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Event Management


Event management refers to the capability of dbx to perform actions when events take place in the program being debugged.

This chapter is organized into the following sections:

Basic Concepts

page 73

Creating Event Handlers

page 74

Manipulating Event Handlers

page 75

Using Event Counters

page 76

Setting Event Specifications

page 76

Parsing and Ambiguity

page 86

Using Predefined Variables

page 87


page 89

Command Reference

page 93

Basic Concepts

Event management is based on the concept of a handler. The name comes from an analogy with hardware interrupt handlers. Each event management command typically creates a handler, which consists of an event specification and a series of side-effect actions need to be specified.

An example of the association of a program event with a dbx action is setting a breakpoint on a particular line. A change in the value of a variable triggers a stop.

The most generic form of creating a handler is through the when command:

when event-specification {action; ... }

Although all event management can be performed through when, dbx has historically had many other commands, which are still retained, either for backward compatibility, or because they are simpler and easier to use.

In many places examples are given on how a command (like stop, step, or ignore) can be written in terms of when. These examples are meant to illustrate the flexibility of when and the underlying handler mechanism, but they are not always exact replacements.

Creating Event Handlers

The commands when, stop, and trace are used to create event handlers. An event-spec is a specification of an event as documented later in this chapter.

Every command returns a number known as a handler id (hid). This number can be accessed via the predefined variable $newhandlerid.

An attempt has been made to make the stop and when commands conform to the handler model. However, backward compatibility with previous dbx releases forces some deviations.

For example, the following samples from an earlier dbx release are equivalent to:

when cond body

when step -if cond body

when cond in func body

when next -if cond -in func body

These examples illustrate that cond is not a pure event; there is no internal handler for conditions.


The when command has a general syntax. When the specified event occurs, the cmds are executed. Once the commands have all executed, the process is automatically continued.

when event-specification [ modifier ] { cmds ... ; }


The stop command has a general syntax. When the specified event occurs, the process is stopped.

 stop event-specification [ modifier ]

stop is shorthand for a common when idiom:

when event-specification { stop -update; whereami; }


When the specified event oc

trace event-specification

curs, a trace message is printed:

Most of the trace commands can be hand-crafted by using the when command, ksh functionality, and event variables. This is especially useful if you want stylized tracing output.

Manipulating Event Handlers

The following list contains commands to manipulate event handlers. For more information on any of the commands, see the section Command Reference at the end of this chapter.

Using Event Counters

Event handlers have trip counters. There is a count limit and the actual counter. Whenever the event occurs, the counter is incremented. The action associated with the handler executes only if the count reaches the limit, at which point the counter is automatically reset to 0. The default limit is 1. Whenever a process is rerun, all event counters are reset.

The count limit can be set using the -count modifier. Otherwise, use the handler to individually manipulate event handlers.

handler [ -count | -reset ] hid new-count new-count-limit

Setting Event Specifications

Event specifiers are used by the stop, when and trace commands to denote event types and parameters. The format is that of a keyword to represent the event type and optional parameters.

Event Specifications

The following are event specifications, syntax and descriptions.

in func

The function has been entered and the first line is about to be executed. If the -instr modifier is used, it is the first instruction of the function about to be executed. (Do not confuse in func with the -in func modifier.) The func specification can take a formal parameter signature to help with overloaded function names, or template instance specification. For example, you can say:

 stop in mumble(int, float, struct Node *)

at lineno

The designated line is about to be executed. If filename is provided, then the designated line in the specified file is about to be executed. The filename can either be the .o, .c, or .cc name of the file. Although quote marks are not required, they may be necessary if the filename contains odd characters.

at filename:lineno

infunction func

Equivalent to in func for all overloaded functions named func, or all template instantiations thereof.

inmember func

inmethod func

Equivalent to in func for the member function named func for every class.

inclass classname

Equivalent to in func for all member functions that are members of classname.

inobject obj-expr

A member function called on the object denoted by obj-expr has been called.

change variable

Fires when the value of variable has changed.

cond cond-expr

The condition denoted by cond-expr evaluates to true. Any expression can be used for cond-expr, but it has to evaluate to an integral type.


