[PATCH] libstdc++: Improve ostream output for std::stacktrace

[Nathan Myers]

On 10/15/25 5:02 PM, Jonathan Wakely wrote:
> Instead of printing "<unknown>" in the ostream output, we could set that
> string in the stacktrace_entry::_Info object, so that description()
> returns that. Returning an empty string from that function seems simpler
> for users to handle and decide on their own behaviour, rather than
> having to parse "<unknown>".
> 
> With this change locations that don't have a source file look like this:
> 
>    18# __libc_start_call_main at [0x7fd6568f6574]
>    19# __libc_start_main@GLIBC_2.2.5 at [0x7fd6568f6627]
>    20# _start at [0x4006a4]
> 
> Instead of:
> 
>    18# __libc_start_call_main at :0
>    19# __libc_start_main@GLIBC_2.2.5 at :0
>    20# _start at :0
> 
> The string representation of a default constructed stacktrace_entry is
> now "<unknown>" instead of an empty string.
> 
> libstdc++-v3/ChangeLog:
> 
> 	* include/std/stacktrace (stacktrace_entry::_Print_pc): New
> 	struct for writing native handle to ostream.
> 	(operator<<(ostream&, const stacktrace_entry&)): Improve output
> 	when description() or source_file() returns an empty string,
> 	or the stacktrace_entry is invalid.
> 	(operator<<(ostream&, const basic_stacktrace<A>&)): Use
> 	size_type of the correct specialization.
> ---
> 
> Tested x86_64-linux.
> 
> Any ideas for improving the output further?
> 
> We need to implement std::formatter specializations for these types too,
> which could just call the to_string overloads, or could do something
> smarter.
> 
>   libstdc++-v3/include/std/stacktrace | 34 +++++++++++++++++++++++++----
>   1 file changed, 30 insertions(+), 4 deletions(-)
> 
> diff --git a/libstdc++-v3/include/std/stacktrace b/libstdc++-v3/include/std/stacktrace
> index b9e260e19f89..61aea57ec0d3 100644
> --- a/libstdc++-v3/include/std/stacktrace
> +++ b/libstdc++-v3/include/std/stacktrace
> @@ -134,6 +134,21 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>       friend ostream&
>       operator<<(ostream&, const stacktrace_entry&);
>   
> +    struct _Print_pc
> +    {
> +      native_handle_type _M_pc;
> +
> +      friend ostream&
> +      operator<<(ostream& __os, _Print_pc __pc)
> +      {
> +	if (__pc._M_pc != (native_handle_type)-1) [[likely]]
> +	  __os << "[0x" << std::hex << __pc._M_pc << std::dec << ']';

Maybe save the output radix, and restore it.
Or just use the facet directly.

> +	else
> +	  __os << "<unknown>";
> +	return __os;
> +      }
> +    };
> +
>       // Type-erased wrapper for the fields of a stacktrace entry.
>       // This type is independent of which std::string ABI is in use.
>       struct _Info
> @@ -683,13 +698,23 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>     inline ostream&
>     operator<<(ostream& __os, const stacktrace_entry& __f)
>     {
> +    using _Pc = stacktrace_entry::_Print_pc;
>       string __desc, __file;
>       int __line;
> -    if (__f._M_get_info(&__desc, &__file, &__line))
> +    if (__f._M_get_info(&__desc, &__file, &__line)) [[likely]]
>         {
> -	__os.width(4);
> -	__os << __desc << " at " << __file << ':' << __line;
> +	if (__desc.empty()) [[unlikely]]
> +	  __os << "<unknown>";
> +	else
> +	  __os << __desc;
> +	__os << string_view(" at ");
> +	if (__file.empty()) [[unlikely]]
> +	  __os << _Pc{__f.native_handle()};
> +	else
> +	  __os << __file << ':' << __line;
>         }
> +    else
> +      __os << _Pc{__f.native_handle()};
>       return __os;
>     }
>   
> @@ -697,7 +722,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>       inline ostream&
>       operator<<(ostream& __os, const basic_stacktrace<_Allocator>& __st)
>       {
> -      for (stacktrace::size_type __i = 0; __i < __st.size(); ++__i)
> +      using size_type = typename basic_stacktrace<_Allocator>::size_type;
> +      for (size_type __i = 0, __size = __st.size(); __i < __size; ++__i)
>   	{
>   	  __os.width(4);
>   	  __os << __i << "# " << __st[__i] << '\n';