如何在Perl调试器中监视表达式的更改？ [英] How can I watch for changes to an expression in the Perl debugger?

问题描述

使用Perl调试器，我知道可以使用 b 命令在某些代码行设置断点。变量的内容更改后，是否可以立即停止调试器？

With the Perl debugger, I know I can set breakpoints at certain lines of code with the b command. Can I get the debugger to stop as soon as the contents of a variable has changed?

推荐答案

您可以使用Perl调试器中的 w 命令。

You can create watch points using the w command in the Perl debugger.

通过键入 w 然后创建将监视其内容的表达式来创建监视表达式更改：

Create a watch-expression by typing w and then an expression that will monitored for changes:

DB<1> w $variablename

输入 c 继续直到观看的表情发生变化。完成后，您将获得类似于以下的输出：

Enter c to continue until the watched expression changes. Once you do, you will get output similar to this:

DB<2> c
Watchpoint 0:   $variablename changed:
    old value:  ''
    new value:  'hi'
main::(ex.pl:6):    $variablename = "";    

请注意，调试器在更改发生后在语句处停止，因此显示的行可能根本不相关。

Note that the debugger stops at the statement after the changed has happened, so the line displayed might not be relevant at all.

还要注意，表达式是 stringized 。因此，例如，将变量更改为 undef 将为您提供以下输出：

Also note that the expression is stringified. So for example, changing a variable to undef will give you this output:

  DB<2> c
Watchpoint 0:   $variablename changed:
    old value:  'hi'
    new value:  ''
main::(ex.pl:7):    $variablename = undef;

如果随后将变量更改为空字符串，调试器将不会停止，如 。

If the variable is subsequently changed to an empty string, the debugger will not stop, as a stringified empty string and a stringified undef is considered equal.

如果watch表达式是一个列表，调试器将比较该列表的字符串化元素：

If the watch expression is a list, the debugger will compare the stringified elements of the list:

  DB<1> w $variablename, "second"

  DB<2> c
Watchpoint 0:   $variablename, "second" changed:
    old value:  'one', 'second'
    new value:  'two', 'second'
main::(hi.pl:6):    $variablename = "three";

您可以将数组变量或哈希变量用作监视表达式，它们将被视为其他变量

You can use array variables or hash variables as watch-expressions, and they will be treated as any other list.

要删除监视表达式，请使用 W 命令，并查看活动监视列表。 -表达式，请使用 L 命令。

To delete a watch-expression, use the W command, and to view a list of active watch-expressions, use the L command.

由于每个表达式都会重新评估watch-expression，因此不能期望使用词法变量的watch-expression超出范围。快速提示是创建对词法的全局引用，并对其进行跟踪：

Since the watch-expression is re-evaluated with every statement, you can't expect a watch-expression that uses a lexical variable to work out of scope. A quick tip is to create a global reference to the lexical, and track that instead:

DB<1> $main::my_debug_variable = $hashref_lexical_variable

DB<2> w $main::my_debug_variable->{key_im_watching}

提示：使用 Data :: Dumper

使用 Data :: Dumper 观看内容非标量：

DB<1> w Data::Dumper->Dump([$hashref])

w $ hashref ，因为它会在哈希值更改时停止，而不是简单地指向引用指向的地址（因为hashref字符串化为<$ c $之类的东西） c> HASH（0x2a07a90））。

This is preferable to a simple w $hashref, because it will stop when the values of the hash change, rather than simply the address the reference is pointing (since a hashref stringifies to something like HASH(0x2a07a90)).

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