A Proposal to add stack trace library

Significant changes to P0881R1 are marked with blue.

Show deleted lines from P0881R1.

I. Motivation

In the current working draft [N4741] 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).

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:

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)

That's not enough information in the assert message to locate the problem without debugger.

This paper proposes classes that could simplify debugging and may change the assertion message into the following:

Expression 'i < N' is false in function 'T& boost::array<T, N>::operator[](boost::array<T, N>::size_type)': 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

II. Impact on the Standard

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++.

II. Design Decisions

The design is based on the Boost.Stacktrace library, a popular library that does not depend on any non-standard library components.

Note about signal safety: 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 extensible solution, that may be made signal safe some day by providing a signal safe allocator and changing the stacktrace implementation details.

Note on function_info name: P0881R1 had a class named stack_frame. LWG found out that stack_frame is a bad name because stack_frame is usable not only in stacktraces but it also could be constructed from function pointer (not some pointer to frame) to provide information about the function.

Type name function_info reminds of the type_info which is not a very popular class. Unlike function_info the type_info class is not copyable, not assignable, not constructible by users, not hashable and not less comparable. Nevertheless we find that functionality of those types is quite common and think that function_info is the suitable name. Additional care was taken to not shut the door closed for making the interfaces more common in the future. Here's a list of changes that may be done later for making those types closer:

• add string type_info::pretty_name()
• add strong_ordering type_info::operator<=>(const type_info& other) const noexcept = default;
• add string type_info::source_file() and size_t type_info::source_line()
• add const cahr* function_info::name()
• enable hash specialization for type_info

Note on performance: 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. All the function_info functions and constructors are lazy and won't decode the pointer information if there was no explicit request from class user.

Note on allocations: initial implementations of Boost.Stacktrace were not using allocator and all the frames were placed inside a fixed size internal storage. That was a mistake! Sometimes the most important information is located at the bottom of the stack. For example if you run Boost.Test, then the test name will be located low on the stack. With a fixed size storage the bottom of the stack could be lost along with the information.

Current design assumes that by default users wish to see the whole stack and OK with dynamic allocations, because do not construct stacktrace in performance critical places. For those users, who wish to use stacktrace on a hot path or in embedded environments basic_stacktrace allows to provide a custom allocator that allocates on the stack or in some other place, where users thinks it is appropriate.

Note on returning std::string and not having noexcept on function_info::source_line(): Unfortunately this is a necessarity on some platforms, where getting source line requires allocating or where source file name returned into a storage provided by user.

Note on expected implementation: We assume that Standard Library implementations would allow to disable/enable gathering stack traces by a compiler switch that does not require recompiling the whole project. In other words, we expect to see a compiler option like -fno-stacktrace or libstacktrace/lib_stacktrace_noop libraries with the same ABI that would force the constructor of the basic_stacktrace to do nothing. This feature is implemented in Boost.Stacktrace and is highly requested in big projects.

III. Significant changes to review

LEWG:

• stack_frame was renamed to function_info.
• native_ptr_t is now an object pointer rather than a function pointer. The intent is to require some kind of const void* that is not callable but could store function pointers.
• Added hash support.
• LWG requested rbegin like functions for basic_stacktrace, so now basic_stacktrace satisfies the requirements of an allocator-aware container, of a sequence container and reversible container except that only operations defined for const-qualified sequence containers are supported and that the semantics of comparison functions and default constructor are different from those required for a container.
• Added template<class MemberFunc> explicit function_info(MemberFunc addr) noexcept;. Does LEWG wish to see this function?
• How about moving function_info into a separate header <function_info>?

Points of special interest for CWG:

• native_ptr_t description
• stacktrace definition in [stacktrace] and member functions in [function_info.query]. Wording should not prevent any optimizations.

