1 | use crate::task::JoinHandle; |
2 | |
3 | use std::future::Future; |
4 | |
5 | cfg_rt! { |
6 | /// Spawns a new asynchronous task, returning a |
7 | /// [`JoinHandle`](super::JoinHandle) for it. |
8 | /// |
9 | /// The provided future will start running in the background immediately |
10 | /// when `spawn` is called, even if you don't await the returned |
11 | /// `JoinHandle`. |
12 | /// |
13 | /// Spawning a task enables the task to execute concurrently to other tasks. The |
14 | /// spawned task may execute on the current thread, or it may be sent to a |
15 | /// different thread to be executed. The specifics depend on the current |
16 | /// [`Runtime`](crate::runtime::Runtime) configuration. |
17 | /// |
18 | /// It is guaranteed that spawn will not synchronously poll the task being spawned. |
19 | /// This means that calling spawn while holding a lock does not pose a risk of |
20 | /// deadlocking with the spawned task. |
21 | /// |
22 | /// There is no guarantee that a spawned task will execute to completion. |
23 | /// When a runtime is shutdown, all outstanding tasks are dropped, |
24 | /// regardless of the lifecycle of that task. |
25 | /// |
26 | /// This function must be called from the context of a Tokio runtime. Tasks running on |
27 | /// the Tokio runtime are always inside its context, but you can also enter the context |
28 | /// using the [`Runtime::enter`](crate::runtime::Runtime::enter()) method. |
29 | /// |
30 | /// # Examples |
31 | /// |
32 | /// In this example, a server is started and `spawn` is used to start a new task |
33 | /// that processes each received connection. |
34 | /// |
35 | /// ```no_run |
36 | /// use tokio::net::{TcpListener, TcpStream}; |
37 | /// |
38 | /// use std::io; |
39 | /// |
40 | /// async fn process(socket: TcpStream) { |
41 | /// // ... |
42 | /// # drop(socket); |
43 | /// } |
44 | /// |
45 | /// #[tokio::main] |
46 | /// async fn main() -> io::Result<()> { |
47 | /// let listener = TcpListener::bind("127.0.0.1:8080").await?; |
48 | /// |
49 | /// loop { |
50 | /// let (socket, _) = listener.accept().await?; |
51 | /// |
52 | /// tokio::spawn(async move { |
53 | /// // Process each socket concurrently. |
54 | /// process(socket).await |
55 | /// }); |
56 | /// } |
57 | /// } |
58 | /// ``` |
59 | /// |
60 | /// To run multiple tasks in parallel and receive their results, join |
61 | /// handles can be stored in a vector. |
62 | /// ``` |
63 | /// # #[tokio::main(flavor = "current_thread")] async fn main() { |
64 | /// async fn my_background_op(id: i32) -> String { |
65 | /// let s = format!("Starting background task {}.", id); |
66 | /// println!("{}", s); |
67 | /// s |
68 | /// } |
69 | /// |
70 | /// let ops = vec![1, 2, 3]; |
71 | /// let mut tasks = Vec::with_capacity(ops.len()); |
72 | /// for op in ops { |
73 | /// // This call will make them start running in the background |
74 | /// // immediately. |
75 | /// tasks.push(tokio::spawn(my_background_op(op))); |
76 | /// } |
77 | /// |
78 | /// let mut outputs = Vec::with_capacity(tasks.len()); |
79 | /// for task in tasks { |
80 | /// outputs.push(task.await.unwrap()); |
81 | /// } |
82 | /// println!("{:?}", outputs); |
83 | /// # } |
84 | /// ``` |
85 | /// This example pushes the tasks to `outputs` in the order they were |
86 | /// started in. If you do not care about the ordering of the outputs, then |
87 | /// you can also use a [`JoinSet`]. |
