1 //! Support for capturing a stack backtrace of an OS thread
3 //! This module contains the support necessary to capture a stack backtrace of a
4 //! running OS thread from the OS thread itself. The `Backtrace` type supports
5 //! capturing a stack trace via the `Backtrace::capture` and
6 //! `Backtrace::force_capture` functions.
8 //! A backtrace is typically quite handy to attach to errors (e.g. types
9 //! implementing `std::error::Error`) to get a causal chain of where an error
12 //! > **Note**: this module is unstable and is designed in [RFC 2504], and you
13 //! > can learn more about its status in the [tracking issue].
15 //! [RFC 2504]: https://github.com/rust-lang/rfcs/blob/master/text/2504-fix-error.md
16 //! [tracking issue]: https://github.com/rust-lang/rust/issues/53487
20 //! Backtraces are attempted to be as accurate as possible, but no guarantees
21 //! are provided about the exact accuracy of a backtrace. Instruction pointers,
22 //! symbol names, filenames, line numbers, etc, may all be incorrect when
23 //! reported. Accuracy is attempted on a best-effort basis, however, and bugs
24 //! are always welcome to indicate areas of improvement!
26 //! For most platforms a backtrace with a filename/line number requires that
27 //! programs be compiled with debug information. Without debug information
28 //! filenames/line numbers will not be reported.
30 //! ## Platform support
32 //! Not all platforms that libstd compiles for support capturing backtraces.
33 //! Some platforms simply do nothing when capturing a backtrace. To check
34 //! whether the platform supports capturing backtraces you can consult the
35 //! `BacktraceStatus` enum as a result of `Backtrace::status`.
37 //! Like above with accuracy platform support is done on a best effort basis.
38 //! Sometimes libraries may not be available at runtime or something may go
39 //! wrong which would cause a backtrace to not be captured. Please feel free to
40 //! report issues with platforms where a backtrace cannot be captured though!
42 //! ## Environment Variables
44 //! The `Backtrace::capture` function may not actually capture a backtrace by
45 //! default. Its behavior is governed by two environment variables:
47 //! * `RUST_LIB_BACKTRACE` - if this is set to `0` then `Backtrace::capture`
48 //! will never capture a backtrace. Any other value this is set to will enable
49 //! `Backtrace::capture`.
51 //! * `RUST_BACKTRACE` - if `RUST_LIB_BACKTRACE` is not set, then this variable
52 //! is consulted with the same rules of `RUST_LIB_BACKTRACE`.
54 //! * If neither of the above env vars are set, then `Backtrace::capture` will
57 //! Capturing a backtrace can be a quite expensive runtime operation, so the
58 //! environment variables allow either forcibly disabling this runtime
59 //! performance hit or allow selectively enabling it in some programs.
61 //! Note that the `Backtrace::force_capture` function can be used to ignore
62 //! these environment variables. Also note that the state of environment
63 //! variables is cached once the first backtrace is created, so altering
64 //! `RUST_LIB_BACKTRACE` or `RUST_BACKTRACE` at runtime may not actually change
65 //! how backtraces are captured.
67 #![unstable(feature = "backtrace", issue = "53487")]
72 // NB: A note on resolution of a backtrace:
74 // Backtraces primarily happen in two steps, one is where we actually capture
75 // the stack backtrace, giving us a list of instruction pointers corresponding
76 // to stack frames. Next we take these instruction pointers and, one-by-one,
77 // turn them into a human readable name (like `main`).
79 // The first phase can be somewhat expensive (walking the stack), especially
80 // on MSVC where debug information is consulted to return inline frames each as
81 // their own frame. The second phase, however, is almost always extremely
82 // expensive (on the order of milliseconds sometimes) when it's consulting debug
85 // We attempt to amortize this cost as much as possible by delaying resolution
86 // of an address to a human readable name for as long as possible. When
87 // `Backtrace::create` is called to capture a backtrace it doesn't actually
88 // perform any symbol resolution, but rather we lazily resolve symbols only just
89 // before they're needed for printing. This way we can make capturing a
90 // backtrace and throwing it away much cheaper, but actually printing a
91 // backtrace is still basically the same cost.
93 // This strategy comes at the cost of some synchronization required inside of a
94 // `Backtrace`, but that's a relatively small price to pay relative to capturing
95 // a backtrace or actually symbolizing it.
97 use crate::backtrace_rs::{self, BytesOrWideString};
99 use crate::ffi::c_void;
101 use crate::sync::atomic::{AtomicUsize, Ordering::SeqCst};
102 use crate::sync::Mutex;
103 use crate::sys_common::backtrace::{lock, output_filename};
106 /// A captured OS thread stack backtrace.
