This is a function "cheat sheet" for the catch_error extension by YellowAfterlife.
The extension can be acquired via itch.io.
A most up-to-date version of the manual is always available online.

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General functions

catch_error_init()

Should be called for any other functions to have any effect.

Returns whether successful (> 0).

Generally, can only fail is the DLL is missing/failed to load.

catch_error_is_ready()

Returns whether the initialization is done and everything is good to go.

catch_error_is_loaded()

Returns whether the DLL is successfully loaded.

If this is false, it is either missing entirely or the user has some serious setup issues.

Error handling

Normal errors

"Normal" errors are the ones that have an "ignore" button in their dialog.

These are captured consistently and pretty future-proof - the only way the capture mechanism could break is if error dialog handling gets rewritten completely.

catch_error_set_normal(mode)

Defines how non-fatal errors should be treated.

mode can be the following:

catch_error_normal_show

Shows the error box (default GM behaviour)

catch_error_normal_quit

Treats non-fatal errors as fatal

catch_error_normal_ignore

Ignores non-fatal errors completely

catch_error_normal_queue

Stores non-fatal errors for retrieval via catch_error_dequeue.

Examples:

catch_error_init();
//
catch_error_set_normal(catch_error_normal_ignore);
show_error("1!", false); // (absolutely nothing happens)
//
catch_error_set_normal(catch_error_normal_queue);
show_error("2!", false);
show_debug_message("Last error: " + catch_error_dequeue());
//
catch_error_set_normal(catch_error_normal_show);
show_error("3!", false); // (a normal error box appears)
//
catch_error_set_normal(catch_error_normal_quit);
catch_error_set_prompt(catch_error_prompt_message, "It would appear like we've crashed");
show_error("4!", false); // (a dialog box with above text appears)
catch_error_get_normal()

Returns the current mode for normal errors (as per constants).

Fatal errors

"Fatal" errors are the ones that do not have an "ignore" button in their dialogs.

Usually these put an end to your game, and you could let that happen (either letting them show or passing them to a log file/external app, skipping the error box), but the extension also offers an option to catch fatal errors as well.

catch_error_set_fatal(mode)

Defines how fatal errors should be treated.

mode can be the following:

catch_error_fatal_show

Shows the error box (default GM behaviour)

catch_error_fatal_quit

Skips the message box

catch_error_fatal_ignore

Ignores fatal errors completely

catch_error_fatal_queue

Stores fatal errors for retrieval via catch_error_dequeue.

Treating fatal errors as non-fatal is obviously a little more experimental. While my tests indicate that the approach works with a variety of GMS/GMS2 versions released in past two years, I would expect this to break once GMS2 gets built-in error handling. But also, you wouldn't need this feature once that's a thing.

Example:

catch_error_init();
catch_error_set_fatal(catch_error_fatal_quit);
catch_error_set_dump_path("error.log");
show_error("oh no", true); // quietly saves the error to `error.log` and exits
catch_error_get_fatal(mode)

Returns the current mode for fatal errors (as per constants).

Newer-format fatal errors

Note: as of 1.0.1 / Feb 10, 2019 update newer-format errors are handled together with normal fatal errors, but you can still use these functions if you want to ignore them entirely for some reason.

Explanation

So, here's a thing: recent (~2018) versions of GM:S/GMS2 have a third way of throwing errors. If you've ever taken a peek at YYC YYGML.h, it's called YYError there.

It is used for "low-level" actions (like + or inst.vari) and a random few built-in functions (like vertex_begin).

The deal with it is that its code goes like this (pseudocode):

global_flag_1 = true;
if (global_flag_2) {
	// (skip to the end of the current script/event if not using YYC)
	return;
}
var error_msg = ...;
if (!global_flag_2) {
	show_error(error_msg, true); // (sort of)
	exit(-1);
}

where exit is stdlib.h exit - it immediately and unconditionally causes the game process to quit. Obviously, a minor inconvenience for queuing errors, as it queues the error but then your game closes.

To make things better, there is no way of knowing that your error comes from such a source, as the function is small enough to get optimized and not appear in low-level call stack at all.

However, I've found that global_flag_2 is also referenced inside show_error, thus was able to add a pair of functions to flip it back and forth, which means that you can omit any such errors, but can't queue their messages (because the function quits early).

This might be addressed in future.

catch_error_set_newer(mode)

Defines how newer-format fatal errors should be treated.

mode can be the following:

catch_error_newer_allow

Allows newer-format fatal errors to dispatch (default GM behaviour).

As per above, if you are queuing/ignoring fatal errors, this will still result in your game ending.

Otherwise these are the same as regular fatal errors and will trigger the rest of the helper functions their way out.

catch_error_newer_ignore

Suppresses newer-format fatal errors completely - your game won't quit, but also the errors won't get added to the queue.

As per above, on non-YYC newer-format errors skip to the end of the current script/event, which can be a good or a bad thing, depending on what you are trying to do.

catch_error_get_newer()catch_error_newer_

Returns the current mode for newer-format fatal errors.

Aside of the default constants, the return value can be:

catch_error_newer_not_ready

No fatal errors were requested for ignoring / queuing yet nor was catch_error_set_newer called, so we don't know yet if we can ignore them or not.

catch_error_newer_unavailable

The global flag required to make this feature work couldn't be found.

