Installing dependencies

In Getting started we used the conan install command to download the Poco library and build an example.

If you inspect the conanbuildinfo.cmake file that was created when running conan install, you can see there that there are many CMake variables declared. For example CONAN_INCLUDE_DIRS_ZLIB, that defines the include path to the zlib headers, and CONAN_INCLUDE_DIRS that defines include paths for all dependencies headers.

If you check the full path that each of these variables defines, you will see that it points to a folder under your <userhome> folder. Together, these folders are the local cache. This is where package recipes and binary packages are stored and cached, so they don’t have to be retrieved again. You can inspect the local cache with conan search, and remove packages from it with conan remove command.

If you navigate to the folders referenced in conanbuildinfo.cmake you will find the headers and libraries for each package.

If you execute a conan install Poco/1.9.0@pocoproject/stable command in your shell, Conan will download the Poco package and its dependencies (OpenSSL/1.0.2l@conan/stable and zlib/1.2.11@conan/stable) to your local cache and print information about the folder where they are installed. While you can handle them manually, the recommended approach is to use a conanfile.txt.

Requires

The required dependencies should be specified in the [requires] section. Here is an example:

[requires]
Poco/1.9.0@pocoproject/stable

Where:

Poco is the name of the package which is usually the same as the project/library.

1.9.0 is the version which usually matches that of the packaged project/library. This can be any string; it does not have to be a number, so, for example, it could indicate if this is a “develop” or “master” version. Packages can be overwritten, so it is also OK to have packages like “nightly” or “weekly”, that are regenerated periodically.

pocoproject is the owner of this package. It is basically a namespace that allows different users to have their own packages for the same library with the same name.

stable is the channel. Channels provide another way to have different variants of packages for the same library and use them interchangeably. They usually denote the maturity of the package as an arbitrary string such as “stable” or “testing”, but they can be used for any purpose such as package revisions (e.g., the library version has not changed, but the package recipe has evolved).

Overriding requirements

You can specify multiple requirements and override transitive “require’s requirements”. In our example, Conan installed the Poco package and all its requirements transitively:

OpenSSL/1.0.2l@conan/stable

zlib/1.2.11@conan/stable

Tip

This is a good example of overriding requirements given the importance of keeping the OpenSSL library updated.

Consider that a new release of the OpenSSL library has been released, and a new corresponding Conan package is available. In our example, we do not need to wait until pocoproject (the author) generates a new package of POCO that includes the new OpenSSL library.

We can simply enter the new version in [requires] section:

[requires]
Poco/1.9.0@pocoproject/stable
OpenSSL/1.0.2p@conan/stable

The second line will override the OpenSSL/1.0.2l required by POCO with the currently non-existent OpenSSL/1.0.2p.

Another example in which we may want to try some new zlib alpha features, we could replace the zlib requirement with one from another user or channel.

[requires]
Poco/1.9.0@pocoproject/stable
OpenSSL/1.0.2p@conan/stable
zlib/1.2.11@otheruser/alpha

Note

You can use environment variable CONAN_ERROR_ON_OVERRIDE to raise an error for every overriden requirement not marked explicitly with the override keyword.

Generators

Conan reads the [generators] section from conanfile.txt and creates files for each generator with all the information needed to link your program with the specified requirements. The generated files are usually temporary, created in build folders and not committed to version control, as they have paths to local folders that will not exist in another machine. Moreover, it is very important to highlight that generated files match the given configuration (Debug/Release, x86/x86_64, etc) specified when running conan install. If the configuration changes, the files will change accordingly.

For a full list of generators, please refer to the complete generators reference.

Options

We have already seen that there are some settings that can be specified during installation. For example, conan install . -s build_type=Debug. These settings are typically a project-wide configuration defined by the client machine, so they cannot have a default value in the recipe. For example, it doesn’t make sense for a package recipe to declare “Visual Studio” as a default compiler because that is something defined by the end consumer, and unlikely to make sense if they are working in Linux.

On the other hand, options are intended for package specific configuration that can be set to a default value in the recipe. For example, one package can define that its default linkage is static, and this is the linkage that should be used if consumers don’t specify otherwise.

