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As you have seen, you can tell the Perl debugger to execute one statement at a time. Another way of controlling program execution is to tell the debugger to execute up to a certain specified point in the program, called a breakpoint.
The following sections describe the commands that create breakpoints, and the command that executes until a breakpoint is detected.
To set a breakpoint in your program, use the b command. This command tells the debugger to halt program execution whenever it is about to execute the specified line. For example, the following command tells the debugger to halt when it is about to execute line 10:
DB<11> b 10
(If the line is not breakable, the debugger will return Line 10 is not breakable.)
| NOTE |
You can have as many breakpoints in your program as you want. The debugger will halt program execution if it is about to execute any of the statements at which a breakpoint has been defined. |
The b command also accepts subroutine names:
DB<12> b menudir
This sets a breakpoint at the first executable statement of the subroutine menudir.
You can use the b command to tell the program to halt only when a specified condition is true. For example, the following command tells the debugger to halt if it is about to execute line 10 and the variable $curdir is equal to the null string:
DB<12> b 10 ($curdir eq "")
The condition specified with the b statement can be any legal Perl conditional
expression.
| Note: |
If a statement is longer than a single line, you can set a breakpoint only at the first line of the statement: 71: print ("Test", Here, you can set a breakpoint at line 71, but not line 72. |
After you have set a breakpoint, you can tell the debugger to execute until it reaches either the breakpoint or the end of the program. To do this, use the c command:
DB<13> c main::(debugtest:10): $curdir =~ s/^\s+|\s+$//g; DB<14>
When the debugger detects that it is about to execute line 10-the line at which the breakpoint was set-it halts and displays the line. (Recall that the debugger always displays the line it is about to execute.)
The debugger now prompts you for another debugging command. This action enables you to start executing one statement at a time using n or s, continue execution using c, set more breakpoints using b, or perform any other debugging operation.
You can specify a temporary (one-time-only) breakpoint with the c command by supplying a line number:
DB<15> c 12 main::(debugtest:12): &readsubdirs($curdir);
The argument 12 supplied with the c command tells the debugger to define a temporary breakpoint at line 12 and then resume execution. When the debugger reaches line 12, it halts execution, displays the line, and deletes the breakpoint. (The line itself still exists, of course.)
Using c to define a temporary breakpoint is useful if you want to skip a few lines without wasting your time executing the program one statement at a time. Using c also means that you don't have to bother defining a breakpoint using b and deleting it using d (described in the following section).
| TIP | If you intend to define breakpoints using c or b,
it is a good idea to ensure that each line of your program contains at most
one statement. If you are in the habit of writing lines that contain more
than one statement, such as
$x++; $y++;you won't get as much use out of the debugger, because it can't stop in the middle of a line |
To list all of your breakpoints, use the L command. This command lists the last few lines executed, the current line, the breakpoints you have defined, and the conditions under which the breakpoints go into effect.
DB<16> L
3: $dircount = 0;
4: $curdir = "";
5: while (1) {
7: if ($curdir eq "") {
10: $curdir =~ s/^\s+|\s+$//g;
break if (1)
Here, the program has executed lines 3-7, and a breakpoint is defined for line 10. (Line 6 is not listed because it is a comment.) You can distinguish breakpoints from executed lines by looking for the breakpoint conditional expression, which immediately follows the breakpoint. Here, the conditional expression is (1), which indicates that the breakpoint is always in effect.
When you are finished with a breakpoint, you can delete it using the d command.
DB<16> d 10
This command tells the debugger to delete the breakpoint at line 10. The line itself remains in the program.
If you do not specify a breakpoint to delete, the debugger assumes that a breakpoint is defined for the next line to be executed, and deletes it.
main::(debugtest:12): &readsubdirs($curdir); DB<17> d
Here, line 12 is the next line to be executed, so the debugger deletes the breakpoint at line 12.
To delete all your breakpoints, use the D command.
DB<18> D
This command deletes all the breakpoints you have defined with the b command.
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