diff --git a/crates/mun_abi/src/autogen_impl.rs b/crates/mun_abi/src/autogen_impl.rs
index 66ad3f220..cde280b1e 100644
--- a/crates/mun_abi/src/autogen_impl.rs
+++ b/crates/mun_abi/src/autogen_impl.rs
@@ -108,6 +108,20 @@ impl fmt::Display for FunctionSignature {
     }
 }
 
+impl PartialEq for FunctionSignature {
+    fn eq(&self, other: &Self) -> bool {
+        self.return_type() == other.return_type()
+            && self.arg_types().len() == other.arg_types().len()
+            && self
+                .arg_types()
+                .iter()
+                .zip(other.arg_types().iter())
+                .all(|(a, b)| PartialEq::eq(a, b))
+    }
+}
+
+impl Eq for FunctionSignature {}
+
 unsafe impl Send for FunctionSignature {}
 unsafe impl Sync for FunctionSignature {}
 
diff --git a/crates/mun_runtime/src/assembly.rs b/crates/mun_runtime/src/assembly.rs
index 7e3f135fa..8fe8acf09 100644
--- a/crates/mun_runtime/src/assembly.rs
+++ b/crates/mun_runtime/src/assembly.rs
@@ -65,22 +65,33 @@ impl Assembly {
             .map(|f| f.prototype.name())
             .collect();
 
-        for (fn_ptr, fn_sig) in self.info.dispatch_table.iter() {
+        for (fn_ptr, fn_prototype) in self.info.dispatch_table.iter() {
             // Only take signatures into account that do *not* yet have a function pointer assigned
             // by the compiler.
             if !fn_ptr.is_null() {
                 continue;
             }
 
-            let fn_name = fn_sig.name();
-            // ASSUMPTION: If we removed a function from the `Assembly`, we expect the compiler to
-            // have failed for any old internal references to it. Therefore it is safe to leave the
-            // `old_assembly`'s functions in the `runtime_dispatch_table` as we perform this check.
-            if !fn_names.contains(fn_name) && runtime_dispatch_table.get_fn(fn_name).is_none() {
-                return Err(io::Error::new(
-                    io::ErrorKind::NotFound,
-                    format!("Failed to link: function `{}` is missing.", fn_name),
-                ));
+            // Ensure that the required function is in the runtime dispatch table and that its signature
+            // is the same.
+            match runtime_dispatch_table.get_fn(fn_prototype.name()) {
+                Some(fn_definition) => {
+                    if fn_prototype.signature != fn_definition.prototype.signature {
+                        return Err(io::Error::new(
+                            io::ErrorKind::NotFound,
+                            format!("Failed to link: function '{}' is missing. A function with the same name does exist, but the signatures do not match (expected: {}, found: {}).", fn_prototype.name(), fn_prototype, fn_definition.prototype),
+                        ));
+                    }
+                }
+                None => {
+                    return Err(io::Error::new(
+                        io::ErrorKind::NotFound,
+                        format!(
+                            "Failed to link: function `{}` is missing.",
+                            fn_prototype.name()
+                        ),
+                    ))
+                }
             }
         }
 
@@ -102,19 +113,7 @@ impl Assembly {
                     .get(fn_definition.prototype.name())
                     .expect("The dependency must exist after the previous check.");
 
-                // TODO: This is a hack
-                if fn_definition.prototype.signature.return_type()
-                    != fn_prototype.signature.return_type()
-                    || fn_definition.prototype.signature.arg_types().len()
-                        != fn_prototype.signature.arg_types().len()
-                    || !fn_definition
-                        .prototype
-                        .signature
-                        .arg_types()
-                        .iter()
-                        .zip(fn_prototype.signature.arg_types().iter())
-                        .all(|(a, b)| PartialEq::eq(a, b))
-                {
+                if fn_prototype.signature != fn_definition.prototype.signature {
                     return Err(io::Error::new(
                         io::ErrorKind::NotFound,
                         format!("Failed to link: function '{}' is missing. A function with the same name does exist, but the signatures do not match (expected: {}, found: {}).", fn_prototype.name(), fn_prototype, fn_definition.prototype),
diff --git a/crates/mun_runtime/tests/marshalling.rs b/crates/mun_runtime/tests/marshalling.rs
index c04b7e1d1..a2f25a8d1 100644
--- a/crates/mun_runtime/tests/marshalling.rs
+++ b/crates/mun_runtime/tests/marshalling.rs
@@ -505,6 +505,24 @@ fn extern_fn_missing() {
     assert_invoke_eq!(isize, 16, driver, "main");
 }
 
