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ftmemsim-valgrind/docs/xml/manual-core.xml
Julian Seward 739bc05601 gdbserver: misc fixes (#214909 c 77) 
Fix some tests on ppc-debian6,s390x + handled Nick Nethercote, Josef
Weidendorfer comments

* improved testing & related doc
  - added option --vex-iropt-precise-memory-exns=yes to mcsig(no)pass.vgtest
    + updated manual-core.xml
  - cleanup some comments in *.vgtest 
  - modified filter_gdb and filter_memcheck_monitor to
    handle specific ppc/debian6.0 mcsig(no)pass output
    handle specific s390x 'missing debug info'
  - added more information in README_DEVELOPPERS on how to
    investigate failing gdbserver tests.

* handled Nick Nethercote comment:
  Replaced kludgy ms.snapshot detailed
              by  ms.detailed_snaphot
  Updated documentation and test.

* handled Josef Weindendorfer comments:
   - do not report an error if ptrace_scope file can't be read.
     Instead, a debug trace is done if -d (debug) option given
   - added an option -l to give the list of active Valgrind
     gdbserver. Useful a.o. to support callgrind_control.
     Updated documentation
   - added ref. to  vgdb help in the vgdb --help message

(Philippe Waroquiers, philippe.waroquiers@skynet.be)



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@11770
2011-05-17 17:15:07 +00:00

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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.</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 both the older "stabs" debugging format, used
by GCC versions prior to 3.1, and the newer DWARF2/3/4 formats
used by GCC
3.1 and later. We continue to develop our debug-info readers,
although the majority of effort will naturally enough go into the newer
DWARF readers.</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>
<sect1 id="manual-core.comment" xreflabel="The Commentary">
<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>
<itemizedlist>
<listitem>
<para><option>-e</option> or <option>--exit-at-zero</option>:
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>
<listitem>
<para><option>portnumber</option>: 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>
</itemizedlist>
<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 its name
and the number of times it got used. Here's the suppressions used by a
run of <computeroutput>valgrind --tool=memcheck ls -l</computeroutput>:</para>
<programlisting><![CDATA[
--27579-- supp: 1 socketcall.connect(serv_addr)/__libc_connect/__nscd_getgrgid_r
--27579-- supp: 1 socketcall.connect(serv_addr)/__libc_connect/__nscd_getpwuid_r
--27579-- supp: 6 strrchr/_dl_map_object_from_fd/_dl_map_object]]></programlisting>
<para>Multiple suppressions files are allowed. By default, Valgrind
uses <filename>$PREFIX/lib/valgrind/default.supp</filename>. You can
ask to add suppressions from another file, by specifying
<option>--suppressions=/path/to/file.supp</option>.
</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>The single most important option.</para>
<variablelist>
<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, 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 enable its embedded gdbserver if value yes
or full is given. This allows an
external <computeroutput>gdb</computeroutput> debuggger to debug
your program running under Valgrind. See
<xref linkend="manual-core.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.vgdb"/>
command line utility can send "monitor commands" to Valgrind
from a shell. The Valgrind core provides a set of
<xref linkend="manual-core.valgrind-monitor-commands"/>. A tool
can optionally provide tool specific monitor commands, which are
documented in the tool specific chapter.
</para>
<para>The value 'full' has a significant overhead
</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</option> yes or full value. Tools that report
errors will invoke the embedded gdbserver for each error above
number. The value 0 will cause gdbserver to be invoked before
executing your program. This is typically used to insert gdb
breakpoints before execution, and will also work with tools that
do not report errors, such as Massif.
</para>
</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. 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>
</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="manual-core.comment">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 and Ptrcheck. 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 50. Note that higher settings
will make Valgrind run a bit more slowly and take a bit more
memory, but can be useful when working with programs with
deeply-nested call chains.</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.stack-traces" 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.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, a more powerful debugging support is
provided by the <option>--vgdb</option> yes or full value,
allowing among others to insert breakpoints, continue from
inside the debugger, etc.
</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. On AIX5 the allowed stack size is restricted to
128MB.</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 and
Massif), 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>
</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 these.</para>
<variablelist id="uncommon.opts.list">
<varlistentry id="opt.smc-check" xreflabel="--smc-check">
<term>
<option><![CDATA[--smc-check=<none|stack|all> [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 three levels of self-modifying code detection:
no detection, detect self-modifying code on the stack (which is used by
GCC to implement nested functions), or detect self-modifying code
everywhere. 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
effort but is much faster.
<!-- commented out because it causes broken links in the man page
; see <xref
linkend="manual-core-adv.clientreq"/> for more details.
