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diff --git a/doc/manual.cli b/doc/manual.cli index 5aea9c1..159ac3e 100644 --- a/doc/manual.cli +++ b/doc/manual.cli @@ -565,15 +565,15 @@ summary: hello C++ executable \ The \c{config} module provides support for persistent configurations. While -project configuration is a large topic that we will discuss in detail later, -in a nutshell \c{build2} support for configuration is an integral part of the -build system with the same mechanisms available to the build system core, -modules, and your projects. However, without \c{config}, the configuration -information is \i{transient}. That is, whatever configuration information was -automatically discovered or that you have supplied on the command line is -discarded after each build system invocation. With the \c{config} module, -however, we can \i{configure} a project to make the configuration -\i{persistent}. We will see an example of this shortly. +build configuration is a large topic that we will be discussing in more detail +later, in a nutshell \c{build2} support for configuration is an integral part +of the build system with the same mechanisms available to the build system +core, modules, and your projects. However, without \c{config}, the +configuration information is \i{transient}. That is, whatever configuration +information was automatically discovered or that you have supplied on the +command line is discarded after each build system invocation. With the +\c{config} module, however, we can \i{configure} a project to make the +configuration \i{persistent}. We will see an example of this shortly. Next up are the \c{test}, \c{install}, and \c{dist} modules. As their names suggest, they provide support for testing, installation and preparation of @@ -583,9 +583,11 @@ operations, and the \c{dist} module defines the \c{dist} (meta-)operation. Again, we will try them out in a moment. Moving on, the \c{root.build} file is optional though most projects will have -it. This is the place where we normally establish project-wide settings as -well as load build system modules that provide support for the languages/tools -that we use. Here is what it could look like for our \c{hello} example: +it. This is the place where we define project's configuration knobs (subject +of \l{#proj-config Project Configuration}), establish project-wide settings, +as well as load build system modules that provide support for the +languages/tools that we use. Here is what it could look like for our \c{hello} +example: \ cxx.std = latest @@ -1391,7 +1393,7 @@ if ($c.target.class != 'windows') c.libs += -lpthread # only C \ -Additionally, as we will see in \l{#intro-operations-config Configuration}, +Additionally, as we will see in \l{#intro-operations-config Configuring}, there are also the \c{config.cc.*}, \c{config.c.*}, and \c{config.cxx.*} sets which are used by the users of our projects to provide external configuration. The initial values of the \c{cc.*}, \c{c.*}, and \c{cxx.*} variables are taken @@ -1620,11 +1622,11 @@ Other than \c{version}, all the modules we load define new operations. Let's examine each of them starting with \c{config}. -\h2#intro-operations-config|Configuration| +\h2#intro-operations-config|Configuring| As mentioned briefly earlier, the \c{config} module provides support for persisting configurations by having us \i{configure} our projects. At first it -may feel natural to call \c{configure} another operation. There is, however, a +may feel natural to call \c{configure} an operation. There is, however, a conceptual problem: we don't really configure a target. And, perhaps after some meditation, it should become clear that what we are really doing is configuring operations on targets. For example, configuring updating a C++ @@ -2102,7 +2104,7 @@ For details on the unit test support implementation see \l{#intro-unit-test Implementing Unit Testing}.| -\h2#intro-operations-install|Installation| +\h2#intro-operations-install|Installing| The \c{install} module defines the \c{install} and \c{uninstall} operations. As the name suggests, this module provides support for project installation. @@ -2287,7 +2289,7 @@ header would have been installed as \c{.../include/libhello/details/utility.hxx}. -\h2#intro-operations-dist|Distribution| +\h2#intro-operations-dist|Distributing| The last module that we load in our \c{bootstrap.build} is \c{dist} which provides support for the preparation of distributions and defines the \c{dist} @@ -3063,7 +3065,7 @@ libhello/ \ And we want to build them with several compilers, let's say GCC and Clang. As -we have already seen in \l{#intro-operations-config Configuration}, we can +we have already seen in \l{#intro-operations-config Configuring}, we can configure several out of source builds for each compiler, for example: \ @@ -4336,6 +4338,509 @@ the number of jobs to perform in parallel, the stack size, queue depths, etc. See the \l{b(1)} man pages for details. +\h1#proj-config|Project Configuration| + +As discussed in the introduction (specifically, \l{#intro-proj-struct Project +Structure}) support for build configurations is an integral part of the build +system with the same mechanism used by the build system core (for example, +for project importation via the \c{config.import.