[libc][c11] implement ctime

[zimirza]

This is an implementation of ctime and includes ctime_r.

According to documentation, ctime and ctime_r are defined as the following:

char *ctime(const time_t *timep);
char *ctime_r(const time_t *restrict timep, char buf[restrict 26]);

closes #86567

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[michaelrj-google]

This is a good start, but I think we can simplify the code a bit. You'll also need to update the build system in a few places:

• add ctime and ctime_r to libc/config/linux/x86_64 as well as any other targets you want to enable these on.
• add ctime and ctime_r to the headergen files
  • for newheadergen you can run the "add_function" command described here: https://libc.llvm.org/dev/header_generation.html (the standard is stdc and the guard and attribute should each be null).
  • for old headergen you'll still have to add them manually in libc/spec/stdc.td

Feel free to reach out if you have questions on how to do any of this, and thank you for the contribution!

[zimirza]

Thank you for the thorough feedback. I apologize for going back and forth in this pull request. I will try to double check my work in future pull requests.

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[michaelrj-google]

All good, review is a normal part of LLVM development. Now that this is approved, do you need me to merge it for you?

[zimirza]

That would be great, since I cannot merge in LLVM.

[michaelrj-google]

I ran the tests locally and ran into some issues, these need to be fixed before we can merge this.

[zimirza]

I am so sorry about this. I have now fixed them all.

[michaelrj-google]

it's fine, just make sure you run the tests to see what's working and what's broken. I usually just run ninja check-libc but if you're using make it should be make check-libc. There are a couple more places where things are broken, I'll add comments in a bit when I get a chance.

[michaelrj-google]

also before I forget: If the tests aren't running for you it might be because these tests only run in full build mode: https://libc.llvm.org/full_host_build.html

When switching to full build mode you'll need to clean out your build folder because otherwise there are some confusing errors.

[zimirza]

Sure, I will try to run the full build mode. I will let you know when this is ready.

[zimirza]

I am testing locally and did find some issues. I will be fixing it now.

[zimirza]

Tests for ctime and ctime_r are now good.

[michaelrj-google]

Thanks for fixing this, it's all working on my machine now so I'll plan on landing it on Monday (since I don't want to land right before I get off work on Friday).

I really appreciate you working through this and getting it completed. I know it took a lot of effort but it's worth it at the end to have something high quality like this.

[zimirza]

Thank you for the thorough review. I really do appreciate it. I have a question. Since this pull request is a new API feature implementation, should I also update documentation for ctime and ctime_r?

[michaelrj-google]

You're welcome, and yes updating the docs for ctime and ctime_r would be good. I don't know if there's an existing page tracking the status of the time functions (it'd be in libc/docs). If there isn't then you can skip it. Adding a whole new page for the time functions should probably be a separate patch.

[github-actions]

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[nickdesaulniers]

Looks like #109892 was a follow up to this PR.

[zimirza]

That is correct. Unfortunately, I did not compare my implementation with glibc's ctime. So, I created a new issue for this. Hope it is ok.