-->
</para>
<para>Some architectures (including ppc32, ppc64 and ARM) 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 and x86/Darwin that you need to use this
option.</para>
</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 down and makes it use 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[
==15516== Uninitialised byte(s) found during client check request
==15516== at 0x400633: croak (varinfo1.c:28)
==15516== by 0x4006B2: main (varinfo1.c:55)
==15516== Address 0x60103b is 7 bytes inside data symbol "global_i2"
==15516==
==15516== Uninitialised byte(s) found during client check request
==15516== at 0x400633: croak (varinfo1.c:28)
==15516== by 0x4006BC: main (varinfo1.c:56)
==15516== Address 0x7fefffefc is on thread 1's stack]]></programlisting>
<para>And here are the same errors with
<option>--read-var-info=yes</option>:</para>
<programlisting><![CDATA[
==15522== Uninitialised byte(s) found during client check request
==15522== at 0x400633: croak (varinfo1.c:28)
==15522== by 0x4006B2: main (varinfo1.c:55)
==15522== Location 0x60103b is 0 bytes inside global_i2[7],
==15522== a global variable declared at varinfo1.c:41
==15522==
==15522== Uninitialised byte(s) found during client check request
==15522== at 0x400633: croak (varinfo1.c:28)
==15522== by 0x4006BC: main (varinfo1.c:56)
==15522== Location 0x7fefffefc is 0 bytes inside local var "local"
==15522== 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>
<para> GDBTD??? unclear why we have sometimes slightly more BB:
it seems that from time to time, some BB are run outside of
run_thread_for_a_while. Maybe this is due to block chasing ? I
do not think this is a problem, as I never saw more than a few
additional basic blocks being run without being visible in the
blocks executed by run_thread_for_a_while.
</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 shadows registers only works with a gdb version
&gt;= 7.1.
</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>enable-inner: </option> Enable some special
magic needed when the program being run is itself
Valgrind.</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.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>
</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.gdbserver"
xreflabel="Debugging your program using Valgrind gdbserver and gdb">
<title>Debugging your program using Valgrind gdbserver and gdb</title>
<para>A program running under Valgrind is not executed directly by the
CPU. It rather runs on a synthetic CPU provided by Valgrind. This is
why a debugger cannot debug your program under Valgrind the usual way.
</para>
<para>
This section describes the special way gdb can interact with the
Valgrind gdbserver to provide a fully debuggable program under
Valgrind. Used in this way, gdb also provides an interactive usage of
Valgrind core or tool functionalities (such as incremental leak search
under Memcheck, on-demand Massif snapshot production, ...).
</para>
<sect2 id="manual-core.gdbserver-simple"
xreflabel="gdbserver simple example">
<title>Quick Start : debugging in 3 steps</title>
<para>If you want to debug a program with gdb when using Memcheck
tool, start Valgrind the following way:
<screen><![CDATA[
valgrind --vgdb=yes --vgdb-error=0 prog
]]></screen></para>
<para>In another window, start a gdb the following way:
<screen><![CDATA[
gdb prog
]]></screen></para>
<para>Then give the following command to gdb:
<screen><![CDATA[
(gdb) target remote | vgdb
]]></screen></para>
<para>You can now debug your program e.g. by inserting a breakpoint
and then using the gdb 'continue' command.</para>
<para> The above quick start is enough for a basic usage of the
Valgrind gdbserver. Read the sections below to learn about the
advanced functionalities provided by the combination of Valgrind and
gdb. Note that the option --vgdb=yes can be omitted, as this is the
default value.
</para>
</sect2>
<sect2 id="manual-core.gdbserver-concept"
xreflabel="gdbserver">
<title>Valgrind gdbserver concept</title>
<para>The gdb debugger is typically used to debug a process running on
the same machine : gdb uses system calls to do actions such as read
the values of the process variables or registers. This technique only
allows gdb to debug a program running on the same computer.
</para>
<para>Gdb can also debug processes running on a different computer.
For this, gdb defines a protocol (i.e. a set of query and reply
packets) that allows to e.g. fetch the value of memory or registers,
to set breakpoints, etc. A gdbserver is an implementation of this
'gdb remote debugging' protocol. To debug a process running on a
remote computer, a gdbserver (sometimes also called a gdb stub) must
run at the remote computer side.
</para>
<para>The Valgrind core integrates an embedded gdbserver
implementation, which is activated using <option>--vgdb=yes</option>
or <option>--vgdb=full</option>. This gdbserver allows the process
running on the Valgrind synthetic CPU to be debugged 'remotely' by gdb
: gdb sends protocol query packets (such as 'get registers values') to
the Valgrind embedded gdbserver. The embedded gdbserver executes the
queries (for example, it will get the registers values of the
synthetic CPU) and give the result back to gdb.
</para>
<para> Gdb can use various ways (tcp/ip, serial line, ...) to send and
receive the remote protocol packets to a gdbserver. In the case of the
Valgrind gdbserver, gdb communicates using a pipe and the
<xref linkend="manual-core.vgdb"/> command as a relay application. If
no gdb is currently being used, vgdb can also be used to send monitor
commands to the Valgrind gdbserver from the shell command line.
