Debugging a Quick Look plugin

Question

I'm writing an app which has its own (cross-platform) custom XML-based file type.

I want to write a quick look plugin so that things look good in the finder, and have found the tutorial on how this is supposed to work, but apparently I must be doing something wrong since I see in /var/log/system.log that I get a segfault when my Quick Look plugin is ran. How do I see where it's crashing?

Where does macOS store core dumps (if it even does that)? Do I need to set some system option or something to enable that?

How do I get Xcode to look at these core dumps?

Answer 1

EDIT SIP prevents you from debugging protected processes. Because of that, it is currently (at least since SIP was introduced, through Mojave) impossible to debug QuickLook plugins without turning off SIP, at least partially.

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As you've probably discovered, since your QuickLook plugin is a plug-in instead of a standalone executable, you need to debug the process that hosts the plugin. To do that, you can hook yourself to the qlmanage executable.

The first step is to make your .qlgenerator plugin available to the Quick Look server. To do that, you need to copy it to ~/Library/QuickLook and run qlmanage -r. The first can be implemented as a post-build action, the second has to be specified in the debug options.

For the post-build action, you should follow these steps:

1. hit Alt+Cmd+R to open the run configuration window;
2. unfold the "Build" tab on the left and go to post-actions;
3. click the plus at the bottom of the window and select "New Run Script Action", and use the following code.
4. absolutely ensure the "Provide build settings from" is set to your QuickLook extension target.

rm -Rf "~/Library/QuickLook/$FULL_PRODUCT_NAME"
cp -R "$BUILT_PRODUCTS_DIR/$FULL_PRODUCT_NAME" ~/Library/QuickLook
qlmanage -r

Then, you need to configure Xcode to launch qlmanage:

1. in the same window, go to the "Run" tab, and select "Info";
2. in the executable drop-down menu, pick "Other...";
3. hit Shift+Cmd+G to enter a path, put in "/usr/bin/qlmanage", and select that file;
4. check "Debug executable" if it isn't already;
5. move to the "Arguments" tab, add a "-p" argument, and then add arguments as you see fit for the files that you need to preview.

Now, when you use the Run action, you'll be able to debug your plugin through qlmanage.

Answer 2

I haven't tried this personally, but this page:

Debugging Quicklook Plugin in Xcode 4.6

has a description of how to debug your plugin in Xcode 4 - 6. That will probably be much more useful than trying to grub through the core file after the fact.