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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html><head>
<title>A Proposal to add stack trace library</title>
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<address>Document number: D0881R0</address>
<address>Project: Programming Language C++</address>
<address>Audience: Library Evolution</address>
<address> </address>
<address>Alexey Gorgurov <<a href="mailto:[email protected]">[email protected]</a>>, <<a href="mailto:[email protected]">[email protected]</a>></address>
<address>Antony Polukhin <<a href="mailto:[email protected]">[email protected]</a>>, <<a href="mailto:[email protected]">[email protected]</a>></address>
<address> </address>
<address>Date: 2018-01-23</address>
<h1>A Proposal to add stack trace library</h1>
<!--p class='notes'>Green lines are notes for the <b>editor</b> or for the <b>SG14</b> that must not be treated as part of the wording.</p-->
<h2>I. Motivation</h2>
<p>In the current working draft [<a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4713.pdf">N4713</a>] there is no way to get,store and decode the current call sequence.
Such call sequences are useful for debugging and post mortem debugging. They are popular in other programming languages (like Java, C#, Python).</p>
<p>Pretty often assertions can't describe the whole picture of a bug and do not provide enough information to locate the problem.
For example, you can see the following message on out-of-range access:</p>
<pre>
boost/array.hpp:123: T& boost::array<T, N>::operator[](boost::array<T, N>::size_type): Assertion '(i < N)&&("out of range")' failed.
Aborted (core dumped)</pre>
<p>That's not enough information in the assert message to locate the problem without debugger.</p>
<p>This paper proposes classes that could simplify debugging and may change the assertion meassage into the following:</p>
<pre>
Expression 'i < N' is false in function 'T& boost::array<T, N>::operator[](boost::array<T, N>::size_type) [with T = int; long unsigned int N = 5ul; boost::array<T, N>::reference = int&; boost::array<T, N>::size_type = long unsigned int]': out of range.
Backtrace:
0# boost::assertion_failed_msg(char const*, char const*, char const*, char const*, long) at ../example/assert_handler.cpp:39
1# boost::array<int, 5ul>::operator[](unsigned long) at ../../../boost/array.hpp:124
2# bar(int) at ../example/assert_handler.cpp:17
3# foo(int) at ../example/assert_handler.cpp:25
4# bar(int) at ../example/assert_handler.cpp:17
5# foo(int) at ../example/assert_handler.cpp:25
6# main at ../example/assert_handler.cpp:54
7# 0x00007F991FD69F45 in /lib/x86_64-linux-gnu/libc.so.6
8# 0x0000000000401139
</pre>
<h2>II. Impact on the Standard</h2>
<p>This proposal is a pure library extension and it does not break the existing code and does not degrade performance.
It does not require any changes in the core language and could be implemented in the standard C++.</p>
<h2>III. Design Decisions</h2>
<p>The design is based on the Boost.Stacktrace library, a popular library that does not depend on any non-standard library components.</p>
<p><b>Note about signal safety:</b> this proposal does not attempt to provide a signal-safe solution for capturing and decoding stacktraces.
Such functionality currently is not implementable on some of the popular platforms. However, the paper attempts to provide extendable solution, that may be made signal safe some day.</p>
<p><b>Note on performance:</b> during Boost.Stacktrace development phase many users requested a fast way to store stack trace, without decoding the function names. This functionality is preserved in the paper.</p>
<h2>IV. Proposed Interface</h2>
<h3>Header <stacktrace> synopsis</h3>
<pre>
namespace std {
class stack_frame;
template<typename Allocator>
class basic_stacktrace;
// free functions
// This is the alias to use unless you'd like to provide a specific allocator to basic_stacktrace.
using stacktrace = basic_stacktrace<allocator<stack_frame>>;
// Outputs stacktrace in a human readable format to output stream.
template<typename CharT, typename TraitsT, typename Allocator>
basic_ostream< CharT, TraitsT > & operator<<(basic_ostream<CharT, TraitsT>& os, const basic_stacktrace<Allocator>& bt);
// Outputs stacktrace in a human readable format to string.
template<typename Allocator>
string to_string(const basic_stacktrace<Allocator>& f);
// Outputs frame in a human readable format to string.
string to_string(const stack_frame& f);
// Outputs frame in a human readable format to output stream.
