Valgrind is licensed under the GNU General Public License,
version 2
An open-source tool for supervising execution of Linux-x86 executables.
Although writing a skin is not easy, and requires learning quite a few things about Valgrind, it is much easier than instrumenting a program from scratch yourself.
The core provides the common low-level infrastructure to support program instrumentation, including the x86-to-x86 JIT compiler, low-level memory manager, signal handling and a scheduler (for pthreads). It also provides certain services that are useful to some but not all skins, such as support for error recording and suppression.
But the core leaves certain operations undefined, which must be filled by skins. Most notably, skins define how program code should be instrumented. They can also define certain variables to indicate to the core that they would like to use certain services, or be notified when certain interesting events occur.
Each skin that is written defines a new program supervision tool. Writing a new tool just requires writing a new skin. The core takes care of all the hard work.
Code executed in user space includes all the program code, almost all of the C library (including things like the dynamic linker), and almost all parts of all other libraries.
malloc()
etc.)A skin has no control over these operations; it never ``sees'' the code doing this work and thus cannot instrument it. However, the core provides hooks so a skin can be notified when certain interesting events happen, for example when when dynamic memory is allocated or freed, the stack pointer is changed, or a pthread mutex is locked, etc.
Note that these hooks only notify skins of events relevant to user space. For example, when the core allocates some memory for its own use, the skin is not notified of this, because it's not directly part of the supervised program's execution.
It should be noted that a skin only has direct control over code executed in user space. This is the vast majority of code executed, but it is not absolutely all of it, so any profiling information recorded by a skin won't be totally accurate.
_dl_runtime_resolve()
); the number of
basic blocks, x86 instruction, UCode instructions executed; the number
of branches executed and the proportion of those which were taken.
This is achieved by packaging each skin into a separate shared object which is
then loaded ahead of the core shared object valgrind.so
, using the
dynamic linker's LD_PRELOAD
variable. Any functions defined in
the skin that share the name with a function defined in core (such as
the instrumentation function SK_(instrument)()
) override the
core's definition. Thus the core can call the necessary skin functions.
This magic is all done for you; the shared object used is chosen with the
To check out the code from the CVS repository, first login:
In what follows, all filenames are relative to Valgrind's top-level directory
Now that we've setup, built and tested the simplest possible skin, onto the
interesting stuff...
--skin
option to the valgrind
startup script. The
default skin used is memcheck
, Valgrind's original memory checker.
2.3 Getting the code
To write your own skin, you'll need to check out a copy of Valgrind from the
CVS repository, rather than using a packaged distribution. This is because it
contains several extra files needed for writing skins.
Then checkout the code. To get a copy of the current development version
(recommended for the brave only):
cvs -d:pserver:anonymous@cvs.valgrind.sourceforge.net:/cvsroot/valgrind login
To get a copy of the stable released branch:
cvs -z3 -d:pserver:anonymous@cvs.valgrind.sourceforge.net:/cvsroot/valgrind co valgrind
where
cvs -z3 -d:pserver:anonymous@cvs.valgrind.sourceforge.net:/cvsroot/valgrind co -r TAG valgrind
TAG
has the form VALGRIND_X_Y_Z
for
version X.Y.Z.
2.4 Getting started
Valgrind uses GNU automake
and autoconf
for the
creation of Makefiles and configuration. But don't worry, these instructions
should be enough to get you started even if you know nothing about those
tools.valgrind/
.
These steps don't have to be followed exactly - you can choose different names
for your source files, and use a different foobar
and fb
as an
example.
foobar/
which will hold the skin.
example/Makefile.am
into foobar/
.
Edit it by replacing all occurrences of the string
``example
'' with ``foobar
'' and the one
occurrence of the string ``ex_
'' with ``fb_
''.
It might be worth trying to understand this file, at least a little; you
might have to do more complicated things with it later on. In
particular, the name of the vgskin_foobar_so_SOURCES
variable
determines the name of the skin's shared object, which determines what
name must be passed to the --skin
option to use the skin.
example/ex_main.c
into
foobar/
, renaming it as fb_main.c
.
Edit it by changing the five lines in SK_(pre_clo_init)()
to something appropriate for the skin. These fields are used in the
startup message, except for bug_reports_to
which is used
if a skin assertion fails.
Makefile.am
, adding the new directory
foobar
to the SUBDIRS
variable.
configure.in
, adding foobar/Makefile
to the
AC_OUTPUT
list.
autogen.sh
./configure --prefix=`pwd`/inst
make install
It should automake, configure and compile without errors, putting copies
of the skin's shared object vgskin_foobar.so
in
foobar/
and
inst/lib/valgrind/
.
inst/bin/valgrind --skin=foobar date
(almost any program should work; date
is just an example).
The output should be something like this:
==738== foobar-0.0.1, a foobarring tool for x86-linux.
