[libc-commits] [libc] [libc][docs] Add guide for implementing a function (PR #188499)

[Jeff Bailey via libc-commits]

https://github.com/kaladron created https://github.com/llvm/llvm-project/pull/188499

Added implementing_a_function.rst providing a checklist for adding a new function to LLVM-libc.

Updated dev/index.rst to include the new guide in the toctree.

>From f7f93deb81268760b14369477da13c73e7c20f65 Mon Sep 17 00:00:00 2001
From: Jeff Bailey <jbailey at raspberryginger.com>
Date: Wed, 25 Mar 2026 14:34:41 +0000
Subject: [PATCH] [libc][docs][NFC] Add guide for implementing a function

Added implementing_a_function.rst providing a checklist for adding a new function to LLVM-libc.

Updated dev/index.rst to include the new guide in the toctree.
---
 libc/docs/dev/implementing_a_function.rst | 73 +++++++++++++++++++++++
 libc/docs/dev/index.rst                   |  1 +
 2 files changed, 74 insertions(+)
 create mode 100644 libc/docs/dev/implementing_a_function.rst

diff --git a/libc/docs/dev/implementing_a_function.rst b/libc/docs/dev/implementing_a_function.rst
new file mode 100644
index 0000000000000..a5bca9e980ef6
--- /dev/null
+++ b/libc/docs/dev/implementing_a_function.rst
@@ -0,0 +1,73 @@
+.. _implementing_a_function:
+
+===========================
+Implementing a New Function
+===========================
+
+This guide provides a step-by-step walkthrough for adding a new function to LLVM-libc.
+
+.. contents:: Table of Contents
+   :depth: 2
+   :local:
+
+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
+------------------------
+
+Register the new entrypoint for the target platforms to include it in the build.
+
+*   **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.
diff --git a/libc/docs/dev/index.rst b/libc/docs/dev/index.rst
index 615db030d7edd..b61e46a59a43a 100644
--- a/libc/docs/dev/index.rst
+++ b/libc/docs/dev/index.rst
@@ -13,6 +13,7 @@ Navigate to the links below for information on the respective topics:
    code_style
    source_tree_layout
    entrypoints
+   implementing_a_function
    cmake_build_rules
    config_options
    clang_tidy_checks