Event Handler Examples

The following are some examples of setting event handlers.

Setting a Breakpoint for Store to an Array Member

To set a data change breakpoint on array[99], type:

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

Implementing a Simple Trace

To implement a simple trace, type:

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

Enabling a Handler While Within a Function (in function)

To enable a handler while within a function, type:

<dbx> trace step -in foo

This is equivalent to:

    # create handler in disabled state
    when step -disable { echo Stepped to $line; }
    t=$newhandlerid    # 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"; };
    }

Determining the Number of Lines Executed

To see how many lines have been executed in a small program, type:

(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. The number of lines executed is 133. This process is very slow. It is most useful with breakpoints on functions that are called many times.

Determining the Number of Instructions Executed by a Source Line

To count how many instructions a line of code executes, type:

(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, the lines in the function are counted as well. You can use the next event instead of step to count instructions, excluding called functions.

Enabling a Breakpoint After an Event Occurs

Enable a breakpoint only after another event has occurred. For example, if your program begins to execute incorrectly in function hash, but only after the 1300’th symbol lookup, you would type:

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

$newhandlerid is referring to the just executed stop incommand.

Resetting 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 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

Checking Program Status

To see quickly where the program is while it is running, type:

(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). 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, type:

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