A Proposal to add stack trace library

Green lines are notes for the editor or for the SG14 that must not be treated as part of the wording.

I. Motivation

At this time there is no standard solution to get the calls sequence that results in unhandled exception, std::terminate or assertion failure. That sequence is widely used in another programming languages (like Java, C#, Python) for debugging and post mortem debugging.

Pretty often assertions can't describe the whole picture of bug and does not provide enough information to locate the problem. For example, you can see the following message on out-of-range access:

../../../boost/array.hpp:123: T& boost::array<T, N>::operator[](boost::array<T, N>::size_type) [with T = int; long unsigned int N = 5ul]: Assertion '(i < N)&&("out of range")' failed. Aborted (core dumped)

This assert can be in any translation unit including the boost/array.hpp header.

With adding of the proposed classes you may see the following message:

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
	

We propose two headers (<stacktrace> and <frame>), two classes (template <typename Allocator>class basic_stacktrace and class frame) and several functions for safe stack frames dump.

II. Impact on the Standard

This proposal is a pure library extension and it do not break the existing code and do not degrade performance. It is does not require any changes in the core language and could be implemented in standard C++.

III. Design Decisions

The design is based on Boost.Stacktrace, an popular library that does not depend on any non-standard library components and provides the STD-like interface.

Note about signal safety: we can't offer the proposal which could be signal-safe on any platform because it's not possible to implement.

The stack frame sequence is stored inside the basic_stacktrace class, the one stack frame is stored inside the frame class.

basic_stacktrace constructors

basic_stacktrace() noexcept;

Stores the current function call sequence inside *this without any decoding or any other heavy platform specific operations.

explicit basic_stacktrace(const allocator_type & a) noexcept;

Stores the current function call sequence inside *this without any decoding or any other heavy platform specific operations.

basic_stacktrace(std::size_t skip, std::size_t max_depth, const allocator_type & a = allocator_type()) noexcept;

Stores [skip; skip + max_depth) of the current function call sequence inside *this without any decoding or any heavy platform specific operations.

basic_stacktrace member functions

size_type size() const noexcept;

const_reference operator[](std::size_t frame_no) const noexcept;

Returns frame that references the actual frame info, stored inside *this.

Parameters: frame_no - 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().

const_iterator begin() const noexcept;

const_iterator end() const noexcept;

explicit operator bool() const noexcept;

Allows to check that stack trace capturing was successful.

bool empty() const noexcept;

Allows to check that stack trace failed.

const std::vector< std::stacktrace::frame, Allocator > & as_vector() const noexcept;

basic_stacktrace operator member functions

template <typename Allocator1, typename Allocator2>
	  std::strong_ordering operator <=>(const basic_stacktrace< Allocator1 > & lhs,
                     const basic_stacktrace< Allocator2 > & rhs) = default;

Auto-generated comparison operators that provide platform dependent ordering and have amortized O(1) complexity; O(size()) worst case complexity; are Async-Handler-Safe.

basic_stacktrace public static functions

template<typename Char, typename Trait> static basic_stacktrace from_dump(std::basic_istream< Char, Trait > & in, const allocator_type & a = allocator_type());

Constructs stacktrace from basic_istreamable that references the dumped stacktrace. Terminating zero frame is discarded.

static basic_stacktrace from_dump(const void * begin, std::size_t buffer_size_in_bytes, const allocator_type & a = allocator_type());

Constructs stacktrace from raw memory dump. Terminating zero frame is discarded.

frame constructors

frame() noexcept;

explicit frame(native_frame_ptr_t addr) noexcept;

Constructs frame that references NULL address. Calls to source_file() and source_line() will return empty string. Calls to source_line() will return 0.

frame(const frame &) = default;

explicit frame(native_frame_ptr_t addr) noexcept;

Constructs frame that references addr and could later generate information about that address using platform specific features.

template<typename T> explicit frame(T * function_addr) noexcept;

Constructs frame that references function_addr and could later generate information about that function using platform specific features.

constexpr frame & operator=(const frame &) = default;

frame public member functions

std::string name() const;

Returns platform specific name of the frame (function name in a human readable form). Throws std::bad_alloc if not enough memory to construct resulting string.

constexpr native_frame_ptr_t address() const noexcept;

Returns address of the frame function.

std::string source_file() const;

Returns 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.

std::string source_line() const;

Returns code line in the source line, where the function of the frame is defined. Throws std::bad_alloc if not enough memory to construct resulting string.

constexpr bool empty() const;

Checks that frame is not references NULL address.

frame operator member functions

std::strong_ordering operator <=>(const frame & lhs, const frame & rhs) = default;

Comparison operators that provide platform dependent ordering and have O(1) complexity; are Async-Handler-Safe.

IV. Proposed Interface

Header <stacktrace>

namespace std {
  namespace stacktrace {
    template<typename Allocator> class basic_stacktrace;

    typedef basic_stacktrace stacktrace;  // This is the typedef to use unless you'd like to provide a specific allocator to std::stacktrace::basic_stacktrace.
    

    // Fast hashing support, O(st.size()) complexity; Async-Handler-Safe. 
    template<typename Allocator>
      std::size_t hash_value(const basic_stacktrace< Allocator > & st);

    // Outputs stacktrace in a human readable format to output stream; unsafe to use in async handlers.
    template<typename CharT, typename TraitsT, typename Allocator>
      std::basic_ostream< CharT, TraitsT > &
      operator<<(std::basic_ostream< CharT, TraitsT > & os,
                 const basic_stacktrace< Allocator > & bt);

    // Fast hashing support, O(1) complexity; Async-Handler-Safe. 
    std::size_t hash_value(const frame & f);

    // Outputs stacktrace::frame in a human readable format to string; unsafe to use in async handlers. 
    std::string to_string(const frame & f);

    // Outputs stacktrace::frame in a human readable format to output stream; unsafe to use in async handlers. 
    template<typename CharT, typename TraitsT> 
      std::basic_ostream< CharT, TraitsT > & 
      operator<<(std::basic_ostream< CharT, TraitsT > & os, const frame & f);
  }
}
		

V. Feature-testing macro

For the purposes of SG10 we recommend the feature-testing macro name __cpp_lib_stacktrace.