This event is just a breakpoint at the return point of the current visited function. The visited function is used so that you can use the returns event spec after doing a number of up's. The plain return event is always -temp and can only be created in the presence of a live process.

returns func

This event fires each time the given function returns to its call site. This is not a temporary event. The return value is not provided, but you can find it through:





Power PC


It is another way of saying:

when in func { stop returns; }


The step event fires when execution reaches the first instruction of a source line. For example, you can get simple tracing with:

when step { echo $lineno: $line; }

When enabling a step event you instruct dbx to single-step automatically next time cont is used. The step command can be implemented as follows:

alias step="when step -temp { whereami; stop; }; cont"


Similar to step except that functions are not stepped into.

timer second

Fires when the debugee has been running for seconds. The timer used with this is shared with the collector command.

sig sig

When the signal is first delivered to the debugee, this event fires. sig can either be a decimal number or the signal name in upper or lower case; the prefix is optional. This is completely independent of the catch/ignore commands, although the catch command can be implemented as follows:

function simple_catch {
	when sig $1 {
			echo Stopped due to $sigstr $sig

Note - When the sig event is received, the process has'nt seen it yet. Only if you cont the process with the given signal is the signal forwarded to it.

sig sig sub-code

When the specified signal with the specified sub-code is first delivered to the child, this event fires. Just as with signals, the sub-code can be entered as a decimal number, in capital or lower case; the prefix is optional.

fault fault

This event fires when the specified fault occurs. The faults are architecture dependent, but a set of them is known to dbx as defined by proc (4):


Illegal instruction


Privileged instruction


Breakpoint instruction


Trace trap (single step)


Memory access (such as alignment)


Memory bounds (invalid address)


Integer overflow


Integer zero divide


Floating-point exception


Irrecoverable stack fault


Recoverable page fault

These faults are taken from /sys/fault.h. fault can be any of those listed above, in upper or lower case, with or without the FLT- prefix, or the actual numerical code. Be aware that BPT, TRACE, and BOUNDS are used by dbx to implement breakpoints, single-stepping, and watchpoints. Handling them may interfere with the inner workings of dbx.

modify addr-exp [ , byte-size ]

The specified address range has been modified. This is the general watchpoint facility. The syntax of the event-specification for watchpoints is:

modify addr-exp [, byte-size-exp ]

addr-exp is any expression that can be evaluated to produce an address. If a symbolic expression is used, the size of the region to be watched is automatically deduced, or you can override that with the \Q,` syntax. You can also use nonsymbolic, typeless address expressions; in which case, the size is mandatory. For example:

stop modify 0x5678, sizeof(Complex)

Limitations of modify event-spec
Addresses on the stack cannot be watched.

The event is not fired if the address being watched is modified by a system call.

Shared memory (MAP_SHARED) cannot be watched, because dbx cannot catch the other processes stores into shared memory. Also, dbx cannot properly deal with SPARC swap and ldstub instructions.

sysin code|name

The specified system call has just been initiated and the process has entered kernel mode.

The concept of system call supported by dbx is that provided by procfs(4). These are traps into the kernel as enumerated in /usr/include/sys/syscall.h.

This is not the same as the ABI notion of system calls. Some ABI system calls are partially implemented in user mode and use non-ABI kernel traps. However, most of the generic system calls (the main exception being signal handling) are the same between syscall.h and the ABI.

sysout code|name

The specified system call is finished and the process is about to return to user mode.

sysin | sysout

Without arguments, all system calls are traced. Note that certain dbx features, for example modify event and RTC, cause the child to execute system calls for their own purposes and show up if traced.


Fired when a new program has been loaded as a result of follow exec.

Note - Handlers for this event are always permanent.


