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Reid Kleckner 821afb2ed8 [docs] Rewrite libc docs from reST to markdown (#208802 )

Tracking issue: https://github.com/llvm/llvm-project/issues/201242
Migration guide docs:
https://llvm.org/docs/SphinxQuickstartTemplate.html#markdown-migration-guidelines
RFC:
https://discourse.llvm.org/t/rfc-make-myst-markdown-the-llvm-docs-format-rip-rest/90840

This was prepared with rst2myst plus LLM-assisted cleanup. I paged
through all the generated HTML looking for migration artifacts, and all
of the differences I could find appear to be formatting error
corrections. Please spot check my work and approve if it looks good.

2026-07-10 11:45:43 -07:00

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(implementing_a_function)=

Implementing a New Function

This guide provides a step-by-step walkthrough for adding a new function to LLVM-libc.

Overview

Adding a new function involves several steps, from updating the public specification to implementing and testing the code. Below is the standard checklist for contributors.

Step-by-Step Checklist

1. Header Entry

Update the standard YAML file that describes the public header to ensure the function is included in the generated public header.

File: libc/include/<header>.yaml (or libc/include/sys/<header>.yaml for system headers)
Add the new function to the functions list.
Specify its name, return type, and arguments.
List the standards it complies with (e.g., stdc, POSIX).

2. Header Declaration

Declare the function in the internal implementation header file. This file is used by other internal code.

File: libc/src/<header>/<func>.h
Follow the structure defined in {ref}implementation_standard.
Ensure the declaration is inside the LIBC_NAMESPACE_DECL namespace.

3. Implementation

Write the actual code for the function.

File: libc/src/<header>/<func>.cpp (or libc/src/<header>/<os>/<func>.cpp for platform-specific implementations)
Use the LLVM_LIBC_FUNCTION macro.
Refer to {ref}code_style for naming and layout conventions.

4. CMake Rule

Add a CMake target for the new function so it can be compiled.

File: libc/src/<header>/CMakeLists.txt
Add an add_entrypoint_object rule for the new file.
List all internal dependencies correctly to ensure proper build order.

5. Platform Registration

File: libc/config/<os>/<arch>/entrypoints.txt
Add the new function to the list of active entrypoints.

6. Testing

Create tests to verify the implementation.

File: libc/test/src/<header>/<func>_test.cpp
Add corresponding tests using the internal testing framework.
Update the CMakeLists.txt in the test directory (libc/test/src/<header>/CMakeLists.txt) to include the new test target.

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