Question

I’m writing a preference pane to control a script I created, and I want to show a sheet on first launch so the user can tell the pane where they installed the script. The sheet appears fine, the problem is that it appears right after you click the button for my pane. This causes System Preferences to Segmentation Fault immediately.

I’m using the command to show the sheet inside awakeFromNib, which may have something to do with it, but I can’t get anything inside of mainViewDidLoad to actually execute. Any ideas on what I can do?

Code:

- (void) awakeFromNib
{
NSLog(@"hi");
[sheetController setParentWindow:[NSApp mainWindow]];
BashScript *script = [[BashScript alloc] init];
if (![script loadScriptFromLocation:[self retrieveScriptLocation]])
{
    NSLog(@"Error loading script.");
}
else {
    [advancedEditor setString:[script getScript]];
}
}


- (NSString*) retrieveScriptLocation
{
NSUserDefaults *preferences = [[NSUserDefaults standardUserDefaults] retain];
NSString *location = [preferences stringForKey:@"scriptLocation"];
if (location != nil)
{
    return location;
}
else
{
    return [self askForScript];
}
}

- (NSString*) askForScript
{
[sheetController openSheet:nil];
}

EDIT: Here’s the stack trace from the crash: http://files.spherecat1.com/prefpanestacktrace.txt

Answer 1

The stack trace reveals the cause of your crash:

- (void) awakeFromNib
{
  if (![script loadScriptFromLocation:[self retrieveScriptLocation]])

- (NSString*) retrieveScriptLocation
{
  return [self askForScript];

- (NSString*) askForScript
{
  [sheetController openSheet:nil];
}

You haven’t returned a value from this method that you’ve declared as returning a value. The compiler should be warning you about this.

The crash happens because the method returns with effectively random bytes (possibly a valid object pointer, possibly not) where the return value should be. You can tell that this return value was supposed to be a string by looking at the attempted message in the stack trace:

Application Specific Information:
objc_msgSend() selector name: getFileSystemRepresentation:maxLength:

That’s an NSString method, so something—in this case, the implementation behind -[NSData initWithContentsOfFile:] and -[NSFileManager contentsAtPath:]—tried to treat the address as that of an NSString. It isn’t, which is what caused the crash, and is because you didn’t return a string from a method where you said you would.

Of course, you can’t just make the method return a string, because you don’t have it to return yet. You need to refactor your code to reflect the asynchronous nature of the Open panel and sheets in general. Recognize that you do not have a string yet and will not for one or more minutes; just begin the Open panel sheet and don’t attempt to load the script until it succeeds.

In a nutshell, try to load the script, and if you don’t have a path yet, just begin the Open panel sheet and then continue on with other initialization.

You also shouldn’t assume the Open panel will succeed. The user may cancel, especially if this is their first time opening the prefpane and they want to take a look around first. In fact, you shouldn’t show an Open panel immediately on prefpane load in the first place.

Instead, have a file-picker view showing where the current script is, if there is one, and make clear, by its placement and explanatory text underneath it and possibly judicious application of color, that your program cannot work until the user chooses a script. Let the user choose the script and thereby enable your program whenever they’re ready to.