corruption interview questions
Top corruption frequently asked interview questions
I have had a hard disk failure which resulted in some files of a Git repository getting damaged. When running git fsck --full I get the following output:
error: .git/objects/pack/pack-6863e0a0e4b4ded6090fac5d12eba6ca7346b19c.pack SHA1 checksum mismatch
error: index CRC mismatch for object 6c8cae4994b5ec7891ccb1527d30634997a978ee from .git/objects/pack/pack-6863e0a0e4b4ded6090fac5d12eba6ca7346b19c.pack at offset 97824129
error: inflate: data stream error (invalid code lengths set)
error: cannot unpack 6c8cae4994b5ec7891ccb1527d30634997a978ee from .git/objects/pack/pack-6863e0a0e4b4ded6090fac5d12eba6ca7346b19c.pack at offset 97824129
error: inflate: data stream error (invalid stored block lengths)
error: failed to read object 0dcf6723cc69cc7f91d4a7432d0f1a1f05e77eaa at offset 276988017 from .git/objects/pack/pack-6863e0a0e4b4ded6090fac5d12eba6ca7346b19c.pack
fatal: object 0dcf6723cc69cc7f91d4a7432d0f1a1f05e77eaa is corrupted
I have backups of the repository, but the only backup that includes the pack file has it already damaged. So I think that I have to find out a way to retrieve the single objects from different backups and somehow instruct Git to produce a new pack with only correct objects.
Can you please give me hints how to fix my repository?
Source: (StackOverflow)
When I try to open my .sln file, Visual Studio quits on me. I can open it by opening up TFS, getting a specific version, choosing the latest version, forcing an overrwrite, opening up the .sln file, and then ignoring an 'incompatible doc message".
However, I would like to know what's wrong and how it can be fixed, in case some day I can't do this.
How can this be fixed?
Source: (StackOverflow)
I'm debugging some random crash bugs, but actually very difficult to go deep into. Because when i open crash dump, only find one error:
0:000> .exr -1
ExceptionAddress: 00000000
ExceptionCode: 80000003 (Break instruction exception)
ExceptionFlags: 00000000
NumberParameters: 0
Actually i haven't set any hard-code breakpoint in code, so i search about this exception in google, some people said this exception may be caused by heap corruption.
So my question is,
Is there any other reason why cause this exception, except hard-code breakpoint, manual breakpoint while debugging, heap corruption?
Another question is, i try to use Application Verifier to check heap corruption, i understand how does it work, app verifier will trigger break instruction exception while heap corruption. But currently, i run without app verifier, who will raise the break instruction exception?
Additional info: call stack for current thread.
*0:000> k
ChildEBP RetAddr
0012f96c 7c827d19 ntdll!KiFastSystemCallRet
0012f970 77e6202c ntdll!NtWaitForMultipleObjects+0xc
0012fa18 7739bbd1 kernel32!WaitForMultipleObjectsEx+0x11a
0012fa74 3b288523 user32!RealMsgWaitForMultipleObjectsEx+0x141
0012fab8 3b32b9bd msenv!EnvironmentMsgLoop+0x1ea
0012fae4 3b32b94d msenv!CMsoCMHandler::FPushMessageLoop+0x86
0012fb0c 3b32b8e9 msenv!SCM::FPushMessageLoop+0xb7
0012fb28 3b32b8b8 msenv!SCM_MsoCompMgr::FPushMessageLoop+0x28
0012fb48 3b32be4e msenv!CMsoComponent::PushMsgLoop+0x28
0012fbe0 3b327561 msenv!VStudioMainLogged+0x482
0012fc0c 3000a4a6 msenv!VStudioMain+0xc1
0012fc38 30007301 devenv!util_CallVsMain+0xff
0012ff14 3000760c devenv!CDevEnvAppId::Run+0x91f
0012ff30 30007680 devenv!WinMain+0x74
0012ffc0 77e6f23b devenv!License::GetPID+0x258
0012fff0 00000000 kernel32!BaseProcessStart+0x23*
Our application is a Visual Studio Package.