==738== Copyright (C) 2002, and GNU GPL'd, by J. Random Hacker.
==738== Built with valgrind-1.1.0, a program execution monitor.
==738== Copyright (C) 2000-2002, and GNU GPL'd, by Julian Seward.
==738== Estimated CPU clock rate is 1400 MHz
==738== For more details, rerun with: -v
==738==
Wed Sep 25 10:31:54 BST 2002
==738==
The skin does nothing except run the program uninstrumented.
--prefix
for
./configure
.2.5 Writing the code
A skin must define at least these four functions:
SK_(pre_clo_init)()
SK_(post_clo_init)()
SK_(instrument)()
SK_(fini)()
Also, it must use the macro VG_DETERMINE_INTERFACE_VERSION
exactly once in its source code. If it doesn't, you will get a link error
involving VG_(skin_interface_major_version)
. This macro is
used to ensure the core/skin interface used by the core and a plugged-in
skin are binary compatible.
In addition, if a skin wants to use some of the optional services provided by
the core, it may have to define other functions.
SK_(pre_clo_init)()
.
Only use SK_(post_clo_init)()
if a skin provides command line
options and must do some initialisation after option processing takes place
(``clo
'' stands for ``command line options'').
First of all, various ``details'' need to be set for a skin, using the
functions VG_(details_*)()
. Some are all compulsory, some aren't.
Some are used when constructing the startup message,
detail_bug_reports_to
is used if VG_(skin_panic)()
is
ever called, or a skin assertion fails. Others have other uses.
Second, various ``needs'' can be set for a skin, using the functions
VG_(needs_*)()
. They are mostly booleans, and can be left
untouched (they default to False
). They determine whether a skin
can do various things such as: record, report and suppress errors; process
command line options; wrap system calls; record extra information about
malloc'd blocks, etc.
For example, if a skin wants the core's help in recording and reporting errors,
it must set the skin_errors
need to True
, and then
provide definitions of six functions for comparing errors, printing out errors,
reading suppressions from a suppressions file, etc. While writing these
functions requires some work, it's much less than doing error handling from
scratch because the core is doing most of the work. See the type
VgNeeds
in include/vg_skin.h
for full details of all
the needs.
Third, the skin can indicate which events in core it wants to be notified
about, using the functions VG_(track_*)()
. These include things
such as blocks of memory being malloc'd, the stack pointer changing, a mutex
being locked, etc. If a skin wants to know about this, it should set the
relevant pointer in the structure to point to a function, which will be called
when that event happens.
For example, if the skin want to be notified when a new block of memory is
malloc'd, it should call VG_(track_new_mem_heap)()
with an
appropriate function pointer, and the assigned function will be called each
time this happens.
More information about ``details'', ``needs'' and ``trackable events'' can be
found in include/vg_skin.h
.
SK_(instrument)()
is the interesting one. It allows you to
instrument UCode, which is Valgrind's RISC-like intermediate language.
UCode is described in the technical docs.
The easiest way to instrument UCode is to insert calls to C functions when
interesting things happen. See the skin ``lackey''
(lackey/lk_main.c
) for a simple example of this, or
Cachegrind (cachegrind/cg_main.c
) for a more complex
example.A much more complicated way to instrument UCode, albeit one that might result in faster instrumented programs, is to extend UCode with new UCode instructions. This is recommended for advanced Valgrind hackers only! See the ``memcheck'' skin for an example.
The file include/vg_skin.h
contains all the types,
macros, functions, etc. that a skin should (hopefully) need, and is the only
.h
file a skin should need to #include
.
In particular, you probably shouldn't use anything from the C library (there
are deep reasons for this, trust us). Valgrind provides an implementation of a
reasonable subset of the C library, details of which are in
vg_skin.h
.
Similarly, when writing a skin, you shouldn't need to look at any of the code in Valgrind's core. Although it might be useful sometimes to help understand something.
vg_skin.h
has a reasonable amount of documentation in it that
should hopefully be enough to get you going. But ultimately, the skins
distributed (memcheck, addrcheck, cachegrind, lackey, etc.) are probably the
best documentation of all, for the moment.
Note that the VG_
and SK_
macros are used heavily.
These just prepend longer strings in front of names to avoid potential
namespace clashes. We strongly recommend using the SK_
macro
for any global functions and variables in your skin.
If you are getting segmentation faults in C functions used by your skin, the
usual GDB command:
If you want to debug C functions used by your skin, you can attach GDB to
Valgrind with some effort:
and rebuild Valgrind.
If you just want to know whether a program point has been reached, using the
If you are having problems with your UCode instrumentation, it's likely that
GDB won't be able to help at all. In this case, Valgrind's
The other debugging command line options can be useful too (run
The profiler is stack-based; you can register a profiling event with
If you add any scripts to your skin (see Cachegrind for an example) you need to
add them to the
The interface version number has the form X.Y. Changes in Y indicate binary
compatible changes. Changes in X indicate binary incompatible changes. If
the core and skin has the same major version number X they should work
together. If X doesn't match, Valgrind will abort execution with an
explanation of the problem.
