diff --git a/libc/config/linux/header_conf.td b/libc/config/linux/header_conf.td
new file mode 100644
--- /dev/null
+++ b/libc/config/linux/header_conf.td
@@ -0,0 +1,87 @@
+include "config/public_api.td"
+
+def LinuxStandards : Standards {
+  // On Linux, we support Public API specified by
+  // the C standard library and the POSIX extensions.
+  let Standards = [
+    "stdc",
+    "posix",
+  ];
+}
+
+def FloatT : TypeDecl<"float_t"> {
+  let Decl = [{
+    #if __FLT_EVAL_METHOD__ == 1
+      typedef float float_t
+    #elseif __FLT_EVAL_METHOD__ == 2
+      ...
+    #else
+      ...
+    #endif
+  }]; // This is only an example and not exactly how it will appear
+}
+
+def SizeT : TypeDecl<"size_t"> {
+  let Decl = [{
+    #define __need_size_t
+    #include <stddef.h>
+  }];
+}
+
+def NullMacro : MacroDef<"NULL"> {
+  let Defn = [{
+    #define __need_NULL
+    #include <stddef.h>
+  }];
+}
+
+def Math : PublicAPI<"math.h"> {
+  let Functions = [
+    FunctionDecl<"acos">,
+    FunctionDecl<"acosl">,
+  ];
+
+  let TypeDeclarations = [
+    FloatT,
+  ];
+}
+
+def String : PublicAPI<"string.h"> {
+  let Functions = [
+    FunctionDecl<"strcat">,
+    FunctionDecl<"strcpy">
+  ];
+
+  let TypeDeclarations = [
+    SizeT,
+  ];
+
+  let Macros = [
+    NullMacro,
+  ];
+}
+
+def StdIO : PublicAPI<"stdio.h"> {
+  let TypeDeclarations = [
+    SizeT,
+  ];
+
+  let Functions = [
+    FunctionDecl<
+        "snprintf",
+
+        // The function attribute below is only valid with clang and gcc.
+        // It is included here as an example.
+        "__attribute__((format(__printf__(3, 4))))",
+
+        // The annotations below are only for illustration purpose and are valid
+        // under msvc I think.
+        // Since there are three argument annotations listed here, the function
+        // should take atleast 3 formal arguments including the varargs.
+        // The header generator will check the arg count mismatch if any.
+        [ArgAnn<["_Out_writes_opt_(_BufferCount)", "_Always_(_Post_z_)"], "_Buffer">,
+         ArgAnn<["_In_"], "_BufferCount">,
+         ArgAnn<["_In_z_", "_Printf_format_string_"], "_Format">]
+    >,
+  ];
+}
diff --git a/libc/config/linux/headers.txt b/libc/config/linux/headers.txt
new file mode 100644
--- /dev/null
+++ b/libc/config/linux/headers.txt
@@ -0,0 +1,6 @@
+
+add_libc_headers(
+  llvmlibc_headers
+  math_h
+  string_h
+)
diff --git a/libc/lib/CMakeLists.txt b/libc/config/linux/library_targets.txt
copy from libc/lib/CMakeLists.txt
copy to libc/config/linux/library_targets.txt
diff --git a/libc/config/public_api.td b/libc/config/public_api.td
new file mode 100644
--- /dev/null
+++ b/libc/config/public_api.td
@@ -0,0 +1,31 @@
+class Standards {
+  list<string> Standards;
+}
+
+class TypeDecl<string name> {
+  string Name = name;
+  string Decl = "";
+}
+
+class MacroDef<string name> {
+  string Name = name;
+  string Defn = "";
+}
+
+class ArgAnn<list<string> annotations, string name = ""> {
+  list<string> Annotations = annotations;
+  string Name = name;
+}
+
+class FunctionDecl<string name, string attr = "", list<ArgAnn> argAnnotations = []> {
+  string Name = name;
+  string Attr = attr;
+  list<ArgAnn> ArgAnnotations = argAnnotations;
+}
+
+class PublicAPI<string header> {
+  list<FunctionDecl> Functions;
+  list<TypeDecl> TypeDeclarations;
+  list<MacroDef> Macros;
+  list<string> Structs;
+}
diff --git a/libc/docs/build_system.rst b/libc/docs/build_system.rst
--- a/libc/docs/build_system.rst
+++ b/libc/docs/build_system.rst
@@ -22,3 +22,18 @@
 ``add_entrypoint_library`` target  takes a list of ``add_entrypoint_object``
 targets and produces a static library containing the object files corresponding
 to the ``add_entrypoint_targets``.