Note

You can see the available options for a package by inspecting the recipe with conan get <reference> command:

$ conan get Poco/1.9.0@pocoproject/stable

To see only specific fields of the recipe you can use the conan inspect command instead:

$ conan inspect Poco/1.9.0@pocoproject/stable -a=options
$ conan inspect Poco/1.9.0@pocoproject/stable -a=default_options

For example, we can modify the previous example to use dynamic linkage instead of the default one, which was static, by editing the conanfile.txt:

[requires]
Poco/1.9.0@pocoproject/stable

[generators]
cmake

[options]
Poco:shared=True # PACKAGE:OPTION=VALUE
OpenSSL:shared=True

Install the requirements and compile from the build folder (change the CMake generator if not in Windows):

$ conan install ..
$ cmake .. -G "Visual Studio 14 Win64"
$ cmake --build . --config Release

As an alternative to defining options in the conanfile.txt file, you can specify them directly in the command line:

$ conan install .. -o Poco:shared=True -o OpenSSL:shared=True
# or even with wildcards, to apply to many packages
$ conan install .. -o *:shared=True

Conan will install the binaries of the shared library packages, and the example will link with them. You can again inspect the different binaries installed. For example, conan search zlib/1.2.8@lasote/stable.

Finally, launch the executable:

$ ./bin/timer

What happened? It fails because it can’t find the shared libraries in the path. Remember that shared libraries are used at runtime, so the operating system, which is running the application, must be able to locate them.

We could inspect the generated executable, and see that it is using the shared libraries. For example, in Linux, we could use the objdump tool and see the Dynamic section:

$ cd bin
$ objdump -p timer
...
Dynamic Section:
 NEEDED               libPocoUtil.so.31
 NEEDED               libPocoXML.so.31
 NEEDED               libPocoJSON.so.31
 NEEDED               libPocoMongoDB.so.31
 NEEDED               libPocoNet.so.31
 NEEDED               libPocoCrypto.so.31
 NEEDED               libPocoData.so.31
 NEEDED               libPocoDataSQLite.so.31
 NEEDED               libPocoZip.so.31
 NEEDED               libPocoFoundation.so.31
 NEEDED               libpthread.so.0
 NEEDED               libdl.so.2
 NEEDED               librt.so.1
 NEEDED               libssl.so.1.0.0
 NEEDED               libcrypto.so.1.0.0
 NEEDED               libstdc++.so.6
 NEEDED               libm.so.6
 NEEDED               libgcc_s.so.1
 NEEDED               libc.so.6

Imports

There are some differences between shared libraries on Linux (*.so), Windows (*.dll) and MacOS (*.dylib). The shared libraries must be located in a folder where they can be found, either by the linker, or by the OS runtime.

You can add the libraries’ folders to the path (dynamic linker LD_LIBRARY_PATH path in Linux, DYLD_LIBRARY_PATH in OSX, or system PATH in Windows), or copy those shared libraries to some system folder where they can be found by the OS. But these operations are are typical operations deployments or final installation of apps; they are not desired during development, and Conan is intended for developers, so it avoids manipulations on the OS.

In Windows and OSX, the simplest approach is to copy the shared libraries to the executable folder, so they are found by the executable, without having to modify the path.

This is done using the [imports] section in conanfile.txt.

To demonstrate this, edit the conanfile.txt file and paste the following [imports] section:

[requires]
Poco/1.9.0@pocoproject/stable

[generators]
cmake

[options]
Poco:shared=True
OpenSSL:shared=True

[imports]
bin, *.dll -> ./bin # Copies all dll files from packages bin folder to my "bin" folder
lib, *.dylib* -> ./bin # Copies all dylib files from packages lib folder to my "bin" folder

Note

You can explore the package folder in your local cache (~/.conan/data) and see where the shared libraries are. It is common that *.dll are copied to /bin. The rest of the libraries should be found in the /lib folder, however, this is just a convention, and different layouts are possible.

Install the requirements (from the mytimer/build folder), and run the binary again:

$ conan install ..
$ ./bin/timer

Now look at the mytimer/build/bin folder and verify that the required shared libraries are there.

As you can see, the [imports] section is a very generic way to import files from your requirements to your project.

This method can be used for packaging applications and copying the resulting executables to your bin folder, or for copying assets, images, sounds, test static files, etc. Conan is a generic solution for package management, not only (but focused in) for C/C++ or libraries.