Perl基础教程：代码调试

perl调试教程

一、DESCRIPTION

A (very) lightweight introduction in the use of the perl debugger, and a pointer to existing,

deeper sources of information on the subject of debugging perl programs.

There's an extraordinary number of people out there who don't appear to know anything about

using the perl debugger, though they use the language every day. This is for them.

这是一个简单的Perl调试器的使用介绍.

这对那些虽然经常用Perl编程，但是并没有使用PERL调试器的人很有用。

二、use strict

First of all, there's a few things you can do to make your life a lot more straightforward

when it comes to debugging perl programs, without using the debugger at all.

To demonstrate, here's a simple script, named "hello", with a problem:

比起不使用调试器，使用调试器能更快捷些。

例如，我们从“hello”脚本程序开始：

#!/usr/bin/perl

$var1 = 'Hello World'; # always wanted to do that :-)

$var2 = "$varl\n";

print $var2;

exit;

While this compiles and runs happily, it probably won't do what's expected, namely it doesn't

print "Hello World\n" at all; It will on the other hand do exactly what it was told to do,

computers being a bit that way inclined. That is, it will print out a newline character,

and you'll get what looks like a blank line. It looks like there's 2 variables when (because

of the typo) there's really 3:

上面的程序并不会按想要的方式运行，是因为设想的是使用两个变量，但实际上使用三个。

$var1 = 'Hello World';

$varl = undef;

$var2 = "\n";

To catch this kind of problem, we can force each variable to be declared before use by pulling

in the strict module, by putting 'use strict;' after the first line of the script.

上面的程序则会出现错误提示。

为了捕捉这类问题，我们可以通过开启限制模块来使得变量使用之前必须先声明。

语法为"use strict"。

Now when you run it, perl complains about the 3 undeclared variables and we get four error

messages because one variable is referenced twice:

这样的，在运行时，Perl会给出相应的错误信息。

Global symbol "$var1" requires explicit package name at ./t1 line 4.

Global symbol "$var2" requires explicit package name at ./t1 line 5.

Global symbol "$varl" requires explicit package name at ./t1 line 5.

Global symbol "$var2" requires explicit package name at ./t1 line 7.

Execution of ./hello aborted due to compilation errors.

Luvverly! and to fix this we declare all variables explicitly and now our script looks like this:

对于这种错误，我们需要显示地声明所有变量，程序如下：

#!/usr/bin/perl

use strict;

my $var1 = 'Hello World';

my $varl = undef;

my $var2 = "$varl\n";

print $var2;

exit;

We then do (always a good idea) a syntax check before we try to run it again:

并进行一下语法检查：

> perl -c hello

hello syntax OK

And now when we run it, we get "\n" still, but at least we know why. Just getting this script

to compile has exposed the '$varl' (with the letter 'l') variable, and simply changing $varl

to $var1 solves the problem.

此时仍只会得到一个换行符，而不会有"hello world",

是因为将 $var1 写成了 $varl。

三、Looking at data and -w and v

Ok, but how about when you want to really see your data, what's in that dynamic variable,

just before using it?

如何查看动态变量的值呢？先来看下面的代码：

#!/usr/bin/perl

use strict;

my $key = 'welcome';

my %data = (

'this' => qw(that),

'tom' => qw(and jerry),

'welcome' => q(Hello World),

'zip' => q(welcome),

);

my @data = keys %data;

print "$data{$key}\n";

exit;

Looks OK, after it's been through the syntax check (perl -c scriptname), we run it and

all we get is a blank line again! Hmmmm.

上面的代码运行后只会得到一个空行。

One common debugging approach here, would be to liberally sprinkle a few print statements,

to add a check just before we print out our data, and another just after:

通常的调试方式，是在代码中插入一些print语句，如变量的前后插入，以检查变量值:

print "All OK\n" if grep($key, keys %data);

print "$data{$key}\n";

print "done: '$data{$key}'\n";

And try again:

然后再去运行，测试:

> perl data

All OK

done: ''

After much staring at the same piece of code and not seeing the wood for the trees for some time,

we get a cup of coffee and try another approach. That is, we bring in the cavalry by giving perl

the '-d' switch on the command line:

盯着代码看了半天仍不明所以后，还是来用亲的方法:

> perl -d data

Default die handler restored.