Obviously, this is something that can only change upon GM updates, so you wouldn't usually need to check for this at all.

Error queues

If you've requested your errors to be queued, you can later retrieve them.

catch_error_dequeue()error_text

If non-fatal error handling is set to catch_error_normal_queue, this function returns the next stored error message from the queue.

If there are no more error messages left, returns "".

Example:

catch_error_init();
catch_error_set_normal(catch_error_normal_queue);
show_error("1!", false);
show_error("2!", false);
var s = catch_error_dequeue();
while (s != "") {
    s = string_replace_all(s, "#", chr(92)+"#"); // GMS1 only - escape "#"s
    show_message("Error: " + chr(13) + chr(10) + s);
    s = catch_error_dequeue();
}

This would display two dialog boxes with caught error messages in order.

catch_error_size()size

Returns the current number of queued error messages.

Example:

catch_error_init();
catch_error_set_normal(catch_error_normal_queue);
show_error("1!", false);
show_error("2!", false);
show_debug_message(catch_error_size()); // 2
catch_error_clear()old_size

Clears the error message queue and returns how many messages there were in it. Example:

catch_error_init();
catch_error_set_normal(catch_error_normal_queue);
show_error("1!", false);
show_error("2!", false);
show_debug_message(catch_error_size()); // 2
catch_error_clear();
show_debug_message(catch_error_size()); // 0

Dumps upon error exit

When your game exits due to an error (user-initiated or automatic via _quit), you can opt in to save the last error message to a file.

You can later access that file from in-game (if you saved it on a relative path) or external software.

catch_error_set_dump_path(?path)

Changes the current log file path.

Call with no arguments (or "") to disable.

Example:

catch_error_init();
catch_error_set_fatal(catch_error_fatal_quit);
catch_error_set_dump_path("error.log");
show_error("oh no", true); // quietly saves the error to `error.log` and exits
catch_error_get_dump_path()path

Returns the current log file path

Executing external programs upon error exit

You can opt in to execute an external program when your game exits due to an error. This can be used to open an error reporting tool, error log location, or even restart the game and show the options in-game.

catch_error_set_exec(?path, ?params)

path is an absolute (C:/..., etc.) path to application to be ran.

params is a string of parameters to pass to the application.

If called without arguments, reverts to not running anything.

Example:

catch_error_init();
catch_error_set_prompt(catch_error_prompt_question,
    "We've hit an error. Would you like to restart the app?",
    "Oh no,");
// let the game restart on error:
var argv = "", argc = parameter_count();
for (var i = 1; i < argc; i++) {
    if (i > 1) argv += " ";
    var v = parameter_string(i);
    if (string_pos(" ", v)) argv += '"' + v + '"'; else argv += v;
}
catch_error_exec(parameter_string(0), argv);
//
show_error("oh no", true); // shows the restart question prompt

Custom error prompt upon error exit

You can show a dialog box prior to the game exiting (and error handler application being ran, if any). If you're choosing to let your game restart on error, this is a little more clear to the user than the game just suddenly restarting.

catch_error_set_prompt(kind, ?message, ?title, ?messageBoxType)

kind (required) can be one of the following:

catch_error_prompt_none

Default - doesn't show any prompts.
When using this, other arguments can be omitted.

catch_error_prompt_message

Shows a message box with an OK button.

catch_error_prompt_question

Shows a message box with Yes/No buttons.
If the user answers "no", application chosen via catch_error_exec will not be ran.

message (optional) is the content of the message box.
This is where you tell the user that you regret to inform that the game/app encountered a serious error and will now close.

title (optional) is the caption of the message box.
If not specified, will be blank ("").

messageBoxType (optional) is what is called uType on this MSDN page.
This allows you to further customize the looks of the dialog box.
0x00000010L is 0x00000010 in GMS2 (no L) or $00000010 in GMS2 (no L; $ prefix).
Leaving out this parameter will not change the existing flags.
Base "type" (MB_OK/MB_YESNO) is automatically filled out based on kind argument.

Example:

catch_error_init();
catch_error_set_prompt(catch_error_prompt_message,
    "We regret to inform that the game encountered a fatal error and will now close.",
    "Error");

for example of catch_error_prompt_question, see catch_error_exec.

FAQ

How does this work?

The extension works by "catching" and operating the built-in error box.

While the implementation details are full of unexciting low-level WinAPI nonsense (which is probably the reason why no one did this yet), the gist is as following - the DLL steps in as a dialogue box processor, gets the information from the GameMaker runtime, and then evaluates what to do (show the actual dialog, store error, exit game...) based on instructions provided via the functions.

What platforms are supported?

The extension works on Windows and Windows (YYC) in both GameMaker: Studio and GameMaker Studio 2.

While it is possible that additional platforms will be supported in future, each would require a completely different implementation, and differ in features (for instance, you cannot launch any apps on mobile/consoles/HTML5), so it would be unwise to make any particular promises here.

What errors are caught?

The extension works for any errors that result in a familiar "code error" box.

It cannot catch access violations (or other low-level errors that cause a "<name> has stopped working" Windows message) as these halt execution abruptly.

This may be partially addressed in future (to allow the game to be restarted in such scenario), but there isn't much information that you can gather in such case.

Can you use this as try-catch?

If Windows is the only platform of concern, absolutely - clear the queue before performing something problematic, then check it's size/contents after finishing.