Valgrind使用入门与用户手册

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"Valgrind是一款强大的内存调试、内存错误检测和性能分析工具，适用于Linux、Unix和macOS等操作系统。它的使用手册包含了丰富的信息，帮助用户理解和有效地利用Valgrind进行程序的诊断和优化。手册由多个部分组成，包括快速入门指南、用户手册、常见问题解答和技术文档等。" Valgrind是一个开源软件，其主要目的是帮助开发者在运行时检查程序中的错误，如内存泄漏、无效内存访问和栈溢出等问题。通过模拟CPU指令，Valgrind可以在不修改源代码的情况下对程序进行动态分析。《Valgrind Quick Start Guide》是为初学者准备的，它会引导用户快速了解如何配置和使用Valgrind进行基本的错误检测。该指南可能包含以下内容：如何安装Valgrind、如何运行简单的命令行来启动分析、以及如何解读分析结果。《Valgrind User Manual》是详尽的使用指南，它深入介绍了Valgrind的各种子工具，如Memcheck（内存错误检测）、Helgrind（多线程错误检测）和DRD（数据竞争检测）。用户手册通常会涵盖每个工具的选项、配置文件的使用、以及如何定制报告格式等内容。《Valgrind FAQ》则提供了常见问题的答案，帮助解决用户在使用过程中可能遇到的问题，如性能影响、特定错误类型的解释以及如何排除某些类型的误报。《Valgrind Technical Documentation》是技术性的参考资料，可能包含关于Valgrind内部工作原理的详细信息，这对于想要深入理解Valgrind或者进行扩展开发的开发者尤其有价值。《Valgrind Distribution Documents》可能包含了Valgrind的发布说明、版权信息以及与软件分发相关的文档，对于那些关心软件版本历史和许可协议的用户来说，这部分内容是必不可少的。最后，手册还包括了GNU Free Documentation License，这是Valgrind文档所遵循的许可协议，允许用户自由复制、分发和修改文档。 Valgrind使用手册是一份全面的资源，涵盖了从基础使用到高级技术的所有方面，对于任何希望提高代码质量和性能的开发者来说，都是不可或缺的工具。通过阅读和实践手册中的内容，开发者可以更有效地定位和修复程序中的错误，提升软件的可靠性和效率。

Using and understanding the Valgrind core

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. Alternatively, the Valgrind option --

read-inline-info=yes instructs Valgrind to read the debug information describing inlining information. With

this, function call chain will be properly shown, even when your application is compiled with inlining.

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 the DWARF2/3/4 formats used by GCC 3.1 and later. The reader for "stabs" debugging format

(used by GCC versions prior to 3.1) has been disabled in Valgrind 3.9.0.

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.

You can direct the commentary to three different places:

Using and understanding the Valgrind core

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 three command-line options:

-e --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.

--max-connect=INTEGER

By default, the listener can connect to up to 50 processes. Occasionally, that number is too small. Use this

option to provide a different limit. E.g. --max-connect=100.

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.

Using and understanding the Valgrind core

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

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.

Using and understanding the Valgrind core

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 the

number of times it got used, its name and the filename and line number where the suppression is defined. Depending

on the suppression kind, the filename and line number are optionally followed by additional information (such as the

number of blocks and bytes suppressed by a Memcheck leak suppression). Here's the suppressions used by a run of

valgrind -v --tool=memcheck ls -l:

--1610-- used_suppression: 2 dl-hack3-cond-1 /usr/lib/valgrind/default.supp:1234

--1610-- used_suppression: 2 glibc-2.5.x-on-SUSE-10.2-(PPC)-2a /usr/lib/valgrind/default.supp:1234

Multiple suppressions files are allowed. Valgrind loads suppression patterns from $PREFIX/lib/valgrind/

default.supp unless --default-suppressions=no has been specified. You can ask to add suppressions

from additional files by specifying --suppressions=/path/to/file.supp one or more times.

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.

Blank and comment lines in a suppression file are ignored. Comment lines are made of 0 or more blanks followed

by a # character followed by some text.

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, functions, or source lines. They begin with obj:, fun:, or src:

respectively. Function, object, and file names to match against may use the wildcard characters * and ?. Source

lines are specified using the form filename[:lineNumber].

Using and understanding the Valgrind core

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/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.

An example of the src: specification, again for the Memcheck tool:

{

libX11.so.6.2/libX11.so.6.2/libXaw.so.7.0

Memcheck:Value4

src:valid.c:321

}

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