# The Rust runtime

This section documents features that define some aspects of the Rust runtime.

## The `panic_handler` attribute

The `panic_handler` attribute can only be applied to a function with signature `fn(&PanicInfo) -> !`. The function marked with this attribute defines the behavior of panics. The `PanicInfo` struct contains information about the location of the panic. There must be a single `panic_handler` function in the dependency graph of a binary, dylib or cdylib crate.

Below is shown a `panic_handler` function that logs the panic message and then halts the thread.

    #![no_std]

    use core::fmt::{self, Write};
    use core::panic::PanicInfo;

    struct Sink {
        // ..
       _0: (),
    }

    impl Sink {
        fn new() -> Sink { Sink { _0: () }}
    }

    impl fmt::Write for Sink {
        fn write_str(&mut self, _: &str) -> fmt::Result { Ok(()) }
    }

    #[panic_handler]
    fn panic(info: &PanicInfo) -> ! {
        let mut sink = Sink::new();

        // logs "panicked at '$reason', src/main.rs:27:4" to some `sink`
        let _ = writeln!(sink, "{}", info);

        loop {}
    }

### Standard behavior

The standard library provides an implementation of `panic_handler` that defaults to unwinding the stack but that can be changed to abort the process. The standard library's panic behavior can be modified at runtime with the set_hook function.

## The `global_allocator` attribute

The `global_allocator` attribute is used on a static item implementing the `GlobalAlloc` trait to set the global allocator.

## The `windows_subsystem` attribute

The `windows_subsystem` attribute may be applied at the crate level to set the subsystem when linking on a Windows target. It uses the MetaNameValueStr syntax to specify the subsystem with a value of either `console` or `windows`. This attribute is ignored on non-Windows targets, and for non-`bin` crate types.

    #![allow(unused_variables)]
    #![windows_subsystem = "windows"]
    fn main() {
    }