Making Sense of HRESULTS

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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

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

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.