[libc] Implement fcntl function

[SchrodingerZhu]

This is currently missing. It is useful when checking fd flags.

[llvmbot]

Hi!

This issue may be a good introductory issue for people new to working on LLVM. If you would like to work on this issue, your first steps are:

1. In the comments of the issue, request for it to be assigned to you.
2. Fix the issue locally.
3. Run the test suite locally. Remember that the subdirectories under test/ create fine-grained testing targets, so you can e.g. use make check-clang-ast to only run Clang's AST tests.
4. Create a Git commit.
5. Run git clang-format HEAD~1 to format your changes.
6. Open a pull request to the upstream repository on GitHub. Detailed instructions can be found in GitHub's documentation.

If you have any further questions about this issue, don't hesitate to ask via a comment in the thread below.

[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: Schrodinger ZHU Yifan (SchrodingerZhu)

This is currently missing. It is useful when checking fd flags.

[l32zhao]

@nickdesaulniers I'd like to work on this.

[SchrodingerZhu]

The function itself is defined in:

https://man7.org/linux/man-pages/man2/fcntl.2.html

https://pubs.opengroup.org/onlinepubs/007904975/functions/fcntl.html

For Linux, it should be simply a syscall wrapper.

https://elixir.bootlin.com/linux/latest/source/fs/fcntl.c

[SchrodingerZhu]

You can refer to #84974. It is still a WIP PR, but it shows what files would a syscall wrapper implementation typically involve.

[vinayakdsci]

Hi @l32zhao, are you still working on this? Otherwise I'd be happy to take this issue up. Thanks!