Below is the result from !analyze -v
0:000> !analyze -v
*******************************************************************************
* *
* Exception Analysis *
* *
*******************************************************************************
*** WARNING: Unable to verify checksum for mscorlib.ni.dll
*************************************************************************
*** ***
*** ***
*** Your debugger is not using the correct symbols ***
*** ***
*** In order for this command to work properly, your symbol path ***
*** must point to .pdb files that have full type information. ***
*** ***
*** Certain .pdb files (such as the public OS symbols) do not ***
*** contain the required information. Contact the group that ***
*** provided you with these symbols if you need this command to ***
*** work. ***
*** ***
*** Type referenced: kernel32!pNlsUserInfo ***
*** ***
*************************************************************************
*************************************************************************
*** ***
*** ***
*** Your debugger is not using the correct symbols ***
*** ***
*** In order for this command to work properly, your symbol path ***
*** must point to .pdb files that have full type information. ***
*** ***
*** Certain .pdb files (such as the public OS symbols) do not ***
*** contain the required information. Contact the group that ***
*** provided you with these symbols if you need this command to ***
*** work. ***
*** ***
*** Type referenced: kernel32!pNlsUserInfo ***
*** ***
*************************************************************************
FAULTING_IP:
+0
00000000 ?? ???
EXCEPTION_RECORD: ffffffff -- (.exr 0xffffffffffffffff)
ExceptionAddress: 00000000
ExceptionCode: 80000003 (Break instruction exception)
ExceptionFlags: 00000000
NumberParameters: 0
FAULTING_THREAD: 00001f1c
DEFAULT_BUCKET_ID: STATUS_BREAKPOINT
PROCESS_NAME: devenv.exe
ERROR_CODE: (NTSTATUS) 0x80000003 - {EXCEPTION} Breakpoint A breakpoint has been reached.
EXCEPTION_CODE: (HRESULT) 0x80000003 (2147483651) - One or more arguments are invalid
NTGLOBALFLAG: 0
APPLICATION_VERIFIER_FLAGS: 0
MANAGED_STACK:
SP IP Function
0012E584 09C8A903 Microsoft_VisualStudio_Design!Microsoft.VisualStudio.NativeMethods.ThrowOnFailure(Int32, Int32[])+0x3b
0012E590 09C8C604 Microsoft_VisualStudio_Design!Microsoft.VisualStudio.Design.VSDesignSurfaceManager.Microsoft.VisualStudio.Shell.Interop.IVsSelectionEvents.OnElementValueChanged(UInt32, System.Object, System.Object)+0x144
StackTraceString: <none>
HResult: 80004005
EXCEPTION_OBJECT: !pe 3115d464
Exception object: 3115d464
Exception type: System.Runtime.InteropServices.COMException
Message: Error HRESULT E_FAIL has been returned from a call to a COM component.
InnerException: <none>
StackTrace (generated):
SP IP Function
0012E584 09C8A903 Microsoft_VisualStudio_Design!Microsoft.VisualStudio.NativeMethods.ThrowOnFailure(Int32, Int32[])+0x3b
0012E590 09C8C604 Microsoft_VisualStudio_Design!Microsoft.VisualStudio.Design.VSDesignSurfaceManager.Microsoft.VisualStudio.Shell.Interop.IVsSelectionEvents.OnElementValueChanged(UInt32, System.Object, System.Object)+0x144
StackTraceString: <none>
HResult: 80004005
MANAGED_OBJECT: !dumpobj 3201988
Name: System.String
MethodTable: 79330a00
EEClass: 790ed64c
Size: 158(0x9e) bytes
(C:\WINDOWS\assembly\GAC_32\mscorlib\2.0.0.0__b77a5c561934e089\mscorlib.dll)
String: Error HRESULT E_FAIL has been returned from a call to a COM component.
Fields:
MT Field Offset Type VT Attr Value Name
79332c4c 4000096 4 System.Int32 1 instance 71 m_arrayLength
79332c4c 4000097 8 System.Int32 1 instance 70 m_stringLength
793316e0 4000098 c System.Char 1 instance 45 m_firstChar
79330a00 4000099 10 System.String 0 shared static Empty
>> Domain:Value 00219c28:03031198 <<
79331630 400009a 14 System.Char[] 0 shared static WhitespaceChars
>> Domain:Value 00219c28:03031798 <<
EXCEPTION_MESSAGE: Error HRESULT E_FAIL has been returned from a call to a COM component.
