Constant EXIT_CODE_WAIT_FAILED 

pub(crate) const EXIT_CODE_WAIT_FAILED: i32 = -3;

The exit code of a process whose exit status the blocking wait, used without pidfds, could not get, for example because it got reaped already. It is the same as EXIT_CODE_TIMED_OUT for compatibility, the pidfd wait reports EXIT_CODE_UNKNOWN instead.