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Valgrind User Manual | |
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Table of Contents
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 Valgrind's core: advanced topics.
A point of terminology: most references to "Valgrind" in this chapter refer to the Valgrind core services.
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.
You invoke Valgrind like this:
valgrind [valgrind-options] your-prog [your-prog-options]
The most important option is --tool which dictates
which Valgrind tool to run. For example, if want to run the command
ls -l using the memory-checking tool
Memcheck, issue this command:
valgrind --tool=memcheck ls -l
However, Memcheck is the default, so if you want to use it you can
omit the --tool option.
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.
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.
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.
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.
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
--gen-suppressions=yes option. This tells Valgrind to
print out a suppression for each reported error, which you can then
copy into a suppressions file.
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.
First off, consider whether it might be beneficial to recompile
your application and supporting libraries with debugging info enabled
(the -g 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 -g, you'll get
messages which point directly to the relevant source code lines.
Another option you might like to consider, if you are working with
C++, is -fno-inline. 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.
If you are planning to use Memcheck: On rare
occasions, compiler optimisations (at -O2
and above, and sometimes -O1) 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
-O. 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 -Wall because
it can identify some or all of the problems that Valgrind can miss at the
higher optimisation levels. (Using -Wall
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.
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.
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
/bin/sh,
/usr/bin/perl, 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
--trace-children=yes, but confusion is still
likely.
Valgrind tools write a commentary, a stream of text, detailing error reports and other significant events. All lines in the commentary have following form:
==12345== some-message-from-Valgrind
The 12345 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.
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 -v option to Valgrind. A second
-v gives yet more detail.
You can direct the commentary to three different places:
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
--log-fd=9.
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.
A less intrusive
option is to write the commentary to a file, which you specify by
--log-file=filename. 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 basic options section
for more details.
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: --log-socket=192.168.0.1:12345 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:
--log-socket=192.168.0.1, in which case a default
port of 1500 is used. This default is defined by the constant
VG_CLO_DEFAULT_LOGPORT in the
sources.
Note, unfortunately, that you have to use an IP address here, rather than a hostname.
Writing to a network socket is pointless if you don't
have something listening at the other end. We provide a simple
listener program,
valgrind-listener, 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.
valgrind-listener 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.
valgrind-listener accepts two
command-line options:
-e or --exit-at-zero:
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.
portnumber: 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 --log-socket to
Valgrind itself.
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.
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
--log-* 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.
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:
==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
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
bogon.cpp, 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.
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.
If you want to know how many times each error occurred, run with
the -v 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.
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.
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.
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 m_errormgr.c and can be increased
if necessary.
To avoid this cutoff you can use the
--error-limit=no 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.
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
./configure script when the system is
built.
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.
Note: By far the easiest way to add
suppressions is to use the --gen-suppressions=yes option
described in Core Command-line Options. This generates
suppressions automatically. For best results,
though, you may want to edit the output
of --gen-suppressions=yes by hand, in which
case it would be advisable to read through this section.
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.
If you use the -v 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 valgrind --tool=memcheck ls -l:
--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
Multiple suppressions files are allowed. By default, Valgrind
uses $PREFIX/lib/valgrind/default.supp. You can
ask to add suppressions from another file, by specifying
--suppressions=/path/to/file.supp.
If you want to understand more about suppressions, look at an
existing suppressions file whilst reading the following documentation.
The file glibc-2.3.supp, in the source
distribution, provides some good examples.
Each suppression has the following components:
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.
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:
tool_name1,tool_name2:suppression_name
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.
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.
Next line: a small number of suppression types have extra
information after the second line (eg. the Param
suppression for Memcheck)
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.
Locations may be names of either shared objects or
functions. They begin
obj: and
fun: respectively. Function and
object names to match against may use the wildcard characters
* and
?.
Important note: C++ function names must be
mangled. If you are writing suppressions by
hand, use the --demangle=no option to get the
mangled names in your error messages. An example of a mangled
C++ name is _ZN9QListView4showEv.
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, QListView::show(),
is what you see at the C++ source code level.
A location line may also be
simply "..." (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.
Finally, the entire suppression must be between curly braces. Each brace must be the first character on its own line.
A suppression only suppresses an error when the error matches all the details in the suppression. Here's an example:
{
__gconv_transform_ascii_internal/__mbrtowc/mbtowc
Memcheck:Value4
fun:__gconv_transform_ascii_internal
fun:__mbr*toc
fun:mbtowc
}
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
__gconv_transform_ascii_internal, when
that is called from any function of name matching
__mbr*toc, when that is called from
mbtowc. It doesn't apply under any
other circumstances. The string by which this suppression is identified
to the user is
__gconv_transform_ascii_internal/__mbrtowc/mbtowc.
(See Writing suppression files for more details on the specifics of Memcheck's suppression kinds.)