</para>
</sect2>
<sect2 id="manual-core.gdbserver-gdb"
xreflabel="Connecting gdb to a Valgrind gdbserver">
<title>Connecting gdb to a Valgrind gdbserver</title>
<para>To debug a program <filename>prog</filename> running under
Valgrind, ensures that the Valgrind gdbserver is activated
(i.e. --vgdb=yes or --vgdb=full). The option
<![CDATA[--vgdb-error=<number> ]]> can be used to ask an invocation of
the gdbserver for each error above number. A zero value will cause an
invocation of the Valgrind gdbserver at startup, allowing to insert
breakpoints before starting the execution. Example:
<screen><![CDATA[
valgrind --tool=memcheck --vgdb=yes --vgdb-error=0 ./prog
]]></screen></para>
<para>With the above command, the Valgrind gdbserver is invoked at startup
and indicates it is waiting for a connection from a gdb:</para>
<programlisting><![CDATA[
==2418== Memcheck, a memory error detector
==2418== Copyright (C) 2002-2010, and GNU GPL'd, by Julian Seward et al.
==2418== Using Valgrind-3.7.0.SVN and LibVEX; rerun with -h for copyright info
==2418== Command: ./prog
==2418==
==2418== (action at startup) vgdb me ...
]]></programlisting>
<para>A gdb in another window can then be connected to the Valgrind gdbserver.
For this, gdb must be started on the program <filename>prog</filename>:
<screen><![CDATA[
gdb ./prog
]]></screen></para>
<para>You then indicate to gdb that a remote target debugging is to be done:
<screen><![CDATA[
(gdb) target remote | vgdb
]]></screen>
gdb then starts a vgdb relay application to communicate with the
Valgrind embedded gdbserver:</para>
<programlisting><![CDATA[
(gdb) target remote | vgdb
Remote debugging using | vgdb
relaying data between gdb and process 2418
Reading symbols from /lib/ld-linux.so.2...done.
Reading symbols from /usr/lib/debug/lib/ld-2.11.2.so.debug...done.
Loaded symbols for /lib/ld-linux.so.2
[Switching to Thread 2418]
0x001f2850 in _start () from /lib/ld-linux.so.2
(gdb)
]]></programlisting>
<para> In case vgdb detects that multiple Valgrind gdbserver can be connected
to, it will exit after reporting the list of the debuggable Valgrind
processes and their PIDs. You can then relaunch the gdb 'target' command, but
specifying the process id of the process you want to debug:
</para>
<programlisting><![CDATA[
(gdb) target remote | vgdb
Remote debugging using | vgdb
no --pid= arg given and multiple valgrind pids found:
use --pid=2479 for valgrind --tool=memcheck --vgdb=yes --vgdb-error=0 ./prog
use --pid=2481 for valgrind --tool=memcheck --vgdb=yes --vgdb-error=0 ./prog
use --pid=2483 for valgrind --vgdb=yes --vgdb-error=0 ./another_prog
Remote communication error: Resource temporarily unavailable.
(gdb) target remote | vgdb --pid=2479
Remote debugging using | vgdb --pid=2479
relaying data between gdb and process 2479
Reading symbols from /lib/ld-linux.so.2...done.
Reading symbols from /usr/lib/debug/lib/ld-2.11.2.so.debug...done.
Loaded symbols for /lib/ld-linux.so.2
[Switching to Thread 2479]
0x001f2850 in _start () from /lib/ld-linux.so.2
(gdb)
]]></programlisting>
<para>Once gdb is connected to the Valgrind gdbserver, gdb can be used
similarly to a native debugging session:</para>
<itemizedlist>
<listitem>
<para> Breakpoints can be inserted or deleted. </para>
</listitem>
<listitem>
<para> Variables and registers values can be examined or modified.
</para>
</listitem>
<listitem>
<para> Signal handling can be configured (printing, ignoring, ...).
</para>
</listitem>
<listitem>
<para> Execution can be controlled (continue, step, next, stepi, ...).
</para>
</listitem>
<listitem>
<para> Program execution can be interrupted using Control-C. </para>
</listitem>
<listitem>
<para> ... </para>
</listitem>
</itemizedlist>
<para> Refer to the gdb user manual for a complete list of gdb functionalities.
</para>
</sect2>
<sect2 id="manual-core.gdbserver-commandhandling"
xreflabel="Monitor command handling by the Valgrind gdbserver">
<title>Monitor command handling by the Valgrind gdbserver</title>
<para> The Valgrind gdbserver provides a set of additional specific
functionalities through "monitor commands". Such monitor commands can
be sent from the gdb command line or from the shell command line. See
<xref linkend="manual-core.valgrind-monitor-commands"/> for the list
of the Valgrind core monitor commands.
