diff --git a/sim/mcuboot-sys/src/api.rs b/sim/mcuboot-sys/src/api.rs
index db8564d..7014d5c 100644
--- a/sim/mcuboot-sys/src/api.rs
+++ b/sim/mcuboot-sys/src/api.rs
@@ -189,42 +189,42 @@
 // This isn't meant to call directly, but by a wrapper.
 
 #[no_mangle]
-pub extern fn sim_get_flash_areas() -> *const CAreaDesc {
+pub extern "C" fn sim_get_flash_areas() -> *const CAreaDesc {
     THREAD_CTX.with(|ctx| {
         ctx.borrow().flash_areas.ptr
     })
 }
 
 #[no_mangle]
-pub extern fn sim_set_flash_areas(areas: *const CAreaDesc) {
+pub extern "C" fn sim_set_flash_areas(areas: *const CAreaDesc) {
     THREAD_CTX.with(|ctx| {
         ctx.borrow_mut().flash_areas.ptr = areas;
     });
 }
 
 #[no_mangle]
-pub extern fn sim_reset_flash_areas() {
+pub extern "C" fn sim_reset_flash_areas() {
     THREAD_CTX.with(|ctx| {
         ctx.borrow_mut().flash_areas.ptr = ptr::null();
     });
 }
 
 #[no_mangle]
-pub extern fn sim_get_context() -> *const CSimContext {
+pub extern "C" fn sim_get_context() -> *const CSimContext {
     SIM_CTX.with(|ctx| {
         ctx.borrow().ptr
     })
 }
 
 #[no_mangle]
-pub extern fn sim_set_context(ptr: *const CSimContext) {
+pub extern "C" fn sim_set_context(ptr: *const CSimContext) {
     SIM_CTX.with(|ctx| {
         ctx.borrow_mut().ptr = ptr;
     });
 }
 
 #[no_mangle]
-pub extern fn sim_reset_context() {
+pub extern "C" fn sim_reset_context() {
     SIM_CTX.with(|ctx| {
         ctx.borrow_mut().ptr = ptr::null();
     });
@@ -257,7 +257,7 @@
 }
 
 #[no_mangle]
-pub extern fn sim_flash_erase(dev_id: u8, offset: u32, size: u32) -> libc::c_int {
+pub extern "C" fn sim_flash_erase(dev_id: u8, offset: u32, size: u32) -> libc::c_int {
     let mut rc: libc::c_int = -19;
     THREAD_CTX.with(|ctx| {
         if let Some(flash) = ctx.borrow().flash_map.get(&dev_id) {
@@ -269,7 +269,7 @@
 }
 
 #[no_mangle]
-pub extern fn sim_flash_read(dev_id: u8, offset: u32, dest: *mut u8, size: u32) -> libc::c_int {
+pub extern "C" fn sim_flash_read(dev_id: u8, offset: u32, dest: *mut u8, size: u32) -> libc::c_int {
     let mut rc: libc::c_int = -19;
     THREAD_CTX.with(|ctx| {
         if let Some(flash) = ctx.borrow().flash_map.get(&dev_id) {
@@ -282,7 +282,7 @@
 }
 
 #[no_mangle]
-pub extern fn sim_flash_write(dev_id: u8, offset: u32, src: *const u8, size: u32) -> libc::c_int {
+pub extern "C" fn sim_flash_write(dev_id: u8, offset: u32, src: *const u8, size: u32) -> libc::c_int {
     let mut rc: libc::c_int = -19;
     THREAD_CTX.with(|ctx| {
         if let Some(flash) = ctx.borrow().flash_map.get(&dev_id) {
@@ -295,14 +295,14 @@
 }
 
 #[no_mangle]
-pub extern fn sim_flash_align(id: u8) -> u32 {
+pub extern "C" fn sim_flash_align(id: u8) -> u32 {
     THREAD_CTX.with(|ctx| {
         ctx.borrow().flash_params.get(&id).unwrap().align
     })
 }
 
 #[no_mangle]
-pub extern fn sim_flash_erased_val(id: u8) -> u8 {
+pub extern "C" fn sim_flash_erased_val(id: u8) -> u8 {
     THREAD_CTX.with(|ctx| {
         ctx.borrow().flash_params.get(&id).unwrap().erased_val
     })
@@ -325,7 +325,7 @@
 /// or
 ///     RUST_LOG=bootsim=info cargo run --release runall
 #[no_mangle]
-pub extern fn sim_log_enabled(level: libc::c_int) -> libc::c_int {
+pub extern "C" fn sim_log_enabled(level: libc::c_int) -> libc::c_int {
     let res = match level {
         1 => log_enabled!(Level::Error),
         2 => log_enabled!(Level::Warn),