diff options
Diffstat (limited to 'cortex-m-rt/src/lib.rs')
| -rw-r--r-- | cortex-m-rt/src/lib.rs | 30 |
1 files changed, 24 insertions, 6 deletions
diff --git a/cortex-m-rt/src/lib.rs b/cortex-m-rt/src/lib.rs index f7ecb91..d89df2a 100644 --- a/cortex-m-rt/src/lib.rs +++ b/cortex-m-rt/src/lib.rs @@ -360,13 +360,31 @@ //! } //! ``` //! -//! ## `pre_init!` +//! ## Uninitialized static variables //! -//! A user-defined function can be run at the start of the reset handler, before RAM is -//! initialized. The macro `pre_init!` can be called to set the function to be run. The function is -//! intended to perform actions that cannot wait the time it takes for RAM to be initialized, such -//! as disabling a watchdog. As the function is called before RAM is initialized, any access of -//! static variables will result in undefined behavior. +//! The `.uninit` linker section can be used to leave `static mut` variables uninitialized. One use +//! case of unitialized static variables is to avoid zeroing large statically allocated buffers (say +//! to be used as thread stacks) -- this can considerably reduce initialization time on devices that +//! operate at low frequencies. +//! +//! The only correct way to use this section is by placing `static mut` variables with type +//! [`MaybeUninit`] in it. +//! +//! [`MaybeUninit`]: https://doc.rust-lang.org/core/mem/union.MaybeUninit.html +//! +//! ``` ignore +//! use core::mem::MaybeUninit; +//! +//! const STACK_SIZE: usize = 8 * 1024; +//! const NTHREADS: usize = 4; +//! +//! #[link_section = ".uninit.STACKS"] +//! static mut STACKS: MaybeUninit<[[u8; STACK_SIZE]; NTHREADS]> = MaybeUninit::uninit(); +//! ``` +//! +//! Be very careful with the `link_section` attribute because it's easy to misuse in ways that cause +//! undefined behavior. At some point in the future we may add an attribute to safely place static +//! variables in this section. // # Developer notes // |