*} variables), by the build +system modules (for example, for supplying compile options such as +\c{config.cxx.coptions}), as well as by our projects to provide any +project-specific configurability. Project configuration is the topic of this +chapter. + +But before we delve into the technical details, let's discuss the overall need +for project configurability. While it may seem that making ones project more +configurable is always a good idea, there are costs: by having a choice we +increase the complexity and open the door for potential incompatibility. +Specifically, we may end up with two projects in the same build needing a +shared dependency with incompatible configurations. + +\N|While some languages, such as Rust, support having multiple +differently-configured projects in the same build, this is not something that +is often done in C/C++. This ability is also not without its drawbacks, most +notably code bloat.| + +As a result, our recommendation is to strive for simplicity and avoid +configurability whenever possible. For example, there is a common desire to +make certain functionality optional in order not to make the user pay for +things they don't need. This, however, is often better addressed either by +always providing the optional functionality if it's fairly small or by +factoring it into a separate project if it's substantial. If a configuration +knob 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. + + +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). + +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}. + +\N|More precisely, a project configuration variable must match the +\c{config[.**].<project>.**} pattern where additional components may be +present after \c{config.} in case of subprojects. Overall, the recommendation +is to use hierarchical names, such as \c{config.libcurl.tests.remote} for +subprojects, similar to build system submodules. + +If a build system module for a tool (such as a source code generator) and the +tool itself share a name, then they may need to coordinate their configuration +variable names in order to avoid clashes. + +The build system core reserves \c{build} and \c{import} as the second +component in configuration variables as well as \c{configured} as the third +and further components.| + + +\h#proj-config-directive|\c{config} Directive| + +To define a project configuration variable we place the \c{config} directive +into the project's \c{build/root.build} file (see \l{#intro-proj-struct +Project Structure}). For example: + + +\ +config [bool] config.libhello.fancy ?= false +config [string] config.libhello.greeting ?= 'Hello' +\ + +\N|The irony does not escape us: these configuration knobs are exactly of the +kind that we advocate against. However, finding reasonable example of +build-time configurability in a \i{\"Hello, World!\"} library is not easy. In +fact, it probably shouldn't have any. So, for this chapter, do as we say, not +as we do.| + +Similar to \c{import} (see \l{#intro-import Target Importation}), it is a +special kind of variable assignment. Let's examine all its parts in turn. + +First comes the optional list of variable attributes inside \c{[\ ]}. The only +attribute that we have in the above example is the variable type, \c{bool} and +\c{string}, respectively. It is generally a good idea to assign static types to +configuration variables because their values will be specified by the users of +our project and the more automatic validation we provide the better. For +example, this is what happens if we misspell the value of the \c{fancy} +variable: + +\ +$ b configure config.libhello.fancy=fals +error: invalid bool value 'fals' in variable config.libhello.fancy +\ + +After the attribute list we have the variable name. As mentioned above, the +\c{config} directive will validate that it matches the +\c{config[.**].<project>.**} pattern (with one exception discussed in +\l{#proj-config-report Configuration Report}). + +Finally, after the variable name comes optional default value. Note that +unlike normal variables, default assignment (\c{?=}) is the only valid +form of assignment in the \c{config} directive. + +The semantics of the \c{config} directive is as follows: First an overridable +variable is entered with the specified name, type (if any), and global +visibility. Then, if the variable is undefined and the default value is +specified, it is assigned the default value. After this, if the variable is +defined (either as user-defined or default), it is marked for persistence. +Finally, a defined variable is also marked for reporting as discussed in +\l{#proj-config-report Configuration Report}. Note that if the variable +is user-defined, then the default value is not evaluated. + +Note also that if the configuration value is not specified by the user and you +haven't provided the default, the variable will be undefined, not \c{null}, +and, as a result, omitted from the persistent configuration +(\c{build/config.build} file). However, \c{null} is a valid default value: + +\ +config [string] config.libhello.fallback_name ?