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 (in decimal) 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
FACILITY_METADIRECTORY 35

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 (remember that out of memory means you ran out of address space, not RAM!)
  • E_INVALIDARG 0x80070057 you passed a bad argument to a method
  • E_NOINTERFACE 0x80004002 COM is asking an object for an interface it does not support. 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.


Commentary from 2020

First off: write-protected floppies were a real thing! Honest!

There were a number of good user comments on this article with advice extending mine:

  • FACILITY_ITF means “the interface you’re calling defines the meaning of the error you’re getting” which can be confusing
  • You can use Windows Calculator in scientific mode to quickly convert decimals to hex DWORDs
  • Look in winerror.h for more predefined error codes
  • The HRPLUS utility is good for HRESULT analysis
  • Visual Studio has an “hr” format specifier that will convert numeric values to their text equivalents. Making a watch on @EAX,hr and @ERR,hr is useful! @ERR shows the value of a call to GetLastError.
  • For a great explanation of how the script engines propagate errors around, see this SO question.

 

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