MANAGED_OBJECT_NAME: System.Runtime.InteropServices.COMException
LAST_CONTROL_TRANSFER: from 7c827d19 to 7c82860c
PRIMARY_PROBLEM_CLASS: STATUS_BREAKPOINT
BUGCHECK_STR: APPLICATION_FAULT_STATUS_BREAKPOINT
STACK_TEXT:
09c8a903 Microsoft_VisualStudio_Design!Microsoft.VisualStudio.NativeMethods.ThrowOnFailure
09c8c604 Microsoft_VisualStudio_Design!Microsoft.VisualStudio.Design.VSDesignSurfaceManager.Microsoft.VisualStudio.Shell.Interop.IVsSelectionEvents.OnElementValueChanged
STACK_COMMAND: dds 12e584 ; kb
FOLLOWUP_IP:
+9c8a903
09c8a903 8bc6 mov eax,esi
SYMBOL_STACK_INDEX: 0
SYMBOL_NAME: Microsoft_VisualStudio_Design!Microsoft.VisualStudio.NativeMethods.ThrowOnFailure+9c8a903
FOLLOWUP_NAME: MachineOwner
MODULE_NAME: Microsoft_VisualStudio_Design
IMAGE_NAME: Microsoft.VisualStudio.Design.dll
DEBUG_FLR_IMAGE_TIMESTAMP: 0
FAILURE_BUCKET_ID: STATUS_BREAKPOINT_80000003_Microsoft.VisualStudio.Design.dll!Microsoft.VisualStudio.NativeMethods.ThrowOnFailure
BUCKET_ID: APPLICATION_FAULT_STATUS_BREAKPOINT_Microsoft_VisualStudio_Design!Microsoft.VisualStudio.NativeMethods.ThrowOnFailure+9c8a903
Followup: MachineOwner
...
In the managed stack, there is a explicit error,Microsoft.VisualStudio.NativeMethods.ThrowOnFailure..
But that means the com exception cause the break instruction exception?
!analyze seems just dump the managed level, the com exception maybe the last error in the managed level.
I also search something about interrupt and exception from google, Normally, the break instruction exception can be triggered in following conditions:
1. Hardcode interrupt request, like: __asm int 3 (ASM), System.Diagnostics.Debugger.Break (C#), DebugBreak() (WinAPI).
2. OS enable memory runtime check, like Application Verifier can trigger after heap corruption, memory overrun.
3. Compiler can have some configuration to enble what should be filled for the uninitialized memory block and end of function(blank area, after retun..). For example, Microsoft VC complier can fill 0xCC if enable /GZ. 0xCC is actually a opcode of __asm int 3. So if some error cause the application run into such block, will trigger a break point.
Correct?
If that, I think Application Verifier should be best choice to find the root cause.
Source: (StackOverflow)
In C++, in which way the stack may get corrupted. One way I guess is to overwriting the stack variables by accessing an array beyond its boundaries. Is there any other way that it can get corrupted?
Source: (StackOverflow)
I haven't done any C++ in a while, but decided to finish a big project I was working on for someone. I am getting the following error message now though...
HEAP CORRUPTION DETECTED: after Normal Block (#1761) at 0x17DEB940.
CRT Detected that the application wrote to memory after end of heap buffer.
I have been stepping through all of the functions I thought might have caused it but I am at a loss. Is there any way using the more advanced debugging features to hunt this down?
Source: (StackOverflow)
Running git fsck on my repo, I get this output:
$ git fsck
Checking object directories: 100% (256/256), done.
warning in tree bde551ba2d6882ac7614c25305c24ddc1c75b1c4: contains zero-padded file modes
warning in tree 7cac28aefa67ff63e5ca163de382a3e08b8a7ba5: contains zero-padded file modes
warning in tree c24803abe783decd96c1dbf05d3ac45dbf3ff372: contains zero-padded file modes
warning in tree 51393697adb908ddb5fac540a86ea5a331fc1da5: contains zero-padded file modes
Checking objects: 100% (40275/40275), done.
Should I be worried about these warnings? Can they damage my repo?
