked in a system call.
</para>
</listitem>
</varlistentry>
<varlistentry id="opt.vgdb-shadow-registers" xreflabel="--vgdb-shadow-registers">
<term>
<option><![CDATA[--vgdb-shadow-registers=no|yes [default: no] ]]></option>
</term>
<listitem>
<para> When activated, gdbserver will expose the Valgrind shadow registers
to GDB. With this, the value of the Valgrind shadow registers can be examined
or changed using GDB. Exposing shadow registers only works with GDB version
7.1 or later.
</para>
</listitem>
</varlistentry>
<varlistentry id="opt.vgdb-prefix" xreflabel="--vgdb-prefix">
<term>
<option><![CDATA[--vgdb-prefix=<prefix> [default: /tmp/vgdb-pipe] ]]></option>
</term>
<listitem>
<para> To communicate with gdb/vgdb, the Valgrind gdbserver
creates 3 files (2 named FIFOs and a mmap shared memory
file). The prefix option controls the directory and prefix for
the creation of these files.
</para>
</listitem>
</varlistentry>
<varlistentry id="opt.run-libc-freeres" xreflabel="--run-libc-freeres">
<term>
<option><![CDATA[--run-libc-freeres=<yes|no> [default: yes] ]]></option>
</term>
<listitem>
<para>This option is only relevant when running Valgrind on Linux.</para>
<para>The GNU C library (<function>libc.so</function>), which is
used by all programs, may allocate memory for its own uses.
Usually it doesn't bother to free that memory when the program
ends&mdash;there would be no point, since the Linux kernel reclaims
all process resources when a process exits anyway, so it would
just slow things down.</para>
<para>The glibc authors realised that this behaviour causes leak
checkers, such as Valgrind, to falsely report leaks in glibc, when
a leak check is done at exit. In order to avoid this, they
provided a routine called <function>__libc_freeres</function>
specifically to make glibc release all memory it has allocated.
Memcheck therefore tries to run
<function>__libc_freeres</function> at exit.</para>
<para>Unfortunately, in some very old versions of glibc,
<function>__libc_freeres</function> is sufficiently buggy to cause
segmentation faults. This was particularly noticeable on Red Hat
7.1. So this option is provided in order to inhibit the run of
<function>__libc_freeres</function>. If your program seems to run
fine on Valgrind, but segfaults at exit, you may find that
<option>--run-libc-freeres=no</option> fixes that, although at the
cost of possibly falsely reporting space leaks in
<filename>libc.so</filename>.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.sim-hints" xreflabel="--sim-hints">
<term>
<option><![CDATA[--sim-hints=hint1,hint2,... ]]></option>
</term>
<listitem>
<para>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 hints
are enabled. Use with caution! Currently known hints are:</para>
<itemizedlist>
<listitem>
<para><option>lax-ioctls: </option> Be very lax about ioctl
handling; the only assumption is that the size is
correct. Doesn't require the full buffer to be initialized
when writing. Without this, using some device drivers with a
large number of strange ioctl commands becomes very
tiresome.</para>
</listitem>
<listitem>
<para><option>fuse-compatible: </option> Enable special
handling for certain system calls that may block in a FUSE
file-system. This may be necessary when running Valgrind
on a multi-threaded program that uses one thread to manage
a FUSE file-system and another thread to access that
file-system.
</para>
</listitem>
<listitem>
<para><option>enable-outer: </option> Enable some special
magic needed when the program being run is itself
Valgrind.</para>
</listitem>
<listitem>
<para><option>no-inner-prefix: </option> Disable printing
a prefix <option>&gt;</option> in front of each stdout or
stderr output line in an inner Valgrind being run by an
outer Valgrind. This is useful when running Valgrind
regression tests in an outer/inner setup. Note that the
prefix <option>&gt;</option> will always be printed in
front of the inner debug logging lines.</para>
</listitem>
<listitem>
<para><option>no-nptl-pthread-stackcache: </option>
This hint is only relevant when running Valgrind on Linux.</para>
<para>The GNU glibc pthread library
(<function>libpthread.so</function>), which is used by
pthread programs, maintains a cache of pthread stacks.