+#[test]
+fn extern_fn_invalid_signature() {
+    extern "C" fn add_int() -> i32 {
+        0
+    }
+
+    let result = TestDriver::new(
+        r#"
+    extern fn add(a: i32, b: i32) -> i32;
+    pub fn main() -> i32 { add(3,4) }
+    "#,
+    )
+    .insert_fn("add", add_int as extern "C" fn() -> i32)
+    .spawn();
+
+    assert!(result.is_err());
+}
+
 #[test]
 #[should_panic]
 fn extern_fn_invalid_sig() {
@@ -596,7 +614,7 @@ fn test_primitive_types() {
 
 #[test]
 fn can_add_external_without_return() {
-    extern "C" fn foo(a: i64) {
+    extern "C" fn foo(a: i32) {
         println!("{}", a);
     }
 
@@ -606,7 +624,7 @@ fn can_add_external_without_return() {
     pub fn main(){ foo(3); }
     "#,
     )
-    .insert_fn("foo", foo as extern "C" fn(i64) -> ());
+    .insert_fn("foo", foo as extern "C" fn(i32) -> ());
     let _: () = invoke_fn!(driver.runtime_mut(), "main").unwrap();
 }
 
diff --git a/crates/mun_runtime_capi/ffi b/crates/mun_runtime_capi/ffi
index 6534434b7..695326aaf 160000
--- a/crates/mun_runtime_capi/ffi
+++ b/crates/mun_runtime_capi/ffi
@@ -1 +1 @@
-Subproject commit 6534434b7dfdbfac1fc766736094a7419e6fd890
+Subproject commit 695326aaf2e7997c4fe0f2601de63e65b303f5d4
diff --git a/crates/mun_runtime_capi/src/lib.rs b/crates/mun_runtime_capi/src/lib.rs
index 9ef5d4f4f..f5ac3da6c 100644
--- a/crates/mun_runtime_capi/src/lib.rs
+++ b/crates/mun_runtime_capi/src/lib.rs
@@ -17,7 +17,7 @@ use std::{os::raw::c_char, time::Duration};
 use crate::error::ErrorHandle;
 use crate::hub::HUB;
 use failure::err_msg;
-use runtime::{Runtime, RuntimeOptions};
+use runtime::Runtime;
 
 pub(crate) type Token = usize;
 
@@ -36,6 +36,40 @@ pub trait TypedHandle {
 #[derive(Clone, Copy)]
 pub struct RuntimeHandle(*mut c_void);
 
+/// Options required to construct a [`RuntimeHandle`] through [`mun_runtime_create`]
+///
+/// # Safety
+///
+/// This struct contains raw pointers as parameters. Passing pointers to invalid data, will lead to
+/// undefined behavior.
+#[repr(C)]
+#[derive(Clone, Copy)]
+pub struct RuntimeOptions {
+    /// The interval at which changes to the disk are detected. `0` will initialize this value to
+    /// default.
+    pub delay_ms: u32,
+
+    /// Function definitions that should be inserted in the runtime before a mun library is loaded.
+    /// This is useful to initialize `extern` functions used in a mun library.
+    ///
+    /// If the [`num_functions`] fields is non-zero this field must contain a pointer to an array
+    /// of [`abi::FunctionDefinition`]s.
+    pub functions: *const abi::FunctionDefinition,
+
+    /// The number of functions in the [`functions`] array.
+    pub num_functions: u32,
+}
+
+impl Default for RuntimeOptions {
+    fn default() -> Self {
+        RuntimeOptions {
+            delay_ms: 0,
+            functions: std::ptr::null(),
+            num_functions: 0,
+        }
+    }
+}
+
 /// Constructs a new runtime that loads the library at `library_path` and its dependencies. If
 /// successful, the runtime `handle` is set, otherwise a non-zero error handle is returned.
 ///
@@ -51,6 +85,7 @@ pub struct RuntimeHandle(*mut c_void);
 #[no_mangle]
 pub unsafe extern "C" fn mun_runtime_create(
     library_path: *const c_char,
+    options: RuntimeOptions,
     handle: *mut RuntimeHandle,
 ) -> ErrorHandle {
     if library_path.is_null() {
@@ -59,6 +94,12 @@ pub unsafe extern "C" fn mun_runtime_create(
             .register(err_msg("Invalid argument: 'library_path' is null pointer."));
     }
 
