The purpose of this post is to document the high-level structure and the relevant interfaces that come into play on the Rust side of this.
The actual mechanism of unwinding is a totally different matter (and one that I am not qualified to speak about).
+*Note:* This post describes panicking as of [this commit](https://github.com/rust-lang/rust/commit/4007d4ef26eab44bdabc2b7574d032152264d3ad).
+Many of the interfaces described here are unstable internal details of libstd, and subject to change any time.
+
## High-level structure
When trying to figure out how panicking works by reading the code in libstd, one can easily get lost in the maze.
## Panic Runtime
-The [interface to the panic runtime](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libstd/panicking.rs#L48) (introduced by [this RFC](https://github.com/rust-lang/rfcs/blob/master/text/1513-less-unwinding.md)) is a function `__rust_start_panic(payload: usize) -> u32` that gets imported by libstd, and that is later resolved by the linker.
+The [interface to the panic runtime](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libstd/panicking.rs#L53) (introduced by [this RFC](https://github.com/rust-lang/rfcs/blob/master/text/1513-less-unwinding.md)) is a function `__rust_start_panic(payload: usize) -> u32` that gets imported by libstd, and that is later resolved by the linker.
The `usize` argument here actually is a `*mut &mut dyn core::panic::BoxMeUp` -- this is where the "payload" of the panic (the information available when it gets caught) gets passed in.
-`BoxMeUp` is an unstable internal implementation detail, but [looking at the trait](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libcore/panic.rs#L268-L271) we can see that all it really does is wrap a `dyn Any + Send`, which is the [type of the panic payload](https://doc.rust-lang.org/std/thread/type.Result.html) as returned by `catch_unwind` and `thread::spawn`.
-`BoxMeUp::box_me_up` returns a `Box<dyn Any + Send>`, but as a raw pointer (because `Box` is not available in the context where this trait is defined); `BoxMeUp::get` just borrows the contents.
+`BoxMeUp` is an unstable internal implementation detail, but [looking at the trait](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libcore/panic.rs#L268-L281) we can see that all it really does is wrap a `dyn Any + Send`, which is the [type of the panic payload](https://doc.rust-lang.org/std/thread/type.Result.html) as returned by `catch_unwind` and `thread::spawn`.
+`BoxMeUp::take_box` returns a `Box<dyn Any + Send>`, but as a raw pointer (because `Box` is not available in the context where this trait is defined); `BoxMeUp::get` just borrows the contents.
-The two implementations of this interface Rust ships with are [`libpanic_unwind`](https://github.com/rust-lang/rust/tree/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libpanic_unwind) for `-C panic=unwind` (the default on most platforms) and [`libpanic_abort`](https://github.com/rust-lang/rust/tree/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libpanic_abort) for `-C panic=abort`.
+The two implementations of this interface Rust ships with are [`libpanic_unwind`](https://github.com/rust-lang/rust/tree/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libpanic_unwind) for `-C panic=unwind` (the default on most platforms) and [`libpanic_abort`](https://github.com/rust-lang/rust/tree/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libpanic_abort) for `-C panic=abort`.
## `std::panic!`
-On top of the panic *runtime* interface, libstd implements the default Rust panic machinery in the internal [`std::panicking`](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libstd/panicking.rs) module.
+On top of the panic *runtime* interface, libstd implements the default Rust panic machinery in the internal [`std::panicking`](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libstd/panicking.rs) module.
#### `rust_panic_with_hook`
-The key function that everything passes through is [`rust_panic_with_hook`](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libstd/panicking.rs#L435-L437):
+The key function that almost everything passes through is [`rust_panic_with_hook`](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libstd/panicking.rs#L439-L441):
{% highlight rust %}
fn rust_panic_with_hook(
payload: &mut dyn BoxMeUp,
This matches `rust_panic_with_hook`'s arguments quite nicely!
`file_line_col` and `message` can be used directly for the first two elements; `payload` gets turned into `&(dyn Any + Send)` through the `BoxMeUp` interface.
-Interestingly, the *default* panic hook entirely ignores the `message`; what you actually see printed is [the `payload` downcast to `&str` or `String`](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libstd/panicking.rs#L171-L177) (whatever works).
+Interestingly, the *default* panic hook entirely ignores the `message`; what you actually see printed is [the `payload` downcast to `&str` or `String`](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libstd/panicking.rs#L176-L182) (whatever works).