When a pthread terminates, the memory used for the pthread
stack and some thread local storage related data structure
are not always directly released. This memory is kept in
a cache (up to a certain size), and is re-used if a new
thread is started.</para>
<para>This cache causes the helgrind tool to report some
false positive race condition errors on this cached
memory, as helgrind does not understand the internal glibc
cache synchronisation primitives. So, when using helgrind,
disabling the cache helps to avoid false positive race
conditions, in particular when using thread local storage
variables (e.g. variables using the
<function>__thread</function> qualifier).</para>
<para>When using the memcheck tool, disabling the cache
ensures the memory used by glibc to handle __thread
variables is directly released when a thread
terminates.</para>
<para>Note: Valgrind disables the cache using some internal
knowledge of the glibc stack cache implementation and by
examining the debug information of the pthread
library. This technique is thus somewhat fragile and might
not work for all glibc versions. This has been succesfully
tested with various glibc versions (e.g. 2.11, 2.16, 2.18)
on various platforms.</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry id="opt.fair-sched" xreflabel="--fair-sched">
<term>
<option><![CDATA[--fair-sched=<no|yes|try> [default: no] ]]></option>
</term>
<listitem> <para>The <option>--fair-sched</option> option controls
the locking mechanism used by Valgrind to serialise thread
execution. The locking mechanism controls the way the threads
are scheduled, and different settings give different trade-offs
between fairness and performance. For more details about the
Valgrind thread serialisation scheme and its impact on
performance and thread scheduling, see
<xref linkend="&vg-pthreads-perf-sched-id;"/>.</para>
<itemizedlist>
<listitem> <para>The value <option>--fair-sched=yes</option>
activates a fair scheduler. In short, if multiple threads are
ready to run, the threads will be scheduled in a round robin
fashion. This mechanism is not available on all platforms or
Linux versions. If not available,
using <option>--fair-sched=yes</option> will cause Valgrind to
terminate with an error.</para>
<para>You may find this setting improves overall
responsiveness if you are running an interactive
multithreaded program, for example a web browser, on
Valgrind.</para>
</listitem>
<listitem> <para>The value <option>--fair-sched=try</option>
activates fair scheduling if available on the
platform. Otherwise, it will automatically fall back
to <option>--fair-sched=no</option>.</para>
</listitem>
<listitem> <para>The value <option>--fair-sched=no</option> activates
a scheduler which does not guarantee fairness
between threads ready to run, but which in general gives the
highest performance.</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry id="opt.kernel-variant" xreflabel="--kernel-variant">
<term>
<option>--kernel-variant=variant1,variant2,...</option>
</term>
<listitem>
<para>Handle system calls and ioctls arising from minor variants
of the default kernel for this platform. This is useful for
running on hacked kernels or with kernel modules which support
nonstandard ioctls, for example. Use with caution. If you don't
understand what this option does then you almost certainly don't
need it. Currently known variants are:</para>
<itemizedlist>
<listitem>
<para><option>bproc: </option> Support the
<function>sys_broc</function> system call on x86. This is for
running on BProc, which is a minor variant of standard Linux which
is sometimes used for building clusters.</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
<varlistentry id="opt.merge-recursive-frames" xreflabel="--merge-recursive-frames">
<term>
<option><![CDATA[--merge-recursive-frames=<number> [default: 0] ]]></option>
</term>
<listitem>
<para>Some recursive algorithms, for example balanced binary
tree implementations, create many different stack traces, each
containing cycles of calls. A cycle is defined as two identical
program counter values separated by zero or more other program
counter values. Valgrind may then use a lot of memory to store
all these stack traces. This is a poor use of memory
considering that such stack traces contain repeated
uninteresting recursive calls instead of more interesting
information such as the function that has initiated the
recursive call.
</para>
<para>The option <option>--merge-recursive-frames=&lt;number&gt;</option>
instructs Valgrind to detect and merge recursive call cycles
having a size of up to <option>&lt;number&gt;</option>
frames. When such a cycle is detected, Valgrind records the
cycle in the stack trace as a unique program counter.
</para>
<para>
The value 0 (the default) causes no recursive call merging.
A value of 1 will cause stack traces of simple recursive algorithms
(for example, a factorial implementation) to be collapsed.
A value of 2 will usually be needed to collapse stack traces produced
by recursive algorithms such as binary trees, quick sort, etc.
Higher values might be needed for more complex recursive algorithms.
</para>
<para>Note: recursive calls are detected by analysis of program
counter values. They are not detected by looking at function
names.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.num-transtab-sectors" xreflabel="--num-transtab-sectors">
<term>
<option><![CDATA[--num-transtab-sectors=<number> [default: 6
for Android platforms, 16 for all others] ]]></option>
</term>
<listitem>
<para>Valgrind translates and instruments your program's machine
code in small fragments. The translations are stored in a
translation cache that is divided into a number of sections
(sectors). If the cache is full, the sector containing the
oldest translations is emptied and reused. If these old
translations are needed again, Valgrind must re-translate and
re-instrument the corresponding machine code, which is
expensive. If the "executed instructions" working set of a
program is big, increasing the number of sectors may improve
performance by reducing the number of re-translations needed.
