How To Build a Cross-Platform C++ Project With Autotools
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How to build a cross-platform C++ project with Autotools from your Mac or Linux box

How To Build a Cross-Platform C++ Project With Autotools

In this article, I want to discuss how to create a cross-platform C++ project from your Mac or Linux Box using Autotools.

Wait, this is not about iOS?

Well, not necessarily. However, a good understanding of C and C++ will come in really handy for you if you develop iOS or macOS applications.

Even though Apple is trying to remove any C or C++ resemblance from Swift, there’s still a lot of libraries (like the Security framework and the crypto library) heavily relying on it.

Thus, learning and being proficient in C and C++ can make you a better, more valuable iOS and macOS developer. Plus, it can help you find other interesting job opportunities too.

Why?

C++ is one of the most professionally used languages out there, from big CRM and ERP software vendors to the Bitcoin core client. It’s everywhere.

However, it’s actually not that easy to write a portable, cross-platform C++ project. Different compilers from different systems, and the presence of a myriad of different libraries, make it really difficult.

Some initiatives exist out there trying to help setting a compiling and linking environment that works on most systems. Some of them include GNU’s Autotools and CMake.

Nevertheless, and perhaps ironically, these platforms are really hard to understand and master. As a result, it’s sometimes easier for developers to generate their own Makefile structure than understanding and using them.

In this post, I want to help you getting started by creating a very simple Hello World project including a binary and a dynamic library that can be ported to any GNU-compliant environment to be compiled and installed.

As always, you can download the code from my Github repository.

Project structure

Our project will build a very simple “hello world” program that will just show a message on screen. It contains a shared library: libHelloWorld.so, and a main program that calls this shared library.

Additionally, there will be some extra scripts (to be installed but not compiled), a man page for the main application, some documentation, examples and sample data.

The project should be structured in the following way:

  • sources in src/
  • documentation in doc/
  • man pages in man/
  • some scripts in scripts/ (in this directory, we will add files that need to be executed but not compiled)
  • examples in examples/

Our src/ directory will contain just three source files:

main.cpp

helloWorld.h

helloWorld.cpp:

Test It First

The first thing you should do is download the code from the Github repository to a local folder.

Then, get into the directory in a terminal, and invoke the autogen.sh script:

This will generate the required links to configure and build the project, and create the Makefile structure.

Now it’s time to configure the project. Run the local configure script:

The configure script will check that the environment is correct. Given that you have Xcode installed on your Mac, or the build-essentials package on most Linux distributions, you should have no problem here.

Next, time to build our project, calling make:

This will compile all the source files and generate the required dynamic libraries.

Finally, time to install and run the hello world sample app.

You can uninstall it anytime by running make uninstall.

The project includes some cool stuff, like an associated man page and some documentation and dummy scripts:

Thus, now that you know what we want to achieve, it’s time to start.

Prerequisites

Depending on our operating system, we need to follow a different process for installing autotools.

On linux based systems (i.e: Debian), all you need to do is installing the required packages (as root):

On Mac OS X, we can use *brew* for this:

In order to install Brew on Mac OS X, you just need to run this command:

More detailed instructions and information on Brew can be found here.

Generating The Project Configuration

In this step, we will generate the required configuration files and the Makefile.am files for the different parts of our project.

These files will set the rules to indicate autoconf and automake which files should be included and how to process them.

Run autoscan:

Autoscan tries to produce a suitable configure.ac file (autoconf’s driver) by performing simple analyses on the files in the package. This is enough for the moment (many people are just happy with it as permanent).

Use AutoScan file as initial Configuration file

Autoscan actually **produces a configure.scan file**, so let it have the name autoconf will look for:

Modify the Configuration file

Now we need to adjust the generated configuration file to add some info, like the name and version of our package:

We will modify this initial file to adapt it to our project.

First, we will modify the *AC_INIT* property that defines the package name, version and bug reporting email address. Replace:

With:

Also, we will change the *AC_CONFIG_SRCDIR* to point to our main.cpp file. Replace:

With:

Additionally, we will add some properties that will create the link between autoconf and automake. One of them will be the *AM_INIT_AUTOMAKE* variable, where you will specify the package name and version again. Also, *AC_OUTPUT*, where we will specify the Makefile files to be generated by autoconf in all of our directories.

Last, we will add some checks to make sure that the system where our project is going to be built has a proper compiling environment for C++ and can link shared libraries.

The final file should look like this:

Create the root Makefile.am:

Now it’s time to create the root Makefile.am, the seed for our root Makefile. It will specify the directories where automake should look for Makefile files.