Supposedly, the caller should ensure that formatting `message`, if present, gives the same result.
(And the ones we discuss below do ensure this.)
#### libstd panicking entry points
-`rust_panic_with_hook` is a private function to `std::panicking`; the module provides three separate entry points on top of this central function:
+`rust_panic_with_hook` is a private function to `std::panicking`; the module provides three entry points on top of this central function, and one that circumvents it:
-* the [default panic handler implementation](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libstd/panicking.rs#L301), backing (as we will see) panics from `core::panic!` and built-in panics (from arithmetic overflow or out-of-bounds array/slice indexing).
+* the [`begin_panic_handler`](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libstd/panicking.rs#L332), the default panic handler implementation that backs (as we will see) panics from `core::panic!` and built-in panics (from arithmetic overflow or out-of-bounds array/slice indexing).
This obtains as input a [`PanicInfo` ](https://doc.rust-lang.org/core/panic/struct.PanicInfo.html), and it has to turn that into arguments for `rust_panic_with_hook`.
Curiously, even though the components of `PanicInfo` and the arguments of `rust_panic_with_hook` match up pretty well and seem like they could just be forwarded, that is *not* what happens.
- Instead, libstd entirely *ignores* the `payload` component of the `PanicInfo`, and sets up the actual payload (passed to `rust_panic_with_hook`) such that it contains [the formatted `message`](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libstd/panicking.rs#L348).
+ Instead, libstd entirely *ignores* the `payload` component of the `PanicInfo`, and sets up the actual payload (passed to `rust_panic_with_hook`) such that it contains [the formatted `message`](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libstd/panicking.rs#L350).
In particular, this means that the panic *runtime* is irrelevant for `no_std` applications.
It only comes into play when libstd's panic handler implementation is used.
(The panic *strategy* selected via `-C panic` still has an effect as it also influences code generation.
For example, with `-C panic=abort` code can become simpler as it does not need to support unwinding.)
-* [`begin_panic_fmt`](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libstd/panicking.rs#L319), backing the [format string version of `std::panic!`](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libstd/macros.rs#L22-L23) (i.e., this is used when you pass multiple arguments to the macro).
- This basically just packages the format string arguments into a `PanicInfo` (with a [dummy payload](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libcore/panic.rs#L56)) and calls the default panic handler that we just discussed.
+* [`begin_panic_fmt`](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libstd/panicking.rs#L316), backing the [format string version of `std::panic!`](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libstd/macros.rs#L22-L23) (i.e., this is used when you pass multiple arguments to the macro).
+ This basically just packages the format string arguments into a `PanicInfo` (with a [dummy payload](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libcore/panic.rs#L56)) and calls the default panic handler that we just discussed.
-* [`begin_panic`](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libstd/panicking.rs#L388), backing the [single-argument version of `std::panic!`](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libstd/macros.rs#L16).
+* [`begin_panic`](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libstd/panicking.rs#L392), backing the [single-argument version of `std::panic!`](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libstd/macros.rs#L16).
Interestingly, this uses a very different code path than the other two entry points!
In particular, this is the only entry point that permits passing in an *arbitrary payload*.
- That payload is just [converted into a `Box<dyn Any + Send>`](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libstd/panicking.rs#L415) so that it can be passed to `rust_panic_with_hook`, and that's it.
+ That payload is just [converted into a `Box<dyn Any + Send>`](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libstd/panicking.rs#L419) so that it can be passed to `rust_panic_with_hook`, and that's it.
In particular, a panic hook that looks at the `message` field of the `PanicData` it is passed will *not* be able to see the message in a `std::panic!("do panic")`, but it *will* see the message in a `std::panic!("panic with data: {}", data)` as the latter passes through `begin_panic_fmt` instead.
That seems quite surprising. (But also note that `PanicData::message()` is not stable yet.)
+* [`rust_panic_without_hook`](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libstd/panicking.rs#L496) is the odd one out: this entry point backs [`resume_unwind`](https://doc.rust-lang.org/nightly/std/panic/fn.resume_unwind.html), and it actually does *not* call the panic hook.
+ Instead, it dispatches to the panic runtime immediately.
+ Like, `begin_panic`, it lets the caller pick an arbitrary payload.
+ Unlike `begin_panic`, the caller is responsible for boxing and unsizing the payload; `update_count_then_panic` just forwards that pretty much verbatim to the panic runtime.
