如何在Perl调试器中查看表达式的更改？

Question

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?

Answer 1

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

Crash course on the w debugger command:

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

DB<1> w $variablename

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 = "";    

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

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;

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 .

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";

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

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

Tip: Use temporary global variables

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}

Tip: Use Data::Dumper

Use Data::Dumper to watch the contents of a non-scalar:

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

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) ).

Answer 2

此外，您可以在Linux中使用"ddd your_script.pl&" ，并在GUI中观察C / C ++调试器等变量。