Source: (StackOverflow)
I'm new to MySQL and I'm figuring out the best way to perform an on-line hot logical backup using mysqldump. This page suggests this command line:
mysqldump --single-transaction --flush-logs --master-data=2
--all-databases > backup_sunday_1_PM.sql
but... if you read the documentation carefully you find that:
While a --single-transaction dump is in process, to ensure a valid dump file
(correct table contents and binary log position), no other connection should use
the following statements: ALTER TABLE, DROP TABLE, RENAME TABLE, TRUNCATE TABLE. A
consistent read is not isolated from those statements, so use of them on a table to
be dumped can cause the SELECT performed by mysqldump to retrieve the table contents
to obtain incorrect contents or fail.
So, is there any way to prevent this possible dump corruption scenario?
I.e. a commands that could block those statements temporarily.
PS: MySQL bug entry on this subject http://bugs.mysql.com/bug.php?id=27850
Source: (StackOverflow)
Im getting these errors while doing git fsck --full --no-dangling on the remote repository. (GIT's server)
user@server:/var/git/REPOSITORY.git$ git fsck --full --no-dangling
bad sha1 file: ./objects/15/19d8eeacc8d0cd603bd87d2034508b1ffaffa8_28537991145d7e9d87b68335e9b82c2f788cb4fc
bad sha1 file: ./objects/32/4f425bcfe23d9f38b154427eeb9c800d109365_6df2d745fff22839759b25cc83f8d742c2e64086
bad sha1 file: ./objects/42/9225fd8d895084051189dc6478343d54fe41c8_28537991145d7e9d87b68335e9b82c2f788cb4fc
bad sha1 file: ./objects/46/798e4a01a5a7eaf2b203f6d1634d603497041b_6df2d745fff22839759b25cc83f8d742c2e64086
bad sha1 file: ./objects/bc/fb9d62ac05d5203249caa0e7e9cb5d1c32daba_6df2d745fff22839759b25cc83f8d742c2e64086
bad sha1 file: ./objects/cd/477947092b4f20feba8c37df974027b1864215_6df2d745fff22839759b25cc83f8d742c2e64086
Checking object directories: 100% (256/256), done.
Checking objects: 100% (136737/136737), done.
missing commit 590ebc3ac022491d7f11c483480fa9530adc91e8
missing tree bab1d768f0d4f6a02e0a30a6c446afaeabc4aa71
For every bad sha1 file there is another file that starts with the same name but doesnt have an underscore _ and then some other text. the files have the exact same file size but different date or time.
Example of the two files:
./objects/15/19d8eeacc8d0cd603bd87d2034508b1ffaffa8_28537991145d7e9d87b68335e9b82c2f788cb4fc
./objects/15/19d8eeacc8d0cd603bd87d2034508b1ffaffa8
I'm attaching screen shots of two cases out of the 6 because they are all the same:
Screenshot 1:
Screenshot 2:
Can i delete the duplicates, what will happen?
*any ideas on the missing commit\tree?
Source: (StackOverflow)
What is the most suitable hash function for file integrity checking (checksums) to detect corruption?
I need to consider the following:
Wide range of file size (1 kb to 10GB+)
Lots of different file types
Large collection of files (+/-100 TB and growing)
Do larger files require higher digest sizes (SHA-1 vs SHA 512)?
I see that the SHA-family is referred to as cryptographic hash functions. Are they ill-suited for "general purpose" use such as detecting file corruption? Will something like MD5 or Tiger be better?
If malicious tampering is also a concern, will your answer change w.r.t the most suitable hash function?
External libraries are not an option, only whats available on Win XP SP3+.
Naturally performance is also of concern.
(Please excuse my terminology if it is incorrect, my knowledge on this subject is very limited).
Source: (StackOverflow)
I have a client that is having problems with our the .msi installer for our application. WiX was used to create this installer. The application has installed just fine on dozens of other machines, but on his machine it displays the message:
This installation package could not be
opened. Contact the application vendor
to verify that this is a valid Windows
Installer Package.
My guess is that it is one of these two possibilities:
- The version of Windows Installer on the client's machine is out of date.
- The .msi he has could be corrupted.
What is the most likely possibility, or are there other possibilities that I'm not aware of?
