# Making Sense of HRESULTS

### Making Sense of HRESULTS

Every now and then -- like, say, this morning -- someone sends me this mail:

I'm getting an error in my JScript program.  The error number is -2147024877.  No description.  Help!

Making sense of those error numbers requires some delving into the depths of how COM represents errors -- the HRESULT.

An HRESULT is a 32 bit unsigned integer where the high bit indicates whether it is an error or a success.  The remaining bits in the high word indicate the "facility" of the error -- into what broad category does this error fall?  The low word indicates the specific error for that facility.

HRESULTS are therefore usually talked about in hex, as the bit structure is a lot easier to read in hex!  Consider 0x80070013, for example.  The high bit is set, so this is an error.  The facility code is 7 and the error code is 0x0013 = 19 in decimal.

Unfortunately, JScript interprets the 32 bit error code as a signed integer and displays it in decimal.  No problem -- just convert that thing back to hex, right?

var x = -2147024877;

print(x.toString(16))

Whoops, not quite.  JScript doesn't know that you want this as an unsigned number, so it converts it to a signed hex number, -0x7ff8ffed.  We need to convert this thing to the value it would have been had JScript interpreted it as an unsigned number in the first place.  A handy fact to know is that the difference between an unsigned number interpreted as a signed number and the same number interpreted as an unsigned number is always 0x100000000 if the high bit is set, 0 otherwise.

var x = -2147024877;

print((x<0?x+0x100000000:x).toString(16))

There we go.  That prints out 80070013.  Or, even better, we could just write a program that takes the error apart:

function DumpHR(hr)

{

if (hr < 0 )

hr += 0x100000000;

if (hr & 0x80000000)

print("Error code");

else

print("Success code");

var facility = (hr & 0x7FFF0000) >> 16;

print("Facility " + facility);

var scode = hr & 0x0000FFFF;

print("SCode " + scode);

}

DumpHR(-2147024877);

The facility codes are as follows

FACILITY_NULL                    0

FACILITY_RPC                     1

FACILITY_DISPATCH                2

FACILITY_STORAGE                 3

FACILITY_ITF                     4

FACILITY_WIN32                   7

FACILITY_WINDOWS                 8

FACILITY_SECURITY                9

FACILITY_CONTROL                 10

FACILITY_CERT                    11

FACILITY_INTERNET                12

FACILITY_MEDIASERVER             13

FACILITY_MSMQ                    14

FACILITY_SETUPAPI                15

FACILITY_SCARD                   16

FACILITY_COMPLUS                 17

FACILITY_AAF                     18

FACILITY_URT                     19

FACILITY_ACS                     20

FACILITY_DPLAY                   21

FACILITY_UMI                     22

FACILITY_SXS                     23

FACILITY_WINDOWS_CE              24

FACILITY_HTTP                    25

FACILITY_BACKGROUNDCOPY          32

FACILITY_CONFIGURATION           33

FACILITY_STATE_MANAGEMENT        34

So you can see that our example is a Windows operating system error (facility 7), and looking up error 19 we see that this is ERROR_WRITE_PROTECT -- someone is trying to write to a write-protected floppy probably.

All the errors generated by the script engines -- syntax errors, for example -- are FACILITY_CONTROL, and the error numbers vary between script engines.  VB also uses FACILITY_CONTROL, but fortunately VBScript assigns the same meanings to the errors as VB does.  But in general, if you get a FACILITY_CONTROL error you need to know what control generated the error -- VBScript, JScript, a third party control, what?  Because each control can define their own errors, and there may be collisions.

Finally, here are some commonly encountered HRESULTs:

E_UNEXPECTED  0x8000FFFF "Catestrophic failure" -- something completely unexpected has happened.

E_NOTIMPL     0x80004001 "Not implemented" -- the developer never got around to writing the method you just called!

E_OUTOFMEMORY 0x8007000E -- pretty obvious what happened here

E_INVALIDARG  0x80070057 -- you passed a bad argument to a method

E_NOINTERFACE 0x80004002 -- COM is asking an object for an interface.  This can happen if you try to script an object that doesn't support IDispatch.

E_ABORT       0x80004004 -- whatever you were doing was terminated

E_FAIL        0x80004005 -- something failed and we don't know what.

And finally, here are three that you should see only rarely from script, but script hosts may see them moving around in memory and wonder what is going on:

SCRIPT_E_RECORDED   0x86664004 -- this is how we internally track whether the details of an error have been recorded in the error object or not.  We need a way to say "yes, there was an error, but do not attempt to record information about it again."

SCRIPT_E_PROPAGATE  0x80020102 -- another internal code that we use to track the case where a recorded error is being propagated up the call stack to a waiting catch handler.

SCRIPT_E_REPORTED   0x80020101 -- the script engines return this to the host when there has been an unhandled error that the host has already been informed about via OnScriptError.

That's a pretty bare-bones look at error codes, but it should at least get you started next time you have a confusing error number.

• Reader Shaka comments on my post about error messages that "catastrophic failure" really does take the

• Excellent, thx.

Thx to Jay Hugard too for his code

• I have a scriptable COM component (implements IDispatchEx) which has a method that takes a script-function as a parameter (an IDispatch* as a parameter).  My COM component calls back into script via that supplied IDispatch parameter.

If the script-function happens to throw an exception, then my call to Invoke will return SCRIPT_E_PROPAGATE.  If I return that HRESULT back to the caller of my COM component, then the script engine throws an error.

However, the *ACTUAL ERROR* is nowhere to be found.  It's not returned in the Invoke HRESULT (that's SCRIPT_E_PROPAGATE).  It's not returned via the EXCEPINFO supplied to Invoke (the structure remains empty).  AND, it's not available via GetErrorInfo (the call returns S_FALSE)!

I'd *really* like to get my hands on that error, because it would be useful to cache it and return the same error on subsequent calls (getting to the error often involves an expensive operation that is defined by the script-function passed as a parameter, but I do know how to cache the error).  Is there a way for a scripted COM component to get to an exception thrown during a call-back into a supplied script-function???

Hmm. I have not looked at that code for ten years (and I didn't understand it real well then; I didn't write the exception handling logic), so I'm a bit rusty. The EXCEPINFO ought to be filled in with the details. Do you know what exactly is being thrown by the script? Do you know if there are any possible handlers up the stack? The engine should interrogate the IDispatchEx to see if it can determine a call stack that has a catch block. Is there any such call stack?

• Thanks for responding, Eric!

Worked this out after tracing into the assembly for the JScript engine.  See the following Question & Answer on StackOverflow.

stackoverflow.com/.../3167705

Man, was that a tough mystery to unravel ;-)  Seriously underdocumented!

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