+
+Rules for header files
+======================
+
+There are two rules for header files. These rules can be used for internal as
+well as public header files. They are explained in detail below:
+
+1. ``add_hdr`` - This rule should be used for public header files whose content
+   is guaranteed to be platform independent and available on all platforms that
+   LLVM libc supports.
+
+2. ``gen_hdr`` - This rule should be used to generate header files from
+   ``.h.def`` files. The format of the ``h.def`` files is explained in the
+   header generation document.
+
diff --git a/libc/docs/ground_truth_specification.rst b/libc/docs/ground_truth_specification.rst
new file mode 100644
--- /dev/null
+++ b/libc/docs/ground_truth_specification.rst
@@ -0,0 +1,11 @@
+The ground truth of standards
+=============================
+
+Like any modern libc, LLVM libc also supports a wide number of standards and
+extensions. To avoid developing headers, wrappers and sources in a disjointed
+fashion, LLVM libc employs ground truth files. These files live under the
+``spec`` directory and list ground truth corresponding the ISO C standanrd, the
+POSIX extension standard, etc. For example, the path to the ground truth file
+for the ISO C standard is ``spec/stdc.td``. Tools like the header generator
+(described in the header generation document), docs generator, etc. use the
+ground truth files to generate headers, docs etc.
diff --git a/libc/docs/header_generation.rst b/libc/docs/header_generation.rst
--- a/libc/docs/header_generation.rst
+++ b/libc/docs/header_generation.rst
@@ -96,3 +96,21 @@
 
   The header generator will only include content starting from the line after the
   line on which this command is listed.
+
+``public_api``
+~~~~~~~~~~~~~~
+
+This is a replacement command which should be listed in an input ``.h.def``
+file. The header file generator will replace this command with the public API of
+the target platform. See the build system document for more information on the
+relevant build rules. Also, see "Mechanics of public_api" to learn the mechanics
+of how the header generator replaces this command with the public API.
+
+Arguments
+
+  None.
+
+Action
+
+  The header generator will replace this command with the public API to be exposed
+  from the generated header file.
diff --git a/libc/docs/mechanics_of_public_api.rst b/libc/docs/mechanics_of_public_api.rst
new file mode 100644
--- /dev/null
+++ b/libc/docs/mechanics_of_public_api.rst
@@ -0,0 +1,30 @@
+The mechanics of the ``public_api`` command
+===========================================
+
+The build system, in combination with the header generation mechanism,
+facilitates the fine grained ability to pick and choose the public API one wants
+to expose on their platform. The public header files are always generated from
+the corresponding ``.h.def`` files. A header generation command ``%%public_api``
+is listed in these files. In the generated header file, the header generator
+replaces this command with the public API relevant for the target platform.
+
+Under the hood
+--------------
+
+When the header generator sees the ``%%public_api`` command, it looks up the
+header config file for the platform in the path
+``config/<platform>/header_config.td``. The header config file lists two kinds
+of items:
+
+1. The list of standards from which the public entities available on the platform
+   are derived from.
+2. For each header file exposed on the platfrom, the list of public members
+   provided in that header file.
+
+Note that, the header generator only learns the names of the public entities
+from the header config file (the 2nd item from above.) The exact manner in which
+the entities are to be declared is got from the standards (the 1st item from
+above.)
