89 lines
3.2 KiB
ReStructuredText
89 lines
3.2 KiB
ReStructuredText
=======================================================
|
|
Capturing configuration information during installation
|
|
=======================================================
|
|
|
|
.. contents::
|
|
:local:
|
|
|
|
The Problem
|
|
===========
|
|
|
|
Currently the libc++ supports building the library with a number of different
|
|
configuration options. Unfortunately all of that configuration information is
|
|
lost when libc++ is installed. In order to support "persistent"
|
|
configurations libc++ needs a mechanism to capture the configuration options
|
|
in the INSTALLED headers.
|
|
|
|
|
|
Design Goals
|
|
============
|
|
|
|
* The solution should not INSTALL any additional headers. We don't want an extra
|
|
#include slowing everybody down.
|
|
|
|
* The solution should not unduly affect libc++ developers. The problem is limited
|
|
to installed versions of libc++ and the solution should be as well.
|
|
|
|
* The solution should not modify any existing headers EXCEPT during installation.
|
|
It makes developers lives harder if they have to regenerate the libc++ headers
|
|
every time they are modified.
|
|
|
|
* The solution should not make any of the libc++ headers dependant on
|
|
files generated by the build system. The headers should be able to compile
|
|
out of the box without any modification.
|
|
|
|
* The solution should not have ANY effect on users who don't need special
|
|
configuration options. The vast majority of users will never need this so it
|
|
shouldn't cost them.
|
|
|
|
|
|
The Solution
|
|
============
|
|
|
|
When you first configure libc++ using CMake we check to see if we need to
|
|
capture any options. If we haven't been given any "persistent" options then
|
|
we do NOTHING.
|
|
|
|
Otherwise we create a custom installation rule that modifies the installed __config
|
|
header. The rule first generates a dummy "__config_site" header containing the required
|
|
#defines. The contents of the dummy header are then prependend to the installed
|
|
__config header. By manually prepending the files we avoid the cost of an
|
|
extra #include and we allow the __config header to be ignorant of the extra
|
|
configuration all together. An example "__config" header generated when
|
|
-DLIBCXX_ENABLE_THREADS=OFF is given to CMake would look something like:
|
|
|
|
.. code-block:: cpp
|
|
|
|
//===----------------------------------------------------------------------===//
|
|
//
|
|
// The LLVM Compiler Infrastructure
|
|
//
|
|
// This file is dual licensed under the MIT and the University of Illinois Open
|
|
// Source Licenses. See LICENSE.TXT for details.
|
|
//
|
|
//===----------------------------------------------------------------------===//
|
|
|
|
#ifndef _LIBCPP_CONFIG_SITE
|
|
#define _LIBCPP_CONFIG_SITE
|
|
|
|
/* #undef _LIBCPP_HAS_NO_GLOBAL_FILESYSTEM_NAMESPACE */
|
|
/* #undef _LIBCPP_HAS_NO_STDIN */
|
|
/* #undef _LIBCPP_HAS_NO_STDOUT */
|
|
#define _LIBCPP_HAS_NO_THREADS
|
|
/* #undef _LIBCPP_HAS_NO_MONOTONIC_CLOCK */
|
|
/* #undef _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS */
|
|
|
|
#endif
|
|
// -*- C++ -*-
|
|
//===--------------------------- __config ---------------------------------===//
|
|
//
|
|
// The LLVM Compiler Infrastructure
|
|
//
|
|
// This file is dual licensed under the MIT and the University of Illinois Open
|
|
// Source Licenses. See LICENSE.TXT for details.
|
|
//
|
|
//===----------------------------------------------------------------------===//
|
|
|
|
#ifndef _LIBCPP_CONFIG
|
|
#define _LIBCPP_CONFIG
|