`extern "C"` tells Rust to use C calling conventions instead of Rust’s. A calling convention governs details such as how function arguments are laid out in memory. This matters when code outside ordinary Rust execution must call a function: if the caller and function disagree, the call may fail. section 11.2.3
A kernel entry point
`#[no_mangle] pub extern "C" fn _start() -> !` defines an entry point for a small operating-system kernel. `extern "C"` selects the calling convention; the separate `#[no_mangle]` attribute prevents Rust from changing the function’s symbol name. Both details matter to booting the kernel. section 11.2.3
Listing 11.4 Creating an OS kernel that paints a block of color
1 #![no_std] #1
2 #![no_main] #1
3 #![feature(core_intrinsics)] #2
4
5 use core::intrinsics; #2
6 use core::panic::PanicInfo; #3
7
8 #[panic_handler]
9 #[no_mangle]
10 pub fn panic(_info: &PanicInfo) -> ! {
11 intrinsics::abort(); #4
12 }
13
14 #[no_mangle]
15 pub extern "C" fn _start() -> ! {
16 let framebuffer = 0xb8000 as *mut u8;
17
18 unsafe {
19 framebuffer
20 .offset(1) #5
21 .write_volatile(0x30); #6
22 }
23
24 loop {}
25 }
See `_start` in a working kernel that writes to the display’s frame buffer. section 11.2.3
`extern "C"` can also surround declarations of functions supplied from elsewhere. In the LLVM intrinsic declarations, `#[link_name]` identifies each function for the linker. Rust knows their declared type signatures, but calls to them must occur in an `unsafe` block. section 12.9.2
Listing 12.15 Declaring the LLVM intrinsic functions within listing 12.17
extern "C" {
#[link_name = "llvm.eh.sjlj.setjmp"] #1
pub fn setjmp(_: *mut i8) -> i32; #2
#[link_name = "llvm.eh.sjlj.longjmp"] #1
pub fn longjmp(_: *mut i8);
}
Compare the block form of `extern "C"` with the kernel’s single-function form. section 12.9.2