</para>
<para> Each tool can also provide tool specific monitor commands. An
example of a tool specific monitor command is the Memcheck monitor
command <computeroutput>mc.leak_check any full
reachable</computeroutput>. This requests a full reporting of the
allocated memory blocks. To have this leak check executed, use the gdb
command:
<screen><![CDATA[
(gdb) monitor mc.leak_check any full reachable
]]></screen>
</para>
<para> gdb will send the mc.leak_check command to the Valgrind gdbserver. The
Valgrind gdbserver will either execute the monitor command itself (if
it recognises a Valgrind core monitor command) or let the tool execute the
tool specific monitor commands:
</para>
<programlisting><![CDATA[
(gdb) monitor mc.leak_check any full reachable
==2418== 100 bytes in 1 blocks are still reachable in loss record 1 of 1
==2418== at 0x4006E9E: malloc (vg_replace_malloc.c:236)
==2418== by 0x804884F: main (prog.c:88)
==2418==
==2418== LEAK SUMMARY:
==2418== definitely lost: 0 bytes in 0 blocks
==2418== indirectly lost: 0 bytes in 0 blocks
==2418== possibly lost: 0 bytes in 0 blocks
==2418== still reachable: 100 bytes in 1 blocks
==2418== suppressed: 0 bytes in 0 blocks
==2418==
(gdb)
]]></programlisting>
<para> Like for the gdb commands, the Valgrind gdbserver will accept
abbreviated monitor command names and arguments, as long as the given
abbreviation is non ambiguous. For example, the above mc.leak_check
command can also be typed as:
<screen><![CDATA[
(gdb) mo mc.l a f r
]]></screen>
The letters <computeroutput>mo</computeroutput> are recognised by gdb as being
<computeroutput>monitor</computeroutput>. So, gdb sends the
string <computeroutput>mc.l a f r</computeroutput> to the Valgrind
gdbserver. The letters provided in this string are unambiguous for the
Valgrind gdbserver. So, this will give the same output as the non
abbreviated command and arguments. If the provided abbreviation is
ambiguous, the Valgrind gdbserver will report the list of commands (or
argument values) that can match:
<programlisting><![CDATA[
(gdb) mo mc. a r f
mc. can match mc.get_vbits mc.leak_check mc.make_memory mc.check_memory
(gdb)
]]></programlisting>
</para>
<para> Instead of sending a monitor command from gdb, you can also
send these from a shell command line. For example, the below command lines
given in a shell will cause the same leak search to be executed by the
process 3145:
<screen><![CDATA[
vgdb --pid=3145 mc.leak_check any full reachable
vgdb --pid=3145 mc.l a f r
]]></screen></para>
<para>Note that the Valgrind gdbserver automatically continues the
execution of the program after a standalone invocation of
vgdb. Monitor commands sent from gdb do not cause the program to
continue: the program execution is controlled explicitely using gdb
commands such as 'continue' or 'next'.</para>
</sect2>
<sect2 id="manual-core.gdbserver-threads"
xreflabel="Valgrind gdbserver thread info">
<title>Valgrind gdbserver thread info</title>
<para> The Valgrind gdbserver enriches the output of the
gdb <computeroutput>info threads</computeroutput> with Valgrind
specific information. The operating system thread number is followed
by the Valgrind 'tid' and the Valgrind scheduler thread state:</para>
<programlisting><![CDATA[
(gdb) info threads
4 Thread 6239 (tid 4 VgTs_Yielding) 0x001f2832 in _dl_sysinfo_int80 () from /lib/ld-linux.so.2
* 3 Thread 6238 (tid 3 VgTs_Runnable) make_error (s=0x8048b76 "called from London") at prog.c:20
2 Thread 6237 (tid 2 VgTs_WaitSys) 0x001f2832 in _dl_sysinfo_int80 () from /lib/ld-linux.so.2
1 Thread 6234 (tid 1 VgTs_Yielding) main (argc=1, argv=0xbedcc274) at prog.c:105
(gdb)
]]></programlisting>
</sect2>
<sect2 id="manual-core.gdbserver-shadowregisters"
xreflabel="Examining and modifying Valgrind shadow registers">
<title>Examining and modifying Valgrind shadow registers</title>
<para> When the option <![CDATA[--vgdb-shadow-registers=yes ]]> is
given, the Valgrind gdbserver will let gdb examine and/or modify the
Valgrind shadow registers. A gdb version &gt;= 7.1 is needed for this
to work.</para>
<para> For each CPU register, the Valgrind core maintains two
shadow registers. These shadow registers can be accessed from
gdb by giving a postfix s1 or s2 for respectively the first
and second shadow registers. As an example, the x86 register
<computeroutput>eax</computeroutput> and its two shadow
registers can be examined using the following commands:</para>
<programlisting><![CDATA[
(gdb) p $eax
$1 = 0
(gdb) p $eaxs1
$2 = 0
(gdb) p $eaxs2
$3 = 0
(gdb)
]]></programlisting>
</sect2>
<sect2 id="manual-core.gdbserver-limitations"
xreflabel="Limitations of the Valgrind gdbserver">
<title>Limitations of the Valgrind gdbserver</title>
<para>Debugging with the Valgrind gdbserver is very similar to native
debugging. The implementation of the Valgrind gdbserver is quite
complete, and so provides most of the gdb debugging facilities. There
are however some limitations or particularities described in details
in this section:</para>
<itemizedlist>
<listitem>
<para> Precision of 'stopped at instruction'.</para>
<para>Gdb commands such as 'step', 'next', 'stepi', breakpoints,
watchpoints, ... will stop the execution of the process. With
the option --vgdb=yes, the process might not stop at the exact
instruction needed. Instead, it might continue execution of the
current block and stop at one of the following blocks. This is
linked to the fact that Valgrind gdbserver has to instrument a
block to allow stopping at the exact instruction requested.