Another example, again for the Memcheck tool:
{
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
}
This suppresses any size 4 uninitialised-value error which occurs
anywhere in libX11.so.6.2, when called from
anywhere in the same library, when called from anywhere in
libXaw.so.7.0. 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.
Although the above two examples do not make this clear, you can
freely mix obj: and
fun: lines in a suppression.
Finally, here's an example using three frame-level wildcards:
{
a-contrived-example
Memcheck:Leak
fun:malloc
...
fun:ddd
...
fun:ccc
...
fun:main
}
This suppresses Memcheck memory-leak errors, in the case where
the allocation was done by main
calling (though any number of intermediaries, including zero)
ccc,
calling onwards via
ddd and eventually
to malloc..
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.
Valgrind's default settings succeed in giving reasonable behaviour in most cases. We group the available options by rough categories.
The single most important option.
These options work with all tools.
-h --help
Show help for all options, both for the core and for the
selected tool. If the option is repeated it is equivalent to giving
--help-debug.
--help-debug
Same as --help, but also lists debugging
options which usually are only of use to Valgrind's
developers.
--version
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.
-q, --quiet
Run silently, and only print error messages. Useful if you are running regression tests or have some other automated test machinery.
-v, --verbose
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.
--trace-children=<yes|no> [default: no]
When enabled, Valgrind will trace into sub-processes
initiated via the exec system call. This is
necessary for multi-process programs.
Note that Valgrind does trace into the child of a
fork (it would be difficult not to, since
fork makes an identical copy of a process), so this
option is arguably badly named. However, most children of
fork calls immediately call exec
anyway.
--trace-children-skip=patt1,patt2
This option only has an effect when
--trace-children=yes 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 ?
and *, which have the usual
meaning.
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.
--child-silent-after-fork=<yes|no> [default: no]
When enabled, Valgrind will not show any debugging or
logging output for the child process resulting from
a fork 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 --trace-children=. Use of this option is also
strongly recommended if you are requesting XML output
(--xml=yes), since otherwise the XML from child and
parent may become mixed up, which usually makes it useless.
--track-fds=<yes|no> [default: no]
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.
--time-stamp=<yes|no> [default: no]
When enabled, each message is preceded with an indication of the elapsed wallclock time since startup, expressed as days, hours, minutes, seconds and milliseconds.
--log-fd=<number> [default: 2, stderr]
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.
--log-file=<filename>
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.
%p is replaced with the current process ID.
This is very useful for program that invoke multiple processes.
WARNING: If you use --trace-children=yes and your
program invokes multiple processes OR your program forks without
calling exec afterwards, and you don't use this specifier
(or the %q specifier below), the Valgrind output from
all those processes will go into one file, possibly jumbled up, and
possibly incomplete.
%q{FOO} is replaced with the contents of the
environment variable FOO. If the
{FOO} 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 BPROC_RANK 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
{ and } characters may need to be
escaped with a backslash.
%% is replaced with %.
If an % is followed by any other character, it
causes an abort.
--log-socket=<ip-address:port-number>
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
valgrind-listener program. For
further details, see
the commentary
in the manual.
These options are used by all tools that can report errors, e.g. Memcheck, but not Cachegrind.
--xml=<yes|no> [default: no]
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
--xml-fd, --xml-file or
--xml-socket to specify where the XML is to be sent.
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 --log-fd, --log-file
and --log-socket) this should not cause problems.
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
docs/internals/xml-output-protocol4.txt
in the source tree for Valgrind 3.5.0 or later.
The recommended options for a GUI to pass, when requesting
XML output, are: --xml=yes to enable XML output,
--xml-file to send the XML output to a (presumably
GUI-selected) file, --log-file to send the plain
text output to a second GUI-selected file,
--child-silent-after-fork=yes, and
-q 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.
--xml-fd=<number> [default: -1, disabled]
Specifies that Valgrind should send its XML output to the
specified file descriptor. It must be used in conjunction with
--xml=yes.
--xml-file=<filename>
Specifies that Valgrind should send its XML output
to the specified file. It must be used in conjunction with
--xml=yes. Any %p or
%q sequences appearing in the filename are expanded
in exactly the same way as they are for --log-file.
See the description of --log-file for details.
