diff options
authorBoris Kolpackov <boris@codesynthesis.com>2020-04-07 10:14:19 +0200
committerBoris Kolpackov <boris@codesynthesis.com>2020-04-07 10:14:19 +0200
commit524df81a705c75a1111b58f61aae3b04850bfc46 (patch)
parentf63b3274d6d79398b700eb1e2342a77f7d0ab3b7 (diff)
Draft 3config-doc
1 files changed, 93 insertions, 4 deletions
diff --git a/doc/manual.cli b/doc/manual.cli
index 543ce57..b011a4f 100644
--- a/doc/manual.cli
+++ b/doc/manual.cli
@@ -4371,18 +4371,98 @@ value is to be provided, it should have a sensible default with a bias for
simplicity and compatibility rather than the optimal result. For example, in
the optional functionality case, the default should probably be to provide it.
+\N|The \c{build2} build system currently provides no support for
+\c{autoconf}-style probing of the build environment in order to automatically
+discover available libraries, functions, features, etc.
+The main reason for omitting this support is the fundamental ambiguity and the
+resulting brittleness of such probing due to the reliance on compiler, linker,
+or test execution failures. Specifically, in many such tests it is impossible
+for build system to distinguish between the missing feature, the broken test,
+and the mis-configured build environment situations. This leads to requiring a
+user intervention in the best case and to a mis-configured build in the
+worst. Other issues with this approach include portability, speed (compiling
+and linking take time), as well as limited applicability during
+cross-compilation (specifically, inability to run tests).
+As a result, we recommend using build-time, \i{expectation-based}
+configuration where your project expects a feature to be available if certain
+conditions are met. Examples of such conditions at source code level include
+C++ feature test macros, platform macros, runtime library macros, compiler
+macros, etc., with the build system modules exposing some of the same
+information via variables to allow making similar decisions in
+\c{buildfiles}. Another alternative is to automatically adapt to missing
+features using more advanced techniques such as C++ SFINAE. And where none of
+this is possible, delegate the decision to the user via a configuration value.
+Our experience with \c{build2} as well as those of other large cross-platform
+projects such as Boost shows that this is a viable strategy.
+Having said that, \c{build2} does provide the ability to extract configuration
+information from the environment (\c{$getenv()} function) or other tools
+(\c{$process.run*()} family of functions). Note, however, that for this to work
+reliably there should be no ambiguity between the \"no configuration
+available\" case (if such a case is possible) and the \"something went
+wrong\" case. We show a realistic example of this in \l{#proj-config-report
+Configuration Report} where we extract the plugin directory from GCC
+which may have been configured without plugin support.|
As discussed in the introduction, the central part of the build configuration
functionality are the \i{configuration variables}. They are automatically
treated as overridable with global visibility and persisted by the \c{config}
-module (see \l{#intro-operations-config Configuring} for details).
+module (see \l{#intro-operations-config Configuring} for details). The
+following example, based on the \c{libhello} project from the introduction,
+provides an overview of the project configuration functionality with the
+remainder of the chapter providing the detailed explanation of all the
+parts shown as well as the alternative approaches.
+├── build/
+│ ├── root.build
+│ └── ...
+├── libhello/
+│ ├── hello.cxx
+│ ├── buildfile
+│ └── ...
+└── ...
+# build/root.build
+config [string] config.libhello.greeting ?= 'Hello'
+# libhello/buildfile
+cxx.poptions += \"-DLIBHELLO_GREETING=\\\"$config.libhello.greeting\\\"\"
+// lihello/hello.cxx
+void say_hello (ostream& o, const string& n)
+ o << LIBHELLO_GREETING \", \" << n << '!' << endl;
+$ b configure config.libhello.greeting=Hi -v
+config libhello@/tmp/libhello/
+ greeting Hi
+$ b -v
+g++ ... -DLIBHELLO_GREETING=\"Hi\" ...
By (enforced) convention, configuration variables start with \c{config.}, for
example, \c{config.import.libhello}. In case of a build system module, the
second component in its configuration variables should be the module name, for
example, \c{config.cxx}, \c{config.cxx.coptions}. Similarly, project-specific
configuration variables should have the project name as their second
-component, for example, \c{config.libhello.fancy}.
+component, for example, \c{config.libhello.greeting}.
\N|More precisely, a project configuration variable must match the
\c{config[.**].<project>.**} pattern where additional components may be
@@ -4846,7 +4926,11 @@ on how this works). The rest is changed as follows:
# libhello/buildfile
lib{hello}: {hxx ixx txx cxx}{** -version -config} hxx{version config}
hxx{config}: in{config}
+ install = false
@@ -4865,8 +4949,8 @@ void say_hello (ostream& o, const string& n)
\N|With this setup, the way to export configuration information to our
-library's users is to install the configuration header, similar to how we do
-it for the version header.|
+library's users is to make the configuration header public and install it,
+similar to how we do it for the version header.|
Now that the macro-based version is working, let's see how we can take
advantage of modern C++ features to hopefully improve on some of their
@@ -4961,7 +5045,12 @@ namespace hello
# libhello/buildfile
lib{hello}: {hxx ixx txx cxx}{** -config} {hxx cxx}{config}
hxx{config}: in{config}
+ install = false
cxx{config}: in{config}