Declare callback delegates as using Cdecl calling convention, so it's consistent with the rest of the code and also consistent across all platforms.

Assets/Scripts/SysCalls.cs

@@ -35,6 +35,7 @@ public class SysCalls: CallbackHandler<SysCalls>
         public IntPtr SysFloatTime;
     }
     
+    [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
     private delegate void SysPrintCallback(IntPtr target, [MarshalAs(UnmanagedType.LPStr)] string message);
     
     [AOT.MonoPInvokeCallback(typeof(SysPrintCallback))]
@@ -48,6 +49,7 @@ public class SysCalls: CallbackHandler<SysCalls>
         Debug.Log(message);
     }
     
+    [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
     private delegate void SysErrorCallback(IntPtr target, [MarshalAs(UnmanagedType.LPStr)] string message);
 
     [AOT.MonoPInvokeCallback(typeof(SysErrorCallback))]
@@ -63,6 +65,7 @@ public class SysCalls: CallbackHandler<SysCalls>
         Application.Quit(1);
     }
 
+    [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
     private delegate void SysQuitCallback(IntPtr target);
     
     [AOT.MonoPInvokeCallback(typeof(SysQuitCallback))]
@@ -77,6 +80,7 @@ public class SysCalls: CallbackHandler<SysCalls>
         Application.Quit(0);
     }
 
+    [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
     private delegate double SysFloatTimeCallback(IntPtr target);
     
     [AOT.MonoPInvokeCallback(typeof(SysFloatTimeCallback))]

engine/projects/uniquake/uniquake.h

@@ -13,10 +13,10 @@ typedef struct unity_syscalls_s
 {
 	void *target;
 
-	void(__stdcall *SysPrint)(void *target, const char *msg);
-	void(__stdcall *SysError)(void *target, const char *msg);
-	void(__stdcall *SysQuit)(void *target);
-	double(__stdcall *SysFloatTime)(void *target);
+	void(*SysPrint)(void *target, const char *msg);
+	void(*SysError)(void *target, const char *msg);
+	void(*SysQuit)(void *target);
+	double(*SysFloatTime)(void *target);
 } unity_syscalls_t;
 
 extern const unity_syscalls_t *unity_syscalls;