Anything changed around fmt::print(ostream&) ?

[mark-99]

This code worked in 8.1.1, and works if I uncomment the lines to use ostream op<<() instead, but with trunk the fmt::print() gets an assert fail inside the CRT:

template<typename Container>
inline bool saveStatsToCSV(std::filesystem::path const& filename, std::string_view header, Container&& samples)
{
    std::ofstream of(filename);
    if (!of)
    {
        SPDLOG_ERROR("Couldn't open file '{}' for writing", filename);
        return false;
    }
    else
    {
        fmt::print(of, "{}\n", header);
        //of << header << "\n";

        for (auto const& sample : samples)
        {
            fmt::print(of, "{}\n", sample);
            //of << sample << "\n";
        }
    }

    return true;
}

Call stack:

ucrtbased.dll!_fileno(_iobuf * public_stream) Line 17
	at minkernel\crts\ucrt\src\appcrt\stdio\fileno.cpp(17)
MdpCpp.exe!fmt::v8::detail::print(_iobuf * f, fmt::v8::basic_string_view<char> text) Line 1502
	at C:\Temp\cpm-source-cache\fmt\a27d11503e3ed8119b8700c55ac8a5449cdb5537\include\fmt\format-inl.h(1502)
MdpCpp.exe!fmt::v8::detail::write(std::basic_filebuf<char,std::char_traits<char>> & buf, fmt::v8::basic_string_view<char> data) Line 86
	at C:\Temp\cpm-source-cache\fmt\a27d11503e3ed8119b8700c55ac8a5449cdb5537\include\fmt\ostream.h(86)
MdpCpp.exe!fmt::v8::detail::write_buffer<char>(std::basic_ostream<char,std::char_traits<char>> & os, fmt::v8::detail::buffer<char> & buf) Line 98
	at C:\Temp\cpm-source-cache\fmt\a27d11503e3ed8119b8700c55ac8a5449cdb5537\include\fmt\ostream.h(98)
MdpCpp.exe!fmt::v8::vprint<char>(std::basic_ostream<char,std::char_traits<char>> & os, fmt::v8::basic_string_view<char> format_str, fmt::v8::basic_format_args<fmt::v8::basic_format_context<fmt::v8::appender,char>> args) Line 158
	at C:\Temp\cpm-source-cache\fmt\a27d11503e3ed8119b8700c55ac8a5449cdb5537\include\fmt\ostream.h(158)
MdpCpp.exe!fmt::v8::print<std::basic_string_view<char,std::char_traits<char>> &>(std::basic_ostream<char,std::char_traits<char>> & os, fmt::v8::basic_format_string<char,std::basic_string_view<char,std::char_traits<char>> &> fmt, std::basic_string_view<char,std::char_traits<char>> & args) Line 174
	at C:\Temp\cpm-source-cache\fmt\a27d11503e3ed8119b8700c55ac8a5449cdb5537\include\fmt\ostream.h(174)

_fileno() asserts in format-inl.h because f==nullptr:

namespace detail {
FMT_FUNC void print(std::FILE* f, string_view text) {
#ifdef _WIN32
  auto fd = _fileno(f);

Any ideas? Clearly there's an easy workaround in this particular case, but would rather know why it's failing.

[mwinterb]

8751a03 is the commit that changed the behavior. If you can find out why get_file is returning nullptr that'd be helpful. I may be able to look into it this weekend, but since you have a repro set up...

[mark-99]

Minimal repro:

int main()
{
    std::ofstream of("c:\\Temp\\Test.txt");
    if (!of)
        return 1;

    fmt::print(of, "{}", "Hello\n");
    return 0;
}

It gets here ok (ostream.h):

inline bool write(std::filebuf& buf, fmt::string_view data) {
  print(get_file(buf), data);
  return true;
}

Then get_file(buf) calls this:

// Formatting of built-in types and arrays is intentionally disabled because
// it's handled by standard (non-ostream) formatters.
template <typename Char> FILE* get_file(std::basic_filebuf<Char>&) {
  return nullptr;
}

so now it's print(nullptr, data) and bang.

So something is screwy. Also I'd suggest return nullptr would be better as e.g. static_assert() (or even regular assert), or similar.

[vitaut]

It's mostly a matter of adding a null check in

fmt/include/fmt/ostream.h

Line 85 in 8e47cfd

print(get_file(buf), data);

and a test case. A PR is welcome.

[mark-99]

How does that fix the actual problem, ie the code does not work? I want it to write the string to the file, not fail in a different way.

[vitaut]

It should fallback to writing to ostream (by returning false from write on null), not fail. This is already what happens in other cases when there is no file descriptor.

[vitaut]

Fixed in eaa8efb. Thanks for reporting.