Loading DB routines from perl5db.pl version 1.07

Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(./data:4): my $key = 'welcome';

Now, what we've done here is to launch the built-in perl debugger on our script. It's stopped

at the first line of executable code and is waiting for input.Before we go any further, you'll

want to know how to quit the debugger:

use just the letter 'q', not the words 'quit' or 'exit':

这样，我们就启动了Perl内建的调试器，并停在可执行代码的第一行。

要退出调试器使用命令'q'.

DB<1> q

>

That's it, you're back on home turf again.

四、help

Fire the debugger up again on your script and we'll look at the help menu. There's a couple of

ways of calling help:

有很多方法都能调出帮助菜单：

a simple 'h' will get the summary help list,

'|h' (pipe-h) will pipe the help through your pager (which is (probably 'more' or 'less'),

and finally, 'h h' (h-space-h) will give you the entire help screen.

Here is the summary page:

D1h

List/search source lines: Control script execution:

l [ln|sub] List source code T Stack trace

- or . List previous/current line s [expr] Single step [in expr]

v [line] View around line n [expr] Next, steps over subs

f filename View source in file <CR/Enter> Repeat last n or s

/pattern/ ?patt? Search forw/backw r Return from subroutine

M Show module versions c [ln|sub] Continue until position

Debugger controls: L List break/watch/actions

o [...] Set debugger options t [expr] Toggle trace [trace expr]

<[<]|{[{]|>[>] [cmd] Do pre/post-prompt b [ln|event|sub] [cnd] Set breakpoint

! [N|pat] Redo a previous command B ln|* Delete a/all breakpoints

H [-num] Display last num commands a [ln] cmd Do cmd before line

= [a val] Define/list an alias A ln|* Delete a/all actions

h [db_cmd] Get help on command w expr Add a watch expression

h h Complete help page W expr|* Delete a/all watch exprs

|[|]db_cmd Send output to pager ![!] syscmd Run cmd in a subprocess

q or ^D Quit R Attempt a restart

Data Examination: expr Execute perl code, also see: s,n,t expr

x|m expr Evals expr in list context, dumps the result or lists methods.

p expr Print expression (uses script's current package).

S [[!]pat] List subroutine names [not] matching pattern

V [Pk [Vars]] List Variables in Package. Vars can be ~pattern or !pattern.

X [Vars] Same as "V current_package [Vars]".

y [n [Vars]] List lexicals in higher scope <n>. Vars same as V.

For more help, type h cmd_letter, or run man perldebug for all docs.

More confusing options than you can shake a big stick at! It's not as bad as it looks and

it's very useful to know more about all of it, and fun too!

There's a couple of useful ones to know about straight away. You wouldn't think we're using

any libraries at all at the moment, but 'M' will show which modules are currently loaded,

and their version number, while 'm' will show the methods, and 'S' shows all subroutines

(by pattern) as shown below. 'V' and 'X' show variables in the program by package scope

and can be constrained by pattern.

'M' : 显示当前加载的模块;

'm' : 显示方法；

'S' : 显示所有子例程

'V' : 显示变量

'X'

DB<2>S str

dumpvar::stringify

strict::bits

strict::import

strict::unimport

Using 'X' and cousins requires you not to use the type identifiers ($@%), just the 'name':

DM<3>X ~err

FileHandle(stderr) => fileno(2)

Remember we're in our tiny program with a problem, we should have a look at where we are,

and what our data looks like. First of all let's view some code at our present position

(the first line of code in this case), via 'v':

回到刚才代码我们再来做调试分析，

'v' : 查看当前位置的代码

DB<4> v

1 #!/usr/bin/perl

2: use strict;

3

4==> my $key = 'welcome';

5: my %data = (

6 'this' => qw(that),

7 'tom' => qw(and jerry),

8 'welcome' => q(Hello World),

9 'zip' => q(welcome),

10 );

At line number 4 is a helpful pointer, that tells you where you are now. To see more code,

type 'v' again:

要查看更多的代码，再使用'v'命令:

DB<4> v

8 'welcome' => q(Hello World),

9 'zip' => q(welcome),

10 );

11: my @data = keys %data;

12: print "All OK\n" if grep($key, keys %data);

13: print "$data{$key}\n";

14: print "done: '$data{$key}'\n";

15: exit;

And if you wanted to list line 5 again, type 'l 5', (note the space):

查看五行代码，可以使用命令'l 5',

DB<4> l 5

5: my %data = (

In this case, there's not much to see, but of course normally there's pages of stuff to wade through,

and 'l' can be very useful. To reset your view to the line we're about to execute,

type a lone period '.':

'.' : 查看刚才运行的行。

DB<5> .

main::(./data_a:4): my $key = 'welcome';

The line shown is the one that is about to be executed next, it hasn't happened yet.

