Panic_nounwind line numbers: is it technical restriction or design decision

compiler
1

If panic going to cross extern "C" boundaries then panic_nounwind was called and process aborted, but why crash dumps not contain line number where panic was called, only line where extern "C" function was defined?

I mean with such code I got backtrace that point to line extern "C" fn c_func(abort: bool) { as source of panic:

extern "C" fn c_func(abort: bool) {
    if abort {
        panic!("something realy goes wrong");
    }
}

fn main() {
    c_func(true);
}

it is useful of course, but much more information I can get, if backtrace would contain line with panic source (panic!("something realy goes wrong");).

So exact line with panic! not included because of some technical problem, or it is by design?

2

By the time that the abort happens, the stack frame of the initial panic!() is already unwound and thus permanently lost.

1 Like
3

If you use the default panic hook with RUST_BACKTRACE=1, the original panic will be printed along with its stacktrace before unwinding starts.

For example (enable backtraces using the "..." button next to "stable")

thread 'main' panicked at src/main.rs:3:9:
something realy goes wrong
stack backtrace:
   0: __rustc::rust_begin_unwind
             at /rustc/29483883eed69d5fb4db01964cdf2af4d86e9cb2/library/std/src/panicking.rs:697:5
   1: core::panicking::panic_fmt
             at /rustc/29483883eed69d5fb4db01964cdf2af4d86e9cb2/library/core/src/panicking.rs:75:14
   2: playground::c_func
             at ./src/main.rs:3:9
   3: playground::main
             at ./src/main.rs:8:5
   4: core::ops::function::FnOnce::call_once
             at ./.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

thread 'main' panicked at library/core/src/panicking.rs:225:5:
panic in a function that cannot unwind
stack backtrace:
[...]
  18:     0x60ebd76aa185 - core::panicking::panic_cannot_unwind::h4320389209385803
                               at /rustc/29483883eed69d5fb4db01964cdf2af4d86e9cb2/library/core/src/panicking.rs:330:5
  19:     0x60ebd76aa920 - playground::c_func::h0a787a9bd077b646
                               at /playground/src/main.rs:1:1
  20:     0x60ebd76aa94b - playground::main::h420381be4f74d8d6
                               at /playground/src/main.rs:8:5
[...]