Sectors are allocated on demand. Once allocated, a sector can
never be freed, and occupies considerable space, depending on the tool
(about 40 MB per sector for Memcheck). Use the
option <option>--stats=yes</option> to obtain precise
information about the memory used by a sector and the allocation
and recycling of sectors.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.aspace-minaddr" xreflabel="----aspace-minaddr">
<term>
<option><![CDATA[--aspace-minaddr=<address> [default: depends
on the platform] ]]></option>
</term>
<listitem>
<para>To avoid potential conflicts with some system libraries,
Valgrind does not use the address space
below <option>--aspace-minaddr</option> value, keeping it
reserved in case a library specifically requests memory in this
region. So, some "pessimistic" value is guessed by Valgrind
depending on the platform. On linux, by default, Valgrind avoids
using the first 64MB even if typically there is no conflict in
this complete zone. You can use the
option <option>--aspace-minaddr</option> to have your memory
hungry application benefitting from more of this lower memory.
On the other hand, if you encounter a conflict, increasing
aspace-minaddr value might solve it. Conflicts will typically
manifest themselves with mmap failures in the low range of the
address space. The
provided <computeroutput>address</computeroutput> must be page
aligned and must be equal or bigger to 0x1000 (4KB). To find the
default value on your platform, do something such as
<computeroutput>valgrind -d -d date 2&gt;&amp;1 | grep -i minaddr</computeroutput>. Values lower than 0x10000 (64KB) are known to create problems
on some distributions.
</para>
</listitem>
</varlistentry>
<varlistentry id="opt.show-emwarns" xreflabel="--show-emwarns">
<term>
<option><![CDATA[--show-emwarns=<yes|no> [default: no] ]]></option>
</term>
<listitem>
<para>When enabled, Valgrind will emit warnings about its CPU
emulation in certain cases. These are usually not
interesting.</para>
</listitem>
</varlistentry>
<varlistentry id="opt.require-text-symbol"
xreflabel="--require-text-symbol">
<term>
<option><![CDATA[--require-text-symbol=:sonamepatt:fnnamepatt]]></option>
</term>
<listitem>
<para>When a shared object whose soname
matches <varname>sonamepatt</varname> is loaded into the
process, examine all the text symbols it exports. If none of
those match <varname>fnnamepatt</varname>, print an error
message and abandon the run. This makes it possible to ensure
that the run does not continue unless a given shared object
contains a particular function name.
</para>
<para>
Both <varname>sonamepatt</varname> and
<varname>fnnamepatt</varname> can be written using the usual
<varname>?</varname> and <varname>*</varname> wildcards. For
example: <varname>":*libc.so*:foo?bar"</varname>. You may use
characters other than a colon to separate the two patterns. It
is only important that the first character and the separator
character are the same. For example, the above example could
also be written <varname>"Q*libc.so*Qfoo?bar"</varname>.
Multiple <varname> --require-text-symbol</varname> flags are
allowed, in which case shared objects that are loaded into
the process will be checked against all of them.
</para>
<para>
The purpose of this is to support reliable usage of marked-up
libraries. For example, suppose we have a version of GCC's
<varname>libgomp.so</varname> which has been marked up with
annotations to support Helgrind. It is only too easy and
confusing to load the wrong, un-annotated
<varname>libgomp.so</varname> into the application. So the idea
is: add a text symbol in the marked-up library, for
example <varname>annotated_for_helgrind_3_6</varname>, and then
give the flag
<varname>--require-text-symbol=:*libgomp*so*:annotated_for_helgrind_3_6</varname>
so that when <varname>libgomp.so</varname> is loaded, Valgrind
scans its symbol table, and if the symbol isn't present the run
is aborted, rather than continuing silently with the
un-marked-up library. Note that you should put the entire flag
in quotes to stop shells expanding up the <varname>*</varname>
and <varname>?</varname> wildcards.
</para>
</listitem>
</varlistentry>
<varlistentry id="opt.soname-synonyms"
xreflabel="--soname-synonyms">
<term>
<option><![CDATA[--soname-synonyms=syn1=pattern1,syn2=pattern2,...]]></option>
</term>
<listitem>
<para>When a shared library is loaded, Valgrind checks for
functions in the library that must be replaced or wrapped.
For example, Memcheck replaces all malloc related
functions (malloc, free, calloc, ...) with its own versions.
Such replacements are done by default only in shared libraries whose
soname matches a predefined soname pattern (e.g.
<varname>libc.so*</varname> on linux).
By default, no replacement is done for a statically linked
library or for alternative libraries such as tcmalloc.
In some cases, the replacements allow
<option>--soname-synonyms</option> to specify one additional
synonym pattern, giving flexibility in the replacement. </para>
<para>Currently, this flexibility is only allowed for the
malloc related functions, using the
synonym <varname>somalloc</varname>. This synonym is usable for
all tools doing standard replacement of malloc related functions
(e.g. memcheck, massif, drd, helgrind, exp-dhat, exp-sgcheck).