template<typename CharT, typename TraitsT>
basic_ostream< CharT, TraitsT >& operator<<(basic_ostream<CharT, TraitsT>& os, const stack_frame& f);
}
</pre>
<h3>Class stack_frame</h3>
<p>The stack_frame class stores a pointer to function and allows querying information about that function.</p>
<pre>
namespace std {
class stack_frame {
public:
using native_frame_ptr_t = <i>unspecified</i>;
// construct/copy/destruct
constexpr stack_frame() noexcept;
constexpr stack_frame(const stack_frame&) noexcept = default;
constexpr stack_frame& operator=(const stack_frame&) = default;
constexpr explicit stack_frame(native_frame_ptr_t f) noexcept;
template<typename T> explicit stack_frame(T* address) noexcept;
constexpr native_frame_ptr_t address() const noexcept;
constexpr explicit operator bool() const noexcept;
constexpr strong_ordering operator <=>(const stack_frame& rhs) = default;
// functions that querying information about stored address
string name() const;
string source_file() const;
size_t source_line() const;
private:
native_frame_ptr_t data; // exposition only
};
}
</pre>
<h3><code>stack_frame</code> constructors</h3>
<pre>stack_frame() noexcept;</pre>
<div class="desc">Constructs stack_frame that references nullptr address. Calls to <code>source_file()</code> and <code>source_line()</code> will return empty string. Calls to <code>source_line()</code> will return 0.</div>
<pre>explicit stack_frame(native_frame_ptr_t addr) noexcept;
template<typename T> explicit stack_frame(T * addr) noexcept;</pre>
<div class="desc">Constructs stack_frame that references addr and could later generate information about that address using platform specific features.</div>
<h3><code>stack_frame</code> member functions</h3>
<pre>std::string name() const;</pre>
<div class="desc">Returns empty string or platform specific name of the function that the stored address is pointing to. Throws std::bad_alloc if not enough memory to construct resulting string.</div>
<pre>constexpr native_frame_ptr_t address() const noexcept;</pre>
<div class="desc">Returns address stored by this.</div>
<pre>std::string source_file() const;</pre>
<div class="desc">Returns empty string or path to the source file, where the function of the frame is defined. Returns empty string if this->source_line() == 0. Throws std::bad_alloc if not enough memory to construct resulting string.</div>
<pre>std::string source_line() const;</pre>
<div class="desc">Returns 0 or code line in the source file, where the function of the stored address is defined. Throws std::bad_alloc if not enough memory to construct resulting string.</div>
<pre>explicit operator bool() const noexcept;</pre>
<div class="desc">Returns true if this stores and address other than NULL.</div>
<h3>Class template <basic_stacktrace></h3>
<p>The basic_stacktrace template class stores current call sequence on construction and provides functions for querying information about the call sequence.</p>
<pre>
namespace std {
template<typename Allocator>
class basic_stacktrace {
public:
using value_type = stack_frame;
using const_reference = const value_type &;
using size_type = <i>implementation-defined</i>;
using const_iterator = <i>implementation-defined</i>;
using allocaotr_type = Allocator;
// functions that capture current call sequence
basic_stacktrace() noexcept;
explicit basic_stacktrace(const allocator_type& a) noexcept;
basic_stacktrace(size_type skip, size_type max_depth, const allocator_type& a = allocator_type()) noexcept;
// construct/copy/destruct
basic_stacktrace(const basic_stacktrace &);
basic_stacktrace(basic_stacktrace &&) noexcept;
basic_stacktrace & operator=(const basic_stacktrace &);
basic_stacktrace & operator=(basic_stacktrace &&);
~basic_stacktrace();
// public member functions
size_type size() const noexcept;
const_reference operator[](size_type ) const noexcept;
const_iterator begin() const noexcept;
const_iterator end() const noexcept;
explicit operator bool() const noexcept;
template <typename Allocator2>
strong_ordering operator <=>(const basic_stacktrace< Allocator2 >& rhs) = default;
private:
vector<value_type> stack_frames; // exposition only
};
}
</pre>
<h3><code>basic_stacktrace</code> constructors</h3>
<pre>basic_stacktrace() noexcept;
explicit basic_stacktrace(const allocator_type & a) noexcept;</pre>
<div class="desc">Stores the current call sequence inside *this without querying information about each call.</div>
<div class="desc">Any exception raised during this operation is
silently ignored. In case of exception <code>(bool)*this</code> is <code>false</code></div>
<pre>basic_stacktrace(size_type skip, size_type max_depth, const allocator_type& a = allocator_type()) noexcept;</pre>
<div class="desc">Stores [skip; skip + max_depth) of the current function call sequence inside *this without querying information about each call.</div>
<div class="desc">Any exception raised during this operation is
silently ignored. In case of exception <code>(bool)*this</code> is <code>false</code></div>
<h3><code>basic_stacktrace</code> member functions</h3>
<pre>const_reference operator[](size_type frame_no) const noexcept;</pre>
<div class="desc">Returns frame that references the actual frame info, stored inside *this.</div>
<div class="desc">Parameters: <code>frame_no</code> - zero-based index of frame to return. 0 is the function index where stacktrace was constructed and index close to this->size() contains function main().</div>
<pre>explicit operator bool() const noexcept;</pre>
<div class="desc">Returns true if call sequence was successfully stored.</div>
<h2>V. Feature-testing macro</h2>
<p>For the purposes of SG10 we recommend the feature-testing macro name <code>__cpp_lib_stacktrace</code>.</p>
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