The process has stopped. Whenever the process stops such that the user gets a prompt, particularly in response to a stop handler, this event is fired. For example, the following are equivalent:

display x
when stop {print x;}


The process being debugged has just been exec()'ed. All memory specified in a.out is valid and present but pre-loaded shared libraries have not been loaded yet. For example, printf, although known to dbx, has not been mapped into memory yet.

A stop on this event is ineffective; however, you can use this event with the when command.


This event is fired after a sync (or attach if the process being debugged has not yet processed shared libraries). It fires after the startup code has executed and the symbol tables of all preloaded shared libraries have been loaded.

A stop on this event is ineffective; however, you can use this event with the when command.


dbx has successfully attached to a process.


The debugee has been detached from.


Fired when lwp has been exited. $lwp contains the id of the exited LWP.


Fired when dbx is no longer associated with a debugged process. The predefined variable $reason will be signal, exit, kill, or detach.


The process being debugged is about to expire. There are only three reasons this can happen:

dlopen [ lib-path ] | dlclose [ lib-path ]

These events are fired after a dlopen() or a dlclose() call succeeds. A dlopen() or dlclose() call can cause more than one library to be loaded. The list of these libraries is always available in the predefined variable $dllist. The first word in $dllist is actually a "+" or a "-", indicating whether the list of libraries is being added or deleted.

lib-path is the name of a shared library you are interested in. If it is specified, the event only fires if the given library was loaded or unloaded. In that case $dlobj contains the name of the library. $dllist is still available.

If lib-path begins with a /, a full string match is performed. Otherwise, only the tails of the paths are compared. If lib-path is not specified, then the events always fire whenever there is any dl-activity. $dlobj is empty but $dllist is valid.

Event Specification Modifiers

An event specification modifier sets additional attributes of a handler, the most common kind being event filters. Modifiers have to appear after the keyword portion of an event spec. They all begin with a hyphen (-), preceded by blanks. Modifiers consist of the following:

-if cond

The condition is evaluated when the event specified by the event-spec occurs. The event is fired only if the condition evaluates to nonzero. The condition is sometimes called a filter. A handler created with a filter is known as a filtered handler.

If -if is used with a location-based event, like in or at, cond is evaluated in the scope corresponding to that location, otherwise it should be properly qualified with the desired scope.

-in func

The handler is enabled only while within the given function, or any function called from func. The number of times the function is entered is reference counted so as to properly deal with recursion. The handler that has been modified by the -in modifier is said to be "bounded by func."


Create the handler in the disabled state.

--count n-count infinity

Have the handler count from 0. Each time the event occurs, the count is incremented until it reaches n. Once that happens, the handler fires and the counter is reset to zero.

Counts of all enabled handlers are reset when a program is run or rerun. More specifically, they are reset when the sync event occurs.


Create a temporary handler. Once the event is fired it is automatically deleted. By default, handlers are not temporary. If the handler is a counting handler, it is automatically deleted only when the count reaches 0 (zero).

Use the delete -temp command to delete all temporary handlers.


Makes the handler act at an instruction level. This replaces the traditional 'i' suffix of most commands. It usually modifies two aspects of the event handler.

-thread tid

The event is fired only if the thread that caused it matches tid.

--lwp lid

The event is fired only if the thread that caused it matches lid.


Makes the handler not show up in a regular status command. Use status -h to see hidden handlers.


Normally all handlers get thrown away when a new program is loaded. Using this modifier causes the handler to be retained across debuggings. A plain delete command will not delete a permanent handler. Use delete -p to delete a permanent handler.

Parsing and Ambiguity

Syntax for event-specs and modifiers is:

Since expressions can have spaces embedded in them, this can cause ambiguous situations. For example, consider the following two commands:

when a -temp
when a-temp

In the first example, even though the application might have a variable named temp, the dbx parser resolves the event-spec in favor of -temp being a modifier. In the second example, a-temp is collectively passed to a language specific expression parser and there must be variables named a and temp or an error occurs. Use parentheses to force parsing.