Currently, re-instrumenting the current block being executed is
not supported. So, if the action requested by gdb (e.g. single
stepping or inserting a breakpoint) implies to re-instrument the
current block, the gdb action might not be executed precisely.
</para>
<para> This limitation will be triggered when the current block
being executed has not (yet) been instrumented for debugging.
This typically happens when the gdbserver is activated due to the
tool reporting an error or to a watchpoint. If the gdbserver
block has been activated following a breakpoint (or if a
breakpoint has been inserted in the block before its execution),
then the block has already been instrumented for debugging.
</para>
<para> If you use the option --vgdb=full, then gdb 'stop actions'
will always be obeyed precisely, but this implies that each
instruction will be instrumented with an additional call to a
gdbserver helper function, which implies some overhead compared
to --vgdb=no. Option --vgdb=yes has neglectible overhead compared
to --vgdb=no.
</para>
</listitem>
<listitem>
<para>Hardware watchpoint support by the Valgrind
gdbserver.</para>
<para> The Valgrind gdbserver can simulate hardware watchpoints
(but only if the tool provides the support for this). Currently,
only Memcheck provides hardware watchpoint simulation. The
hardware watchpoint simulation provided by Memcheck is much
faster that gdb software watchpoints (which are implemented by
gdb checking the value of the watched zone(s) after each
instruction). Hardware watchpoint simulation also provides read
watchpoints. The hardware watchpoint simulation by Memcheck has
some limitations compared to the real hardware
watchpoints. However, the number and length of simulated
watchpoints are not limited.
</para>
<para> Typically, the number of (real) hardware watchpoint is
limited. For example, the x86 architecture supports a maximum of
4 hardware watchpoints, each watchpoint watching 1, 2, 4 or 8
bytes. The Valgrind gdbserver does not have a limitation on the
number of simulated hardware watchpoints. It also has no
limitation on the length of the memory zone being
watched. However, gdb currently does not (yet) understand that
Valgrind gdbserver watchpoints have no length limit. A gdb patch
providing a command 'set remote hardware-watchpoint-length-limit'
has been developped. The integration of this patch in gdb would
allow to fully use the flexibility of the Valgrind gdbserver
simulated hardware watchpoints (is there a gdb developper reading
this ?).
</para>
<para> Memcheck implements hardware watchpoint simulation by
marking the watched zone(s) as being unaddressable. In case a
hardware watchpoint is removed, the zone is marked as addressable
and defined. Hardware watchpoint simulation of addressable
undefined memory zones will properly work, but will have as a
side effect to mark the zone as defined when the watchpoint is
removed.</para>
<para> Write watchpoints might not be reported at the instruction
which is modifying the value unless option --vgdb=full is
given. Read watchpoints will always be reported at the exact
instruction reading the watched memory.</para>
<para> It is better to avoid using hardware watchpoint of not
addressable (yet) memory: in such a case, gdb will fallback to
extremely slow software watchpoints. Also, if you do not quit gdb
between two debugging sessions, the hardware watchpoints of the
previous sessions will be re-inserted as software watchpoints if
the watched memory zone is not addressable at program startup.
</para>
</listitem>
<listitem>
<para> Stepping inside shared libraries on ARM.</para>
<para> For a not (yet?) clear reason, stepping inside a shared
library on ARM might fail. The bypass is to use the ldd command
to find the list of shared libraries and their loading address
and inform gdb of the loading address using the gdb command
'add-symbol-file'. Example (for a ./p executable):
<programlisting><![CDATA[
(gdb) shell ldd ./p
libc.so.6 => /lib/libc.so.6 (0x4002c000)
/lib/ld-linux.so.3 (0x40000000)
(gdb) add-symbol-file /lib/libc.so.6 0x4002c000
add symbol table from file "/lib/libc.so.6" at
.text_addr = 0x4002c000
(y or n) y
Reading symbols from /lib/libc.so.6...(no debugging symbols found)...done.
(gdb)
]]></programlisting>
</para>
</listitem>
<listitem>
<para> gdb version needed for ARM and PPC32/64.</para>
<para> You must use a gdb version which is able to read XML
target description sent by gdbserver (this is the standard setup
if the gdb was configured on a computer with the expat
library). If your gdb was not configured with XML support, it
will report an error message when using the target
command. Debugging will not work because gdb will then not be
able to fetch the registers from the Valgrind gdbserver.
</para>
</listitem>
<listitem>
<para> Stack unwinding on PPC32/PPC64. </para>
<para> On PPC32/PPC64, stack unwinding for leaf functions
(i.e. functions not calling other functions) does work properly
only with <option>--vex-iropt-precise-memory-exns=yes</option>.
You must also pass this option to have a precise stack when
a signal is trapped by gdb.
</para>
</listitem>
<listitem>
<para> Breakpoint encountered multiple times. </para>
<para> Some instructions (e.g. the x86 "rep movsb")
are translated by Valgrind using a loop. If a breakpoint is placed
on such an instruction, the breakpoint will be encountered
multiple times (i.e. once for each step of the "implicit" loop
implementing the instruction).
