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Re: Difficulty integrating with Swift/Objective-C

From: Taylan Kammer
Subject: Re: Difficulty integrating with Swift/Objective-C
Date: Sun, 5 Sep 2021 10:26:49 +0200
User-agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.13.0

On 05.09.2021 08:03, paul wrote:
> Good day,
> I have an existing app which is written in Swift and runs on macOS 10.15.  I 
> would like to provide users a way of customising the app (initially just 
> simple things like modifying keybindings for example, later hopefully more) 
> and as a keen Emacs user, i'm inspired by the idea of providing a 
> Schemey/Lispy interface for such extensions.  Guile looks like it'd be great 
> for this.  If i understand correctly, it'd be best if i could bundle the 
> Guile runtime as a static library with the app, and call out to it to 
> evaluate user-provided code.  I haven't thought deeply about this interface 
> yet; i thought i'd get a proof-of-concept working first. I wonder if i might 
> humbly ask for some guidance on how to get it working, because after a couple 
> of days i seem to have failed. I'm no C/threads/low-level guru, so my 
> apologies if i'm doing something very dumb.
> I had some difficulty getting my app to compile against Guile, but i 
> eventually managed to link against a version of Guile installed with Homebrew 
> (guile: stable 3.0.7 (bottled)), however when trying to boot it up i seemed 
> to run into the same issue described by Jeffrey Walton [1].  My app would 
> boot, and as soon as it hit the Guile initialisation calls, it would error 
> with "allocating JIT code buffer failed: Permission denied, jit.c:5804: 
> fatal: assertion failed".  While that person seems to imply the problem is 
> with Apple's M1 silicon, i'm actually running an older machine (2.9 GHz 
> Dual-Core Intel Core i5, macOS 11.5.2).  I then managed to get further by 
> downloading the Guile release tarball version 3.0.7 and and building with 
> `./configure --enable-jit=no`; this got me a bit further, however it still 
> didn't work: i think it is because some assumption Guile has about the thread 
> it runs on, or when it's invoked, is violated.. but i'm unsure how to find 
> out.
> What i currently have, is this snippet.  It's being called from Swift land, 
> in the `applicationDidFinishLaunching(_ aNotification: Notification)` 
> function.  As far as i can tell, that _is_ the main thread.
> ```
> #include "libguile.h"
> static void* register_functions (void* data)
> {
>    SCM test = scm_c_eval_string("(+ 3 5)");
>    int foo = scm_to_int(test);
>    printf("foo = %d\n", foo);
>    return NULL;
> }
> void run_guile() {
>    printf("hello from C, before Guile\n");
>    scm_init_guile();
>    //scm_with_guile(&register_functions, NULL); // i've tried    only having 
> this line uncommented, too, but that also causes    immediate crashes
>    //scm_shell(0, NULL);
> }
> ```
> This compiles fine, and i see the "hello from C" line printed, but then it 
> crashes.  The error seems to vary, here are some i've seen:
> 1. "Thread 1: EXC_BAD_ACCESS (code=1, address=0x0)" at line 182 of pairs.h,
> 2. "Pre-boot error; key: misc-error, args: ("%search-path" "path is not a 
> proper list: ~a" (("/usr/local/share/guile/3.0" 
> "/usr/local/share/guile/site/3.0" "/usr/local/share/guile/site" . #<program 
> 12503b140 124fc10fc>)) #f)", "Thread 1: signal SIGABRT", line 260 of throw.c
> 3. "Thread 1: EXC_BAD_ACCESS (code=1, address=0x9)", at line 585 of 
> weak-set.c.
> 4. I've also sometimes seen this one, 
> although perhaps that's indeed related to closed stdout.
> Because these errors are different all the time i guess it's some race 
> condition or threading issue?  I wonder if someone knows an avenue i can 
> attempt to use to debug what's going on? 🙏
> All the best,
> paul
> 1.

Hi Paul,

To narrow down the issue, I'd attempt a few things, in order:

1. Compile only the C code, adding a main() function, just to make sure the OS
   and the chosen Guile version and such are working fine with each other.

2. Compile pure Objective-C code, calling that run_guile() function firstly
   directly from the main() function in main.m of the Objective-C program, and
   commenting out the NSApplicationMain() call that would initialize Apple's
   application framework.

3. See if reactivating the NSApplicationMain() call causes problems.  (It should
   be called *after* the Guile initialization.)

4. See if you can use Guile's C functions from -applicationDidFinishLaunching:
   e.g. by doing: scm_c_eval_string("(begin (display 'HelloWorld) (newline))")

If that works, we now have an Objective-C + Guile application, and want to move
to using Swift instead.  This is where my Apple knowledge hits its limits 
I never used Swift. :-)

But I guess Swift should have something equivalent to the main() function of C 
Objective-C, and calling Guile initialization from there might do the trick.

If you hit a problem in step 3 or 4, then it could mean that Guile and Cocoa are
somehow incompatible as they both want to apply some magic to the C program 
initializing themselves.  I wouldn't know how to approach that issue.


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