[deschutes]

My complaint is that it is too easy to create an object with uninitialized members by accident. On account of the syntax being identical. I guess its too late to add affordances there.

I suppose a definition of valid that prohibited uninitialized members would preclude lots of useful stuff like container and buffer types.

From the msan documentation [1], the flaw with my earlier example is that `printf` isn't instrumented.

And to address your other question, I don't know if msan can instrument a function implemented in assembly. It definitely can't deal with something it didn't compile as the instrumentation is added during compilation.

It seems that on godbolt the platform library also isn't instrumented because the equivalent iostream code is msan clean [2]. I suppose that makes sense as it's allowing you to pass arbitrary options to the compiler.

In summary, msan can detect these uninitialized reads but it requires quite a lot of fiddling.

[1]: https://clang.llvm.org/docs/MemorySanitizer.html#handling-ex... [2]: https://godbolt.org/z/Gsxsfn9GT

[jcelerier]

I think there could also be issues with what godbolt prints ; this example: https://godbolt.org/z/x9W3xGeeb

shows nothing on godbolt, but when run on my local machine yields the expected

    $ clang++ bar.cpp -fsanitize=memory -DFMT_HEADER_ONLY=1 -std=c++20
    $ ./a.out
    42
    ==269465==WARNING: MemorySanitizer: use-of-uninitialized-value
        #0 0x55ba20d00c8e in fmt::v8::appender fmt::v8::detail::write<char, fmt::v8::appender, int, 0>(fmt::v8::appender, int) (/tmp/a.out+0xb9c8e)
        #1 0x55ba20cff0ac in fmt::v8::appender fmt::v8::detail::default_arg_formatter<char>::operator()<int>(int) (/tmp/a.out+0xb80ac)
        #2 0x55ba20d6ef67 in char const* fmt::v8::detail::parse_replacement_field<char, void fmt::v8::detail::vformat_to<char>(fmt::v8::detail::buffer<char>&, fmt::v8::basic_string_view<char>, fmt::v8::basic_format_args<fmt::v8::basic_format_context<std::conditional<std::is_same<fmt::v8::type_identity<char>::type, char>::value, fmt::v8::appender, std::back_insert_iterator<fmt::v8::detail::buffer<fmt::v8::type_identity<char>::type> > >::type, fmt::v8::type_identity<char>::type> >, fmt::v8::detail::locale_ref)::format_handler&>(char const*, char const*, void fmt::v8::detail::vformat_to<char>(fmt::v8::detail::buffer<char>&, fmt::v8::basic_string_view<char>, fmt::v8::basic_format_args<fmt::v8::basic_format_context<std::conditional<std::is_same<fmt::v8::type_identity<char>::type, char>::value, fmt::v8::appender, std::back_insert_iterator<fmt::v8::detail::buffer<fmt::v8::type_identity<char>::type> > >::type, fmt::v8::type_identity<char>::type> >, fmt::v8::detail::locale_ref)::format_handler&) (/tmp/a.out+0x127f67)
        #3 0x55ba20cfaca9 in void fmt::v8::detail::vformat_to<char>(fmt::v8::detail::buffer<char>&, fmt::v8::basic_string_view<char>, fmt::v8::basic_format_args<fmt::v8::basic_format_context<std::conditional<std::is_same<fmt::v8::type_identity<char>::type, char>::value, fmt::v8::appender, std::back_insert_iterator<fmt::v8::detail::buffer<fmt::v8::type_identity<char>::type> > >::type, fmt::v8::type_identity<char>::type> >, fmt::v8::detail::locale_ref) (/tmp/a.out+0xb3ca9)
        #4 0x55ba20cf8a57 in fmt::v8::vprint(_IO_FILE*, fmt::v8::basic_string_view<char>, fmt::v8::basic_format_args<fmt::v8::basic_format_context<fmt::v8::appender, char> >) (/tmp/a.out+0xb1a57)
        #5 0x55ba20cf86c2 in fmt::v8::vprint(fmt::v8::basic_string_view<char>, fmt::v8::basic_format_args<fmt::v8::basic_format_context<fmt::v8::appender, char> >) (/tmp/a.out+0xb16c2)
        #6 0x55ba20cf7cce in main (/tmp/a.out+0xb0cce)
        #7 0x7f45c65b330f in __libc_start_call_main libc-start.c
        #8 0x7f45c65b33c0 in __libc_start_main@GLIBC_2.2.5 (/usr/lib/libc.so.6+0x2d3c0)
        #9 0x55ba20c6a444 in _start (/tmp/a.out+0x23444)
    
    SUMMARY: MemorySanitizer: use-of-uninitialized-value (/tmp/a.out+0xb9c8e) in fmt::v8::appender fmt::v8::detail::write<char, fmt::v8::appender, int, 0>(fmt::v8::appender, int)
    Exiting

[deschutes]

`fmt::print` isn't using the uninitialized variable in your example. After correcting that, it produces a report. https://godbolt.org/z/97bo3YTbc

[jcelerier]

wops, good catch