88 | /// |
89 | /// [`JoinSet`]: struct@crate::task::JoinSet |
90 | /// |
91 | /// # Panics |
92 | /// |
93 | /// Panics if called from **outside** of the Tokio runtime. |
94 | /// |
95 | /// # Using `!Send` values from a task |
96 | /// |
97 | /// The task supplied to `spawn` must implement `Send`. However, it is |
98 | /// possible to **use** `!Send` values from the task as long as they only |
99 | /// exist between calls to `.await`. |
100 | /// |
101 | /// For example, this will work: |
102 | /// |
103 | /// ``` |
104 | /// use tokio::task; |
105 | /// |
106 | /// use std::rc::Rc; |
107 | /// |
108 | /// fn use_rc(rc: Rc<()>) { |
109 | /// // Do stuff w/ rc |
110 | /// # drop(rc); |
111 | /// } |
112 | /// |
113 | /// #[tokio::main] |
114 | /// async fn main() { |
115 | /// tokio::spawn(async { |
116 | /// // Force the `Rc` to stay in a scope with no `.await` |
117 | /// { |
118 | /// let rc = Rc::new(()); |
119 | /// use_rc(rc.clone()); |
120 | /// } |
121 | /// |
122 | /// task::yield_now().await; |
123 | /// }).await.unwrap(); |
124 | /// } |
125 | /// ``` |
126 | /// |
127 | /// This will **not** work: |
128 | /// |
129 | /// ```compile_fail |
130 | /// use tokio::task; |
131 | /// |
132 | /// use std::rc::Rc; |
133 | /// |
134 | /// fn use_rc(rc: Rc<()>) { |
135 | /// // Do stuff w/ rc |
136 | /// # drop(rc); |
137 | /// } |
138 | /// |
139 | /// #[tokio::main] |
140 | /// async fn main() { |
141 | /// tokio::spawn(async { |
142 | /// let rc = Rc::new(()); |
143 | /// |
144 | /// task::yield_now().await; |
145 | /// |
146 | /// use_rc(rc.clone()); |
147 | /// }).await.unwrap(); |
148 | /// } |
149 | /// ``` |
150 | /// |
151 | /// Holding on to a `!Send` value across calls to `.await` will result in |
152 | /// an unfriendly compile error message similar to: |
153 | /// |
154 | /// ```text |
155 | /// `[... some type ...]` cannot be sent between threads safely |
156 | /// ``` |
157 | /// |
158 | /// or: |
159 | /// |
160 | /// ```text |
161 | /// error[E0391]: cycle detected when processing `main` |
162 | /// ``` |
163 | #[track_caller ] |
164 | pub fn spawn<F>(future: F) -> JoinHandle<F::Output> |
165 | where |
166 | F: Future + Send + 'static, |
167 | F::Output: Send + 'static, |
168 | { |
169 | // preventing stack overflows on debug mode, by quickly sending the |
170 | // task to the heap. |
171 | if cfg!(debug_assertions) && std::mem::size_of::<F>() > 2048 { |
172 | spawn_inner(Box::pin(future), None) |
173 | } else { |
174 | spawn_inner(future, None) |
175 | } |
176 | } |
177 | |
178 | #[track_caller ] |
179 | pub(super) fn spawn_inner<T>(future: T, name: Option<&str>) -> JoinHandle<T::Output> |
180 | where |
181 | T: Future + Send + 'static, |
182 | T::Output: Send + 'static, |
183 | { |
184 | use crate::runtime::{context, task}; |
185 | |
186 | #[cfg (all( |
187 | tokio_unstable, |
188 | tokio_taskdump, |
189 | feature = "rt" , |
190 | target_os = "linux" , |
191 | any( |
192 | target_arch = "aarch64" , |
193 | target_arch = "x86" , |
194 | target_arch = "x86_64" |
195 | ) |
196 | ))] |
197 | let future = task::trace::Trace::root(future); |
198 | let id = task::Id::next(); |
199 | let task = crate::util::trace::task(future, "task" , name, id.as_u64()); |
200 | |
201 | match context::with_current(|handle| handle.spawn(task, id)) { |
202 | Ok(join_handle) => join_handle, |
203 | Err(e) => panic!("{}" , e), |
204 | } |
205 | } |
206 | } |
207 | |