108 /// This type represents a stack backtrace for an OS thread captured at a
109 /// previous point in time. In some instances the `Backtrace` type may
110 /// internally be empty due to configuration. For more information see
111 /// `Backtrace::capture`.
112 pub struct Backtrace {
116 /// The current status of a backtrace, indicating whether it was captured or
117 /// whether it is empty for some other reason.
119 #[derive(Debug, PartialEq, Eq)]
120 pub enum BacktraceStatus {
121 /// Capturing a backtrace is not supported, likely because it's not
122 /// implemented for the current platform.
124 /// Capturing a backtrace has been disabled through either the
125 /// `RUST_LIB_BACKTRACE` or `RUST_BACKTRACE` environment variables.
127 /// A backtrace has been captured and the `Backtrace` should print
128 /// reasonable information when rendered.
135 Captured(Mutex<Capture>),
141 frames: Vec<BacktraceFrame>,
144 fn _assert_send_sync() {
145 fn _assert<T: Send + Sync>() {}
146 _assert::<Backtrace>();
149 struct BacktraceFrame {
151 symbols: Vec<BacktraceSymbol>,
155 Actual(backtrace_rs::Frame),
160 struct BacktraceSymbol {
161 name: Option<Vec<u8>>,
162 filename: Option<BytesOrWide>,
171 impl fmt::Debug for Backtrace {
172 fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
173 let mut capture = match &self.inner {
174 Inner::Unsupported => return fmt.write_str("<unsupported>"),
175 Inner::Disabled => return fmt.write_str("<disabled>"),
176 Inner::Captured(c) => c.lock().unwrap(),
180 let frames = &capture.frames[capture.actual_start..];
182 write!(fmt, "Backtrace ")?;
184 let mut dbg = fmt.debug_list();
186 for frame in frames {
187 if frame.frame.ip().is_null() {
191 dbg.entries(&frame.symbols);
198 impl fmt::Debug for BacktraceSymbol {
199 fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
202 if let Some(fn_name) = self.name.as_ref().map(|b| backtrace_rs::SymbolName::new(b)) {
203 write!(fmt, "fn: \"{:#}\"", fn_name)?;
205 write!(fmt, "fn: <unknown>")?;
208 if let Some(fname) = self.filename.as_ref() {
209 write!(fmt, ", file: \"{:?}\"", fname)?;
212 if let Some(line) = self.lineno.as_ref() {
213 write!(fmt, ", line: {:?}", line)?;
220 impl fmt::Debug for BytesOrWide {
221 fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
225 BytesOrWide::Bytes(w) => BytesOrWideString::Bytes(w),
226 BytesOrWide::Wide(w) => BytesOrWideString::Wide(w),
228 backtrace_rs::PrintFmt::Short,
229 crate::env::current_dir().as_ref().ok(),
235 /// Returns whether backtrace captures are enabled through environment
237 fn enabled() -> bool {
238 // Cache the result of reading the environment variables to make
239 // backtrace captures speedy, because otherwise reading environment
240 // variables every time can be somewhat slow.
241 static ENABLED: AtomicUsize = AtomicUsize::new(0);
242 match ENABLED.load(SeqCst) {
247 let enabled = match env::var("RUST_LIB_BACKTRACE") {
249 Err(_) => match env::var("RUST_BACKTRACE") {
254 ENABLED.store(enabled as usize + 1, SeqCst);
258 /// Capture a stack backtrace of the current thread.
260 /// This function will capture a stack backtrace of the current OS thread of
261 /// execution, returning a `Backtrace` type which can be later used to print
262 /// the entire stack trace or render it to a string.
264 /// This function will be a noop if the `RUST_BACKTRACE` or
265 /// `RUST_LIB_BACKTRACE` backtrace variables are both not set. If either
266 /// environment variable is set and enabled then this function will actually
267 /// capture a backtrace. Capturing a backtrace can be both memory intensive
268 /// and slow, so these environment variables allow liberally using
269 /// `Backtrace::capture` and only incurring a slowdown when the environment
270 /// variables are set.
272 /// To forcibly capture a backtrace regardless of environment variables, use
273 /// the `Backtrace::force_capture` function.
274 #[inline(never)] // want to make sure there's a frame here to remove
275 pub fn capture() -> Backtrace {
276 if !Backtrace::enabled() {
277 return Backtrace { inner: Inner::Disabled };
279 Backtrace::create(Backtrace::capture as usize)
282 /// Forcibly captures a full backtrace, regardless of environment variable
285 /// This function behaves the same as `capture` except that it ignores the
286 /// values of the `RUST_BACKTRACE` and `RUST_LIB_BACKTRACE` environment
287 /// variables, always capturing a backtrace.