= [null] +\ + +A common approach for representing an enum-like value is to use \c{string} as +a type and pattern matching for validation. In fact, validation and +propagation can often be combined. For example, if our library needed to use a +database for some reason, we could handle it like this: + +\ +config [string] config.libhello.database ?= [null] + +using cxx + +switch $config.libhello.database +{ + case [null] + { + # No database in use. + } + case 'sqlite' + { + cxx.poptions += -DLIBHELLO_WITH_SQLITE + } + case 'pgsql' + { + cxx.poptions += -DLIBHELLO_WITH_PGSQL + } + default + { + fail \"invalid config.libhello.database value \ +'$config.libhello.database'\" + } +} +\ + +While it is generally a good idea to provide a sensible default for all your +configuration variables, if you need to force the user to specify its value +explicitly, this can be achieved with an extra check. For example: + +\ +config [string] config.libhello.database + +if! $defined(config.libhello.database) + fail 'config.libhello.database must be specified' +\ + +And if you want to also disallow \c{null} values, then the above check should +be rewritten as follows: \N{An undefined variable expands into a \c{null} +value.} + +\ +if ($config.libhello.database == [null]) + fail 'config.libhello.database must be specified' +\ + +Other than assigning the default value via the \c{config} directive, +configuration variables should not be modified in the project's +\c{buildfiles}. Instead, if further processing of the configuration value is +necessary, we should assign the configuration value to a different, +non-\c{config.*}, variable and modify that. The two situations where this is +commonly required are post-processing the configuration value to be more +suitable for use in \c{buildfiles} as well as further customization of +configuration values. + +As an example of the first situation, let's say we need to translate the +database identifiers specified by the user: + +\ +config [string] config.libhello.database ?= [null] + +switch $config.libhello.database +{ + case [null] + database = [null] + + case 'sqlite' + database = 'SQLITE' + + case 'pgsql' + database = 'PGSQL' + + case 'mysql' + case 'mariadb' + database = 'MYSQL' + + default + fail \"...\" + } +} + +using cxx + +if ($database != [null]) + cxx.poptions += \"-DLIBHELLO_WITH_$database\" +\ + +For the second situation, the typical pattern looks like this: + +\ +config [strings] config.libhello.options + +options = # Overridable options go here. +options += $config.libhello.options +options += # Non-overridable options go here. +\ + +That is, assuming that the subsequently specified options (for example, +command line options) override any previously specified, we first set default +\c{buildfile} options that are allowed to be overridden by the configuration +values, then append any such options, and finish off by appending any +\c{buildfile} options that should always be in effect. + +As a concrete example of this approach, let's say we want to make the warning +levels of our project configurable (likely a bad idea; also ignores compiler +differences): + +\ +config [strings] config.libhello.woptions + +woptions = -Wall -Wextra +woptions += $config.libhello.woptions +woptions += -Werror + +using cxx + +cxx.coptions += $woptions +\ + +With this arrangement, the users of our project can customize the warning +levels but cannot disable the treatment of warnings as errors. For example: + +\ +$ b -v config.libhello.woptions=-Wno-extra +g++ ... -Wall -Wextra -Wno-extra -Werror ... +\ + +While we have already seen some examples of how to propagate the configuration +values to our source code, this topic is discussed further in +\l{#proj-config-propag Configuration Propagation}. + + +\h#proj-config-report|Configuration Report| + + +\h#proj-config-propag|Configuration Propagation| + +Using configuration values in our \c{buildfiles} is straightforward: they are +like any other \c{buildfile} variables and we can access them directly. For +example, this is how we could provide optional functionality in our library by +conditionally including certain source files: \N{See \l{#intro-if-else +Conditions (\c{if-else})} for why you should not use \c{if} to implement +this.} + +\ +# build/root.build + +config [strings] config.libhello.io ?= true +\ + +\ +# libhello/buildfile + +lib{hello}: {hxx ixx txx cxx}{** -version -hello-io} hxx{version} +lib{hello}: {hxx cxx}{hello-io}: include = $config.libhello.io +\ + +However, it is often required to propagate the configuration information to +our source code. In fact, we have already seen one way to do it: we can +pass this information via preprocessor macros defined on the compiler's +command line. For example: + +\ +# build/root.build + +config [bool] config.libhello.fancy ?= false +config [string] config.libhello.greeting ?