How to debug a runtime stack underflow error?

Question

I'm really struggling to resolve a stack underflow that I'm getting. The traceback I get at runtime is:

VerifyError: Error #1024: Stack underflow occurred.

at flash.events::EventDispatcher/dispatchEventFunction()
at flash.events::EventDispatcher/dispatchEvent()
at flash.net::URLLoader/onComplete()

This is particularly difficult to debug because when I run in debug mode it does not happen at all. It only happens when compiled as a release.

Does anyone have any tips on how to debug a Stack Underflow? Are have a clean explanation of what that means for Flash?

In case it helps, this error is occurring when I click a button whose handler makes an RPC call, which uses a URLLoader, an AsyncToken, and then invokes the set of AsyncResponder instances associated with the AsyncToken. With some server-side logging as well as some logging hacked into the swf, I know that the UrlLoader is successfully doing and GET'ing a crossdomain.xml file, is correctly processing it (ie: if I wreck it, I get a security error), and is also successfully completing the "load" request (the server sends the data). The underflow seems to be happening in the Event.COMPLETE listening/handling process (as is, of course, implied by the traceback as well).

mxmlc used = from flex_sdk_4.5.0.20967

Example player (I've tried a few) = 10.2.153.1

---

UPDATE: My specific problem is solved... but I'm leaving the question as-is since I would like to know how to generally debug such a problem, rather than just getting my specific solution.

In my code I had the following Application definition:

<s:Application height="100%" width="100%"
                              xmlns:fx="http://ns.adobe.com/mxml/2009"
               xmlns:s="library://ns.adobe.com/flex/spark"
               xmlns:mx="library://ns.adobe.com/flex/mx"
               initialize="InitData();">

Note that the code is/was attached to the initialize event.

InitData() and relevant defintions are/were:

import classes.RpcServerProxy;
public var SP:RpcServerProxy;

public function InitData():void {
    SP = new RpcServerProxy("http://192.168.1.102:1234");
}

When I switched the InitData() call to be on the onCompletion event instead of initialize (thanks J_A_X!), the problem goes away entirely. What seems to have been happening was that the Event.COMPLETE event handler (onComplete in the stack trace) was using the global SP object. Something about the release (vs debug) compilation must have been affecting the startup timing of the SP variable initialization. Moving the handler later to the onCompletion event resolved all issues.

As said above, I would still like to know what tricks/tools are available for debugging initialization issues like this.

---

UPDATE 2:

applicationComplete seems to be an even better event than creationComplete to put application initialization code. See this blog entry for some explanation, and and this video (around 4:25) by an Adobe Tech Evangelist for an example of simple "start of application" data initialization.

Answer 1

Stack underflow basically means the compiler messed up.

You can use SWFWire Inspector to look at the bytecode of the event handler, if you want to know exactly how it messed up. You can also use SWFWire Debugger to see which methods were called, but in this case, you already knew where it was happening.

If you post the broken swf, I can give you more info.

Answer 2

I got rid of this error by adding compiler argument:
-omit-trace-statements=false

Answer 3

Sean is right that to debug it you can look at the byte code, but that didn't sound appealing to me.

Based on my experience and research, it is often due to the presence of a trace statement that incorrectly gets compiled out in release mode, and generates invalid byte code. So, I would say to "debug" it, "Look for places where you are using trace. Try commenting them all out in the offending function and see if the issue goes away."

In my case, it was a trace statement as the first line of a catch block:

catch (e:TypeError) {
    trace(e.getStackTrace()); //This line is the problem
    throw new Error("Unexpected type encountered");
}

I found someone else with this exact issue here.

Answer 4

This code also leads to stack underflow only in release mode (flag -debug=false):

true && trace('123');

mxlmc flex sdk version 4.5.0.20967, flashplayer version 10.3.181.14 (linux).

Check your code for similar expressions.

Answer 5

This code caused me issues when I compiled a release candidate from flash builder 4.5

public function set configVO( value:PopupConfigVO ):void
        {trace("CHANGING")

Resolved by inserting a space between the the trace and curly brace

public function set configVO( value:PopupConfigVO ):void
        { trace("CHANGING")

Hope this helps.

Answer 6

For people looking for the same problem, I just got this caused by a trace statement in the 'default' case of a switch statement. Commented out the trace, stack underflow resolved.

Answer 7

Interesting... I was getting this error with a SWF that I'd pulled off the web, an Away3D based graphics demo. At the time I was running this on the Tamarin VM rather than the actual Flash/AIR runtimes, so could stick a breakpoint on the "verifyFailed(kStackUnderflowError)" line and see what was happening.

The -Dverbose flag also helped find the culprit:

typecheck MethodInfo-1480()
  outer-scope = [global]
                       [Object~ Object] {} ()
  0:pop
VERIFY FAILED: Error #1024: Stack underflow occurred.

And looking at the ABC using SWFInvestigator, I found this:

var function(Object):void   /* disp_id=0 method_id=1480 nameIndex = 0 */
{
   // local_count=2 max_scope=0 max_stack=0 code_len=2
   // method position=52968 code position=155063
   0      pop               
   1      returnvoid        
}

So there is an obvious issue where the 'trace' has been removed but the compiler has put a 'pop' in there: I wouldn't have thought this was needed as a trace call should presumably have been made via 'callpropvoid'?

Quite why this doesn't fail on AIR/Flash I don't know..

Anyway: looks to me like an ASC compiler problem i.e perhaps one of the ActionScript3 compilers had a fault with this - hence the workarounds that have been mentioned so far.

Answer 8

It's quite simple, and it doesn't have anything to do with spaces before or after brackets, trace commands or whatever else: it's just 1 really simple thingy:

DO NOT LOOP EMPTY!

Meaning, while developing, we all //comment some lines sometimes, and when that results in

 for (...) { 
             // skip for now
         }

the compiler gets :

      for(...){}

and that my good friends, is something the compiler doesn't like!

so, NO empty loops, and you're on your way again...

Happy hunting, P.

Answer 9

I had the exact same problem, but in my case the cause of the problem was a trace statement in a place where the compiler didn't expect it to find it, right after a package declaration at the beginning of the class:

package utils 
{

trace ("trace something here");

And that's why compiling in debug mode removed the problem.