Configure settings and options in recipes

We already explained Conan settings and options and how to use them to build your projects for different configurations like Debug, Release, with static or shared libraries, etc. In this section, we explain how to configure these settings and options in the case that one of them does not apply to a Conan package. We will introduce briefly how Conan models binary compatibility and how that relates to options and settings.

Please, first clone the sources to recreate this project. You can find them in the examples2 repository on GitHub:

$ git clone https://github.com/conan-io/examples2.git
$ cd examples2/tutorial/creating_packages/configure_options_settings

You will notice some changes in the conanfile.py file from the previous recipe. Let’s check the relevant parts:

class helloRecipe(ConanFile):
    name = "hello"
    version = "1.0"

    ...
    options = {"shared": [True, False],
               "fPIC": [True, False],
               "with_fmt": [True, False]}

    default_options = {"shared": False,
                       "fPIC": True,
                       "with_fmt": True}
    ...

    def config_options(self):
        if self.settings.os == "Windows":
            del self.options.fPIC

    def configure(self):
        if self.options.shared:
            # If os=Windows, fPIC will have been removed in config_options()
            # use rm_safe to avoid double delete errors
            self.options.rm_safe("fPIC")
    ...

You can see that we added a configure() method to the recipe. Let’s explain what’s the objective of this method and how it’s different from the config_options() method we already had defined in the recipe:

• configure(): use this method to configure which options or settings of the recipe are available. For example, in this case, we delete the fPIC option, because it should only be True if we are building the library as shared (in fact, some build systems will add this flag automatically when building a shared library).

• config_options(): This method is used to constrain the available options in a package before they take a value. If a value is assigned to a setting or option that is deleted inside this method, Conan will raise an error. In this case we are deleting the fPIC option in Windows because that option does not exist for that operating system. Note that this method is executed before the configure() method.

Be aware that deleting an option using the config_options() method has a different result from using the configure() method. Deleting the option in config_options() is like we never declared it in the recipe which will raise an exception saying that the option does not exist. However, if we delete it in the configure() method we can pass the option but it will have no effect. For example, if you try to pass a value to the fPIC option in Windows, Conan will raise an error warning that the option does not exist:

Windows

$ conan create . --build=missing -o fPIC=True
...
-------- Computing dependency graph --------
ERROR: option 'fPIC' doesn't exist
Possible options are ['shared', 'with_fmt']

As you have noticed, the configure() and config_options() methods delete an option if certain conditions are met. Let’s explain why we are doing this and the implications of removing that option. It is related to how Conan identifies packages that are binary compatible with the configuration set in the profile. In the next section, we introduce the concept of the Conan package ID.

Conan packages binary compatibility: the package ID

We used Conan in previous examples to build for different configurations like Debug and Release. Each time you create the package for one of those configurations, Conan will build a new binary. Each of them is related to a generated hash called the package ID. The package ID is just a way to convert a set of settings, options and information about the requirements of the package to a unique identifier.

Let’s build our package for Release and Debug configurations and check the generated binaries package IDs.