I think I've recreated the user's problem. If, from a command shell, I run MyFile.msi, then it successfully loads, then I get this in the log file:
=== Verbose logging started: 2/12/2009 10:34:38 Build type: SHIP UNICODE 4.00.6001.00 Calling process: C:\Windows\System32\msiexec.exe ===
MSI (c) (F4:04) [10:34:38:795]: Resetting cached policy values
MSI (c) (F4:04) [10:34:38:795]: Machine policy value 'Debug' is 0
MSI (c) (F4:04) [10:34:38:795]: ******* RunEngine:
******* Product: C:\Users\kelley\Downloads\PixelActiveCityScape_v1_6_Demo.msi
******* Action:
******* CommandLine: **********
MSI (c) (F4:04) [10:34:38:802]: Machine policy value 'DisableUserInstalls' is 0
MSI (c) (F4:04) [10:34:38:830]: Note: 1: 1402 2: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer 3: 2
MSI (c) (F4:04) [10:34:39:140]: SOFTWARE RESTRICTION POLICY: Verifying package --> 'C:\Users\kelley\Downloads\PixelActiveCityScape_v1_6_Demo.msi' against software restriction policy
MSI (c) (F4:04) [10:34:39:141]: Note: 1: 2262 2: DigitalSignature 3: -2147287038
MSI (c) (F4:04) [10:34:39:141]: SOFTWARE RESTRICTION POLICY: C:\Users\kelley\Downloads\PixelActiveCityScape_v1_6_Demo.msi is not digitally signed
MSI (c) (F4:04) [10:34:39:142]: SOFTWARE RESTRICTION POLICY: C:\Users\kelley\Downloads\PixelActiveCityScape_v1_6_Demo.msi is permitted to run at the 'unrestricted' authorization level.
MSI (c) (F4:04) [10:34:39:189]: Cloaking enabled.
MSI (c) (F4:04) [10:34:39:190]: Attempting to enable all disabled privileges before calling Install on Server
MSI (c) (F4:04) [10:34:39:197]: End dialog not enabled
MSI (c) (F4:04) [10:34:39:197]: Original package ==> C:\Users\kelley\Downloads\PixelActiveCityScape_v1_6_Demo.msi
MSI (c) (F4:04) [10:34:39:197]: Package we're running from ==> C:\Users\kelley\AppData\Local\Temp\40a3581.msi
.
.
.
However, if I run msiexec /i MyFile.msi /l*v MyLog.TXT, I get this:
=== Verbose logging started: 2/12/2009 10:32:19 Build type: SHIP UNICODE 4.00.6001.00 Calling process: C:\Windows\sy
stem32\msiexec.exe ===
MSI (c) (FC:F0) [10:32:19:597]: Resetting cached policy values
MSI (c) (FC:F0) [10:32:19:597]: Machine policy value 'Debug' is 0
MSI (c) (FC:F0) [10:32:19:597]: ******* RunEngine:
******* Product: .\PixelActiveCityScape_v1_6_Demo.msi
******* Action:
******* CommandLine: **********
MSI (c) (FC:F0) [10:32:19:599]: Note: 1: 2203 2: .\PixelActiveCityScape_v1_6_Demo.msi 3: -2147287038
MSI (c) (FC:F0) [10:32:19:600]: MainEngineThread is returning 2
=== Verbose logging stopped: 2/12/2009 10:32:19 ===
And this shows this dialog box:
This installation package could not be
opened. Verify that the package
exists and that you can access it, or
contact the application vendor to
verify that this is a valid Windows
Installer package.
So it seems like a permissions issue, but I'm not exactly sure why and how I can fix it. Do I maybe have to digitally sign the .msi file?
Source: (StackOverflow)
I've managed to write a limited video player able to view a .3gp file from internet. The video will be shown centered full screen, maintaining the video aspect ratio. Also, rotations don't interrupt the video, which keeps playing without problems.
Everything seems fine, but... on my HTC Legend when you rotate back to portrait, the video is corrupted, and instead of showing full screen it is displayed at its native pixel size. But rotating again to landscape works and is shown perfectly. Any ideas why? Unfortunately I don't have more hardware to test this on and I've run out of ideas to test.
You can get the full example source code from https://github.com/gradha/Android-video-stream-rotation. Here are screen captures of me opening the application, rotating to landscape, touching the screen to display the video controls, then rotating back to portrait to see the corruption.
Source: (StackOverflow)
EDIT: Due to a comment that was right about my example I removed it and turn this into a generic question:
Some times in my projects I come across stack corruption. No matter how much I fight to write code to avoid it, sometimes it is just unavoidable. But when it happens what are the ways to fight it?
