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    // Copyright 2014-2015 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    use io;
    use io::prelude::*;
    use libc;
    use mem;
    use sync::StaticMutex;

    use super::super::printing::print;

    #[inline(never)] // if we know this is a function call, we can skip it when
                     // tracing
    pub fn write(w: &mut Write) -> io::Result<()> {
        struct Context<'a> {
            idx: isize,
            writer: &'a mut (Write+'a),
            last_error: Option<io::Error>,
        }

        // When using libbacktrace, we use some necessary global state, so we
        // need to prevent more than one thread from entering this block. This
        // is semi-reasonable in terms of printing anyway, and we know that all
        // I/O done here is blocking I/O, not green I/O, so we don't have to
        // worry about this being a native vs green mutex.
        static LOCK: StaticMutex = StaticMutex::new();
        let _g = LOCK.lock();

        writeln!(w, "stack backtrace:")?;

        let mut cx = Context { writer: w, last_error: None, idx: 0 };
        return match unsafe {
            uw::_Unwind_Backtrace(trace_fn,
                                  &mut cx as *mut Context as *mut libc::c_void)
        } {
            uw::_URC_NO_REASON => {
                match cx.last_error {
                    Some(err) => Err(err),
                    None => Ok(())
                }
            }
            _ => Ok(()),
        };

        extern fn trace_fn(ctx: *mut uw::_Unwind_Context,
                           arg: *mut libc::c_void) -> uw::_Unwind_Reason_Code {
            let cx: &mut Context = unsafe { mem::transmute(arg) };
            let mut ip_before_insn = 0;
            let mut ip = unsafe {
                uw::_Unwind_GetIPInfo(ctx, &mut ip_before_insn) as *mut libc::c_void
            };
            if !ip.is_null() && ip_before_insn == 0 {
                // this is a non-signaling frame, so `ip` refers to the address
                // after the calling instruction. account for that.
                ip = (ip as usize - 1) as *mut _;
            }

            // dladdr() on osx gets whiny when we use FindEnclosingFunction, and
            // it appears to work fine without it, so we only use
            // FindEnclosingFunction on non-osx platforms. In doing so, we get a
            // slightly more accurate stack trace in the process.
            //
            // This is often because panic involves the last instruction of a
            // function being "call std::rt::begin_unwind", with no ret
            // instructions after it. This means that the return instruction
            // pointer points *outside* of the calling function, and by
            // unwinding it we go back to the original function.
            let symaddr = if cfg!(target_os = "macos") || cfg!(target_os = "ios") {
                ip
            } else {
                unsafe { uw::_Unwind_FindEnclosingFunction(ip) }
            };

            // Don't print out the first few frames (they're not user frames)
            cx.idx += 1;
            if cx.idx <= 0 { return uw::_URC_NO_REASON }
            // Don't print ginormous backtraces
            if cx.idx > 100 {
                match write!(cx.writer, " ... <frames omitted>\n") {
                    Ok(()) => {}
                    Err(e) => { cx.last_error = Some(e); }
                }
                return uw::_URC_FAILURE
            }

            // Once we hit an error, stop trying to print more frames
            if cx.last_error.is_some() { return uw::_URC_FAILURE }

            match print(cx.writer, cx.idx, ip, symaddr) {
                Ok(()) => {}
                Err(e) => { cx.last_error = Some(e); }
            }

            // keep going
            uw::_URC_NO_REASON
        }
    }

    /// Unwind library interface used for backtraces
    ///
    /// Note that dead code is allowed as here are just bindings
    /// iOS doesn't use all of them it but adding more
    /// platform-specific configs pollutes the code too much
    #[allow(non_camel_case_types)]
    #[allow(non_snake_case)]
    mod uw {
        pub use self::_Unwind_Reason_Code::*;

        use libc;