So while we can print a variable with the letter 'p', at this point all we'd get is

an empty (undefined) value back. What we need to do is to step through the next

executable statement with an 's':

'p' : 查看变量值

's' : 步进执行程序。

DB<6> s

main::(./data_a:5): my %data = (

main::(./data_a:6): 'this' => qw(that),

main::(./data_a:7): 'tom' => qw(and jerry),

main::(./data_a:8): 'welcome' => q(Hello World),

main::(./data_a:9): 'zip' => q(welcome),

main::(./data_a:10): );

Now we can have a look at that first ($key) variable:

查看变量:

DB<7> p $key

welcome

line 13 is where the action is, so let's continue down to there via the letter 'c',

which by the way, inserts a 'one-time-only' breakpoint at the given line or sub routine:

'c' : 设置一次性断点并运行到这行:

DB<8> c 13

All OK

main::(./data_a:13): print "$data{$key}\n";

We've gone past our check (where 'All OK' was printed) and have stopped just

before the meat of our task. We could try to print out a couple of variables

to see what is happening:

变量查看:

DB<9> p $data{$key}

Not much in there, lets have a look at our hash:

查看哈希表:

DB<10> p %data

Hello Worldziptomandwelcomejerrywelcomethisthat

DB<11> p keys %data

Hello Worldtomwelcomejerrythis

Well, this isn't very easy to read, and using the helpful manual (h h),

the 'x' command looks promising:

用'x'命令查看哈希表更清楚:

DB<12> x %data

0 'Hello World'

1 'zip'

2 'tom'

3 'and'

4 'welcome'

5 undef

6 'jerry'

7 'welcome'

8 'this'

9 'that'

That's not much help, a couple of welcomes in there, but no indication of which are keys,

and which are values, it's just a listed array dump and, in this case, not particularly helpful.

The trick here, is to use a reference to the data structure:

还可以以键值对的形式查看哈希表:

DB<13> x \%data

0 HASH(0x8194bc4)

'Hello World' => 'zip'

'jerry' => 'welcome'

'this' => 'that'

'tom' => 'and'

'welcome' => undef

The reference is truly dumped and we can finally see what we're dealing with. Our quoting

was perfectly valid but wrong for our purposes, with 'and jerry' being treated as 2 separate

words rather than a phrase, thus throwing the evenly paired hash structure out of alignment.

通过最后的引用方式查看哈希表，我们很轻松地找到了程序的问题点。

The '-w' switch would have told us about this, had we used it at the start, and saved us

a lot of trouble:

'-w' : 可以做运行前的编译检查。

> perl -w data

Odd number of elements in hash assignment at ./data line 5.

We fix our quoting: 'tom' => q(and jerry), and run it again,

this time we get our expected output:

改正程序的bug:

> perl -w data

Hello World

While we're here, take a closer look at the 'x' command, it's really useful and will merrily

dump out nested references, complete objects, partial objects - just about whatever you throw

at it:

再来仔细研究'x'命令，因为它的作用是很大的。

Let's make a quick object and x-plode it, first we'll start the debugger: it wants some form

of input from STDIN, so we give it something non-committal, a zero:

启动调试器并设置:

> perl -de 0

Default die handler restored.

Loading DB routines from perl5db.pl version 1.07

Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(-e:1): 0

Now build an on-the-fly object over a couple of lines (note the backslash):

还能在调试时创建动态对象:

DB<1> $obj = bless({'unique_id'=>'123', 'attr'=> \

cont: {'col' => 'black', 'things' => [qw(this that etc)]}}, 'MY_class')

And let's have a look at it:

DB<2> x $obj

0 MY_class=HASH(0x828ad98)

'attr' => HASH(0x828ad68)

'col' => 'black'

'things' => ARRAY(0x828abb8)

0 'this'

1 'that'

2 'etc'

'unique_id' => 123

DB<3>

Useful, huh? You can eval nearly anything in there,

and experiment with bits of code or regexes until the cows come home:

还可以进行表达式设置:

DB<3> @data = qw(this that the other atheism leather theory scythe)

DB<4> p 'saw -> '.($cnt += map { print "\t:\t$_\n" } grep(/the/, sort @data))

atheism

leather

other

scythe

the

theory

saw -> 6

If you want to see the command History, type an 'H':

'H' : 查看历史命令;