IV. Wording Intent

Key features that should be preserved by implementations:

• All the functions are lazy and do not query the pointer information if there was no explicit request.
• No fixed max size for trace - all the available invokers must be stored in a stacktrace.
• Implementations should allow to disable/enable gathering stacktraces by a link-time switch.
• Stacktracing does not prevent any of the optimizations.
• No info for a pointer is OK (it's not an error, do not throw!).
• function_info::pretty_name() should return a demangled function signature if possible.
• to_string(stacktrace) should query information from debug symbols, symbol export tables and return demangled signatures if possible.

V. Wording

23.? Stacktrace [stacktrace]

Define INVOKER(x) as a function that returns the function that invoked function x; NTH_INVOKERn(x) as a function NTH_INVOKERn-1(INVOKER(x)) for n > 0 and NTH_INVOKER0(x) as INVOKER(x). A sequence of functions (f₀, ..., f_m) called stacktrace, where:

• f₀ is the current function or the result of applying NTH_INVOKERn1(x) to the current function with implementation-defined n1,
• for each _j in range (0, m] function f_j is the result of applying NTH_INVOKERnj(x) to the f_j-1 with implementation-defined nj and m,
• f_m also is the main function or is the result of applying NTH_INVOKERnm(x) to the main function with implementation-defined nm; m could be zero if current function is the main function.

This subclause describes components that C++ programs may use to store the stacktrace of the current thread of execution, query information about the stored stacktrace or particular function at runtime.

23.?.1 Header <stacktrace> synopsis [stacktrace.syn]

namespace std {
  // 23.?.2, class function_info
  class function_info;

  // 23.?.3, class basic_stacktrace
  template<class Allocator>
  class basic_stacktrace;

  // basic_stacktrace typedef names
  using stacktrace = basic_stacktrace<allocator<function_info>>;

  // 23.?.4, non-member functions
  void swap(function_info& a, function_info& b) noexcept;

  template<class Allocator>
  void swap(basic_stacktrace<Allocator>& a, basic_stacktrace<Allocator>& b);

  template<class Allocator>
  string to_string(const basic_stacktrace<Allocator>& st);

  string to_string(const function_info& f);

  template<class charT, class traits, class Allocator>
  basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const basic_stacktrace<Allocator>& st);

  template<class charT, class traits>
  basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const function_info& f);

  // 23.?.5, hash support
  template<class T> struct hash;
  template<> struct hash<function_info>;
  template<class Allocator> struct hash<basic_stacktrace<Allocator>>;
}
        

23.?.2 Class function_info [function_info]

namespace std {
  class function_info {
  public:
    // member typedefs
    using native_ptr_t = implementation-defined;

    // 23.?.2.1, construct/copy/assign/destroy
    constexpr function_info() noexcept;
    constexpr explicit function_info(native_ptr_t addr) noexcept;
    template<class Func> explicit function_info(Func* addr) noexcept;
    template<class MemberFunc> explicit function_info(MemberFunc addr) noexcept;

    constexpr function_info(const function_info& other) noexcept = default;
    constexpr function_info& operator=(const function_info& other) noexcept = default;
    ~function_info() = default;

    // 23.?.2.2, observers
    constexpr native_ptr_t address() const noexcept;
    constexpr explicit operator bool() const noexcept;

    // 23.?.2.3, query
    string pretty_name() const;
    string source_file() const;
    size_t source_line() const;

    // 23.?.2.4, comparison
    constexpr strong_ordering operator<=>(const function_info& other) const noexcept = default;

    // 23.?.2.5, modifiers
    void swap(function_info& other) noexcept;

  private:
    native_ptr_t ptr_; // exposition only
  };
}
        

An object of class function_info stores an address of any function type or an address of any member function type or it has no value. function_info provides operations for querying information not deducible from function type.

native_ptr_t is an implementation-defined object pointer type that designates a function or member function. Pointer arithmetic on native_ptr_t is ill-formed.

