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Setting Breakpoints and Traces


This chapter describes how to set, clear, and list breakpoints and traces, and how to use watchpoints.

The chapter is organized into the following sections:

Basic Concepts

page 63

Setting Breakpoints

page 63

Tracing Code

page 66

Listing and Clearing Event Handlers

page 68

Setting Breakpoint Filters

page 70

Efficiency Considerations

page 72

Basic Concepts

The stop, when, and trace commands are called event management commands. Event management refers to the general capability of dbx to perform certain actions when certain events take place in the program being debugged.

Setting Breakpoints

There are three types of breakpoint action commands:

Setting a stop Breakpoint at a Line of Source Code

You can set a breakpoint at a line number, using the dbx stop at command:

(dbx) stop at filename: n

where n is a source code line number and filename is an optional program filename qualifier. For example

(dbx) stop at

If the line specified in a stop or when command is not an executable line of source code, dbx sets the breakpoint at the next executable line.

Setting a when Breakpoint at a Line

A when breakpoint command accepts other dbx commands like list, allowing you to write your own version of trace.

(dbx) when at 123 { list $lineno;}

when operates with an implied cont command. In the example above, after listing the source code at the current line, the program continues executing.

Setting a Breakpoint in a Dynamically Linked Library

dbx provides full debugging support for code that makes use of the programmatic interface to the run-time linker; code that calls dlopen(), dlclose() and their associated functions. The run-time linker binds and unbinds shared libraries during program execution. Debugging support for dlopen()/dlclose() allows you to step into a function or set a breakpoint in functions in a dynamically shared library just as you can in a library linked when the program is started.

There are three exceptions:

Setting Multiple Breaks in C++ Programs

You may want to check for problems related to calls to members of different classes, calls to any members of a given class, or calls to overloaded top-level functions. You can use a keyword--inmember, inclass, infunction, or inobject--with a stop, when, or trace command to set multiple breaks in C++ code.

Setting Breakpoints in Member Functions of Different Classes

To set a breakpoint in each of the object-specific variants of a particular member function (same member function name, different classes), use when inmember.

In the Blocks demo program, a member function draw(), is defined in each of five different classes: hand, brick, ball, wedge, table.

Setting Breakpoints in Member Functions of Same Class

To set a breakpoint in all member functions of a specific class, use the stop in class command.

To set a when breakpoint, use when inclass.

In the Blocks demo program, to set a breakpoint in all member functions of the class draw:

(dbx) stop inmember draw

Breakpoints are inserted in only the class member functions defined in the class. It does not include those that it may inherit from base classes.

Due to the large number of breakpoints that may be inserted by stop inclass and other breakpoint selections, you should be sure to set your dbxenv step_events to on to speed up step and next.

Setting Multiple Breakpoints in Nonmember Functions

To set multiple breakpoints in nonmember functions with overloaded names (same name, different type or number of arguments), use the stop in function command.

To set a when breakpoint, use when infunction.

For example, if a C++ program has defined two versions of a function named sort(), one which passes an int type argument, the other a float, then, to place a breakpoint in both functions:

(dbx) when infunction sort {cmd;}

Tracing Code

Tracing displays information about the line of code about to be executed or a function about to be called.

Setting trace Commands

Set trace commands from the command line. The following table shows the command syntax for the types of traces that you can set. The information a trace provides depends on the type of event associated with it.

trace prints ...

trace step

every line in the program as it is about to be executed.

trace next -in function

every line while the program is in the function

trace at line_number

the line number and the line itself, as that line becomes the next line to be executed.

trace in function

the name of the function that called function; line number, parameters passed in, and return value

trace inmember member_function

the name of the function that called member_function of any class; its line number, parameters passed in, and its return value

trace inclass class

the name of the function that called any member_function in class; its line number, parameters passed in, and return value

trace infunction function

the name of the function that called any member_function in class; its line number, parameters passed in, and return value

trace change variable
[-in function]

the new value of variable, if it changes, and the line at which it changed

Controlling the Speed of a Trace

In many programs, code execution is too fast to view the code. The dbxenv trace_speed allows you to control the delay after each trace is printed. The default delay is 0.5 seconds.

To set the interval between execution of each line of code during a trace:

dbxenv trace_speed number

Listing and Clearing Event Handlers

Often, you set more than one breakpoint or trace handler during a debugging session. dbx supports commands for listing and clearing them.

Listing Breakpoints and Traces

To display a list of all active breakpoints, use the status command to print ID numbers in parenthesis, which can then be used by other commands.