$ conan create . --build=missing -s build_type=Release -tf="" # -tf="" will skip ng the test_package
...
[ 50%] Building CXX object CMakeFiles/hello.dir/src/hello.cpp.o
[100%] Linking CXX static library libhello.a
[100%] Built target hello
hello/1.0: Package '738feca714b7251063cc51448da0cf4811424e7c' built
hello/1.0: Build folder /Users/user/.conan2/p/tmp/7fe7f5af0ef27552/b/build/Release
hello/1.0: Generated conaninfo.txt
hello/1.0: Generating the package
hello/1.0: Temporary package folder /Users/user/.conan2/p/tmp/7fe7f5af0ef27552/p
hello/1.0: Calling package()
hello/1.0: CMake command: cmake --install "/Users/user/.conan2/p/tmp/7fe7f5af0ef27552/b/build/Release" --prefix "/Users/user/.conan2/p/tmp/7fe7f5af0ef27552/p"
hello/1.0: RUN: cmake --install "/Users/user/.conan2/p/tmp/7fe7f5af0ef27552/b/build/Release" --prefix "/Users/user/.conan2/p/tmp/7fe7f5af0ef27552/p"
-- Install configuration: "Release"
-- Installing: /Users/user/.conan2/p/tmp/7fe7f5af0ef27552/p/lib/libhello.a
-- Installing: /Users/user/.conan2/p/tmp/7fe7f5af0ef27552/p/include/hello.h
hello/1.0 package(): Packaged 1 '.h' file: hello.h
hello/1.0 package(): Packaged 1 '.a' file: libhello.a
hello/1.0: Package '738feca714b7251063cc51448da0cf4811424e7c' created
hello/1.0: Created package revision 3bd9faedc711cbb4fdf10b295268246e
hello/1.0: Full package reference: hello/1.0#e6b11fb0cb64e3777f8d62f4543cd6b3:738feca714b7251063cc51448da0cf4811424e7c#3bd9faedc711cbb4fdf10b295268246e
hello/1.0: Package folder /Users/user/.conan2/p/5c497cbb5421cbda/p

$ conan create . --build=missing -s build_type=Debug -tf="" # -tf="" will skip building the test_package
...
[ 50%] Building CXX object CMakeFiles/hello.dir/src/hello.cpp.o
[100%] Linking CXX static library libhello.a
[100%] Built target hello
hello/1.0: Package '3d27635e4dd04a258d180fe03cfa07ae1186a828' built
hello/1.0: Build folder /Users/user/.conan2/p/tmp/19a2e552db727a2b/b/build/Debug
hello/1.0: Generated conaninfo.txt
hello/1.0: Generating the package
hello/1.0: Temporary package folder /Users/user/.conan2/p/tmp/19a2e552db727a2b/p
hello/1.0: Calling package()
hello/1.0: CMake command: cmake --install "/Users/user/.conan2/p/tmp/19a2e552db727a2b/b/build/Debug" --prefix "/Users/user/.conan2/p/tmp/19a2e552db727a2b/p"
hello/1.0: RUN: cmake --install "/Users/user/.conan2/p/tmp/19a2e552db727a2b/b/build/Debug" --prefix "/Users/user/.conan2/p/tmp/19a2e552db727a2b/p"
-- Install configuration: "Debug"
-- Installing: /Users/user/.conan2/p/tmp/19a2e552db727a2b/p/lib/libhello.a
-- Installing: /Users/user/.conan2/p/tmp/19a2e552db727a2b/p/include/hello.h
hello/1.0 package(): Packaged 1 '.h' file: hello.h
hello/1.0 package(): Packaged 1 '.a' file: libhello.a
hello/1.0: Package '3d27635e4dd04a258d180fe03cfa07ae1186a828' created
hello/1.0: Created package revision 67b887a0805c2a535b58be404529c1fe
hello/1.0: Full package reference: hello/1.0#e6b11fb0cb64e3777f8d62f4543cd6b3:3d27635e4dd04a258d180fe03cfa07ae1186a828#67b887a0805c2a535b58be404529c1fe
hello/1.0: Package folder /Users/user/.conan2/p/c7796386fcad5369/p

As you can see Conan generated two package IDs:

• Package 738feca714b7251063cc51448da0cf4811424e7c for Release

• Package 3d27635e4dd04a258d180fe03cfa07ae1186a828 for Debug

These two package IDs are calculated by taking the set of settings, options and some information about the requirements (we will explain this later in the documentation) and calculating a hash with them. So, for example, in this case, they are the result of the information depicted in the diagram below.

Those package IDs are different because the build_type is different. Now, when you want to install a package, Conan will:

• Collect the settings and options applied, along with some information about the requirements and calculate the hash for the corresponding package ID.

• If that package ID matches one of the packages stored in the local Conan cache Conan will use that. If not, and we have any Conan remote configured, it will search for a package with that package ID in the remotes.