23.?.2.1 function_info construct/copy/assign/destroy [function_info.cons]

constexpr function_info() noexcept;

constexpr explicit function_info(native_ptr_t addr) noexcept;

template<class Func> explicit function_info(Func* addr) noexcept;

template<class MemberFunc> explicit function_info(MemberFunc addr) noexcept;

    struct base {
        void no_virt();
        virtual void virt() = 0;
    };
    struct derived: base {
        void no_virt();
        void virt() override;
    };

    function_info(&base::no_virt).source_line();    // returns information for base::no_virt if possible
    function_info(&base::virt).source_line();       // returns information for base::virt if possible
    function_info(&derived::no_virt).source_line(); // returns information for derived::no_virt if possible

    using base_ptr_t = void(base::*)();
    base_ptr_t base_ptr = &derived::virt;

    // returns information for base::virt if possible
    function_info(base_ptr).source_line();

23.?.2.2 function_info observers [function_info.obs]

constexpr native_ptr_t address() const noexcept;

constexpr explicit operator bool() const noexcept;

23.?.2.3 function_info query [function_info.query]

[Note: All the function_info query functions treat errors other than memory allocation errors (like errors during filesystem operations) as "no information available" and do not throw in that case. - end note]

string pretty_name() const;

string source_file() const;

size_t source_line() const;

23.?.2.4 function_info comparison [function_info.comp]

constexpr strong_ordering operator<=>(const function_info& other) const noexcept = default;

23.?.2.4 function_info modifiers [function_info.mod]

void swap(function_info& other) noexcept;

23.?.3 Class template basic_stacktrace [stacktrace.basic.template]

namespace std {
  template<class Allocator>
  class basic_stacktrace {
  public:
    using value_type = function_info;
    using const_reference = const value_type&;
    using reference = value_type&;
    using const_iterator = implementation-defined;
    using iterator = const_iterator;
    using reverse_iterator = std::reverse_iterator<iterator>;
    using const_reverse_iterator = std::reverse_iterator<const_iterator>;
    using difference_type = typename iterator_traits<iterator>::difference_type;
    using size_type = typename allocator_traits<Allocator>::size_type;
    using allocator_type = Allocator;

    // 23.?.3.1, construct/copy/destroy
    basic_stacktrace() noexcept;
    explicit basic_stacktrace(const allocator_type& alloc) noexcept;
    basic_stacktrace(size_type skip, size_type max_depth, const allocator_type& alloc = allocator_type()) noexcept;

    basic_stacktrace(const basic_stacktrace& other) = default;
    basic_stacktrace(basic_stacktrace&& other) noexcept = default;
    basic_stacktrace& operator=(const basic_stacktrace& other) = default;
    basic_stacktrace& operator=(basic_stacktrace&& other) = default;
    ~basic_stacktrace() = default;

    // 23.?.3.2, observers
    allocator_type get_allocator() const;

    const_iterator begin() const noexcept;
    const_iterator end() const noexcept;
    const_reverse_iterator rbegin() const noexcept;
    const_reverse_iterator rend() const noexcept;

    const_iterator cbegin() const noexcept;
    const_iterator cend() const noexcept;
    const_reverse_iterator crbegin() const noexcept;
    const_reverse_iterator crend() const noexcept;

    explicit operator bool() const noexcept;
    [[nodiscard]] bool empty() const noexcept;
    size_type size() const noexcept;
    size_type max_size() const noexcept;

    const_reference operator[](size_type ) const;
    const_reference at(size_type ) const;

    // 23.?.3.3, comparisons
    template <class Allocator2>
    strong_ordering operator<=>(const basic_stacktrace< Allocator2 >& rhs) const noexcept;

    // 23.?.3.4, modifiers
    void swap(basic_stacktrace& other);

  private:
    vector<value_type, allocator_type> frames; // exposition only
  };

}
        

The basic_stacktrace template class stores the stacktrace of the current thread of execution on construction and provides access to the stored stacktrace.