</para>
<itemizedlist>
<listitem>
<para>Alternate malloc library: to replace the malloc
related functions in an alternate library with
soname <varname>mymalloclib.so</varname>, give the
option <option>--soname-synonyms=somalloc=mymalloclib.so</option>.
A pattern can be used to match multiple libraries sonames.
For
example, <option>--soname-synonyms=somalloc=*tcmalloc*</option>
will match the soname of all variants of the tcmalloc library
(native, debug, profiled, ... tcmalloc variants). </para>
<para>Note: the soname of a elf shared library can be
retrieved using the readelf utility. </para>
</listitem>
<listitem>
<para>Replacements in a statically linked library are done by
using the <varname>NONE</varname> pattern. For example, if
you link with <varname>libtcmalloc.a</varname>, memcheck
will properly work when you give the
option <option>--soname-synonyms=somalloc=NONE</option>. Note
that a NONE pattern will match the main executable and any
shared library having no soname. </para>
</listitem>
<listitem>
<para>To run a "default" Firefox build for Linux, in which
JEMalloc is linked in to the main executable,
use <option>--soname-synonyms=somalloc=NONE</option>.
</para>
</listitem>
</itemizedlist>
</listitem>
</varlistentry>
</variablelist>
<!-- end of xi:include in the manpage -->
</sect2>
<sect2 id="manual-core.debugopts" xreflabel="Debugging Options">
<title>Debugging Options</title>
<!-- start of xi:include in the manpage -->
<para id="debug.opts.para">There are also some options for debugging
Valgrind itself. You shouldn't need to use them in the normal run of
things. If you wish to see the list, use the
<option>--help-debug</option> option.</para>
<para>If you wish to debug your program rather than debugging
Valgrind itself, then you should use the options
<option>--vgdb=yes</option> or <option>--vgdb=full</option>
or <option>--db-attach=yes</option>.
</para>
<!-- end of xi:include in the manpage -->
</sect2>
<sect2 id="manual-core.defopts" xreflabel="Setting Default Options">
<title>Setting Default Options</title>
<para>Note that Valgrind also reads options from three places:</para>
<orderedlist>
<listitem>
<para>The file <computeroutput>~/.valgrindrc</computeroutput></para>
</listitem>
<listitem>
<para>The environment variable
<computeroutput>$VALGRIND_OPTS</computeroutput></para>
</listitem>
<listitem>
<para>The file <computeroutput>./.valgrindrc</computeroutput></para>
</listitem>
</orderedlist>
<para>These are processed in the given order, before the
command-line options. Options processed later override those
processed earlier; for example, options in
<computeroutput>./.valgrindrc</computeroutput> will take
precedence over those in
<computeroutput>~/.valgrindrc</computeroutput>.
</para>
<para>Please note that the <computeroutput>./.valgrindrc</computeroutput>
file is ignored if it is marked as world writeable or not owned
by the current user. This is because the
<computeroutput>./.valgrindrc</computeroutput> can contain options that are
potentially harmful or can be used by a local attacker to execute code under
your user account.
</para>
<para>Any tool-specific options put in
<computeroutput>$VALGRIND_OPTS</computeroutput> or the
<computeroutput>.valgrindrc</computeroutput> files should be
prefixed with the tool name and a colon. For example, if you
want Memcheck to always do leak checking, you can put the
following entry in <literal>~/.valgrindrc</literal>:</para>
<programlisting><![CDATA[
--memcheck:leak-check=yes]]></programlisting>
<para>This will be ignored if any tool other than Memcheck is
run. Without the <computeroutput>memcheck:</computeroutput>
part, this will cause problems if you select other tools that
don't understand
<option>--leak-check=yes</option>.</para>
</sect2>
</sect1>
<sect1 id="manual-core.pthreads" xreflabel="Support for Threads">
<title>Support for Threads</title>
<para>Threaded programs are fully supported.</para>
<para>The main thing to point out with respect to threaded programs is
that your program will use the native threading library, but Valgrind
serialises execution so that only one (kernel) thread is running at a
time. This approach avoids the horrible implementation problems of
implementing a truly multithreaded version of Valgrind, but it does
mean that threaded apps never use more than one CPU simultaneously,
even if you have a multiprocessor or multicore machine.</para>
<para>Valgrind doesn't schedule the threads itself. It merely ensures
that only one thread runs at once, using a simple locking scheme. The
actual thread scheduling remains under control of the OS kernel. What
this does mean, though, is that your program will see very different
scheduling when run on Valgrind than it does when running normally.
This is both because Valgrind is serialising the threads, and because
the code runs so much slower than normal.</para>
<para>This difference in scheduling may cause your program to behave
differently, if you have some kind of concurrency, critical race,
locking, or similar, bugs. In that case you might consider using the
tools Helgrind and/or DRD to track them down.</para>
<para>On Linux, Valgrind also supports direct use of the
<computeroutput>clone</computeroutput> system call,
<computeroutput>futex</computeroutput> and so on.