Using Predefined Variables

Certain read-only ksh predefined variables are provided. The following are always valid:



Current program counter address (hexadecimal)


Disassembly of the current instruction


Current line number in decimal


Contents of the current line


Name of the current function


Name of the current "visiting" function


Name of the class to which $func belongs


Name of the class to which $vfunc belongs


Name of the current file


Name of the current file being visited


Name of the current loadable object


Name of the current loadable object being visited


Scope of the current PC in back-quote notation


Scope of the visited PC in back-quote notation


Address of $func in hex


Name of the function calling $func


After dlopen or dlclose event, contains the list of load objects just dlopened or dlclosed. The first word of dllist is actually a "+" or a "-" depending on whether a dlopen or a dlclose has occurred.


ID of the most recently created handler


Process id of the current process being debugged


Lwp id of the current LWP


Thread id of the current thread


Full pathname of the program being debugged


Old, or original value of $prog. This is very handy for getting back to what you were debugging following an exec().


Exit status from the last run of the program. The value is an empty string if the process hasn't actually exited.

As an example, consider that whereami can be roughly implemented as:

function whereami {
  echo Stopped in $func at line $lineno in file $(basename $file)
  echo "$lineno\t$line"

Event-Specific Variables

The following event-specific variables are only valid within the body of a when.


During the execution of the body, $handlerid is the id of the when command to which the body belongs. These commands are equivalent:

when X -temp { do_stuff; }
when X  { do_stuff; delete $handlerid; }


Is set to true if the event occurs during the boot process. Whenever a new program is debugged, it is first booted so that the list and location of shared libraries can be ascertained. The process is then killed.

While booting happens, all events are still available. Use this variable to distinguish the sync and the syncrtld events occurring during a debug and the ones occurring during a normal run.

Valid Variables

For Event sig


Signal number that caused the event


Name of $sig


Subcode of $sig if applicable


Name of $sigcode


Process id of sender of the signal, if appropriate

For Event exit


Value of the argument passed to _exit(2) or exit(3) or the return value of main

For Events dlopen and dlclose


Pathname of the load object dlopened or dlclosed

For Events sysin and sysout


System call number


System call name

For Event proc_gone


One of signal, exit, kill, or detach


Use these examples for setting event handlers.

Set Watchpoint for Store to Array Member

To set a watchpoint on array[99]:

(dbx) stop modify &array[99]
(2) stop modify &array[99], 4
Running: watch.x2
watchpoint array[99] (0x2ca88[4]) at line 22 in file "watch.c"
22 array[i] = i;

Simple Trace

To implement a simple trace:

(dbx) when step { echo at line $lineno; }

Enable Handler While Within the Given Function (in func)

For example:

trace step -in foo

is equivalent to:

	# create handler in disabled state
when step -disable { echo Stepped to $line; }
# remember handler id
when in foo {
# when entered foo enable the trace
handler -enable "$t"
# arrange so that upon returning from foo,
# the trace is disabled.
when returns { handler -disable "$t"; };

Determine the Number of Lines Executed in a Program

To see how many lines were executed in a small program:

(dbx) stop step -count infinity	 # step and stop when count=inf
(2) stop step -count 0/infinity
(dbx) run
(dbx) status
(2) stop step -count 133/infinity

The program never stops--the program terminates. 133 is the number of lines executed. This process is very slow though. This technique is more useful with breakpoints on functions that are called many times.

Determine the Number of Instructions Executed by a Source Line

To count how many instructions a line of code executes:

(dbx) ...                        # get to the line in question
(dbx) stop step -instr -count infinity 
(dbx) step ... 
(dbx) status 
(3) stop step -count 48/infinity # 48 instructions were executed

If the line you are stepping over makes a function call, you end up counting those as well. You can use the next event instead of step to count instructions, excluding called functions.