This approach was chosen so that if the interface changes in the future,
old skins won't work and the reason will be clearly explained, instead of
possibly crashing mysteriously. We have attempted to minimise the potential
for binary incompatible changes by means such as minimising the use of naked
structs in the interface.
The first consequence of this is that the core/skin interface is quite
immature. It will almost certainly change in the future; we have no intention
of freezing it and then regretting the inevitable stupidities. Hopefully most
of the future changes will be to add new features, hooks, functions, etc,
rather than to change old ones, which should cause a minimum of trouble for
existing skins, and we've put some effort into future-proofing the interface
to avoid binary incompatibility. But we can't guarantee anything. The
versioning system should catch any incompatibilities. Just something to be
aware of.
The second consequence of this is that we'd love to hear your feedback about
it:
Happy programming.
2.10 Words of Advice
Writing and debugging skins is not trivial. Here are some suggestions for
solving common problems.
usually gives the location of the segmentation fault.gdb prog core
GDB may be able to give you useful information. Note that by default
most of the system is built with coregrind/vg_main.c
by
changing if (0)
into if (1)
:
/* Hook to delay things long enough so we can get the pid and
attach GDB in another shell. */
if (0) {
Int p, q;
for (p = 0; p < 50000; p++)
for (q = 0; q < 50000; q++) ;
}
Valgrind starts the program, printing its process id, and then delays for
a few seconds (you may have to change the loop bounds to get a suitable
delay).valgrind prog
gdb prog pid
-fomit-frame-pointer
,
and you'll need to get rid of this to extract useful tracebacks from
GDB.OINK
macro (in include/vg_skin.h
) can be easier than
using GDB.--trace-codegen
option is invaluable for observing the results of
instrumentation.valgrind
-h
for the list).3 Advanced Topics
Once a skin becomes more complicated, there are some extra things you may
want/need to do.
3.1 Suppressions
If your skin reports errors and you want to suppress some common ones, you can
add suppressions to the suppression files. The relevant files are
valgrind/*.supp
; the final suppression file is aggregated from
these files by combining the relevant .supp
files depending on the
versions of linux, X and glibc on a system.
3.2 Documentation
If you are feeling conscientious and want to write some HTML documentation for
your skin, follow these steps (using foobar
as the example skin
name again):
foobar/docs/
.
foobar/Makefile.am
, adding docs
to
the SUBDIRS
variable.
configure.in
, adding
foobar/docs/Makefile
to the AC_OUTPUT
list.
foobar/docs/Makefile.am
. Use
memcheck/docs/Makefile.am
as an example.
foobar/docs/index.html
.
docs/index.html
to
foobar/docs/index.html
3.3 Regression tests
Valgrind has some support for regression tests. If you want to write
regression tests for your skin:
foobar/tests/
.
foobar/Makefile.am
, adding tests
to
the SUBDIRS
variable.
configure.in
, adding
foobar/tests/Makefile
to the AC_OUTPUT
list.
foobar/tests/Makefile.am
. Use
memcheck/tests/Makefile.am
as an example.
.vgtest
test description files,
.stdout.exp
and .stderr.exp
expected output
files. (Note that Valgrind's output goes to stderr.) Some details
on writing and running tests are given in the comments at the top of the
testing script tests/vg_regtest
.
foobar/tests/filter_stderr
.
It can call the existing filters in tests/
. See
memcheck/tests/filter_stderr
for an example; in particular
note the $dir
trick that ensures the filter works correctly
from any directory.
3.4 Profiling
To do simple tick-based profiling of a skin, include the line
#include "vg_profile.c"
in the skin somewhere, and rebuild (you may have to make clean
first). Then run Valgrind with the --profile=yes
option.VGP_(register_profile_event)()
and then use the
VGP_PUSHCC
and VGP_POPCC
macros to record time spent
doing certain things. New profiling event numbers must not overlap with the
core profiling event numbers. See include/vg_skin.h
for details
and the ``memcheck'' skin for an example.
3.5 Other makefile hackery
If you add any directories under valgrind/foobar/
, you will
need to add an appropriate Makefile.am
to it, and add a
corresponding entry to the AC_OUTPUT
list in
valgrind/configure.in
.bin_SCRIPTS
variable in
valgrind/foobar/Makefile.am
.3.5 Core/skin interface versions
In order to allow for the core/skin interface to evolve over time, Valgrind
uses a basic interface versioning system. All a skin has to do is use the
VG_DETERMINE_INTERFACE_VERSION
macro exactly once in its code.
If not, a link error will occur when the skin is built.
4 Final Words
This whole core/skin business is very new and experimental, and under active
development.
or anything else!