<computeroutput>clone</computeroutput> is supported where either
everything is shared (a thread) or nothing is shared (fork-like); partial
sharing will fail.
</para>
<!-- Referenced from both the manual and manpage -->
<sect2 id="&vg-pthreads-perf-sched-id;" xreflabel="&vg-pthreads-perf-sched-label;">
<title>Scheduling and Multi-Thread Performance</title>
<para>A thread executes code only when it holds the abovementioned
lock. After executing some number of instructions, the running thread
will release the lock. All threads ready to run will then compete to
acquire the lock.</para>
<para>The <option>--fair-sched</option> option controls the locking mechanism
used to serialise thread execution.</para>
<para>The default pipe based locking mechanism
(<option>--fair-sched=no</option>) is available on all
platforms. Pipe based locking does not guarantee fairness between
threads: it is quite likely that a thread that has just released the
lock reacquires it immediately, even though other threads are ready to
run. When using pipe based locking, different runs of the same
multithreaded application might give very different thread
scheduling.</para>
<para>An alternative locking mechanism, based on futexes, is available
on some platforms. If available, it is activated
by <option>--fair-sched=yes</option> or
<option>--fair-sched=try</option>. Futex based locking ensures
fairness (round-robin scheduling) between threads: if multiple threads
are ready to run, the lock will be given to the thread which first
requested the lock. Note that a thread which is blocked in a system
call (e.g. in a blocking read system call) has not (yet) requested the
lock: such a thread requests the lock only after the system call is
finished.</para>
<para> The fairness of the futex based locking produces better
reproducibility of thread scheduling for different executions of a
multithreaded application. This better reproducibility is particularly
helpful when using Helgrind or DRD.</para>
<para>Valgrind's use of thread serialisation implies that only one
thread at a time may run. On a multiprocessor/multicore system, the
running thread is assigned to one of the CPUs by the OS kernel
scheduler. When a thread acquires the lock, sometimes the thread will
be assigned to the same CPU as the thread that just released the
lock. Sometimes, the thread will be assigned to another CPU. When
using pipe based locking, the thread that just acquired the lock
will usually be scheduled on the same CPU as the thread that just
released the lock. With the futex based mechanism, the thread that
just acquired the lock will more often be scheduled on another
CPU.</para>
<para>Valgrind's thread serialisation and CPU assignment by the OS
kernel scheduler can interact badly with the CPU frequency scaling
available on many modern CPUs. To decrease power consumption, the
frequency of a CPU or core is automatically decreased if the CPU/core
has not been used recently. If the OS kernel often assigns the thread
which just acquired the lock to another CPU/core, it is quite likely
that this CPU/core is currently at a low frequency. The frequency of
this CPU will be increased after some time. However, during this
time, the (only) running thread will have run at the low frequency.
Once this thread has run for some time, it will release the lock.
Another thread will acquire this lock, and might be scheduled again on
another CPU whose clock frequency was decreased in the
meantime.</para>
<para>The futex based locking causes threads to change CPUs/cores more
often. So, if CPU frequency scaling is activated, the futex based
locking might decrease significantly the performance of a
multithreaded app running under Valgrind. Performance losses of up to
50% degradation have been observed, as compared to running on a
machine for which CPU frequency scaling has been disabled. The pipe
based locking locking scheme also interacts badly with CPU frequency
scaling, with performance losses in the range 10..20% having been
observed.</para>
<para>To avoid such performance degradation, you should indicate to
the kernel that all CPUs/cores should always run at maximum clock
speed. Depending on your Linux distribution, CPU frequency scaling
may be controlled using a graphical interface or using command line
such as
<computeroutput>cpufreq-selector</computeroutput> or
<computeroutput>cpufreq-set</computeroutput>.
</para>
<para>An alternative way to avoid these problems is to tell the
OS scheduler to tie a Valgrind process to a specific (fixed) CPU using the
<computeroutput>taskset</computeroutput> command. This should ensure
that the selected CPU does not fall below its maximum frequency
setting so long as any thread of the program has work to do.
</para>
</sect2>
</sect1>
<sect1 id="manual-core.signals" xreflabel="Handling of Signals">
<title>Handling of Signals</title>
<para>Valgrind has a fairly complete signal implementation. It should be
able to cope with any POSIX-compliant use of signals.</para>
<para>If you're using signals in clever ways (for example, catching
SIGSEGV, modifying page state and restarting the instruction), you're
probably relying on precise exceptions. In this case, you will need
to use <option>--vex-iropt-register-updates=allregs-at-mem-access</option>
or <option>--vex-iropt-register-updates=allregs-at-each-insn</option>.