</para>
</listitem>
<listitem>
<para> Execution of Inferior function calls by the Valgrind
gdbserver. </para>
<para> gdb allows the user to "call" functions inside the process
being debugged. Such calls are named 'Inferior calls' in the gdb
terminology. A typical usage of an 'Inferior call' is to execute
a function that outputs a readable image of a complex data
structure. To make an Inferior call, use the gdb 'print' command
followed by the function to call and its arguments. As an
example, the following gdb command causes an Inferior call to the
libc printf function to be executed by (and inside) the process
being debugged:
</para>
<programlisting><![CDATA[
(gdb) p printf("process being debugged has pid %d\n", getpid())
$5 = 36
(gdb)
]]></programlisting>
<para>The Valgrind gdbserver accepts Inferior function
calls. During Inferior calls, the Valgrind tool will report
errors as usual. If you do not want to have such errors stopping
the execution of the Inferior call, you can use 'vg.set
vgdb-error' to set a big value before the call, and reset the
value after the Inferior call.</para>
<para>To execute Inferior calls, gdb changes registers such as
the program counter, and then continues the execution of the
program. In a multi-thread program, all threads are continued,
not only the thread instructed to make an Inferior call. If
another thread reports an error or encounters a break, the
evaluation of the Inferior call is abandonned.</para>
<para> Note that Inferior function calls is a powerful gdb
functionality but it has to be used with caution. For example, if
the program being debugged is stopped inside the function printf,
'forcing' a recursive call to printf via an Inferior call will
very probably create problems. The Valgrind tool might also add
another level of complexity to Inferior calls, e.g. by reporting
tool errors during the Inferior call or due to the
instrumentation done.
</para>
</listitem>
<listitem>
<para>Connecting to or interrupting a Valgrind process blocked in
a system call.</para>
<para> Connecting to or interrupting a Valgrind process blocked
in a system call is depending on ptrace system call, which might
be disabled on your kernel. </para>
<para> At regular interval, after having executed some basic
blocks, the Valgrind scheduler checks if some input is to be
handled by the Valgrind gdbserver. However, this check is only
done if at least one thread of the process is executing (enough)
basic blocks. If all the threads of the process are blocked in a
system call, then no basic blocks are being executed, and the
Valgrind scheduler will not invoke the Valgrind gdbserver. In
such a case, the vgdb relay application will 'force' the Valgrind
gdbserver to be invoked, without the intervention of the Valgrind
scheduler.
</para>
<para> Such forced invocation of the Valgrind gdbserver is
implemented by vgdb using ptrace system calls. On a properly
implemented kernel, the ptrace calls done by vgdb will not
influence the behaviour of the program running under Valgrind. In
case of unexpected impact, giving the option --max-invoke-ms=0 to
the vgdb relay application will disable the usage of ptrace
system call. The consequence of disabling ptrace system call in
vgdb is that a Valgrind process blocked in a system call cannot
be waken up or interrupted from gdb till it executes (enough)
basic blocks to let the scheduler poll invoke the gdbserver..
</para>
<para>When ptrace is disabled in vgdb, you might increase the
responsiveness of the Valgrind gdbserver to commands or
interrupts by giving a lower value to the option --vgdb-poll: if
your application is most of the time blocked in a system call,
using a very low value for vgdb-poll will cause a faster
invocation of gdbserver. As the gdbserver poll done by the
scheduler is very efficient, the more frequent check by the
scheduler should not cause significant performance degradation.
</para>
<para>When ptrace is disabled in vgdb, a query packet sent by gdb
might take a significant time to be handled by the Valgrind
gdbserver. In such a case, gdb might encounter a protocol
timeout. To avoid having gdb encountering such a timeout error,
you can increase the value of this timeout by using the gdb
command 'set remotetimeout'.
</para>
<para> Ubuntu version &gt;= 10.10 can also restrict the scope of
ptrace to the children of the process calling ptrace. As the
Valgrind process is not a child of vgdb, such restricted scope
causes ptrace system call to fail. To avoid that, when Valgrind
gdbserver receives the first packet from a vgdb, it calls
prctl(PR_SET_PTRACER, vgdb_pid, 0, 0, 0) to ensure vgdb can use
ptrace. Once vgdb_pid has been set as ptracer, vgdb can then
properly force the invocation of Valgrind gdbserver when
needed. To ensure the vgdb is set as ptracer before the Valgrind
process could be blocked in a system call, connect your gdb to
the Valgrind gdbserver at startup (i.e. use --vgdb-error=0).
Note that this 'set ptracer' is not solving the problem for the
connection of a standalone vgdb: the first command to be sent by
a standalone vgdb must wake up the Valgrind process before
Valgrind gdbserver will set vgdb as ptracer.
</para>
<para> Unblocking a process blocked in a system call is
not implemented on Darwin. So, waiting for vgdb on Darwin to
be enhanced, you cannot connect/interrupt a process blocked
in a system call on Darwin.
