Help: Basic API call randomly crashes. Enlightened experts may know...

Question

Here is the function and it is called once a second. The try-catch was added later.bool UserInfo::IsCurrentUserInteractive()
uid_t uid;
gid_t gid;
/* Getting the username of current consoleuser. Note
* that if the username is NULL then that means that
* no user is presently logged in. We do the lookup
* using the SCDynamicStoreCopyConsoleUser call.
* First Argument: The dynamic store to use to lookup
* the console username. We only need a temporary dynamic
* store so pass null.
* Second Argument: On return this will have the
* UserID (UID) of the new user. We pass in a uid_t variable
* so we can get the return value. This value isn't used but
* is included for demonstration purposes.
* Third Argument: On return this will have the
* GroupID (GID) of the new user. We pass in a gid_t variable
* so we can get the return value. This value isn't used but
* is included for demonstration purposes.
* Return Value: The console username expressed as a CFString.
* This string must be eventually released with a CFRelease call.
CFStringRef Name = NULL;
// This call has crashed before
try
Name = SCDynamicStoreCopyConsoleUser(NULL, &uid, &gid);
catch(...)
return true;
if (NULL == Name)
return false;
CFRelease(Name);
return true;
This can run for hours, being called every second without a problem. However eventually it will crash. Here is the crash log BEFORE I added the try-catch.PID: 148
Thread: 0
Exception: EXCBADACCESS (0x0001)
Codes: KERNPROTECTIONFAILURE (0x0002) at 0x000001f9
Thread 0 Crashed:
0 libSystem.B.dylib 0x90003040 szone_malloc + 590
1 libSystem.B.dylib 0x90002da7 malloczonemalloc + 571
2 com.apple.CoreFoundation 0x9082bb0d CFRunLoopRunSpecific + 196
3 com.apple.CoreFoundation 0x9082ba42 CFRunLoopRunInMode + 61
4 com.apple.HIToolbox 0x92de9878 RunCurrentEventLoopInMode + 285
5 com.apple.HIToolbox 0x92edece0 GetNextEventMatchingMask + 100
6 com.apple.HIToolbox 0x92f58c5a GetNextEvent + 85
7 com.chatter.box 0x00007cba EventPump() + 36
8 libSystem.B.dylib 0x9011072c _sigtramp + 49
9 libSystem.B.dylib 0x90002f9c szone_malloc + 426
10 libSystem.B.dylib 0x90002da7 malloczonemalloc + 571
11 com.apple.CoreFoundation 0x9080b53c _CFRuntimeCreateInstance + 155
12 com.apple.CoreFoundation 0x9080b27c __CFStringCreateImmutableFunnel3 + 1453
13 com.apple.CoreFoundation 0x90817c1b CFStringCreateWithBytes + 94
14 com.apple.CoreFoundation 0x90837622 __CFBinaryPlistCreateObject + 1405
15 com.apple.CoreFoundation 0x90837fc6 __CFBinaryPlistCreateObject + 3873
16 com.apple.CoreFoundation 0x90837b86 __CFBinaryPlistCreateObject + 2785
17 com.apple.CoreFoundation 0x90837fc6 __CFBinaryPlistCreateObject + 3873
18 com.apple.CoreFoundation 0x9082e623 __CFTryParseBinaryPlist + 177
19 com.apple.CoreFoundation 0x9082d990 _CFPropertyListCreateFromXMLData + 150
20 com.apple.CoreFoundation 0x9082d8f7 CFPropertyListCreateFromXMLData + 54
21 com.apple.SystemConfiguration 0x91421152 _SCUnserialize + 99
22 com.apple.SystemConfiguration 0x91420f6b SCDynamicStoreCopyValue + 285
23 com.apple.SystemConfiguration 0x91423e97 SCDynamicStoreCopyConsoleUser + 158
24 com.chatter.box 0x0000a51a UserInfo::IsCurrentUserInteractive() + 32
25 com.chatter.box 0x000087de ClientCore::DoRun() + 808
26 com.chatter.box 0x00007e20 main + 308
27 com.chatter.box 0x0000733e _start + 216
28 com.chatter.box 0x00007265 start + 41Now after the try-catch:PID: 291
Thread: 0
Exception: EXCBADACCESS (0x0001)
Codes: KERNPROTECTIONFAILURE (0x0002) at 0x00000000
Thread 0 Crashed:
0 libobjc.A.dylib 0x90a58387 objc_msgSend + 23
1 com.chatter.box 0x0000a58f UserInfo::IsCurrentUserInteractive() + 61
2 com.chatter.box 0x00008833 ClientCore::DoRun() + 805
3 com.chatter.box 0x00007e5b main + 355
4 com.chatter.box 0x0000733e _start + 216
5 com.chatter.box 0x00007265 start + 41
This leads me to several questions:
1. Why is this crashing my app?
2. Why isn't the try-catch trapping the exception and preventing the crash?
3. How can I prevent this crash?
The purpose is simply to know if there is an interactive user. By getting the console user you can find out. If there is no console user, then clearly, there is no interactive session. I know there can be fast user switching on Macs, although I do not know how to do it, so it is possible for an app to be running as a user who doesn't have a GUI because they didn't log out, they just switched out to another user and all their apps are still running. I am going to make all the parameters null since I do not use them, this was basically a cut and paste job from some API documentation with sample code.

Answer 1

karma-9 wrote:
809648 wrote:
i know i m sucked and really need ur expertise help asap or i would suicide for this hw! pls!
If you can't even start with that assignment, there is no need to go to such extreme lengths as to kill yourself.
Just don't go into programming, find other areas of interest and a different career path.Or, you know, talk to your lecturer, teacher, or tutor. As opposed to just give up.
I failed my first-ever CS assignment. Got 7/20 for my Haskell and then -7 because it didn't compile.
Considered dropping my degree, spoke to the tutor who pointed out a couple of book chapters to read, got 90% or higher in the next three tests, then ended up with an honours degree four years later. Everyone has hurdles or brick walls to negotiate, but it doesn't mean you're not cut out for a career there. It's not a case of natural talent, just persistence and practice.
Oh, and mods - sorry for the 'irrelevant humour' before. I shan't try it again until I've got my post count to my previous levels and I then can join in all the mocking by regulars. ;)