</para>
<para>If your program dies as a result of a fatal core-dumping signal,
Valgrind will generate its own core file
(<computeroutput>vgcore.NNNNN</computeroutput>) containing your program's
state. You may use this core file for post-mortem debugging with GDB or
similar. (Note: it will not generate a core if your core dump size limit is
0.) At the time of writing the core dumps do not include all the floating
point register information.</para>
<para>In the unlikely event that Valgrind itself crashes, the operating system
will create a core dump in the usual way.</para>
</sect1>
<sect1 id="manual-core.install" xreflabel="Building and Installing">
<title>Building and Installing Valgrind</title>
<para>We use the standard Unix
<computeroutput>./configure</computeroutput>,
<computeroutput>make</computeroutput>, <computeroutput>make
install</computeroutput> mechanism. Once you have completed
<computeroutput>make install</computeroutput> you may then want
to run the regression tests
with <computeroutput>make regtest</computeroutput>.
</para>
<para>In addition to the usual
<option>--prefix=/path/to/install/tree</option>, there are three
options which affect how Valgrind is built:
<itemizedlist>
<listitem>
<para><option>--enable-inner</option></para>
<para>This builds Valgrind with some special magic hacks which make
it possible to run it on a standard build of Valgrind (what the
developers call "self-hosting"). Ordinarily you should not use
this option as various kinds of safety checks are disabled.
</para>
</listitem>
<listitem>
<para><option>--enable-only64bit</option></para>
<para><option>--enable-only32bit</option></para>
<para>On 64-bit platforms (amd64-linux, ppc64-linux,
amd64-darwin), Valgrind is by default built in such a way that
both 32-bit and 64-bit executables can be run. Sometimes this
cleverness is a problem for a variety of reasons. These two
options allow for single-target builds in this situation. If you
issue both, the configure script will complain. Note they are
ignored on 32-bit-only platforms (x86-linux, ppc32-linux,
arm-linux, x86-darwin).
</para>
</listitem>
</itemizedlist>
</para>
<para>The <computeroutput>configure</computeroutput> script tests
the version of the X server currently indicated by the current
<computeroutput>$DISPLAY</computeroutput>. This is a known bug.
The intention was to detect the version of the current X
client libraries, so that correct suppressions could be selected
for them, but instead the test checks the server version. This
is just plain wrong.</para>
<para>If you are building a binary package of Valgrind for
distribution, please read <literal>README_PACKAGERS</literal>
<xref linkend="dist.readme-packagers"/>. It contains some
important information.</para>
<para>Apart from that, there's not much excitement here. Let us
know if you have build problems.</para>
</sect1>
<sect1 id="manual-core.problems" xreflabel="If You Have Problems">
<title>If You Have Problems</title>
<para>Contact us at <ulink url="&vg-url;">&vg-url;</ulink>.</para>
<para>See <xref linkend="manual-core.limits"/> for the known
limitations of Valgrind, and for a list of programs which are
known not to work on it.</para>
<para>All parts of the system make heavy use of assertions and
internal self-checks. They are permanently enabled, and we have no
plans to disable them. If one of them breaks, please mail us!</para>
<para>If you get an assertion failure
in <filename>m_mallocfree.c</filename>, this may have happened because
your program wrote off the end of a heap block, or before its
beginning, thus corrupting heap metadata. Valgrind hopefully will have
emitted a message to that effect before dying in this way.</para>
<para>Read the <xref linkend="FAQ"/> for more advice about common problems,
crashes, etc.</para>
</sect1>
<sect1 id="manual-core.limits" xreflabel="Limitations">
<title>Limitations</title>
<para>The following list of limitations seems long. However, most
programs actually work fine.</para>
<para>Valgrind will run programs on the supported platforms
subject to the following constraints:</para>
<itemizedlist>
<listitem>
<para>On x86 and amd64, there is no support for 3DNow!
instructions. If the translator encounters these, Valgrind will
generate a SIGILL when the instruction is executed. Apart from
that, on x86 and amd64, essentially all instructions are supported,
up to and including AVX and AES in 64-bit mode and SSSE3 in 32-bit
mode. 32-bit mode does in fact support the bare minimum SSE4
instructions to needed to run programs on MacOSX 10.6 on 32-bit
targets.
</para>
</listitem>
<listitem>
<para>On ppc32 and ppc64, almost all integer, floating point and
Altivec instructions are supported. Specifically: integer and FP
insns that are mandatory for PowerPC, the "General-purpose
optional" group (fsqrt, fsqrts, stfiwx), the "Graphics optional"
group (fre, fres, frsqrte, frsqrtes), and the Altivec (also known
as VMX) SIMD instruction set, are supported. Also, instructions
from the Power ISA 2.05 specification, as present in POWER6 CPUs,
are supported.</para>
</listitem>
<listitem>
<para>On ARM, essentially the entire ARMv7-A instruction set
is supported, in both ARM and Thumb mode. ThumbEE and Jazelle are
not supported. NEON, VFPv3 and ARMv6 media support is fairly
complete.
