Valgrind实战指南：从入门到精通

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"Valgrind 是一款强大的内存错误检测和性能分析工具，常用于调试和优化C/C++程序。这份文档是Valgrind 3.8.0版本的介绍，包含了一个快速入门指南和用户手册，旨在帮助用户理解和有效地利用Valgrind进行实践操作。文档遵循GNU Free Documentation License，允许自由复制、分发和修改。" Valgrind 是一个开源的软件开发工具，它提供了一套虚拟机监控系统，能够帮助开发者在不改变原有代码的情况下发现程序中的内存错误，如内存泄漏、非法内存访问和无效内存释放等问题。Valgrind 提供了多种子工具，如 Memcheck、Cachegrind 和 Helgrind，分别用于内存错误检测、缓存分析和竞态条件检测。《Valgrind Quick Start Guide》是为初学者准备的，它将引导用户快速了解如何配置和使用Valgrind。在指南中，用户可以找到关于如何编译和运行程序以通过Valgrind进行检查的基本步骤，以及如何解读Valgrind生成的报告。通过这个快速入门，用户能迅速掌握Valgrind的基本用法。《Valgrind User Manual》则提供了更详细的文档，包括Valgrind所有子工具的全面介绍、使用选项、配置文件和自定义事件等。用户手册深入探讨了如何针对特定问题定制Valgrind的使用，以提高调试和分析的效率。例如，Memcheck 工具可以帮助识别未初始化的内存读取、使用后即释放的内存、泄露的内存等常见问题；而Cachegrind则可以模拟CPU缓存的行为，帮助分析程序的缓存效率。《Valgrind FAQ》可能包含了一些常见问题和解决方法，对于遇到使用困难的用户来说，这是一个非常有价值的资源。《Valgrind Technical Documentation》可能包含了Valgrind的内部工作原理和技术细节，适合对Valgrind感兴趣的开发者深入了解其设计和实现。《Valgrind Distribution Documents》可能是关于Valgrind分发包的一些附加信息，如安装指南、更改日志等，这些文档有助于用户保持更新和理解Valgrind的最新变化。 Valgrind 是一个强大的工具，不仅能够帮助开发者找出程序中的内存错误，还能提供性能分析，是C/C++开发过程中不可或缺的一部分。通过阅读和实践提供的文档，用户可以逐步提升在使用Valgrind上的技能，从而提高软件的质量和稳定性。

Using and understanding the Valgrind core

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.

2.2. Getting started

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.

2.3. The Commentary

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.

Using and understanding the Valgrind core

You can direct the commentary to three different places:

1. 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.

2. 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.

3. 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.

Using and understanding the Valgrind core

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.

2.4. Reporting of errors

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.

2.5. Suppressing errors

Using and understanding the Valgrind core

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.

Using and understanding the Valgrind core

• 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 ap-

ply 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/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

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