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    use libc;
    use backtrace_sys::{self, backtrace_state};

    use ffi::CStr;
    use io;
    use mem;
    use ptr;
    use sys::backtrace::BacktraceContext;
    use sys_common::backtrace::Frame;

    pub fn foreach_symbol_fileline<F>(frame: Frame,
                                      mut f: F,
                                      _: &BacktraceContext) -> io::Result<bool>
    where F: FnMut(&[u8], u32) -> io::Result<()>
    {
        // pcinfo may return an arbitrary number of file:line pairs,
        // in the order of stack trace (i.e., inlined calls first).
        // in order to avoid allocation, we stack-allocate a fixed size of entries.
        const FILELINE_SIZE: usize = 32;
        let mut fileline_buf = [(ptr::null(), !0); FILELINE_SIZE];
        let ret;
        let fileline_count = {
            let state = unsafe { init_state() };
            if state.is_null() {
                return Err(io::Error::new(
                    io::ErrorKind::Other,
                    "failed to allocate libbacktrace state")
                )
            }
            let mut fileline_win: &mut [FileLine] = &mut fileline_buf;
            let fileline_addr = &mut fileline_win as *mut &mut [FileLine];
            ret = unsafe {
                backtrace_sys::backtrace_pcinfo(
                    state,
                    frame.exact_position as libc::uintptr_t,
                    pcinfo_cb,
                    error_cb,
                    fileline_addr as *mut libc::c_void,
                )
            };
            FILELINE_SIZE - fileline_win.len()
        };
        if ret == 0 {
            for &(file, line) in &fileline_buf[..fileline_count] {
                if file.is_null() { continue; } // just to be sure
                let file = unsafe { CStr::from_ptr(file).to_bytes() };
                f(file, line)?;
            }
            Ok(fileline_count == FILELINE_SIZE)
        } else {
            Ok(false)
        }
    }

    /// Converts a pointer to symbol to its string value.
    pub fn resolve_symname<F>(frame: Frame,
                              callback: F,
                              _: &BacktraceContext) -> io::Result<()>
        where F: FnOnce(Option<&str>) -> io::Result<()>
    {
        let symname = {
            let state = unsafe { init_state() };
            if state.is_null() {
                return Err(io::Error::new(
                    io::ErrorKind::Other,
                    "failed to allocate libbacktrace state")
                )
            }
            let mut data: *const libc::c_char = ptr::null();
            let data_addr = &mut data as *mut *const libc::c_char;
            let ret = unsafe {
                backtrace_sys::backtrace_syminfo(
                    state,
                    frame.symbol_addr as libc::uintptr_t,
                    syminfo_cb,
                    error_cb,
                    data_addr as *mut libc::c_void,
                )
            };
            if ret == 0 || data.is_null() {
                None
            } else {
                unsafe {
                    CStr::from_ptr(data).to_str().ok()
                }
            }
        };
        callback(symname)
    }

    ////////////////////////////////////////////////////////////////////////
    // helper callbacks
    ////////////////////////////////////////////////////////////////////////

    type FileLine = (*const libc::c_char, u32);

    extern fn error_cb(_data: *mut libc::c_void, _msg: *const libc::c_char,
                       _errnum: libc::c_int) {
        // do nothing for now
    }
    extern fn syminfo_cb(data: *mut libc::c_void,
                         _pc: libc::uintptr_t,
                         symname: *const libc::c_char,
                         _symval: libc::uintptr_t,
                         _symsize: libc::uintptr_t) {
        let slot = data as *mut *const libc::c_char;
        unsafe { *slot = symname; }
    }
    extern fn pcinfo_cb(data: *mut libc::c_void,
                        _pc: libc::uintptr_t,
                        filename: *const libc::c_char,
                        lineno: libc::c_int,
                        _function: *const libc::c_char) -> libc::c_int {
        if !filename.is_null() {
            let slot = data as *mut &mut [FileLine];
            let buffer = unsafe {ptr::read(slot)};

            // if the buffer is not full, add file:line to the buffer
            // and adjust the buffer for next possible calls to pcinfo_cb.
            if !buffer.is_empty() {
                buffer[0] = (filename, lineno as u32);
                unsafe { ptr::write(slot, &mut buffer[1..]); }
            }
        }

        0
    }

    // The libbacktrace API supports creating a state, but it does not
    // support destroying a state. I personally take this to mean that a
    // state is meant to be created and then live forever.
    //
    // I would love to register an at_exit() handler which cleans up this
    // state, but libbacktrace provides no way to do so.
    //
    // With these constraints, this function has a statically cached state
    // that is calculated the first time this is requested. Remember that
    // backtracing all happens serially (one global lock).
    //
    // Things don't work so well on not-Linux since libbacktrace can't track
    // down that executable this is. We at one point used env::current_exe but
    // it turns out that there are some serious security issues with that
    // approach.
    //
    // Specifically, on certain platforms like BSDs, a malicious actor can cause
    // an arbitrary file to be placed at the path returned by current_exe.
    // libbacktrace does not behave defensively in the presence of ill-formed
    // DWARF information, and has been demonstrated to segfault in at least one
    // case. There is no evidence at the moment to suggest that a more carefully
    // constructed file can't cause arbitrary code execution. As a result of all
    // of this, we don't hint libbacktrace with the path to the current process.
    unsafe fn init_state() -> *mut backtrace_state {
        static mut STATE: *mut backtrace_state = ptr::null_mut();
        if !STATE.is_null() { return STATE  }

        let filename = match ::sys::backtrace::gnu::get_executable_filename() {
            Ok((filename, file)) => {
                // filename is purposely leaked here since libbacktrace requires
                // it to stay allocated permanently, file is also leaked so that
                // the file stays locked
                let filename_ptr = filename.as_ptr();
                mem::forget(filename);
                mem::forget(file);
                filename_ptr
            },
            Err(_) => ptr::null(),
        };

        STATE = backtrace_sys::backtrace_create_state(
            filename,
            0,
            error_cb,
            ptr::null_mut(),
        );
        STATE
    }

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