export fn and finch build --shared: libraries a C program loads (game scripts)

export fn name(...) is a C function by that name (numbers, bool, char, pointers in and out).
--shared builds lib<name>.so / <name>.dll with no main. finch_module_on_fatal(handler) lets the
host take runtime errors over (longjmp out) instead of the process exiting.
tests/shared.sh: a C host loads one, calls it and survives an index error.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/shared.sh

@@ -0,0 +1,17 @@
+#!/bin/sh
+# finch build --shared: a library a C program loads, calls by name, and that hands runtime
+# errors to the host instead of exiting.
+set -e
+cd "$(dirname "$0")/shared"
+FINCH=${FINCH:-../../build/finch}
+tmp=$(mktemp -d)
+trap 'rm -rf "$tmp"' EXIT
+"$FINCH" build mod.fch --shared -o "$tmp/libmod.so"
+${CC:-cc} host.c -rdynamic -o "$tmp/host" -ldl
+out=$("$tmp/host" "$tmp/libmod.so")
+expected='add 42
+even 1 0
+host_print: hello Rakya
+caught: mod.fch:17: runtime error: index 7 is out of range (the length is 3)
+still alive'
+if [ "$out" = "$expected" ]; then echo "shared library: passed"; else printf 'shared library: FAILED\n%s\n' "$out"; exit 1; fi

tests/shared/host.c

@@ -0,0 +1,17 @@
+#include <dlfcn.h>
+#include <setjmp.h>
+#include <stdio.h>
+static jmp_buf jb;
+void host_print(const char *s) { printf("host_print: %s\n", s); }
+static void onFatal(const char *m) { printf("caught: %s\n", m); longjmp(jb, 1); }
+int main(int argc, char **argv) {
+    void *h = dlopen(argc > 1 ? argv[1] : "./libmod.so", RTLD_NOW | RTLD_LOCAL);
+    if (!h) { printf("%s\n", dlerror()); return 1; }
+    ((void (*)(void (*)(const char *)))dlsym(h, "finch_module_on_fatal"))(onFatal);
+    printf("add %ld\n", ((long (*)(long, long))dlsym(h, "add"))(2, 40));
+    printf("even %d %d\n", ((_Bool (*)(long))dlsym(h, "is_even"))(4), ((_Bool (*)(long))dlsym(h, "is_even"))(5));
+    ((void (*)(const char *))dlsym(h, "hello"))("Rakya");
+    if (!setjmp(jb)) ((long (*)(long))dlsym(h, "crash"))(7);
+    printf("still alive\n");
+    return 0;
+}

tests/shared/host.h

@@ -0,0 +1 @@
+void host_print(const char *s);

tests/shared/mod.fch

@@ -0,0 +1,22 @@
+import "host.h"
+
+fn greet(str who) -> str {
+    return "hello " + who
+}
+
+export fn add(int a, int b) -> int {
+    return a + b
+}
+
+export fn hello(ptr[char] name) {
+    host_print(greet(str(name)))
+}
+
+export fn crash(int i) -> int {
+    xs := [1, 2, 3]
+    return xs[i]
+}
+
+export fn is_even(int n) -> bool {
+    return n % 2 == 0
+}