diff --git a/libc/cmake/modules/LLVMLibCLibraryRules.cmake b/libc/cmake/modules/LLVMLibCLibraryRules.cmake
--- a/libc/cmake/modules/LLVMLibCLibraryRules.cmake
+++ b/libc/cmake/modules/LLVMLibCLibraryRules.cmake
@@ -1,4 +1,6 @@
 function(collect_object_file_deps target result)
+  # NOTE: This function does add entrypoint targets to |result|.
+  # It is expected that the caller adds them separately.
   set(all_deps "")
   get_target_property(target_type ${target} "TARGET_TYPE")
   if(NOT target_type)
@@ -28,7 +30,6 @@
       endif()
       set(entrypoint_target ${aliasee})
     endif()
-    list(APPEND all_deps ${entrypoint_target})
     get_target_property(deps ${target} "DEPS")
     foreach(dep IN LISTS deps)
       collect_object_file_deps(${dep} dep_targets)
@@ -80,6 +81,21 @@
     endif()
     collect_object_file_deps(${dep} recursive_deps)
     list(APPEND all_deps ${recursive_deps})
+    # Add the entrypoint object target explicitly as collect_object_file_deps
+    # only collects object files from non-entrypoint targets.
+    if(${dep_type} STREQUAL ${ENTRYPOINT_OBJ_TARGET_TYPE})
+      set(entrypoint_target ${dep})
+      get_target_property(is_alias ${entrypoint_target} "IS_ALIAS")
+      if(is_alias)
+        get_target_property(aliasee ${entrypoint_target} "DEPS")
+        if(NOT aliasee)
+          message(FATAL_ERROR
+                  "Entrypoint alias ${entrypoint_target} does not have an aliasee.")
+        endif()
+        set(entrypoint_target ${aliasee})
+      endif()
+    endif()
+    list(APPEND all_deps ${entrypoint_target})
   endforeach(dep)
   list(REMOVE_DUPLICATES all_deps)
   set(objects "")
diff --git a/libc/config/baremetal/entrypoints.txt b/libc/config/baremetal/entrypoints.txt
--- a/libc/config/baremetal/entrypoints.txt
+++ b/libc/config/baremetal/entrypoints.txt
@@ -17,6 +17,9 @@
     libc.src.ctype.tolower
     libc.src.ctype.toupper
 
+    # errno.h entrypoints
+    libc.src.errno.errno
+
     # string.h entrypoints
     libc.src.string.bcmp
     libc.src.string.bcopy
diff --git a/libc/config/linux/aarch64/entrypoints.txt b/libc/config/linux/aarch64/entrypoints.txt
--- a/libc/config/linux/aarch64/entrypoints.txt
+++ b/libc/config/linux/aarch64/entrypoints.txt
@@ -17,6 +17,9 @@
     libc.src.ctype.tolower
     libc.src.ctype.toupper
     
+    # errno.h entrypoints
+    libc.src.errno.errno
+
     # fcntl.h entrypoints
     libc.src.fcntl.creat
     libc.src.fcntl.open
diff --git a/libc/config/linux/arm/entrypoints.txt b/libc/config/linux/arm/entrypoints.txt
--- a/libc/config/linux/arm/entrypoints.txt
+++ b/libc/config/linux/arm/entrypoints.txt
@@ -17,6 +17,9 @@
     libc.src.ctype.tolower
     libc.src.ctype.toupper
 
+    # errno.h entrypoints
+    libc.src.errno.errno
+
     # string.h entrypoints
     libc.src.string.bcmp
     libc.src.string.bcopy
diff --git a/libc/config/linux/x86_64/entrypoints.txt b/libc/config/linux/x86_64/entrypoints.txt
--- a/libc/config/linux/x86_64/entrypoints.txt
+++ b/libc/config/linux/x86_64/entrypoints.txt
@@ -17,6 +17,9 @@
     libc.src.ctype.tolower
     libc.src.ctype.toupper
 
+    # errno.h entrypoints
+    libc.src.errno.errno
+
     # fcntl.h entrypoints
     libc.src.fcntl.creat
     libc.src.fcntl.open
diff --git a/libc/docs/dev/code_style.rst b/libc/docs/dev/code_style.rst
--- a/libc/docs/dev/code_style.rst
+++ b/libc/docs/dev/code_style.rst
@@ -65,3 +65,40 @@
    definitions that are implicitly inline. Examples of such functions are
    class methods (static and non-static) defined inline and ``constexpr``
    functions.
+
+Setting ``errno`` from runtime code
+===================================
+
+Many libc functions set ``errno`` to indicate an error condition. If LLVM's libc
+is being used as the only libc, then the ``errno`` from LLVM's libc is affected.
+If LLVM's libc is being used in the :ref:`overlay_mode`, then the ``errno`` from
+the system libc is affected. When a libc function, which can potentially affect
+the ``errno``, is called from a unit test, we do not want the global ``errno``
+(as in, the ``errno`` of the process thread running the unit test) to be
+affected. If the global ``errno`` is affected, then the operation of the unit
+test infrastructure itself can be affected. To avoid perturbing the unit test
+infrastructure around the setting of ``errno``, the following rules are to be
+followed:
+
+#. A special macro named ``libc_errno`` defined in ``src/errno/libc_errno.h``
+   should be used when setting ``errno`` from libc runtime code. For example,
+   code to set ``errno`` to ``EINVAL`` should be:
+
+   .. code-block:: c++
+
+    libc_errno = EINVAL;
+
+#. ``errno`` should be set just before returning from the implementation of the
+   public function. It should not be set from within helper functions. Helper
+   functions should use idiomatic C++ constructs like
+   `cpp::optional <https://github.com/llvm/llvm-project/blob/main/libc/src/__support/CPP/optional.h>`_
+   and
+   `ErrorOr <https://github.com/llvm/llvm-project/blob/main/libc/src/__support/error_or.h>`_
+   to return error values.
+
+#. The header file ``src/errno/libc_errno.h`` is shipped as part of the target
+   corresponding to the ``errno`` entrypoint ``libc.src.errno.errno``. We do
+   not in general allow dependecies between entrypoints. However, the ``errno``
+   entrypoint is the only exceptional entrypoint on which other entrypoints
+   should explicitly depend on if they set ``errno`` to indicate error
+   conditions.
diff --git a/libc/src/errno/CMakeLists.txt b/libc/src/errno/CMakeLists.txt
--- a/libc/src/errno/CMakeLists.txt
+++ b/libc/src/errno/CMakeLists.txt
@@ -1,4 +1,4 @@
-add_object_library(
+add_entrypoint_object(
   errno
   SRCS
     libc_errno.cpp