</para>
</listitem>
<listitem>
<para> Changing registers of a thread.</para>
<para> The Valgrind gdbserver only accepts to modify the values
of the registers of a thread when the thread is in status
Runnable or Yielding. In other states (typically, WaitSys), changing
registers values will not be accepted. This among others ensures
that Inferior calls are not executed for a thread which is in a
system call : the Valgrind gdbserver does not implement system
call restart.
</para>
</listitem>
<listitem>
<para> gdb functionalities not supported.</para>
<para> gdb provides an awful lot of debugging functionalities.
At least the following are not supported: reversible debugging,
tracepoints.
</para>
</listitem>
<listitem>
<para> Unknown limitations or problems.</para>
<para> The combination of gdb, Valgrind and the Valgrind
gdbserver has for sure some still unknown other
limitations/problems but we do not know about these unknown
limitations/problems :). If you encounter such (annoying)
limitations or problems, feel free to report a bug. But first
verify if the limitation or problem is not inherent to gdb or the
gdb remote protocol e.g. by checking the behaviour with the
standard gdbserver part of the gdb package.
</para>
</listitem>
</itemizedlist>
</sect2>
</sect1>
<sect1 id="manual-core.vgdb"
xreflabel="vgdb">
<title>vgdb command line options</title>
<para> Usage: vgdb [OPTION]... [[-c] COMMAND]...</para>
<para> vgdb (Valgrind to gdb) has two usages:</para>
<orderedlist>
<listitem id="manual-core.vgdb-standalone" xreflabel="vgdb standalone">
<para>As a standalone utility, it is used from a shell command
line to send monitor commands to a process running under
Valgrind. For this usage, the vgdb OPTION(s) must be followed by
the monitor command to send. To send more than one command,
separate them with the -c option.
</para>
</listitem>
<listitem id="manual-core.vgdb-relay" xreflabel="vgdb relay">
<para>In combination with gdb 'target remote |' command, it is
used as the relay application between gdb and the Valgrind
gdbserver. For this usage, only OPTION(s) can be given, no
command can be given.
</para>
</listitem>
</orderedlist>
<para><computeroutput>vgdb</computeroutput> accepts the following
options:</para>
<itemizedlist>
<listitem>
<para><option>--pid=&lt;number&gt;</option>: specifies the pid of
the process to which vgdb must connect to. This option is useful
in case more than one Valgrind gdbserver can be connected to. If
--pid argument is not given and multiple Valgrind gdbserver
processes are running, vgdb will report the list of such processes
and then exit.</para>
</listitem>
<listitem>
<para><option>--vgdb-prefix</option> must be given to both
Valgrind and vgdb utility if you want to change the default prefix
for the FIFOs communication between the Valgrind gdbserver and
vgdb. </para>
</listitem>
<listitem>
<para><option>--max-invoke-ms=&lt;number&gt;</option> gives the
number of milli-seconds after which vgdb will force the invocation
of gdbserver embedded in valgrind. Default value is 100
milli-seconds. A value of 0 disables the forced invocation.
</para>
<para>If you specify a big value here, you might need to increase
the gdb remote timeout. The default value of the gdb remotetimeout
is 2 seconds. You should ensure that the gdb remotetimeout (in
seconds) is bigger than the max-invoke-ms value. For example, for
a 5000 --max-invoke-ms, the following gdb command will set a value
big enough:
<screen><![CDATA[
(gdb) set remotetimeout 6
]]></screen>
</para>
</listitem>
<listitem>
<para><option>--wait=&lt;number&gt;</option> instructs vgdb to
check during the specified number of seconds if a Valgrind
gdbserver can be found. This allows to start a vgdb before the
Valgrind gdbserver is started. This option will be more useful in
combination with a --vgdb-prefix unique for the process you want
to wait for. Also, if you use the --wait argument in the gdb
'target remote' command, you must set the gdb remotetimeout to a
value bigger than the --wait argument value. See option
--max-invoke-ms for an example of setting this remotetimeout
value.</para>
</listitem>
<listitem>
<para><option>-c</option> To give more than one command, separate
the commands by an option -c. Example:
<screen><![CDATA[
vgdb vg.set log_output -c mc.leak_check any
]]></screen></para>
</listitem>
<listitem>
<para><option>-d</option> instructs vgdb to produce debugging
output. Give multiple -d args for more debug info.</para>
</listitem>
<listitem>
<para><option>-D</option> instructs vgdb to show the state of the
shared memory used by the Valgrind gdbserver. vgdb will exit after
having shown the Valgrind gdbserver shared memory state.</para>
</listitem>
<listitem>
<para><option>-l</option> instructs vgdb to report the list of
the Valgrind gdbserver processes running and then exit.</para>
</listitem>
</itemizedlist>
</sect1>
<sect1 id="manual-core.valgrind-monitor-commands"
xreflabel="Valgrind monitor commands">
<title>Valgrind monitor commands</title>
<para>The Valgrind monitor commands are available whatever the
tool. They can be sent either from a shell command line (using a
standalone vgdb) or from gdb (using the gdb 'monitor' command).</para>
<itemizedlist>
<listitem>
<para><varname>help [debug]</varname> instructs Valgrind gdbserver
to give the list of all monitor commands of the Valgrind core and
of the tool. The optional 'debug' argument tells to also give help
for the monitor commands aimed at Valgrind internals debugging.
