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 //! # Example
  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
  59 use prelude::v1::*;
  60
  61 use sync::atomic::{self, AtomicUsize, Ordering};
  62 use sync::{Mutex, Once, ONCE_INIT};
  63
  64 use sys::thread_local as imp;
  65
  66 /// A type for TLS keys that are statically allocated.
  67 ///
  68 /// This type is entirely `unsafe` to use as it does not protect against
  69 /// use-after-deallocation or use-during-deallocation.
  70 ///
  71 /// The actual OS-TLS key is lazily allocated when this is used for the first
  72 /// time. The key is also deallocated when the Rust runtime exits or `destroy`
  73 /// is called, whichever comes first.
  74 ///
  75 /// # Example
  76 ///
  77 /// ```ignore
  78 /// use tls::os::{StaticKey, INIT};
  79 ///
  80 /// static KEY: StaticKey = INIT;
  81 ///
  82 /// unsafe {
  83 ///     assert!(KEY.get().is_null());
  84 ///     KEY.set(1 as *mut u8);
  85 /// }
  86 /// ```
  87 #[stable(feature = "rust1", since = "1.0.0")]
  88 pub struct StaticKey {
  89     /// Inner static TLS key (internals), created with by `INIT_INNER` in this
  90     /// module.
  91     #[stable(feature = "rust1", since = "1.0.0")]
  92     pub inner: StaticKeyInner,
  93     /// Destructor for the TLS value.
  94     ///
  95     /// See `Key::new` for information about when the destructor runs and how
  96     /// it runs.
  97     #[stable(feature = "rust1", since = "1.0.0")]
  98     pub dtor: Option<unsafe extern fn(*mut u8)>,
  99 }
 100
 101 /// Inner contents of `StaticKey`, created by the `INIT_INNER` constant.
 102 pub struct StaticKeyInner {
 103     key: AtomicUsize,
 104 }
 105
 106 /// A type for a safely managed OS-based TLS slot.
 107 ///
 108 /// This type allocates an OS TLS key when it is initialized and will deallocate
 109 /// the key when it falls out of scope. When compared with `StaticKey`, this
 110 /// type is entirely safe to use.
 111 ///
 112 /// Implementations will likely, however, contain unsafe code as this type only
 113 /// operates on `*mut u8`, an unsafe pointer.
 114 ///
 115 /// # Example
 116 ///
 117 /// ```rust,ignore
 118 /// use tls::os::Key;
 119 ///
 120 /// let key = Key::new(None);
 121 /// assert!(key.get().is_null());
 122 /// key.set(1 as *mut u8);
 123 /// assert!(!key.get().is_null());
 124 ///
 125 /// drop(key); // deallocate this TLS slot.
 126 /// ```
 127 pub struct Key {
 128     key: imp::Key,
 129 }
 130
 131 /// Constant initialization value for static TLS keys.
 132 ///
 133 /// This value specifies no destructor by default.
 134 #[stable(feature = "rust1", since = "1.0.0")]
 135 pub const INIT: StaticKey = StaticKey {
 136     inner: INIT_INNER,
 137     dtor: None,
 138 };
 139
 140 /// Constant initialization value for the inner part of static TLS keys.
 141 ///
 142 /// This value allows specific configuration of the destructor for a TLS key.
 143 #[stable(feature = "rust1", since = "1.0.0")]
 144 pub const INIT_INNER: StaticKeyInner = StaticKeyInner {
 145     key: atomic::ATOMIC_USIZE_INIT,
 146 };
 147
 148 static INIT_KEYS: Once = ONCE_INIT;
 149 static mut KEYS: *mut Mutex<Vec<imp::Key>> = 0 as *mut _;
 150
 151 impl StaticKey {
 152     /// Gets the value associated with this TLS key
 153     ///
 154     /// This will lazily allocate a TLS key from the OS if one has not already
 155     /// been allocated.
 156     #[inline]
 157     pub unsafe fn get(&self) -> *mut u8 { imp::get(self.key()) }
 158
 159     /// Sets this TLS key to a new value.
 160     ///
 161     /// This will lazily allocate a TLS key from the OS if one has not already
 162     /// been allocated.
 163     #[inline]
 164     pub unsafe fn set(&self, val: *mut u8) { imp::set(self.key(), val) }
 165
 166     /// Deallocates this OS TLS key.
 167     ///
 168     /// This function is unsafe as there is no guarantee that the key is not
 169     /// currently in use by other threads or will not ever be used again.