--xml-socket=<ip-address:port-number>
Specifies that Valgrind should send its XML output the
specified port at the specified IP address. It must be used in
conjunction with --xml=yes. The form of the argument
is the same as that used by --log-socket.
See the description of --log-socket
for further details.
--xml-user-comment=<string>
Embeds an extra user comment string at the start of the XML
output. Only works when --xml=yes is specified;
ignored otherwise.
--demangle=<yes|no> [default: yes]
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.
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.
--num-callers=<number> [default: 12]
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.
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.
--error-limit=<yes|no> [default: yes]
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.
--error-exitcode=<number> [default: 0]
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.
--show-below-main=<yes|no> [default: no]
By default, stack traces for errors do not show any
functions that appear beneath main because
most of the time it's uninteresting C library stuff and/or
gobbledygook. Alternatively, if main is not
present in the stack trace, stack traces will not show any functions
below main-like functions such as glibc's
__libc_start_main. Furthermore, if
main-like functions are present in the trace,
they are normalised as (below main), in order to
make the output more deterministic.
If this option is enabled, all stack trace entries will be
shown and main-like functions will not be
normalised.
--fullpath-after=<string>
[default: don't show source paths]
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.
--fullpath-after 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 string is found in the path, then the path
up to and including string is omitted, else the
path is shown unmodified. Note that string is
not required to be a prefix of the path.
For example, consider a file named
/home/janedoe/blah/src/foo/bar/xyzzy.c.
Specifying --fullpath-after=/home/janedoe/blah/src/
will cause Valgrind to show the name
as foo/bar/xyzzy.c.
Because the string is not required to be a prefix,
--fullpath-after=src/ will produce the same
output. This is useful when the path contains arbitrary
machine-generated characters. For example, the
path
/my/build/dir/C32A1B47/blah/src/foo/xyzzy
can be pruned to foo/xyzzy
using
--fullpath-after=/blah/src/.
If you simply want to see the full path, just specify an
empty string: --fullpath-after=. This isn't a
special case, merely a logical consequence of the above rules.
Finally, you can use --fullpath-after
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 --fullpath-after-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.
--suppressions=<filename> [default: $PREFIX/lib/valgrind/default.supp]
Specifies an extra file from which to read descriptions of errors to suppress. You may use up to 100 extra suppression files.
--gen-suppressions=<yes|no|all> [default: no]
When set to yes, Valgrind will pause
after every error shown and print the line:
---- Print suppression ? --- [Return/N/n/Y/y/C/c] ----
The prompt's behaviour is the same as for the
--db-attach option (see below).
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.
When set to all, Valgrind will print a
suppression for every reported error, without querying the
user.
This option is particularly useful with C++ programs, as it prints out the suppressions with mangled names, as required.
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.
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
<insert a suppression name
here>; the name doesn't really matter, it's
only used with the -v option which prints out all
used suppression records.
--db-attach=<yes|no> [default: no]
When enabled, Valgrind will pause after every error shown and print the line:
---- Attach to debugger ? --- [Return/N/n/Y/y/C/c] ----
Pressing Ret, or N Ret or
n Ret, causes Valgrind not to start a debugger
for this error.
Pressing Y Ret or
y Ret 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.
C Ret or c Ret causes
Valgrind not to start a debugger, and not to ask again.
--db-command=<command> [default: gdb -nw %f %p]
Specify the debugger to use with the
--db-attach command. The default debugger is
GDB. This option is a template that is expanded by Valgrind at
runtime. %f is replaced with the executable's
file name and %p is replaced by the process ID
of the executable.
This specifies how Valgrind will invoke the debugger. By
default it will use whatever GDB is detected at build time, which
is usually /usr/bin/gdb. Using
this command, you can specify some alternative command to invoke
the debugger you want to use.
The command string given can include one or instances of the
%p and %f expansions. Each
instance of %p expands to the PID of the
process to be debugged and each instance of %f
expands to the path to the executable for the process to be
debugged.
Since <command> is likely
to contain spaces, you will need to put this entire option in
quotes to ensure it is correctly handled by the shell.
--input-fd=<number> [default: 0, stdin]
When using --db-attach=yes or
--gen-suppressions=yes, 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.
--dsymutil=no|yes [no]
This option is only relevant when running Valgrind on Mac OS X.
Mac OS X uses a deferred debug information (debuginfo)
linking scheme. When object files containing debuginfo are
linked into a .dylib or an
executable, the debuginfo is not copied into the final file.
Instead, the debuginfo must be linked manually by
running dsymutil, a
system-provided utility, on the executable
or .dylib. The resulting
combined debuginfo is placed in a directory alongside the
executable or .dylib, but with
the extension .dSYM.