The class template basic_stacktrace satisfies the requirements of an allocator-aware container, of a sequence container and reversible container (21.2.1, 21.2.3) except that only operations defined for const-qualified sequence containers are supported and that the semantics of comparison functions and default constructor are different from those required for a container.

23.?.3.1 basic_stacktrace construct/copy/destroy [stacktrace.basic.cons]

basic_stacktrace() noexcept;
explicit basic_stacktrace(const allocator_type& alloc) noexcept;

basic_stacktrace(size_type skip, size_type max_depth, const allocator_type& alloc = allocator_type()) noexcept;

23.?.3.2 basic_stacktrace observers [stacktrace.basic.obs]

allocator_type get_allocator() const;

const_iterator begin() const noexcept;
const_iterator cbegin() const noexcept;

const_iterator end() const noexcept;
const_iterator cend() const noexcept;

const_iterator rbegin() const noexcept;
const_iterator crbegin() const noexcept;

const_iterator rend() const noexcept;
const_iterator crend() const noexcept;

explicit operator bool() const noexcept;

[[nodiscard]] bool empty() const noexcept;

size_type size() const noexcept;

size_type max_size() const noexcept;

const_reference operator[](size_type frame_no) const;

const_reference at(size_type frame_no) const;

23.?.3.3 basic_stacktrace comparisons [stacktrace.basic.comp]

template <class Allocator2>
strong_ordering operator<=>(const basic_stacktrace< Allocator2 >& rhs) const noexcept;

23.?.3.4 basic_stacktrace modifiers [stacktrace.basic.mod]

void swap(basic_stacktrace& other);

23.?.4 Non-member functions [stacktrace.nonmembers]

void swap(function_info& a, function_info& b) noexcept;

template<class Allocator>
void swap(basic_stacktrace<Allocator>& a, basic_stacktrace<Allocator>& b);

template<class Allocator>
string to_string(const basic_stacktrace<Allocator>& st);

string to_string(const function_info& f);

template<class charT, class traits, class Allocator>
basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const basic_stacktrace<Allocator>& st);

template<class charT, class traits>
basic_ostream<charT, traits>& operator<<(basic_ostream<charT, traits>& os, const function_info& f);

23.?.5 Hash support [stacktrace.hash]

template<> struct hash<function_info>;
template<class Allocator> struct hash<basic_stacktrace<Allocator>>;

Feature-testing macro

Add a row into the "Standard library feature-test macros" table [support.limits.general]:

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);
}
		

namespace std {
  class stack_frame {
  public:
    using native_frame_ptr_t = unspecified;

    // construct/copy/destruct
    constexpr stack_frame() noexcept;
    constexpr stack_frame(const stack_frame&) noexcept = default;
    constexpr stack_frame& operator=(const stack_frame&) noexcept = 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 query information about stored address
    string name() const;
    string source_file() const;
    size_t source_line() const;

  private:
    native_frame_ptr_t data; // exposition only
  };
}
		

stack_frame() noexcept;

explicit stack_frame(native_frame_ptr_t addr) noexcept;
template<typename T> explicit stack_frame(T * addr) noexcept;

std::string name() const;

constexpr native_frame_ptr_t address() const noexcept;

std::string source_file() const;

std::string source_line() const;

explicit operator bool() const noexcept;

namespace std {
  template<typename Allocator>
  class basic_stacktrace {
  public:
    using value_type = stack_frame;
    using const_reference = const value_type &;
    using size_type = implementation-defined;
    using const_iterator = implementation-defined;
    using allocator_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 &&) noexcept;
    ~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) noexcept = default;

  private:
    vector<value_type>    stack_frames; // exposition only
  };

}
		

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;

const_reference operator[](size_type frame_no) const noexcept;

explicit operator bool() const noexcept;

VI. Acknowledgements

Many thanks to Jens Maurer, JF Bastien and Marshall Clow for pointing out many issues in the early wordings.

Many many thanks to all the people who participated in the LWG meeting on 20th of August and reviewed early version of the wording.