+
## Panic Handler
All of the `std::panic!` machinery is really useful, but it relies on heap allocations through `Box` which is not always available.
To give libcore a way to cause panics, [panic handlers were introduced](https://github.com/rust-lang/rfcs/blob/master/text/2070-panic-implementation.md).
As we have seen, if libstd is available, it provides an implementation of that interface to wire `core::panic!` into the libstd panic machinery.
-The [interface to the panic handler](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libcore/panicking.rs#L78) is a function `fn panic(info: &core::panic::PanicInfo) -> !` that libcore imports and that is later resolved by the linker.
+The [interface to the panic handler](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libcore/panicking.rs#L78) is a function `fn panic(info: &core::panic::PanicInfo) -> !` that libcore imports and that is later resolved by the linker.
The [`PanicInfo` ](https://doc.rust-lang.org/core/panic/struct.PanicInfo.html) type is the same as for panic hooks: it contains a panic source location, a panic message, and a payload (a `dyn Any + Send`).
The panic message is represented as [`fmt::Arguments`](https://doc.rust-lang.org/std/fmt/struct.Arguments.html), i.e., a format string with its arguments that has not been formatted yet.
## `core::panic!`
-On top of the panic handler interface, libcore provides a [minimal panic API](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libcore/panicking.rs).
-The [`core::panic!`](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libcore/macros/mod.rs#L10-L26) macro creates a `fmt::Arguments` which is then [passed to the panic handler](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libcore/panicking.rs#L82).
+On top of the panic handler interface, libcore provides a [minimal panic API](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libcore/panicking.rs).
+The [`core::panic!`](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libcore/macros/mod.rs#L10-L26) macro creates a `fmt::Arguments` which is then [passed to the panic handler](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libcore/panicking.rs#L82).
No formatting happens here as that would require heap allocations; this is why `PanicInfo` contains an "uninterpreted" format string with its arguments.
-Curiously, the `payload` field of the `PanicInfo` that gets passed to the panic handler is always set to [a dummy value](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libcore/panic.rs#L56).
+Curiously, the `payload` field of the `PanicInfo` that gets passed to the panic handler is always set to [a dummy value](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libcore/panic.rs#L56).
This explains why the libstd panic handler ignores the payload (and instead constructs a new payload from the `message`), but that makes me wonder why that field is part of the panic handler API in the first place.
-Another consequence of this is that [`core::panic!("message")`](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libcore/macros/mod.rs#L15) and [`std::panic!("message")`](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libstd/macros.rs#L16) (the variants without any formatting) actually result in very different panics: the former gets turned into `fmt::Arguments`, passed through the panic handler interface, and then libstd creates a `String` payload by formatting it.
+Another consequence of this is that [`core::panic!("message")`](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libcore/macros/mod.rs#L15) and [`std::panic!("message")`](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libstd/macros.rs#L16) (the variants without any formatting) actually result in very different panics: the former gets turned into `fmt::Arguments`, passed through the panic handler interface, and then libstd creates a `String` payload by formatting it.
The latter, however, directly uses the `&str` as a payload, and the `message` field remains `None` (as already mentioned).
Some elements of the libcore panic API are lang items because the compiler inserts calls to these functions during code generation:
-* The [`panic` lang item](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libcore/panicking.rs#L39) is called when the compiler needs to raise a panic that does not require any formatting (such as arithmetic overflow); this is the same function that also backs single-argument [`core::panic!`](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libcore/macros/mod.rs#L15).
-* The [`panic_bounds_check`](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libcore/panicking.rs#L55) lang item is called on a failed array/slice bounds check.
- It calls into the same method as [`core::panic!` with formatting](https://github.com/rust-lang/rust/blob/7d761fe0462ba0f671a237d0bb35e3579b8ba0e8/src/libcore/macros/mod.rs#L21-L24).
+* The [`panic` lang item](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libcore/panicking.rs#L39) is called when the compiler needs to raise a panic that does not require any formatting (such as arithmetic overflow); this is the same function that also backs single-argument [`core::panic!`](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libcore/macros/mod.rs#L15).
+* The [`panic_bounds_check`](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libcore/panicking.rs#L55) lang item is called on a failed array/slice bounds check.
+ It calls into the same method as [`core::panic!` with formatting](https://github.com/rust-lang/rust/blob/4007d4ef26eab44bdabc2b7574d032152264d3ad/src/libcore/macros/mod.rs#L21-L24).
## Conclusion
projects / web.git / blobdiff