With --dsymutil=no, Valgrind
will detect cases where the
.dSYM directory is either
missing, or is present but does not appear to match the
associated executable or .dylib,
most likely because it is out of date. In these cases, Valgrind
will print a warning message but take no further action.
With --dsymutil=yes, Valgrind
will, in such cases, automatically
run dsymutil as necessary to
bring the debuginfo up to date. For all practical purposes, if
you always use --dsymutil=yes, then
there is never any need to
run dsymutil manually or as part
of your applications's build system, since Valgrind will run it
as necessary.
Valgrind will not attempt to
run dsymutil on any
executable or library in
/usr/,
/bin/,
/sbin/,
/opt/,
/sw/,
/System/,
/Library/ or
/Applications/
since dsymutil 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.
Be careful when
using --dsymutil=yes, since it will
cause pre-existing .dSYM
directories to be silently deleted and re-created. Also note that
dsymutil is quite slow, sometimes
excessively so.
--max-stackframe=<number> [default: 2000000]
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.
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.
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.
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.
--main-stacksize=<number>
[default: use current 'ulimit' value]
Specifies the size of the main thread's stack.
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.
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.
If you need a larger total stack size,
use --main-stacksize 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.
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.
--main-stacksize 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.
You may need to use both --main-stacksize
and --max-stackframe together. It is important
to understand that --main-stacksize sets the
maximum total stack size,
whilst --max-stackframe specifies the largest
size of any one stack frame. You will have to work out
the --main-stacksize value for yourself
(usually, if your applications segfaults). But Valgrind will
tell you the needed --max-stackframe size, if
necessary.
As discussed further in the description
of --max-stackframe, 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.
For tools that use their own version of
malloc (e.g. Memcheck and
Massif), the following options apply.
--alignment=<number> [default: 8 or 16, depending on the platform]
By default Valgrind's malloc,
realloc, 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.
These options apply to all tools, as they affect certain obscure workings of the Valgrind core. Most people won't need to use these.
--smc-check=<none|stack|all> [default: stack]
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.
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 and
subsequently overwrite part or all of it. Running with
all will slow Valgrind down noticeably. Running with
none will rarely speed things up, since very little
code gets put on the stack for most programs. The
VALGRIND_DISCARD_TRANSLATIONS client request is
an alternative to --smc-check=all that requires more
effort but is much faster.
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.
--read-var-info=<yes|no> [default: no]
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:
==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
And here are the same errors with
--read-var-info=yes:
==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
--run-libc-freeres=<yes|no> [default: yes]
This option is only relevant when running Valgrind on Linux.
The GNU C library (libc.so), 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—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.
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 __libc_freeres
specifically to make glibc release all memory it has allocated.
Memcheck therefore tries to run
__libc_freeres at exit.
Unfortunately, in some very old versions of glibc,
__libc_freeres 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
__libc_freeres. If your program seems to run
fine on Valgrind, but segfaults at exit, you may find that
--run-libc-freeres=no fixes that, although at the
cost of possibly falsely reporting space leaks in
libc.so.
--sim-hints=hint1,hint2,...
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:
lax-ioctls: 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.
enable-inner: Enable some special
magic needed when the program being run is itself
Valgrind.
--kernel-variant=variant1,variant2,...
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:
bproc: Support the
sys_broc 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.
--show-emwarns=<yes|no> [default: no]
When enabled, Valgrind will emit warnings about its CPU emulation in certain cases. These are usually not interesting.
--require-text-symbol=:sonamepatt:fnnamepatt
When a shared object whose soname
matches sonamepatt is loaded into the
process, examine all the text symbols it exports. If none of
those match fnnamepatt, 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.
Both sonamepatt and
fnnamepatt can be written using the usual
? and * wildcards. For
example: ":*libc.so*:foo?bar". 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 "Q*libc.so*Qfoo?bar".
Multiple --require-text-symbol flags are
allowed, in which case shared objects that are loaded into
the process will be checked against all of them.
The purpose of this is to support reliable usage of marked-up
libraries. For example, suppose we have a version of GCC's
libgomp.so which has been marked up with
annotations to support Helgrind. It is only too easy and
confusing to load the wrong, un-annotated
libgomp.so into the application. So the idea
is: add a text symbol in the marked-up library, for
example annotated_for_helgrind_3_6, and then
give the flag
--require-text-symbol=:*libgomp*so*:annotated_for_helgrind_3_6
so that when libgomp.so 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 *
and ? wildcards.