I found one macro given by the good fellow in this blog: http://rxwen.blogspot.com/2009/04/detect-stack-corruption.html which reads the ebp register value to detect corruption.
But there are bound to be more sophisticated tools to help with not shooting yourself on the foot. I am programming in Windows using Codeblocks and the gcc compiler. The reason I make this question is to find tools which I can use under my programming environment to help me detect such mistakes and correct them. Any suggestions?
Thanks for any answers and for taking the time to read my question.
Source: (StackOverflow)
I've just encountered a very annoying problem while deploying new updates to my website. I have two seperate websites whereas one of them is a development version. Now when I want to apply my changes to the production it won't work because of error message:
File
"/usr/lib/python2.6/dist-packages/django/core/handlers/base.py",
line 99, in get_response
request.path_info)
File
"/usr/lib/python2.6/dist-packages/django/core/urlresolvers.py",
line 249, in resolve for pattern in
self.url_patterns:
File
"/usr/lib/python2.6/dist-packages/django/core/urlresolvers.py",
line 278, in _get_url_patterns
patterns =
getattr(self.urlconf_module,
"urlpatterns", self.urlconf_module)
File
"/usr/lib/python2.6/dist-packages/django/core/urlresolvers.py",
line 273, in _get_urlconf_module
self._urlconf_module =
import_module(self.urlconf_name)
File
"/usr/lib/python2.6/dist-packages/django/utils/importlib.py",
line 35, in import_module
import(name)
File
"/srv/websites/spelutveckla_se/urls.py",
line 21, in
(r'^account/login/$',
LoginView.as_view()),
NameError: name 'LoginView' is not
defined
It doesn't matter if I remove that app, view, class or module because it will complain about an other module instead. LoginView IS defined in an import statement. The files are just a fresh copy of the development files (except for settings.py) that is running on an another subdomain without any problem. I've checked the settings.py several times and made sure the correct settings are set (differs by a couple of directory paths). I've also checked that the apache2 www-data user has permission to access the files. i've also restarted apache a couple of times and re-copied the files over and over but nothing works.
I'm desperate and have no clue of what the problem might be...?
Here's what my urls.py looks like at the top:
from django.conf.urls.defaults import *
from django.conf import settings
from django.views.generic import DetailView, ListView, TemplateView
from django.contrib import admin
from project.models import Project
from project.views import Detail, EditView as EditProject, CreateProjectView, EditProjectLinksView, EditProjectFeedsView, EditProjectTagsView, EditMultimediaView, PostForumTopic, AudioFormset
from frontpage.views import FrontpageView
from userprofile.views import UserRegistrationView, UserRegistrationActivationView, LogoutView, UserProfileView, LoginView, EditProfileView, CreateUserProfileView
from registration.views import activate as UserActivatedView
Source: (StackOverflow)
I am trying to make a program in C++ to process a lot of packets in the fastest way possible. All the packets come from the standard should be read as fast as possible, sent to one thread from a pool to do the processing and then handled to an output thread that will write the packet to the standard output.
When you are using the standard input and output in C++, it's recommended that before any input or output you call to the std::ios_base::sync_with_stdio(false) function. In some environments this achieves a great speedup, although you should avoid using standard C functions for input/output after the call.
Well, this seems to work perfectly in a single thread. But as I have said my intention is using one thread for input, one for output and multiple threads for parallel processing. I've observed some problems with the output. This is the output thread (very simplified):
void PacketDispatcher::thread_process_output(OutputQueue& output_queue) {
std::vector<Packet> packet_list;
while(output_queue.get(packet_list)) {
for (const auto& packet: packet_list) {
std::cout << "Packet id = " << packet.id << "\n";
}
}
std::cout.flush();
}
If I used std::endl instead of "\n" there were less corruption, but std::endl forces a flush of the stream, affecting performance in this case (and the problem wasn't solved, only minimized).
That's the only point in the program using std::cout, but if I make the call to std::ios_base::sync_with_stdio(false) at the beggining of the program I get a noticeable speedup, but my output is corrupted always in some way:
Packet id = Packet id = 4
Packet id = 5
Packet id = 6
Packet id = 7
Packet id = 8
Packet id = 9
Packet id = 10
So, where is the problem? Isn't C++ able to do multithreading using fast standard input/output?
Source: (StackOverflow)