src/libstd/sys_common/thread_local.rs

   1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! OS-based thread local storage
  12 //!
  13 //! This module provides an implementation of OS-based thread local storage,
  14 //! using the native OS-provided facilities (think `TlsAlloc` or
  15 //! `pthread_setspecific`). The interface of this differs from the other types
  16 //! of thread-local-storage provided in this crate in that OS-based TLS can only
  17 //! get/set pointers,
  18 //!
  19 //! This module also provides two flavors of TLS. One is intended for static
  20 //! initialization, and does not contain a `Drop` implementation to deallocate
  21 //! the OS-TLS key. The other is a type which does implement `Drop` and hence
  22 //! has a safe interface.
  23 //!
  24 //! # Usage
  25 //!
  26 //! This module should likely not be used directly unless other primitives are
  27 //! being built on. types such as `thread_local::spawn::Key` are likely much
  28 //! more useful in practice than this OS-based version which likely requires
  29 //! unsafe code to interoperate with.
  30 //!
  31 //! # Examples
  32 //!
  33 //! Using a dynamically allocated TLS key. Note that this key can be shared
  34 //! among many threads via an `Arc`.
  35 //!
  36 //! ```rust,ignore
  37 //! let key = Key::new(None);
  38 //! assert!(key.get().is_null());
  39 //! key.set(1 as *mut u8);
  40 //! assert!(!key.get().is_null());
  41 //!
  42 //! drop(key); // deallocate this TLS slot.
  43 //! ```
  44 //!
  45 //! Sometimes a statically allocated key is either required or easier to work
  46 //! with, however.
  47 //!
  48 //! ```rust,ignore
  49 //! static KEY: StaticKey = INIT;
  50 //!
  51 //! unsafe {
  52 //!     assert!(KEY.get().is_null());
  53 //!     KEY.set(1 as *mut u8);
  54 //! }
  55 //! ```
  56
  57 #![allow(non_camel_case_types)]
  58 #![unstable(feature = "thread_local_internals", issue = "0")]
  59 #![allow(dead_code)] // sys isn't exported yet
  60
  61 use sync::atomic::{self, AtomicUsize, Ordering};
  62
  63 use sys::thread_local as imp;
  64
  65 /// A type for TLS keys that are statically allocated.
  66 ///
  67 /// This type is entirely `unsafe` to use as it does not protect against
  68 /// use-after-deallocation or use-during-deallocation.
  69 ///
  70 /// The actual OS-TLS key is lazily allocated when this is used for the first
  71 /// time. The key is also deallocated when the Rust runtime exits or `destroy`
  72 /// is called, whichever comes first.
  73 ///
  74 /// # Examples
  75 ///
  76 /// ```ignore
  77 /// use tls::os::{StaticKey, INIT};
  78 ///
  79 /// static KEY: StaticKey = INIT;
  80 ///
  81 /// unsafe {
  82 ///     assert!(KEY.get().is_null());
  83 ///     KEY.set(1 as *mut u8);
  84 /// }
  85 /// ```
  86 pub struct StaticKey {
  87     /// Inner static TLS key (internals).
  88     key: AtomicUsize,
  89     /// Destructor for the TLS value.
  90     ///
  91     /// See `Key::new` for information about when the destructor runs and how
  92     /// it runs.
  93     dtor: Option<unsafe extern fn(*mut u8)>,
  94 }
  95
  96 /// A type for a safely managed OS-based TLS slot.
  97 ///
  98 /// This type allocates an OS TLS key when it is initialized and will deallocate
  99 /// the key when it falls out of scope. When compared with `StaticKey`, this
 100 /// type is entirely safe to use.
 101 ///
 102 /// Implementations will likely, however, contain unsafe code as this type only
 103 /// operates on `*mut u8`, a raw pointer.
 104 ///
 105 /// # Examples
 106 ///
 107 /// ```rust,ignore
 108 /// use tls::os::Key;
 109 ///
 110 /// let key = Key::new(None);
 111 /// assert!(key.get().is_null());
 112 /// key.set(1 as *mut u8);
 113 /// assert!(!key.get().is_null());
 114 ///
 115 /// drop(key); // deallocate this TLS slot.
 116 /// ```
 117 pub struct Key {
 118     key: imp::Key,
 119 }
 120
 121 /// Constant initialization value for static TLS keys.
 122 ///
 123 /// This value specifies no destructor by default.
 124 pub const INIT: StaticKey = StaticKey::new(None);
 125
 126 impl StaticKey {
 127     pub const fn new(dtor: Option<unsafe extern fn(*mut u8)>) -> StaticKey {
 128         StaticKey {
 129             key: atomic::AtomicUsize::new(0),
 130             dtor: dtor
 131         }
 132     }
 133
 134     /// Gets the value associated with this TLS key
 135     ///
 136     /// This will lazily allocate a TLS key from the OS if one has not already
 137     /// been allocated.