        #[repr(C)]
        pub enum _Unwind_Reason_Code {
            _URC_NO_REASON = 0,
            _URC_FOREIGN_EXCEPTION_CAUGHT = 1,
            _URC_FATAL_PHASE2_ERROR = 2,
            _URC_FATAL_PHASE1_ERROR = 3,
            _URC_NORMAL_STOP = 4,
            _URC_END_OF_STACK = 5,
            _URC_HANDLER_FOUND = 6,
            _URC_INSTALL_CONTEXT = 7,
            _URC_CONTINUE_UNWIND = 8,
            _URC_FAILURE = 9, // used only by ARM EABI
        }

        pub enum _Unwind_Context {}

        pub type _Unwind_Trace_Fn =
                extern fn(ctx: *mut _Unwind_Context,
                          arg: *mut libc::c_void) -> _Unwind_Reason_Code;

        extern {
            // No native _Unwind_Backtrace on iOS
            #[cfg(not(all(target_os = "ios", target_arch = "arm")))]
            pub fn _Unwind_Backtrace(trace: _Unwind_Trace_Fn,
                                     trace_argument: *mut libc::c_void)
                        -> _Unwind_Reason_Code;

            // available since GCC 4.2.0, should be fine for our purpose
            #[cfg(all(not(all(target_os = "android", target_arch = "arm")),
                      not(all(target_os = "linux", target_arch = "arm"))))]
            pub fn _Unwind_GetIPInfo(ctx: *mut _Unwind_Context,
                                     ip_before_insn: *mut libc::c_int)
                        -> libc::uintptr_t;

            #[cfg(all(not(target_os = "android"),
                      not(all(target_os = "linux", target_arch = "arm"))))]
            pub fn _Unwind_FindEnclosingFunction(pc: *mut libc::c_void)
                -> *mut libc::c_void;
        }

        // On android, the function _Unwind_GetIP is a macro, and this is the
        // expansion of the macro. This is all copy/pasted directly from the
        // header file with the definition of _Unwind_GetIP.
        #[cfg(any(all(target_os = "android", target_arch = "arm"),
                  all(target_os = "linux", target_arch = "arm")))]
        pub unsafe fn _Unwind_GetIP(ctx: *mut _Unwind_Context) -> libc::uintptr_t {
            #[repr(C)]
            enum _Unwind_VRS_Result {
                _UVRSR_OK = 0,
                _UVRSR_NOT_IMPLEMENTED = 1,
                _UVRSR_FAILED = 2,
            }
            #[repr(C)]
            enum _Unwind_VRS_RegClass {
                _UVRSC_CORE = 0,
                _UVRSC_VFP = 1,
                _UVRSC_FPA = 2,
                _UVRSC_WMMXD = 3,
                _UVRSC_WMMXC = 4,
            }
            #[repr(C)]
            enum _Unwind_VRS_DataRepresentation {
                _UVRSD_UINT32 = 0,
                _UVRSD_VFPX = 1,
                _UVRSD_FPAX = 2,
                _UVRSD_UINT64 = 3,
                _UVRSD_FLOAT = 4,
                _UVRSD_DOUBLE = 5,
            }

            type _Unwind_Word = libc::c_uint;
            extern {
                fn _Unwind_VRS_Get(ctx: *mut _Unwind_Context,
                                   klass: _Unwind_VRS_RegClass,
                                   word: _Unwind_Word,
                                   repr: _Unwind_VRS_DataRepresentation,
                                   data: *mut libc::c_void)
                    -> _Unwind_VRS_Result;
            }

            let mut val: _Unwind_Word = 0;
            let ptr = &mut val as *mut _Unwind_Word;
            let _ = _Unwind_VRS_Get(ctx, _Unwind_VRS_RegClass::_UVRSC_CORE, 15,
                                    _Unwind_VRS_DataRepresentation::_UVRSD_UINT32,
                                    ptr as *mut libc::c_void);
            (val & !1) as libc::uintptr_t
        }

        // This function doesn't exist on Android or ARM/Linux, so make it same
        // to _Unwind_GetIP
        #[cfg(any(all(target_os = "android", target_arch = "arm"),
                  all(target_os = "linux", target_arch = "arm")))]
        pub unsafe fn _Unwind_GetIPInfo(ctx: *mut _Unwind_Context,
                                        ip_before_insn: *mut libc::c_int)
            -> libc::uintptr_t
        {
            *ip_before_insn = 0;
            _Unwind_GetIP(ctx)
        }

        // This function also doesn't exist on Android or ARM/Linux, so make it
        // a no-op
        #[cfg(any(target_os = "android",
                  all(target_os = "linux", target_arch = "arm")))]
        pub unsafe fn _Unwind_FindEnclosingFunction(pc: *mut libc::c_void)
            -> *mut libc::c_void
        {
            pc
        }
    }

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