</para>
</listitem>
<listitem>
<para>If your program does its own memory management, rather than
using malloc/new/free/delete, it should still work, but Memcheck's
error checking won't be so effective. If you describe your
program's memory management scheme using "client requests" (see
<xref linkend="manual-core-adv.clientreq"/>), Memcheck can do
better. Nevertheless, using malloc/new and free/delete is still
the best approach.</para>
</listitem>
<listitem>
<para>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 weird 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.</para>
</listitem>
<listitem>
<para>Machine 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
report all the details printed out, so we can try and implement the
missing feature.</para>
</listitem>
<listitem>
<para>Memory consumption of your program is majorly increased
whilst running under Valgrind's Memcheck tool. This is due to the
large amount of administrative information maintained behind the
scenes. Another cause is that Valgrind dynamically translates the
original executable. Translated, instrumented code is 12-18 times
larger than the original so you can easily end up with 150+ MB of
translations when running (eg) a web browser.</para>
</listitem>
<listitem>
<para>Valgrind can handle dynamically-generated code just fine. If
you regenerate code over the top of old code (ie. at the same
memory addresses), if the code is on the stack Valgrind will
realise the code has changed, and work correctly. This is
necessary to handle the trampolines GCC uses to implemented nested
functions. If you regenerate code somewhere other than the stack,
and you are running on an 32- or 64-bit x86 CPU, you will need to
use the <option>--smc-check=all</option> option, and Valgrind will
run more slowly than normal. Or you can add client requests that
tell Valgrind when your program has overwritten code.
</para>
<para> On other platforms (ARM, PowerPC) Valgrind observes and
honours the cache invalidation hints that programs are obliged to
emit to notify new code, and so self-modifying-code support should
work automatically, without the need
for <option>--smc-check=all</option>.</para>
</listitem>
<listitem>
<para>Valgrind has the following limitations
in its implementation of x86/AMD64 floating point relative to
IEEE754.</para>
<para>Precision: There is no support for 80 bit arithmetic.
Internally, Valgrind represents all such "long double" numbers in 64
bits, and so there may be some differences in results. Whether or
not this is critical remains to be seen. Note, the x86/amd64
fldt/fstpt instructions (read/write 80-bit numbers) are correctly
simulated, using conversions to/from 64 bits, so that in-memory
images of 80-bit numbers look correct if anyone wants to see.</para>
<para>The impression observed from many FP regression tests is that
the accuracy differences aren't significant. Generally speaking, if
a program relies on 80-bit precision, there may be difficulties
porting it to non x86/amd64 platforms which only support 64-bit FP
precision. Even on x86/amd64, the program may get different results
depending on whether it is compiled to use SSE2 instructions (64-bits
only), or x87 instructions (80-bit). The net effect is to make FP
programs behave as if they had been run on a machine with 64-bit IEEE
floats, for example PowerPC. On amd64 FP arithmetic is done by
default on SSE2, so amd64 looks more like PowerPC than x86 from an FP
perspective, and there are far fewer noticeable accuracy differences
than with x86.</para>
<para>Rounding: Valgrind does observe the 4 IEEE-mandated rounding
modes (to nearest, to +infinity, to -infinity, to zero) for the
following conversions: float to integer, integer to float where
there is a possibility of loss of precision, and float-to-float
rounding. For all other FP operations, only the IEEE default mode
(round to nearest) is supported.</para>
<para>Numeric exceptions in FP code: IEEE754 defines five types of
numeric exception that can happen: invalid operation (sqrt of
negative number, etc), division by zero, overflow, underflow,
inexact (loss of precision).</para>
<para>For each exception, two courses of action are defined by IEEE754:
either (1) a user-defined exception handler may be called, or (2) a
default action is defined, which "fixes things up" and allows the
computation to proceed without throwing an exception.</para>
<para>Currently Valgrind only supports the default fixup actions.
