Use the new @extern field, .is_dll_import, to link the memory allocation functions from a shared library with the -msvc ABI

Author: kcbanner

src/memory.zig

@@ -1,4 +1,5 @@
 const std = @import("std");
+const options = @import("zmesh_options");
 
 pub fn init(alloc: std.mem.Allocator) void {
     std.debug.assert(mem_allocator == null and mem_allocations == null);
@@ -7,10 +8,27 @@ pub fn init(alloc: std.mem.Allocator) void {
     mem_allocations = std.AutoHashMap(usize, usize).init(alloc);
     mem_allocations.?.ensureTotalCapacity(32) catch unreachable;
 
-    zmeshMallocPtr = zmeshMalloc;
-    zmeshCallocPtr = zmeshCalloc;
-    zmeshReallocPtr = zmeshRealloc;
-    zmeshFreePtr = zmeshFree;
+    const zmeshMallocPtr = @extern(*?*const fn (size: usize) callconv(.C) ?*anyopaque, .{
+        .name = "zmeshMallocPtr",
+        .is_dll_import = options.shared,
+    });
+    const zmeshCallocPtr = @extern(*?*const fn (num: usize, size: usize) callconv(.C) ?*anyopaque, .{
+        .name = "zmeshCallocPtr",
+        .is_dll_import = options.shared,
+    });
+    const zmeshReallocPtr = @extern(*?*const fn (ptr: ?*anyopaque, size: usize) callconv(.C) ?*anyopaque, .{
+        .name = "zmeshReallocPtr",
+        .is_dll_import = options.shared,
+    });
+    const zmeshFreePtr = @extern(*?*const fn (maybe_ptr: ?*anyopaque) callconv(.C) void, .{
+        .name = "zmeshFreePtr",
+        .is_dll_import = options.shared,
+    });
+
+    zmeshMallocPtr.* = zmeshMalloc;
+    zmeshCallocPtr.* = zmeshCalloc;
+    zmeshReallocPtr.* = zmeshRealloc;
+    zmeshFreePtr.* = zmeshFree;
     meshopt_setAllocator(zmeshMalloc, zmeshFree);
 }
 
@@ -33,8 +51,6 @@ var mem_allocations: ?std.AutoHashMap(usize, usize) = null;
 var mem_mutex: std.Thread.Mutex = .{};
 const mem_alignment = 16;
 
-extern var zmeshMallocPtr: ?*const fn (size: usize) callconv(.C) ?*anyopaque;
-
 pub fn zmeshMalloc(size: usize) callconv(.C) ?*anyopaque {
     mem_mutex.lock();
     defer mem_mutex.unlock();
@@ -50,8 +66,6 @@ pub fn zmeshMalloc(size: usize) callconv(.C) ?*anyopaque {
     return mem.ptr;
 }
 
-extern var zmeshCallocPtr: ?*const fn (num: usize, size: usize) callconv(.C) ?*anyopaque;
-
 fn zmeshCalloc(num: usize, size: usize) callconv(.C) ?*anyopaque {
     const ptr = zmeshMalloc(num * size);
     if (ptr != null) {
@@ -66,8 +80,6 @@ pub fn zmeshAllocUser(user: ?*anyopaque, size: usize) callconv(.C) ?*anyopaque {
     return zmeshMalloc(size);
 }
 
-extern var zmeshReallocPtr: ?*const fn (ptr: ?*anyopaque, size: usize) callconv(.C) ?*anyopaque;
-
 fn zmeshRealloc(ptr: ?*anyopaque, size: usize) callconv(.C) ?*anyopaque {
     mem_mutex.lock();
     defer mem_mutex.unlock();
@@ -91,8 +103,6 @@ fn zmeshRealloc(ptr: ?*anyopaque, size: usize) callconv(.C) ?*anyopaque {
     return mem.ptr;
 }
 
-extern var zmeshFreePtr: ?*const fn (maybe_ptr: ?*anyopaque) callconv(.C) void;
-
 fn zmeshFree(maybe_ptr: ?*anyopaque) callconv(.C) void {
     if (maybe_ptr) |ptr| {
         mem_mutex.lock();