 138     #[inline]
 139     pub unsafe fn get(&self) -> *mut u8 { imp::get(self.key()) }
 140
 141     /// Sets this TLS key to a new value.
 142     ///
 143     /// This will lazily allocate a TLS key from the OS if one has not already
 144     /// been allocated.
 145     #[inline]
 146     pub unsafe fn set(&self, val: *mut u8) { imp::set(self.key(), val) }
 147
 148     /// Deallocates this OS TLS key.
 149     ///
 150     /// This function is unsafe as there is no guarantee that the key is not
 151     /// currently in use by other threads or will not ever be used again.
 152     ///
 153     /// Note that this does *not* run the user-provided destructor if one was
 154     /// specified at definition time. Doing so must be done manually.
 155     pub unsafe fn destroy(&self) {
 156         match self.key.swap(0, Ordering::SeqCst) {
 157             0 => {}
 158             n => { imp::destroy(n as imp::Key) }
 159         }
 160     }
 161
 162     #[inline]
 163     unsafe fn key(&self) -> imp::Key {
 164         match self.key.load(Ordering::Relaxed) {
 165             0 => self.lazy_init() as imp::Key,
 166             n => n as imp::Key
 167         }
 168     }
 169
 170     unsafe fn lazy_init(&self) -> usize {
 171         // POSIX allows the key created here to be 0, but the compare_and_swap
 172         // below relies on using 0 as a sentinel value to check who won the
 173         // race to set the shared TLS key. As far as I know, there is no
 174         // guaranteed value that cannot be returned as a posix_key_create key,
 175         // so there is no value we can initialize the inner key with to
 176         // prove that it has not yet been set. As such, we'll continue using a
 177         // value of 0, but with some gyrations to make sure we have a non-0
 178         // value returned from the creation routine.
 179         // FIXME: this is clearly a hack, and should be cleaned up.
 180         let key1 = imp::create(self.dtor);
 181         let key = if key1 != 0 {
 182             key1
 183         } else {
 184             let key2 = imp::create(self.dtor);
 185             imp::destroy(key1);
 186             key2
 187         };
 188         assert!(key != 0);
 189         match self.key.compare_and_swap(0, key as usize, Ordering::SeqCst) {
 190             // The CAS succeeded, so we've created the actual key
 191             0 => key as usize,
 192             // If someone beat us to the punch, use their key instead
 193             n => { imp::destroy(key); n }
 194         }
 195     }
 196 }
 197
 198 impl Key {
 199     /// Creates a new managed OS TLS key.
 200     ///
 201     /// This key will be deallocated when the key falls out of scope.
 202     ///
 203     /// The argument provided is an optionally-specified destructor for the
 204     /// value of this TLS key. When a thread exits and the value for this key
 205     /// is non-null the destructor will be invoked. The TLS value will be reset
 206     /// to null before the destructor is invoked.
 207     ///
 208     /// Note that the destructor will not be run when the `Key` goes out of
 209     /// scope.
 210     #[inline]
 211     pub fn new(dtor: Option<unsafe extern fn(*mut u8)>) -> Key {
 212         Key { key: unsafe { imp::create(dtor) } }
 213     }
 214
 215     /// See StaticKey::get
 216     #[inline]
 217     pub fn get(&self) -> *mut u8 {
 218         unsafe { imp::get(self.key) }
 219     }
 220
 221     /// See StaticKey::set
 222     #[inline]
 223     pub fn set(&self, val: *mut u8) {
 224         unsafe { imp::set(self.key, val) }
 225     }
 226 }
 227
 228 impl Drop for Key {
 229     fn drop(&mut self) {
 230         unsafe { imp::destroy(self.key) }
 231     }
 232 }
 233
 234 #[cfg(test)]
 235 mod tests {
 236     use super::{Key, StaticKey};
 237
 238     fn assert_sync<T: Sync>() {}
 239     fn assert_send<T: Send>() {}
 240
 241     #[test]
 242     fn smoke() {
 243         assert_sync::<Key>();
 244         assert_send::<Key>();
 245
 246         let k1 = Key::new(None);
 247         let k2 = Key::new(None);
 248         assert!(k1.get().is_null());
 249         assert!(k2.get().is_null());
 250         k1.set(1 as *mut _);
 251         k2.set(2 as *mut _);
 252         assert_eq!(k1.get() as usize, 1);
 253         assert_eq!(k2.get() as usize, 2);
 254     }
 255
 256     #[test]
 257     fn statik() {
 258         static K1: StaticKey = StaticKey::new(None);
 259         static K2: StaticKey = StaticKey::new(None);
 260
 261         unsafe {
 262             assert!(K1.get().is_null());
 263             assert!(K2.get().is_null());
 264             K1.set(1 as *mut _);
 265             K2.set(2 as *mut _);
 266             assert_eq!(K1.get() as usize, 1);
 267             assert_eq!(K2.get() as usize, 2);
 268         }
 269     }
 270 }