Again, feedback on the importance of exception support would be
appreciated.</para>
<para>When Valgrind detects that the program is trying to exceed any
of these limitations (setting exception handlers, rounding mode, or
precision control), it can print a message giving a traceback of
where this has happened, and continue execution. This behaviour used
to be the default, but the messages are annoying and so showing them
is now disabled by default. Use <option>--show-emwarns=yes</option> to see
them.</para>
<para>The above limitations define precisely the IEEE754 'default'
behaviour: default fixup on all exceptions, round-to-nearest
operations, and 64-bit precision.</para>
</listitem>
<listitem>
<para>Valgrind has the following limitations in
its implementation of x86/AMD64 SSE2 FP arithmetic, relative to
IEEE754.</para>
<para>Essentially the same: no exceptions, and limited observance of
rounding mode. Also, SSE2 has control bits which make it treat
denormalised numbers as zero (DAZ) and a related action, flush
denormals to zero (FTZ). Both of these cause SSE2 arithmetic to be
less accurate than IEEE requires. Valgrind detects, ignores, and can
warn about, attempts to enable either mode.</para>
</listitem>
<listitem>
<para>Valgrind has the following limitations in
its implementation of ARM VFPv3 arithmetic, relative to
IEEE754.</para>
<para>Essentially the same: no exceptions, and limited observance
of rounding mode. Also, switching the VFP unit into vector mode
will cause Valgrind to abort the program -- it has no way to
emulate vector uses of VFP at a reasonable performance level. This
is no big deal given that non-scalar uses of VFP instructions are
in any case deprecated.</para>
</listitem>
<listitem>
<para>Valgrind has the following limitations
in its implementation of PPC32 and PPC64 floating point
arithmetic, relative to IEEE754.</para>
<para>Scalar (non-Altivec): Valgrind provides a bit-exact emulation of
all floating point instructions, except for "fre" and "fres", which are
done more precisely than required by the PowerPC architecture specification.
All floating point operations observe the current rounding mode.
</para>
<para>However, fpscr[FPRF] is not set after each operation. That could
be done but would give measurable performance overheads, and so far
no need for it has been found.</para>
<para>As on x86/AMD64, IEEE754 exceptions are not supported: all floating
point exceptions are handled using the default IEEE fixup actions.
Valgrind detects, ignores, and can warn about, attempts to unmask
the 5 IEEE FP exception kinds by writing to the floating-point status
and control register (fpscr).
</para>
<para>Vector (Altivec, VMX): essentially as with x86/AMD64 SSE/SSE2:
no exceptions, and limited observance of rounding mode.
For Altivec, FP arithmetic
is done in IEEE/Java mode, which is more accurate than the Linux default
setting. "More accurate" means that denormals are handled properly,
rather than simply being flushed to zero.</para>
</listitem>
</itemizedlist>
<para>Programs which are known not to work are:</para>
<itemizedlist>
<listitem>
<para>emacs starts up but immediately concludes it is out of
memory and aborts. It may be that Memcheck does not provide
a good enough emulation of the
<computeroutput>mallinfo</computeroutput> function.
Emacs works fine if you build it to use
the standard malloc/free routines.</para>
</listitem>
</itemizedlist>
</sect1>
<sect1 id="manual-core.example" xreflabel="An Example Run">
<title>An Example Run</title>
<para>This is the log for a run of a small program using Memcheck.
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).</para>
<programlisting><![CDATA[
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==
==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== 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
]]></programlisting>
<para>The GCC folks fixed this about a week before GCC 3.0
shipped.</para>
</sect1>
<sect1 id="manual-core.warnings" xreflabel="Warning Messages">
<title>Warning Messages You Might See</title>
<para>Some of these only appear if you run in verbose mode
(enabled by <option>-v</option>):</para>
<itemizedlist>
<listitem>
<para><computeroutput>More than 100 errors detected. Subsequent
errors will still be recorded, but in less detail than
before.</computeroutput></para>
<para>After 100 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 100. The 100 constant can be changed by recompiling
Valgrind.</para>
</listitem>
<listitem>
<para><computeroutput>More than 1000 errors detected. I'm not
reporting any more. Final error counts may be inaccurate. Go fix
your program!</computeroutput></para>
<para>After 1000 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 1000
number is a compile-time constant.</para>
</listitem>
<listitem>
<para><computeroutput>Warning: client switching stacks?</computeroutput></para>
<para>Valgrind spotted such a large change in the stack pointer
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. At the moment
"large change" is defined as a change of more that 2000000 in the
value of the stack pointer register. If Valgrind guesses wrong,
you may get many bogus error messages following this and/or have
crashes in the stack trace recording code. You might avoid these
problems by informing Valgrind about the stack bounds using
VALGRIND_STACK_REGISTER client request. </para>
</listitem>
<listitem>
<para><computeroutput>Warning: client attempted to close Valgrind's
logfile fd &lt;number&gt;</computeroutput></para>
<para>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
<option>--log-fd=&lt;number&gt;</option> option to specify a
different logfile file-descriptor number.</para>
</listitem>
<listitem>
<para><computeroutput>Warning: noted but unhandled ioctl
&lt;number&gt;</computeroutput></para>
<para>Valgrind observed a call to one of the vast family of
<computeroutput>ioctl</computeroutput> system calls, but did not
modify its memory status info (because nobody has yet written a
suitable wrapper). 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.</para>
</listitem>
<listitem>
<para><computeroutput>Warning: set address range perms: large range
&lt;number></computeroutput></para>
<para>Diagnostic message, mostly for benefit of the Valgrind
developers, to do with memory permissions.</para>
</listitem>
</itemizedlist>
</sect1>
</chapter>