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
--help-debug option.
Note that Valgrind also reads options from three places:
The file ~/.valgrindrc
The environment variable
$VALGRIND_OPTS
The file ./.valgrindrc
These are processed in the given order, before the
command-line options. Options processed later override those
processed earlier; for example, options in
./.valgrindrc will take
precedence over those in
~/.valgrindrc.
Please note that the ./.valgrindrc
file is ignored if it is marked as world writeable or not owned
by the current user. This is because the
./.valgrindrc can contain options that are
potentially harmful or can be used by a local attacker to execute code under
your user account.
Any tool-specific options put in
$VALGRIND_OPTS or the
.valgrindrc 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 ~/.valgrindrc:
--memcheck:leak-check=yes
This will be ignored if any tool other than Memcheck is
run. Without the memcheck:
part, this will cause problems if you select other tools that
don't understand
--leak-check=yes.
Threaded programs are fully supported.
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.
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.
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.
On Linux, Valgrind also supports direct use of the
clone system call,
futex and so on.
clone is supported where either
everything is shared (a thread) or nothing is shared (fork-like); partial
sharing will fail.
Valgrind has a fairly complete signal implementation. It should be able to cope with any POSIX-compliant use of signals.
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 --vex-iropt-precise-memory-exns=yes.
If your program dies as a result of a fatal core-dumping signal,
Valgrind will generate its own core file
(vgcore.NNNNN) 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.
In the unlikely event that Valgrind itself crashes, the operating system will create a core dump in the usual way.
We use the standard Unix
./configure,
make, make
install mechanism. Once you have completed
make install you may then want
to run the regression tests
with make regtest.
In addition to the usual
--prefix=/path/to/install/tree, there are three
options which affect how Valgrind is built:
--enable-inner
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.
--enable-only64bit
--enable-only32bit
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).
The configure script tests
the version of the X server currently indicated by the current
$DISPLAY. 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.
If you are building a binary package of Valgrind for
distribution, please read README_PACKAGERS
Readme Packagers. It contains some
important information.
Apart from that, there's not much excitement here. Let us know if you have build problems.
Contact us at http://www.valgrind.org/.
See Limitations for the known limitations of Valgrind, and for a list of programs which are known not to work on it.
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!
If you get an assertion failure
in m_mallocfree.c, 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.
Read the Valgrind FAQ for more advice about common problems, crashes, etc.
The following list of limitations seems long. However, most programs actually work fine.
Valgrind will run programs on the supported platforms subject to the following constraints:
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.
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.
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.
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 The Client Request mechanism), Memcheck can do better. Nevertheless, using malloc/new and free/delete is still the best approach.
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.
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.
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.
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 --smc-check=all option, and Valgrind will
run more slowly than normal. Or you can add client requests that
tell Valgrind when your program has overwritten code.
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 --smc-check=all.
Valgrind has the following limitations in its implementation of x86/AMD64 floating point relative to IEEE754.
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.
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.
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.
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).
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.
Currently Valgrind only supports the default fixup actions. Again, feedback on the importance of exception support would be appreciated.
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 --show-emwarns=yes to see
them.
The above limitations define precisely the IEEE754 'default' behaviour: default fixup on all exceptions, round-to-nearest operations, and 64-bit precision.
Valgrind has the following limitations in its implementation of x86/AMD64 SSE2 FP arithmetic, relative to IEEE754.
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.
Valgrind has the following limitations in its implementation of ARM VFPv3 arithmetic, relative to IEEE754.
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.
Valgrind has the following limitations in its implementation of PPC32 and PPC64 floating point arithmetic, relative to IEEE754.
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.
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.
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).
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.
Programs which are known not to work are:
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
mallinfo function.
Emacs works fine if you build it to use
the standard malloc/free routines.
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).
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
The GCC folks fixed this about a week before GCC 3.0 shipped.
Some of these only appear if you run in verbose mode
(enabled by -v):
More than 100 errors detected. Subsequent
errors will still be recorded, but in less detail than
before.
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.
More than 1000 errors detected. I'm not
reporting any more. Final error counts may be inaccurate. Go fix
your program!
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.
Warning: client switching stacks?
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.
Warning: client attempted to close Valgrind's
logfile fd <number>
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
--log-fd=<number> option to specify a
different logfile file-descriptor number.
Warning: noted but unhandled ioctl
<number>
Valgrind observed a call to one of the vast family of
ioctl 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.
Warning: set address range perms: large range
<number>
Diagnostic message, mostly for benefit of the Valgrind developers, to do with memory permissions.