• If that calculated package ID does not exist in the local cache and remotes, Conan will fail with a “missing binary” error message, or will try to build that package from sources (this depends on the value of the --build argument). This build will generate a new package ID in the local cache.

These steps are simplified, there is far more to package ID calculation than what we explain here, recipes themselves can even adjust their package ID calculations, we can have different recipe and package revisions besides package IDs and there’s also a built-in mechanism in Conan that can be configured to declare that some packages with a certain package ID are compatible with other.

Maybe you have now the intuition of why we delete settings or options in Conan recipes. If you do that, those values will not be added to the computation of the package ID, so even if you define them, the resulting package ID will be the same. You can check this behaviour, for example with the fPIC option that is deleted when we build with the option shared=True. Regardless of the value you pass for the fPIC option the generated package ID will be the same for the hello/1.0 binary:

$ conan conan create . --build=missing -o shared=True -o fPIC=True -tf=""
...
hello/1.0 package(): Packaged 1 '.h' file: hello.h
hello/1.0 package(): Packaged 1 '.dylib' file: libhello.dylib
hello/1.0: Package '2a899fd0da3125064bf9328b8db681cd82899d56' created
hello/1.0: Created package revision f0d1385f4f90ae465341c15740552d7e
hello/1.0: Full package reference: hello/1.0#e6b11fb0cb64e3777f8d62f4543cd6b3:2a899fd0da3125064bf9328b8db681cd82899d56#f0d1385f4f90ae465341c15740552d7e
hello/1.0: Package folder /Users/user/.conan2/p/8a55286c6595f662/p

$ conan conan create . --build=missing -o shared=True -o fPIC=False -tf=""
...
-------- Computing dependency graph --------
Graph root
    virtual
Requirements
    fmt/8.1.1#601209640bd378c906638a8de90070f7 - Cache
    hello/1.0#e6b11fb0cb64e3777f8d62f4543cd6b3 - Cache

-------- Computing necessary packages --------
Requirements
    fmt/8.1.1#601209640bd378c906638a8de90070f7:d1b3f3666400710fec06446a697f9eeddd1235aa#24a2edf207deeed4151bd87bca4af51c - Skip
    hello/1.0#e6b11fb0cb64e3777f8d62f4543cd6b3:2a899fd0da3125064bf9328b8db681cd82899d56#f0d1385f4f90ae465341c15740552d7e - Cache

-------- Installing packages --------

-------- Installing (downloading, building) binaries... --------
hello/1.0: Already installed!

As you can see, the first run created the 2a899fd0da3125064bf9328b8db681cd82899d56 package, and the second one, regardless of the different value of the fPIC option, said we already had the 2a899fd0da3125064bf9328b8db681cd82899d56 package installed.

C libraries

There are other typical cases where you want to delete certain settings. Imagine that you are packaging a C library. When you build this library, there are settings like the compiler C++ standard (settings.compiler.cppstd) or the standard library used (self.settings.compiler.libcxx) that won’t affect the resulting binary at all. Then it does not make sense that they affect to the package ID computation, so a typical pattern is to delete them in the configure() method:

def configure(self):
    del self.settings.compiler.cppstd
    del self.settings.compiler.libcxx

Please, note that deleting these settings in the configure() method will modify the package ID calculation but will also affect how the toolchain, and the build system integrations work because the C++ settings do not exist.

Header-only libraries

A similar case happens with packages that package header-only libraries. In that case, there’s no binary code we need to link with, but just some header files to add to our project. In this cases the package ID of the Conan package should not be affected by settings or options. For that case, there’s a simplified way of declaring that the generated package ID should not take into account settings, options or any information from the requirements, which is using the self.info.clear() method inside another recipe method called package_id():

def package_id(self):
    self.info.clear()

We will explain the package_id() method later and explain how you can customize the way the package ID for the package is calculated. You can also check the Conanfile’s methods reference if you want to know how this method works in more detail.

Read more

• Header-only packages.

• Check the binary compatibility compatibility.py extension.

• Conan package types.

• Setting package_id_mode for requirements.

• Read the binary model reference for a full view of the Conan binary model.