DB<5> H

4: p 'saw -> '.($cnt += map { print "\t:\t$_\n" } grep(/the/, sort @data))

3: @data = qw(this that the other atheism leather theory scythe)

2: x $obj

1: $obj = bless({'unique_id'=>'123', 'attr'=>

{'col' => 'black', 'things' => [qw(this that etc)]}}, 'MY_class')

DB<5>

And if you want to repeat any previous command, use the exclamation: '!':

'!' : 重复前面的命令。

DB<5> !4

p 'saw -> '.($cnt += map { print "$_\n" } grep(/the/, sort @data))

atheism

leather

other

scythe

the

theory

saw -> 12

For more on references see perlref and perlreftut

五、Stepping through code【步进调试代码】

Here's a simple program which converts between Celsius and Fahrenheit, it too has a problem:

再来看一个华氏温度和摄氏温度的转换程序:

#!/usr/bin/perl -w

use strict;

my $arg = $ARGV[0] || '-c20';

if ($arg =~ /^\-(c|f)((\-|\+)*\d+(\.\d+)*)$/) {

my ($deg, $num) = ($1, $2);

my ($in, $out) = ($num, $num);

if ($deg eq 'c') {

$deg = 'f';

$out = &c2f($num);

} else {

$deg = 'c';

$out = &f2c($num);

}

$out = sprintf('%0.2f', $out);

$out =~ s/^((\-|\+)*\d+)\.0+$/$1/;

print "$out $deg\n";

} else {

print "Usage: $0 -[c|f] num\n";

}

exit;

sub f2c {

my $f = shift;

my $c = 5 * $f - 32 / 9;

return $c;

}

sub c2f {

my $c = shift;

my $f = 9 * $c / 5 + 32;

return $f;

}

For some reason, the Fahrenheit to Celsius conversion fails to return the expected output.

This is what it does:

上面的程序是有bug的:

> temp -c0.72

33.30 f

> temp -f33.3

162.94 c

Not very consistent! We'll set a breakpoint in the code manually and run it under the debugger

to see what's going on. A breakpoint is a flag, to which the debugger will run without interruption,

when it reaches the breakpoint, it will stop execution and offer a prompt for further interaction.

In normal use, these debugger commands are completely ignored, and they are safe - if a little messy,

to leave in production code.

在代码中添加断点行:

my ($in, $out) = ($num, $num);

$DB::single=2; # insert at line 9!

if ($deg eq 'c')

...

> perl -d temp -f33.3

Default die handler restored.

Loading DB routines from perl5db.pl version 1.07

Editor support available.

Enter h or `h h' for help, or `man perldebug' for more help.

main::(temp:4): my $arg = $ARGV[0] || '-c100';

We'll simply continue down to our pre-set breakpoint with a 'c':

执行到断点:

DB<1> c

main::(temp:10): if ($deg eq 'c') {

Followed by a view command to see where we are:

查看代码:

DB<1> v

7: my ($deg, $num) = ($1, $2);

8: my ($in, $out) = ($num, $num);

9: $DB::single=2;

10==> if ($deg eq 'c') {

11: $deg = 'f';

12: $out = &c2f($num);

13 } else {

14: $deg = 'c';

15: $out = &f2c($num);

16 }

And a print to show what values we're currently using:

查看相关变量:

DB<1> p $deg, $num

f33.3

We can put another break point on any line beginning with a colon, we'll use line 17

as that's just as we come out of the subroutine, and we'd like to pause there later on:

再设置断点:

DB<2> b 17

There's no feedback from this, but you can see what breakpoints are set by using the

list 'L' command:

'L' : 查看设置的断点;

DB<3> L

temp:

17: print "$out $deg\n";

break if (1)

Note that to delete a breakpoint you use 'B'.Now we'll continue down into our subroutine,

this time rather than by line number, we'll use the subroutine name, followed by the now

familiar 'v':

'B' : 删除一个断点;

DB<3> c f2c

main::f2c(temp:30): my $f = shift;

DB<4> v

24: exit;

25

26 sub f2c {

27==> my $f = shift;

28: my $c = 5 * $f - 32 / 9;

29: return $c;

30 }

31

32 sub c2f {

33: my $c = shift;

Note that if there was a subroutine call between us and line 29, and we wanted to single-step

through it, we could use the 's' command, and to step over it we would use 'n' which would

execute the sub, but not descend into it for inspection. In this case though, we simply

continue down to line 29:

直接设置行断点以进入子函数:

DB<4> c 29

main::f2c(temp:29): return $c;

And have a look at the return value:

查看返回值:

DB<5> p $c

162.944444444444

This is not the right answer at all, but the sum looks correct. I wonder if it's anything

to do with operator precedence? We'll try a couple of other possibilities with our sum:

查看有可能出错的表达式:

DB<6> p (5 * $f - 32 / 9)

162.944444444444

DB<7> p 5 * $f - (32 / 9)

162.944444444444

DB<8> p (5 * $f) - 32 / 9

162.944444444444

DB<9> p 5 * ($f - 32) / 9

0.722222222222221

:-) that's more like it! Ok, now we can set our return variable and we'll return out of

the sub with an 'r':

'r' : 运行并退出子函数:

DB<10> $c = 5 * ($f - 32) / 9

DB<11> r

scalar context return from main::f2c: 0.722222222222221

Looks good, let's just continue off the end of the script:

运行至程序结束:

DB<12> c

0.72 c

Debugged program terminated. Use q to quit or R to restart,

use O inhibit_exit to avoid stopping after program termination,

h q, h R or h O to get additional info.

A quick fix to the offending line (insert the missing parentheses) in the actual program

and we're finished.

六、Placeholder for a, w, t, T

Actions, watch variables, stack traces etc.: on the TODO list.

a

w

t

T

七、REGULAR EXPRESSIONS

Ever wanted to know what a regex looked like? You'll need perl compiled with

the DEBUGGING flag for this one:

正则表达式的查看:

> perl -Dr -e '/^pe(a)*rl$/i'

Compiling REx `^pe(a)*rl$'

size 17 first at 2

rarest char

at 0

1: BOL(2)

2: EXACTF <pe>(4)

4: CURLYN[1] {0,32767}(14)

6: NOTHING(8)

8: EXACTF <a>(0)

12: WHILEM(0)

13: NOTHING(14)

14: EXACTF <rl>(16)

16: EOL(17)

17: END(0)

floating `'$ at 4..2147483647 (checking floating) stclass `EXACTF <pe>'

anchored(BOL) minlen 4

Omitting $` $& $' support.

EXECUTING...

Freeing REx: `^pe(a)*rl$'

Did you really want to know? :-) For more gory details on getting regular expressions

to work, have a look at perlre, perlretut, and to decode the mysterious labels

(BOL and CURLYN, etc. above), see perldebguts.

八、OUTPUT TIPS

To get all the output from your error log, and not miss any messages via helpful

operating system buffering, insert a line like this, at the start of your script:

获得错误日志中的所有输出，在代码中添加如下行:

$|=1;

To watch the tail of a dynamically growing logfile, (from the command line):

也可以查看错误日志的最新行:

tail -f $error_log

Wrapping all die calls in a handler routine can be useful to see how,

and from where, they're being called, perlvar has more information:

查看所有的已死调用:

BEGIN { $SIG{__DIE__} = sub { require Carp; Carp::confess(@_) } }

Various useful techniques for the redirection of STDOUT and STDERR filehandles

are explained in perlopentut and perlfaq8.

九、CGI

Just a quick hint here for all those CGI programmers who can't figure out how on

earth to get past that 'waiting for input' prompt, when running their CGI script

from the command-line, try something like this:

> perl -d my_cgi.pl -nodebug

Of course CGI and perlfaq9 will tell you more.

十、GUIs

The command line interface is tightly integrated with an emacs extension and there's a vi interface too.

You don't have to do this all on the command line, though, there are a few GUI options out there.

The nice thing about these is you can wave a mouse over a variable and a dump of its data will

appear in an appropriate window, or in a popup balloon, no more tiresome typing of 'x $varname' :-)

In particular have a hunt around for the following: ptkdb perlTK based wrapper for the built-in debugger

ddd data display debugger PerlDevKit and PerlBuilder are NT specific NB.

(more info on these and others would be appreciated).

十一、SUMMARY

We've seen how to encourage good coding practices with use strict and -w.

We can run the perl debugger perl -d scriptname to inspect your data from within the

perl debugger with the p and x commands. You can walk through your code, set breakpoints

with b and step through that code with s or n, continue with c and return from a sub with r.

Fairly intuitive stuff when you get down to it.There is of course lots more to find out about,

this has just scratched the surface. The best way to learn more is to use perldoc to find out

more about the language, to read the on-line help (perldebug is probably the next place to go),

and of course, experiment