= 'Hello' +\ + +\ +# libhello/buildfile + +if $config.libhello.fancy + cxx.poptions += -DLIBHELLO_FANCY + +cxx.poptions += \"-DLIBHELLO_GREETING=\\\"$config.libhello.greeting\\\"\" +\ + +\ +// lihello/hello.cxx + +void say_hello (ostream& o, const string& n) +{ +#ifdef LIBHELLO_FANCY + // TODO: something fancy. +#else + o << LIBHELLO_GREETING \", \" << n << '!' << endl; +#endif +} +\ + +We can even export certain configuration information this way to our library's +users (see \l{#intro-lib Library Exportation and Versioning} for details): + +\ +# libhello/buildfile + +# Export options. +# +if $config.libhello.fancy + lib{hello}: cxx.export.poptions += -DLIBHELLO_FANCY +\ + +This mechanism is simple and works well across compilers so there is no reason +not to use it when the number of configuration values passed and their size is +small. However, it can quickly get unwieldy as these numbers grow. For such +cases, it may make sense to save this information into a separate +auto-generated source file with the help of the \l{#module-in \c{in}} module, +similar to how we do it for the version header. + +The often-used arrangement is to generate a header file and include it into +source files that need access to the configuration information. Historically, +this was a C header full of macros called \c{config.h}. However, for C++ +projects, there is no reason not to make it a C++ header and use modern C++ +features instead of macros if desired. Which is what we will do here. + +As an example of this approach, let's convert the above command line-based +implementation to use the configuration header. We will continue using macros +as a start (or in case this is a C project) and try more modern techniques +later. The \c{build/root.build} file is unchanged except for loading the +\c{in} module: + +\ +# build/root.build + +config [bool] config.libhello.fancy ?= false +config [string] config.libhello.greeting ?= 'Hello' + +using in +\ + +The \c{libhello/config.hxx.in} file is new: + +\ +// libhello/config.hxx.in + +#pragma once + +#define LIBHELLO_FANCY $config.libhello.fancy$ +#define LIBHELLO_GREETING \"$config.libhello.greeting$\" +\ + +As you can see, we can reference our configuration variables directly in the +\c{config.hxx.in} substitutions (see \l{#module-in \c{in} Module} for details +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} +\ + +\ +// lihello/hello.cxx + +#include <libhello/config.hxx> + +void say_hello (ostream& o, const string& n) +{ +#if LIBHELLO_FANCY + // TODO: something fancy. +#else + o << LIBHELLO_GREETING \", \" << n << '!' << endl; +#endif +} +\ + +\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.| + +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 +drawbacks. As a first step, we can replace the \c{LIBHELLO_FANCY} macro with a +compile-time constant and use \c{if\ constexpr} instead of \c{#ifdef} in our +implementation: + +\ +// libhello/config.hxx.in + +namespace hello +{ + inline constexpr bool fancy = $config.libhello.fancy$; +} +\ + +\ +// lihello/hello.cxx + +#include <libhello/config.hxx> + +void say_hello (ostream& o, const string& n) +{ + if constexpr (fancy) + { + // TODO: something fancy. + } + else + o << LIBHELLO_GREETING \", \" << n << '!' << endl; +} +\ + +\N|Note that with \c{if\ constexpr} the branch not taken must still be valid, +parsable code. This is both one of the main benefits of using it instead of +\c{#if} (the code we are not using is still guaranteed to be syntactically +correct) as well as its main drawbacks (it cannot be used, for example, for +platform-specific code without extra efforts, such as shims, etc).| + +Next, we can do the same for \c{LIBHELLO_GREETING}: + +\ +// libhello/config.hxx.in + +namespace hello +{ + inline constexpr char greeting[] = \"$config.libhello.greeting$\"; +} +\ + +\ +// lihello/hello.cxx + +#include <libhello/config.hxx> + +void say_hello (ostream& o, const string& n) +{ + if constexpr (fancy) + { + // TODO: something fancy. + } + else + o << greeting << \", \" << n << '!' << endl; +} +\ + +\N|Note that for \c{greeting} we can achieve the same result without using +inline variables or \c{constexpr} and which would be usable in older C++ and +even C. All we have to do is add the \c{config.cxx.in} source file next to +our header with the definition of the \c{greeting} variable. For example: + +\ +// libhello/config.hxx.in + +namespace hello +{ + extern const char greeting[]; +} +\ + +\ +// libhello/config.cxx.in + +#include <libhello/config.hxx> + +namespace hello +{ + const char greeting[] = \"$config.libhello.greeting$\"; +} +\ + +\ +# libhello/buildfile + +lib{hello}: {hxx ixx txx cxx}{** -config} {hxx cxx}{config} +hxx{config}: in{config} +cxx{config}: in{config} +\ + +As this also shows, the \c{in} module can produce as many auto-generated +source files as we need. For example, we could use this to split the +configuration header into two, one public and installed while the other +private.| + + \h1#attributes|Attributes| \N{This chapter is a work in progress and is incomplete.} |