Enable Breakpoint after Event Occurs

Enable a breakpoint only after another event has fired. Suppose things go bad in function hash, but only after the 1300'th symbol lookup:

(dbx) when in lookup -count 1300 {
	stop in hash
	when proc_gone -temp { delete $hash_bpt; }

Note - $newhandlerid is referring to the just executed stop in command.

Set Automatic Breakpoints for dlopen Objects

To have breakpoints in dlopened objects be managed automatically:

(dbx) when dlopen { 	 # delete old one if it exists.
	if [ -n "$B1" ]
	then	 delete "$B1"
	fi	                         # create new one 
	stop in func

Reset Application Files for replay

If your application processes files that need to be reset during a replay, you can write a handler to do that for you each time you run the program:

(dbx)  when sync { sh regen ./database; }
(dbx)  run < ./database...	# during which database gets clobbered
(dbx) save
...              # implies a RUN, which implies the SYNC event which
(dbx)  restore	   # causes regen to run

Check Program Status

To see quickly where the program is while it's running:

(dbx) ignore sigint
(dbx) when sig sigint { where; cancel; }

Then type ^C to see a stack trace of the program without stopping it.

This is basically what the collector hand sample mode does (and more of course). Use SIGQUIT (^\) to interrupt the program because ^C is now used up.

Catch Floating Point Exceptions

To catch only specific floating-point exceptions, for example, IEEE underflow:

(dbx)ignore FPE	               # turn off default handler 
(dbx) help signals | grep FPE	 # can't remember the subcode name
(dbx) stop sig fpe FPE_FLTUND

Command Reference


To execute command(s) when line is reached

when at line { command(s); }

To execute command(s) when proc is called:

when in proc { command(s); }


To stop execution now and update displays. Whereas normally the process is continued after the body has executed, the stop command prevents that. This form is only valid within the body of a when:

stop -update

Same as above, but does not update displays:

stop -noupdate

To stop execution at line line:

stop at line 

To stop execution when function func is called

stop in func

To set breakpoints on all member functions of a class/struct/union or template class:

stop inclass classname

To set breakpoints on all member functions:

stop inmember name

To set breakpoints on all non-member functions:

stop infunction name


The step command is equivalent to:

when step -temp { stop; }; cont

The step command can take a sig argument. A step by itself cancels the current signal just like cont does. To forward the signal, you must explicitly give the signal. You can use the variable $sig to step the program sending it the current signal:

step -sig $sig


Only valid within the body of when. The cancel command cancels any signal that might have been delivered, and lets the process continue. For example:

when sig SIGINT { echo signal info;  cancel; }


The status command lists handlers, those created by trace, when, and stop. status hid lists the given handler. If the handler is disabled, its hid is printed inside square brackets [ ] instead of parentheses ( ).

status [-s] -h hid

The output of status can be redirected to a file. Nominally, the format is unchanged during redirection. You can use the -s flag to produce output that allows a handler to be reinstated using the source command. If the -h option is used, hidden handlers are also listed.

The original technique of redirecting handlers using status and sourcing the file was a way to compensate for the lack of handler enabling and disabling functionality.


delete hid deletes the specified handler; delete all or delete 0 (zero), and delete -all deletes all handlers including temporary handlers. delete -temp deletes only all temporary handlers.

delete all -all -temp hid [hid ... ]

-h hid is required if hid is a hidden handler. -h all deletes all hidden handlers as well.


clear with no arguments deletes all handlers based on breakpoints at the location where the process stopped. clear line deletes all handlers based on breakpoints on the given line.

clear line


handler -disable hid disables the specified event; handler -disable all disables all handlers. handler -enable hid enables the specified handler; handler -enable all enables all handlers.

handler [ -disable | -enable ] all hid [...]

handler -count hid returns the count of the event in the form current-count/limit (same as printed by status). limit might be the keyword infinity. Use the ksh modifiers ${#} and ${##} to split the printed value.

handler [ -count | -reset ] hid new-count-limit

handler -count hid new-count assigns a new count limit to the given handler. handler -reset hid resets the count of the handler to 0 (zero).

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