r[runtime]
# The Rust runtime

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

<!-- template:attributes -->
r[runtime.global_allocator]
## The `global_allocator` attribute

r[runtime.global_allocator.intro]
The *`global_allocator` [attribute][attributes]* selects a [memory allocator][std::alloc].

> [!EXAMPLE]
> ```rust
> use core::alloc::{GlobalAlloc, Layout};
> use std::alloc::System;
>
> struct MyAllocator;
>
> unsafe impl GlobalAlloc for MyAllocator {
>     unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
>         unsafe { System.alloc(layout) }
>     }
>     unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
>         unsafe { System.dealloc(ptr, layout) }
>     }
> }
>
> #[global_allocator]
> static GLOBAL: MyAllocator = MyAllocator;
> ```

r[runtime.global_allocator.syntax]
The `global_allocator` attribute uses the [MetaWord] syntax.

r[runtime.global_allocator.allowed-positions]
The `global_allocator` attribute may only be applied to a [static item] whose type implements the [`GlobalAlloc`] trait.

r[runtime.global_allocator.duplicates]
The `global_allocator` attribute may only be used once on an item.

r[runtime.global_allocator.single]
The `global_allocator` attribute may only be used once in the crate graph.

r[runtime.global_allocator.stdlib]
The `global_allocator` attribute is exported from the [standard library prelude][core::prelude::v1].

<!-- template:attributes -->
r[runtime.windows_subsystem]
## The `windows_subsystem` attribute

r[runtime.windows_subsystem.intro]
The *`windows_subsystem` [attribute][attributes]* sets the [subsystem] when linking on a Windows target.

> [!EXAMPLE]
> ```rust
> #![windows_subsystem = "windows"]
> ```

r[runtime.windows_subsystem.syntax]
The `windows_subsystem` attribute uses the [MetaNameValueStr] syntax. Accepted values are `"console"` and `"windows"`.

r[runtime.windows_subsystem.allowed-positions]
The `windows_subsystem` attribute may only be applied to the crate root.

r[runtime.windows_subsystem.duplicates]
Only the first use of `windows_subsystem` has effect.

> [!NOTE]
> `rustc` lints against any use following the first. This may become an error in the future.

r[runtime.windows_subsystem.ignored]
The `windows_subsystem` attribute is ignored on non-Windows targets and non-`bin` [crate types].

r[runtime.windows_subsystem.console]
The `"console"` subsystem is the default. If a console process is run from an existing console then it will be attached to that console; otherwise a new console window will be created.

r[runtime.windows_subsystem.windows]
The `"windows"` subsystem will run detached from any existing console.

> [!NOTE]
> The `"windows"` subsystem is commonly used by GUI applications that do not want to display a console window on startup.

[`GlobalAlloc`]: alloc::alloc::GlobalAlloc
[crate types]: linkage.md
[static item]: items/static-items.md
[subsystem]: https://msdn.microsoft.com/en-us/library/fcc1zstk.aspx