[libc][posix 2001] implement fileno

[nickdesaulniers]

libc++ unconditionally depends on posix fileno. We should implement this in libc.

see man 3 fileno.

https://pubs.opengroup.org/onlinepubs/9699919799/functions/fileno.html

[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: Nick Desaulniers (nickdesaulniers)

libc++ depends on posix fileno. We should implement this in libc.

see man 3 fileno.

https://pubs.opengroup.org/onlinepubs/9699919799.2018edition/

[llvmbot]

@llvm/issue-subscribers-libc

Author: Nick Desaulniers (nickdesaulniers)

libc++ depends on posix fileno. We should implement this in libc.

see man 3 fileno.

https://pubs.opengroup.org/onlinepubs/9699919799.2018edition/

[Sh0g0-1758]

Hey, @nickdesaulniers, I would like to work on this. Can you provide me some pointers to get started.

[nickdesaulniers]

Hi @Sh0g0-1758 ! Thanks for your interest.

I think these are the steps you can follow to implement this:

1. define the entrypoint in libc/config/linux/x86_64/entrypoints.txt, under the list of functions from stdio.h, I'd add an entry for libc.src.stdio.fileno. The build should now fail to configure during cmake because that target hasn't been defined anywhere yet.
2. I'd define libc.src.stdio.fileno in libc/src/stdio/CMakeLists.txt. Perhaps using add_stdio_entrypoint_object since files on the GPU require RPC calls. The build should still fail to configure at this point.
3. Define fileno in libc/src/stdio/generic/CMakeLists.txt. The build should fail to configure since fileno.cpp and fileno.h do not yet exist.
4. create libc/src/stdio/generic/fileno.cpp and libc/src/stdio/fileno.h. Make sure to include the license block. At this point the build should configure and succeed.
5. implement fileno in libc/src/stdio/generic/fileno.cpp.
6. write tests in libc/test/src/stdio/fileop_test.cpp. ninja libc_stdio_unittests should run the suite with the newly added tests.
7. update libc/spec/posix.td to add a FunctionSpec for fileno to the HeaderSpec for stdio.h.

Take a crack at it, and let us know if you get stuck!

[Sh0g0-1758]

Hey, @nickdesaulniers, made PR with the changes. Can you please suggest changes in it, if any.