localtime wtih strftime does not seem to work properly

[justinmcgrath]

There's a very good chance I just don't understand what is going on, but this does not work as I'd expect (not my code).

#include <ctime>
#include <chrono>
#include <iostream> 

using namespace std::chrono;
using std::string;

string current_iso8601_datetime()
{
    system_clock::time_point now = system_clock::now();
    time_t timet = system_clock::to_time_t(now);
    std::tm tm{};
    string format = string("%I:%M:%S%z");
    localtime_s(&tm, &timet);
    string result = string(255, 0);
    std::cout << result << std::endl;
    size_t length = std::strftime(&result[0], result.size(), format.c_str(), &tm);
    result.resize(length);
    return result;
}

int main()
{
    std::cout << current_iso8601_datetime() << std::endl;
}

Using MSYS2 (installed with msys2-x86_64-20210725.exe) and g++ version 10.3.0, this outputs something like "11:55:59Central Daylight Time", but it should be "11:55:59-0500".

"%z" is supposed to return the offset from UTC. "%Z" is supposed to return the time zone name. Both are outputting the name. On a similar vein, "%F" and "%T" do not work at all.

Compiling on Ubuntu 20.04 with whatever that latest g++ is works as expected.

Links to strftime descriptions for MS and the standard: https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/strftime-wcsftime-strftime-l-wcsftime-l?view=msvc-160 https://www.cplusplus.com/reference/ctime/strftime/

[lazka]

Unrelated to the problem, do you maybe want to use the mingw gcc instead? pacman -S mingw-w64-x86_64-toolchain -> open a mingw64 shell, use g++ there.

[justinmcgrath]

That is what I've done, so maybe I should I report this to mingw instead?

[justinmcgrath]

Yeah, I think I've confused something things. If I use an MSYS terminal, and change localtime_s(&tm, &timet); to localtime_r(&timet, &tm); in order to match the different libraries, then everything works correctly. The problem appears to be in mingw.

[lazka]

I see. I get under mingw64: 08:53:00W. Europe Daylight Time and under ucrt64: 08:53:23+0200

[lazka]

So it looks to me that msvcrt is lacking those features. This looks related: https://sourceforge.net/p/mingw-w64/bugs/793/

If you depend on your mingw builds behaving like with MSVC I'd suggest to use the ucrt64 (or clang) environment instead. See https://www.msys2.org/docs/environments/ for details

[justinmcgrath]

Ah, I haven't used msys in a while and didn't realize it worked this way. How do I install the ucrt64 environment? I can't seem to figure it out.

[lazka]

It's there by default, C:/msys64/ucrt64.exe. its packages/groups start with mingw-w64-ucrt-x86_64-, for example pacman -S mingw-w64-ucrt-x86_64-toolchain

[justinmcgrath]

Ah, thank you.

On Thu, Aug 26, 2021 at 4:22 PM Christoph Reiter @.***> wrote:

It's there by default, C:/msys64/ucrt64.exe. its packages/groups start with mingw-w64-ucrt-x86_64-, for example pacman -S mingw-w64-ucrt-x86_64-toolchain

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