</para>
</listitem>
<listitem>
<para><varname>vg.info all_errors</varname> shows all errors found
so far.</para>
</listitem>
<listitem>
<para><varname>vg.info last_error</varname> shows the last error
found.</para>
</listitem>
<listitem>
<para><varname>vg.info n_errs_found</varname> shows the nr of
errors found so far and the current value of the --vgdb-error
argument.</para>
</listitem>
<listitem>
<para><varname>vg.set {gdb_output | log_output |
mixed_output}</varname> allows to redirect the Valgrind output
(e.g. the errors detected by the tool). By default, the setting is
mixed_output. </para>
<para>With mixed_output, the Valgrind output goes to the Valgrind
log (typically stderr) while the output of the interactive gdb
monitor commands (e.g. vg.info last_error) is displayed by
gdb.</para>
<para>With gdb_output, both the Valgrind output and the
interactive gdb monitor commands output is displayed by
gdb.</para>
<para>With log_output, both the Valgrind output and the
interactive gdb monitor commands output go to the Valgrind
log.</para>
</listitem>
<listitem>
<para><varname>vg.wait [ms (default 0)]</varname> instructs
Valgrind gdbserver to sleep 'ms' milli-seconds and then
continue. When sent from a standalone vgdb, if this is the last
command, the Valgrind process will continue the execution of the
guest process. The typical usage of this is to use vgdb to send a
"no-op" command to a Valgrind gdbserver so as to continue the
execution of the guess process.
</para>
</listitem>
<listitem>
<para><varname>vg.kill;</varname> requests the gdbserver to kill
the process. This can be used from a standalone vgdb to properly
kill a Valgrind process which is currently expecting a vgdb
connection.</para>
</listitem>
<listitem>
<para><varname>vg.set vgdb-error &lt;errornr&gt;</varname>
dynamically changes the value of the --vgdb-error argument. A
typical usage of this is to start with --vgdb-error=0 on the
command line, then set a few breakpoints, set the vgdb-error value
to a huge value and continue execution.</para>
</listitem>
</itemizedlist>
<para>The below Valgrind monitor commands are useful to investigate
the behaviour of Valgrind or Valgrind gdbserver in case of problem or
bug.</para>
<itemizedlist>
<listitem>
<para><varname>vg.info gdbserver_status</varname> shows the
gdbserver status. In case of problem (e.g. of communications),
this gives the value of some relevant Valgrind gdbserver internal
variables. Note that the variables related to breakpoints and
watchpoints (e.g. the nr of gdbserved addresses and the nr of
watchpoints) will be zero, as gdb by default removes all
watchpoints and breakpoints when execution stops, and re-inserts
them when resuming the execution of the debugged process. You can
change this gdb behaviour by using the gdb command 'set breakpoint
always-inserted on'.
</para>
</listitem>
<listitem>
<para><varname>vg.info memory</varname> shows the statistics of
the Valgrind heap management. If
option <option>--profile-heap=yes=yes</option> was given, detailed
statistics will be output.
</para>
</listitem>
<listitem>
<para><varname>vg.set debuglog &lt;intvalue&gt;</varname> sets the
valgrind debug log level to &lt;intvalue&gt;. This allows to
dynamically change the log level of Valgrind e.g. when a problem
is detected.</para>
</listitem>
<listitem>
<para><varname>vg.translate &lt;address&gt;
[&lt;traceflags&gt;]</varname> traces the translation of the block
containing address with the given trace flags. The traceflags is a
bit pattern similar to the --trace-flags option. It can be given
in hexadecimal (e.g. 0x20) or decimal (e.g. 32) or in binary 1s
and 0s bit (e.g. 0b00100000). The default value of the traceflags
is 0b00100000, corresponding to 'show after instrumentation'. Note
that the output of this command always goes to the Valgrind
log. The additional bit flag 0b100000000 traces in addition the
gdbserver specific instrumentation. Note that bit can only enable
the addition of the gdbserver instrumentation in the trace.
Keeping this flag to 0 will not disable the tracing of the
gdbserver instrumentation if it is active for another reason
(e.g. because there is a breakpoint at this address or because
gdbserver is in single stepping mode). </para>
</listitem>
</itemizedlist>
</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 run only on one CPU, 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>
</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-precise-memory-exns=yes</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 head 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 SSE4.2 in 64-bit mode and SSSE3 in 32-bit mode.
Some exceptions: SSE4.2 AES instructions are not supported in
64-bit mode, and 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 and VFPv3 support is fairly complete. ARMv6
media instruction support is mostly done but not yet 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 100+ 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.
You may get many bogus error messages following this, if Valgrind
guesses wrong. At the moment "large change" is defined as a change
of more that 2000000 in the value of the
stack pointer register.</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>
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