+    if options.num_functions > 0 && options.functions.is_null() {
+        return HUB
+            .errors
+            .register(err_msg("Invalid argument: 'functions' is null pointer."));
+    }
+
     let library_path = match CStr::from_ptr(library_path).to_str() {
         Ok(path) => path,
         Err(_) => {
@@ -77,10 +118,29 @@ pub unsafe extern "C" fn mun_runtime_create(
         }
     };
 
-    let runtime_options = RuntimeOptions {
+    let delay_ms = if options.delay_ms > 0 {
+        options.delay_ms
+    } else {
+        10
+    };
+
+    let user_functions =
+        std::slice::from_raw_parts(options.functions, options.num_functions as usize)
+            .iter()
+            .map(|def| {
+                abi::FunctionDefinitionStorage::new_function(
+                    def.prototype.name(),
+                    def.prototype.signature.arg_types(),
+                    def.prototype.signature.return_type(),
+                    def.fn_ptr,
+                )
+            })
+            .collect();
+
+    let runtime_options = runtime::RuntimeOptions {
         library_path: library_path.into(),
-        delay: Duration::from_millis(10),
-        user_functions: Default::default(),
+        delay: Duration::from_millis(delay_ms.into()),
+        user_functions,
     };
 
     let runtime = match Runtime::new(runtime_options) {
diff --git a/crates/mun_runtime_capi/src/tests.rs b/crates/mun_runtime_capi/src/tests.rs
index a09b4cbe2..b82b43e66 100644
--- a/crates/mun_runtime_capi/src/tests.rs
+++ b/crates/mun_runtime_capi/src/tests.rs
@@ -53,7 +53,13 @@ fn make_runtime(lib_path: &Path) -> RuntimeHandle {
     let lib_path = CString::new(lib_path).unwrap();
 
     let mut handle = RuntimeHandle(ptr::null_mut());
-    let error = unsafe { mun_runtime_create(lib_path.as_ptr(), &mut handle as *mut _) };
+    let error = unsafe {
+        mun_runtime_create(
+            lib_path.as_ptr(),
+            RuntimeOptions::default(),
+            &mut handle as *mut _,
+        )
+    };
     assert_eq!(error.token(), 0, "Failed to create runtime");
     handle
 }
@@ -95,7 +101,8 @@ test_invalid_runtime!(
 
 #[test]
 fn test_runtime_create_invalid_lib_path() {
-    let handle = unsafe { mun_runtime_create(ptr::null(), ptr::null_mut()) };
+    let handle =
+        unsafe { mun_runtime_create(ptr::null(), RuntimeOptions::default(), ptr::null_mut()) };
     assert_ne!(handle.token(), 0);
 
     let message = unsafe { CStr::from_ptr(mun_error_message(handle)) };
@@ -111,8 +118,13 @@ fn test_runtime_create_invalid_lib_path() {
 fn test_runtime_create_invalid_lib_path_encoding() {
     let invalid_encoding = ['�', '\0'];
 
-    let handle =
-        unsafe { mun_runtime_create(invalid_encoding.as_ptr() as *const _, ptr::null_mut()) };
+    let handle = unsafe {
+        mun_runtime_create(
+            invalid_encoding.as_ptr() as *const _,
+            RuntimeOptions::default(),
+            ptr::null_mut(),
+        )
+    };
     assert_ne!(handle.token(), 0);
 
     let message = unsafe { CStr::from_ptr(mun_error_message(handle)) };
@@ -128,7 +140,13 @@ fn test_runtime_create_invalid_lib_path_encoding() {
 fn test_runtime_create_invalid_handle() {
     let lib_path = CString::new("some/path").expect("Invalid library path");
 
-    let handle = unsafe { mun_runtime_create(lib_path.into_raw(), ptr::null_mut()) };
+    let handle = unsafe {
+        mun_runtime_create(
+            lib_path.into_raw(),
+            RuntimeOptions::default(),
+            ptr::null_mut(),
+        )
+    };
     assert_ne!(handle.token(), 0);
 
     let message = unsafe { CStr::from_ptr(mun_error_message(handle)) };