289 /// Note that capturing a backtrace can be an expensive operation on some
290 /// platforms, so this should be used with caution in performance-sensitive
292 #[inline(never)] // want to make sure there's a frame here to remove
293 pub fn force_capture() -> Backtrace {
294 Backtrace::create(Backtrace::force_capture as usize)
297 /// Forcibly captures a disabled backtrace, regardless of environment
298 /// variable configuration.
299 pub const fn disabled() -> Backtrace {
300 Backtrace { inner: Inner::Disabled }
303 // Capture a backtrace which start just before the function addressed by
305 fn create(ip: usize) -> Backtrace {
306 // SAFETY: We don't attempt to lock this reentrantly.
307 let _lock = unsafe { lock() };
308 let mut frames = Vec::new();
309 let mut actual_start = None;
311 backtrace_rs::trace_unsynchronized(|frame| {
312 frames.push(BacktraceFrame {
313 frame: RawFrame::Actual(frame.clone()),
316 if frame.symbol_address() as usize == ip && actual_start.is_none() {
317 actual_start = Some(frames.len());
323 // If no frames came out assume that this is an unsupported platform
324 // since `backtrace` doesn't provide a way of learning this right now,
325 // and this should be a good enough approximation.
326 let inner = if frames.is_empty() {
329 Inner::Captured(Mutex::new(Capture {
330 actual_start: actual_start.unwrap_or(0),
339 /// Returns the status of this backtrace, indicating whether this backtrace
340 /// request was unsupported, disabled, or a stack trace was actually
342 pub fn status(&self) -> BacktraceStatus {
344 Inner::Unsupported => BacktraceStatus::Unsupported,
345 Inner::Disabled => BacktraceStatus::Disabled,
346 Inner::Captured(_) => BacktraceStatus::Captured,
351 impl fmt::Display for Backtrace {
352 fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
353 let mut capture = match &self.inner {
354 Inner::Unsupported => return fmt.write_str("unsupported backtrace"),
355 Inner::Disabled => return fmt.write_str("disabled backtrace"),
356 Inner::Captured(c) => c.lock().unwrap(),
360 let full = fmt.alternate();
361 let (frames, style) = if full {
362 (&capture.frames[..], backtrace_rs::PrintFmt::Full)
364 (&capture.frames[capture.actual_start..], backtrace_rs::PrintFmt::Short)
367 // When printing paths we try to strip the cwd if it exists, otherwise
368 // we just print the path as-is. Note that we also only do this for the
369 // short format, because if it's full we presumably want to print
371 let cwd = crate::env::current_dir();
372 let mut print_path = move |fmt: &mut fmt::Formatter<'_>, path: BytesOrWideString<'_>| {
373 output_filename(fmt, path, style, cwd.as_ref().ok())
376 let mut f = backtrace_rs::BacktraceFmt::new(fmt, style, &mut print_path);
378 for frame in frames {
379 let mut f = f.frame();
380 if frame.symbols.is_empty() {
381 f.print_raw(frame.frame.ip(), None, None, None)?;
383 for symbol in frame.symbols.iter() {
386 symbol.name.as_ref().map(|b| backtrace_rs::SymbolName::new(b)),
387 symbol.filename.as_ref().map(|b| match b {
388 BytesOrWide::Bytes(w) => BytesOrWideString::Bytes(w),
389 BytesOrWide::Wide(w) => BytesOrWideString::Wide(w),
402 fn resolve(&mut self) {
403 // If we're already resolved, nothing to do!
407 self.resolved = true;
409 // Use the global backtrace lock to synchronize this as it's a
410 // requirement of the `backtrace` crate, and then actually resolve
412 // SAFETY: We don't attempt to lock this reentrantly.
413 let _lock = unsafe { lock() };
414 for frame in self.frames.iter_mut() {
415 let symbols = &mut frame.symbols;
416 let frame = match &frame.frame {
417 RawFrame::Actual(frame) => frame,
419 RawFrame::Fake => unimplemented!(),
422 backtrace_rs::resolve_frame_unsynchronized(frame, |symbol| {
423 symbols.push(BacktraceSymbol {
424 name: symbol.name().map(|m| m.as_bytes().to_vec()),
425 filename: symbol.filename_raw().map(|b| match b {
426 BytesOrWideString::Bytes(b) => BytesOrWide::Bytes(b.to_owned()),
427 BytesOrWideString::Wide(b) => BytesOrWide::Wide(b.to_owned()),
429 lineno: symbol.lineno(),
438 fn ip(&self) -> *mut c_void {
440 RawFrame::Actual(frame) => frame.ip(),
442 RawFrame::Fake => 1 as *mut c_void,