+
+See the ground truth document for more information on how the standards are
+formally listed in LLVM libc using LLVM table-gen files.
diff --git a/libc/lib/CMakeLists.txt b/libc/lib/CMakeLists.txt
--- a/libc/lib/CMakeLists.txt
+++ b/libc/lib/CMakeLists.txt
@@ -1,9 +1,4 @@
+# DO NOT list targets directly in this file unless it is guaranteed to be
+# platform independent.
 
-add_entrypoint_library(
-  llvmlibc
-  DEPENDS
-    ### C standard library entrypoints
-    # string.h entrypoints
-    strcpy
-    strcat
-)
+include($LIBC_SOURCE_DIR/config/${LIBC_TARGET_OS}/library_targets.txt)
diff --git a/libc/spec/posix.td b/libc/spec/posix.td
new file mode 100644
diff --git a/libc/spec/spec.td b/libc/spec/spec.td
new file mode 100644
--- /dev/null
+++ b/libc/spec/spec.td
@@ -0,0 +1,52 @@
+class Type {}
+
+class NamedType<string name> : Type {
+  string Name = name;
+}
+
+class Field<string name, Type type> {
+  string Name = name;
+  Type FieldType = type;
+}
+
+// Class to describe concrete structs specified in the standard.
+class Struct<string name> : NamedType<name> {
+  list<Field> Fields;
+}
+
+class PtrType<Type type> : Type {
+  Type PointeeType = type;
+}
+
+class ConstType<Type type> : Type {
+  Type UnqualifiedType = type;
+}
+
+class RestrictedPtrType<Type type> : Type {
+  Type PointeeType = type;
+}
+
+// Builtin types.
+def VarArgType : Type {}
+def VoidType : NamedType<"void">;
+def IntType : NamedType<"int">;
+def FloatType : NamedType<"float">;
+def DoubleType : NamedType<"double">;
+def CharType : NamedType<"char">;
+
+class Macro<string name> {
+  string Name = name;
+}
+
+class Function<string name, Type return_type, list<Type> args> {
+  string Name = name;
+  Type ReturnType = return_type;
+  list<Type> Args = args;
+}
+
+class Header<string name> {
+  string Name = name;
+  list<Function> Functions;
+  list<Type> Types;
+  list<Macro> Macros;
+}
diff --git a/libc/spec/stdc.td b/libc/spec/stdc.td
new file mode 100644
--- /dev/null
+++ b/libc/spec/stdc.td
@@ -0,0 +1,47 @@
+include "spec/spec.td"
+
+def Math : Header<"math.h"> {
+  let Functions = [
+    Function<"acos", DoubleType, [DoubleType]>
+  ];
+
+  let Types = [
+    NamedType<"float_t">,
+    NamedType<"double_t">,
+  ];
+}
+
+def VoidPtr : PtrType<VoidType>;
+def VoidRestrictedPtr : RestrictedPtrType<VoidType>;
+def ConstVoidRestrictedPtr : ConstType<VoidRestrictedPtr>;
+
+def CharPtr : PtrType<CharType>;
+def CharRestrictedPtr : RestrictedPtrType<CharType>;
+def ConstCharRestrictedPtr : ConstType<CharRestrictedPtr>;
+
+def SizeTType : NamedType<"size_t">;
+
+def String : Header<"string.h"> {
+  let Types = [
+    SizeTType,
+  ];
+
+  let Macros = [
+    Macro<"NULL">,
+  ];
+
+  let Functions = [
+    Function<"strcpy", CharPtr, [CharRestrictedPtr, ConstCharRestrictedPtr]>,
+    Function<"memcpy", VoidPtr, [VoidRestrictedPtr, ConstVoidRestrictedPtr, SizeTType]>
+  ];
+}
+
+def StdIO : Header<"stdio.h"> {
+  let Types = [
+    SizeTType,
+  ];
+
+  let Functions = [
+    Function<"snprintf", IntType, [CharPtr, SizeTType, ConstCharRestrictedPtr, VarArgType]>,
+  ];
+}