As noted, dbx reports multiple breakpoints set with the inmember, inclass, and infunction keywords as a single set of breakpoints with one status ID number.

Deleting Specific Breakpoints Using Handler ID Numbers

When you list breakpoints using the status command, dbx prints the ID number assigned to each breakpoint when it was created. Using the delete command, you can remove breakpoints by ID number, or use the keyword all to remove all breakpoints currently set anywhere in the program.

To delete breakpoints by ID number:

(dbx) delete 3 5

To delete all breakpoints set in the program currently loaded in dbx:

(dbx) delete all


Watchpointing is the capability of dbx to note when the value of a variable or expression has changed.

Stopping Execution When Modified

To stop program execution when the contents of an address gets written to:

(dbx) stop modify &variable

Keep these points in mind when using stop modify:

Stopping Execution When Variables Change

To stop program execution if the value of a specified variable changes:

(dbx) stop change variable

Keep these points in mind when using stop change:

dbx implements stop change by causing automatic single stepping together with a check on the value at each step. Stepping skips over library calls. So, if control flows in the following manner:

   user_routine calls
      library_routine, which calls
        user_routine2, which changes variable

dbx does not trace the nested user_ routine2 because tracing skips the library call and the nested call to user_-routine2, so the change in the value of variable appears to have occurred after the return from the library call, not in the middle of user_routine2.

Stopping Execution on a Condition

To stop program execution if a conditional statement evaluates to true:

(dbx) stop cond condition

The Faster modify Event

A faster way of setting watchpoints is to use the modify command. Instead of automatically single-stepping the program, it uses a page protection scheme which is much faster. The speed depends on how many times the page on which the variable you are watching is modified, as well as the overall system call rate of the program being debugged.

Setting Breakpoint Filters

In dbx, most of the event management commands also support an optional event filter modifier statement. The simplest filter instructs dbx to test for a condition after the program arrives at a breakpoint or trace handler, or after a watch condition occurs.

If this filter condition evaluates to true (non 0), the event command applies. If the condition evaluates to false (0), dbx continues program execution as if the event never happened.

To set a breakpoint at a line or in a function that includes a conditional filter, add an optional -if condition modifier statement to the end of a stop or trace command.

The condition can be any valid expression, including function calls, returning Boolean or integer in the language current at the time the command is entered.

Note - With location-based breakpoints like in or at, the scope is that of the breakpoint location. Otherwise, the scope of the condition is the scope at the time of entry, not at the time of the event. You may have to use syntax to specify the scope precisely.
These two filters are not the same:

stop in foo -if a>5
stop cond a>5

The former will breakpoint at foo and test the condition. The latter automatically single-steps and tests for the condition.

This point is emphasized because new users sometimes confuse setting a conditional event command (a watch-type command) with using filters. Conceptually, "watching" creates a precondition that must be checked before each line of code executes (within the scope of the watch). But even a breakpoint command with a conditional trigger can also have a filter attached to it.

Consider this example:

(dbx) stop modify &speed -if speed==fast_enough

This command instructs dbx to monitor the variable, speed; if the variable speed is written to (the "watch" part), then the -if filter goes into effect. dbx checks to see if the new value of speed is equal to fast_enough. If it is not, the program continues on, "ignoring" the stop.

In dbx syntax, the filter is represented in the form of an [-if condition] statement at the end of the formula:

stop in function [-if condition]

Efficiency Considerations

Various events have varying degrees of overhead in respect to the execution time of the program being debugged. Some events, like the simplest breakpoints have practically no overhead. Events based on a single breakpoint, like have minimal overhead.

Multiple breakpoints, such as inclass, that might result in hundreds of breakpoints, have an overhead only during creation time. This is because dbx uses permanent breakpoints; the breakpoints are retained in the process at all times and are not taken out on every stoppage and put in on every cont.

Note - In the case of step and next, by default all breakpoints are taken out before the process is resumed and reinserted once the step completes. If you are using many breakpoints or multiple breakpoints on prolific classes the speed of step and next slows down considerably. Use the dbxenv step_events to control whether breakpoints are taken out and reinserted after each step or next.
The slowest events are those that utilize automatic single stepping. This might be explicit and obvious as in the trace step command, which single steps through every source line. Other events, like the watchpoints stop change expression or trace cond variable not only single step automatically but also have to evaluate an expression or a variable at each step.

These are very slow, but you can often overcome the slowness by bounding the event with a function using the -in modifier. For example:

trace next -in mumble
stop change clobbered_variable -in lookup

Do not use trace -in main because the trace is effective in the functions called by main as well. Do use in the cases where you suspect that the lookup() function is clobbering your variable.

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