 170     ///
 171     /// Note that this does *not* run the user-provided destructor if one was
 172     /// specified at definition time. Doing so must be done manually.
 173     pub unsafe fn destroy(&self) {
 174         match self.inner.key.swap(0, Ordering::SeqCst) {
 175             0 => {}
 176             n => { imp::destroy(n as imp::Key) }
 177         }
 178     }
 179
 180     #[inline]
 181     unsafe fn key(&self) -> imp::Key {
 182         match self.inner.key.load(Ordering::Relaxed) {
 183             0 => self.lazy_init() as imp::Key,
 184             n => n as imp::Key
 185         }
 186     }
 187
 188     unsafe fn lazy_init(&self) -> uint {
 189         // POSIX allows the key created here to be 0, but the compare_and_swap
 190         // below relies on using 0 as a sentinel value to check who won the
 191         // race to set the shared TLS key. As far as I know, there is no
 192         // guaranteed value that cannot be returned as a posix_key_create key,
 193         // so there is no value we can initialize the inner key with to
 194         // prove that it has not yet been set. As such, we'll continue using a
 195         // value of 0, but with some gyrations to make sure we have a non-0
 196         // value returned from the creation routine.
 197         // FIXME: this is clearly a hack, and should be cleaned up.
 198         let key1 = imp::create(self.dtor);
 199         let key = if key1 != 0 {
 200             key1
 201         } else {
 202             let key2 = imp::create(self.dtor);
 203             imp::destroy(key1);
 204             key2
 205         };
 206         assert!(key != 0);
 207         match self.inner.key.compare_and_swap(0, key as uint, Ordering::SeqCst) {
 208             // The CAS succeeded, so we've created the actual key
 209             0 => key as uint,
 210             // If someone beat us to the punch, use their key instead
 211             n => { imp::destroy(key); n }
 212         }
 213     }
 214 }
 215
 216 impl Key {
 217     /// Create a new managed OS TLS key.
 218     ///
 219     /// This key will be deallocated when the key falls out of scope.
 220     ///
 221     /// The argument provided is an optionally-specified destructor for the
 222     /// value of this TLS key. When a thread exits and the value for this key
 223     /// is non-null the destructor will be invoked. The TLS value will be reset
 224     /// to null before the destructor is invoked.
 225     ///
 226     /// Note that the destructor will not be run when the `Key` goes out of
 227     /// scope.
 228     #[inline]
 229     pub fn new(dtor: Option<unsafe extern fn(*mut u8)>) -> Key {
 230         Key { key: unsafe { imp::create(dtor) } }
 231     }
 232
 233     /// See StaticKey::get
 234     #[inline]
 235     pub fn get(&self) -> *mut u8 {
 236         unsafe { imp::get(self.key) }
 237     }
 238
 239     /// See StaticKey::set
 240     #[inline]
 241     pub fn set(&self, val: *mut u8) {
 242         unsafe { imp::set(self.key, val) }
 243     }
 244 }
 245
 246 impl Drop for Key {
 247     fn drop(&mut self) {
 248         unsafe { imp::destroy(self.key) }
 249     }
 250 }
 251
 252 #[cfg(test)]
 253 mod tests {
 254     use prelude::v1::*;
 255     use super::{Key, StaticKey, INIT_INNER};
 256
 257     fn assert_sync<T: Sync>() {}
 258     fn assert_send<T: Send>() {}
 259
 260     #[test]
 261     fn smoke() {
 262         assert_sync::<Key>();
 263         assert_send::<Key>();
 264
 265         let k1 = Key::new(None);
 266         let k2 = Key::new(None);
 267         assert!(k1.get().is_null());
 268         assert!(k2.get().is_null());
 269         k1.set(1 as *mut _);
 270         k2.set(2 as *mut _);
 271         assert_eq!(k1.get() as uint, 1);
 272         assert_eq!(k2.get() as uint, 2);
 273     }
 274
 275     #[test]
 276     fn statik() {
 277         static K1: StaticKey = StaticKey { inner: INIT_INNER, dtor: None };
 278         static K2: StaticKey = StaticKey { inner: INIT_INNER, dtor: None };
 279
 280         unsafe {
 281             assert!(K1.get().is_null());
 282             assert!(K2.get().is_null());
 283             K1.set(1 as *mut _);
 284             K2.set(2 as *mut _);
 285             assert_eq!(K1.get() as uint, 1);
 286             assert_eq!(K2.get() as uint, 2);
 287         }
 288     }
 289 }