How does madsim override libc functions?

[xxchan]

I only noticed code like this. How does it work? Is there any magic? 👀

madsim/madsim/src/sim/time/system_time.rs

Lines 3 to 6 in 0d6a86a

	/// Override the libc `gettimeofday` function. For `SystemTime` on macOS.
	#[no_mangle]
	#[inline(never)]
	unsafe extern "C" fn gettimeofday(tp: *mut libc::timeval, tz: *mut libc::c_void) -> libc::c_int {

[xxchan]

Oh! Is it sth like "strong symbol" vs "weak symbol"?

[wangrunji0408]

Right! The strong symbol we defined would override the weak symbol defined in libc.

[xxchan]

And does this mean that adding madsim to the dependency will immediately override it? (Does this introduce fixed overhead? Or is it only replaced inside madsim crate?) 🤔 (Oh, I think it will replace when 1. adding madsim as dependency 2. program is compiled with RUSTFLAGS="--cfg madsim")

Another question is what will happen if I have the same override in my program? 😄

(I might need to relearn how linking works...)

[wangrunji0408]

Another question is what will happen if I have the same override in my program?

If there are multiple strong symbols, the linker will raise an error. 😄