Add this content:

Apart from specifying the subfolders containing Makefiles, we are adding some commonly used settings for most projects.

Create the Makefile.am in the src/ directory

The src directory is where we keep all our source files (.cpp and .h). We want our application to be called helloWorld. Let’s see how the Makefile.am generator for our source dir looks like.

Add this content:

As you can see, after defining some global options, we set the initial binary for the project, *helloWorld*, with the property *bin_PROGRAMS*.

Next, we define the sources and the libraries that should be included in this *helloWorld* object, with the *_SOURCES* and *_LDADD* properties respectively. Note how we use the extension .la instead of .so for the shared library. This is a convention.

Then, we define our small library (*lib_LTLIBRARIES*), set some flags (*libHelloWorld_la_LDFLAGS*) and specify its source files. That’s all we need. If you were to add additional source files, you will add them to *helloWorld_SOURCES*, and repeat the same process for libHelloWorld in order to add more libraries.

Create the Makefile.am in the man/ directory

Next, we’ll add the man page for our helloWorld executable.

We will create a very simple man page with this content:

Write that down in a file called helloWorld.1 (The man page number 1 is a convention, because our App is considered a “User command”).

Now it’s time to write our man/Makefile.am:

Add this content:

Create the Makefile.am in the scripts/ directory

Next, we will create a makefile.a, for our scripts directory.

Add this content

These scripts are just dummy scripts we will be using for this sample project. Thus, we will create two files, helloWorldScript1.sh…

and helloWorldScript2.sh:

Create the Makefile.am in the examples/ directory

In the examples directory, we can add configuration sample files, data examples, or any other kind of resource we might want to add.

In this example, we will add a dummy sample configuration file called sample.data:

Then, we will generate the Makefile.am:

With this content:

Indeed, we can add as many files as we want (separated by whitespaces) in *example_DATA*.

Create the Makefile.am in the doc/ directory

Finally, in order to set a doc directory, we need to specify a variable that will depend on the name of the project specified in configure.ac, @PACKAGE@.

Then, we add a doc_DATA variable with the files we want to include. We will use two dummy files: README.md and changelog.

Add this content to the file:

That’s all! We have finished generating all configuration files. Now, let’s create a script to generate the proper configuration and make scripts.

Integrating Autoconf and Automake

Last, we need to build our configure file. This file, called autogen.sh, will be called in order to setup all configuration and makefile scripts needed to configure and build the project.

In order to do that, we need to follow a series of steps. First, we need to call libtoolize to generate the linking helper ltmain.sh script, that will assist the linked when generating the shared libraries. Unfortunately, in Mac OS X it’s called glibtoolize, whereas in linux it’s called libtoolize. As a result, we need to address both cases in our script.

Next, we need to call aclocal. This command will generate the aclocal.m4 that contains all necessary information (including macros) needed for automake.

Then, we need to run autoheader to generate the configuration headers. After that, we can call autoconf to generate the configure command and automake to go through all our directory structure and generate, for every Makefile.am file, a corresponding Makefile.in file.

Here’s how the final autogen.sh script looks like:

Voila! 👏🏻 Our multiplatform configure file is generated and ready to use in any GNU-compliant environment. You went through the whole process before, but let’s see how it works.

Invoking Configure And Make

First of all, we call autogen.sh. As we saw, this script will call libtool and autotools to generate the configure script and the Makefile structure in our project:

After that, we call the generated configuration script.

It will read our configure.ac file and scan all the required libraries and dependencies. If a dependency or library is missing, the proper error message will be delivered.

For every Makefile.in we generated previously, as specified in AC_OUTPUT, it will generate the plaform-dependent Makefile file.

Now, we just can run make to build our project:

Makes reverses all the folder structure specified in the root Makefile.am and call make on that directory. If everything goes well, it will generate all the libraries and executable files. Now we can install our project to be available for the entire system (as superuser).

This command will typically copy the binaries in /usr/local/bin, the libraries in /usr/local/lib, the man pages in /usr/local/man and the docs in our @PACKAGE@ defined directory: /usr/local/share/helloWorld.

You can also uninstall everything by calling *make uninstall* (as superuser). This will do the reverse process, removing the files in the installation directory.

Congratulations! You have built a cross-platform C++ project 😎.

Conclusion

In this post I discussed how to build a cross-platorm C++ project from your Mac or Linux box using Autotools. C++ is a very interesting language to learn and master for iOS and macOS developers. However, building cross-platform projects that can be ported to different environments (like macOS, Linux, Unix, etc) can be painful. As a result, I wanted to share a sample project with detailed instructions on how to do that to easily get you started.

I hope you enjoyed this post. If you have any questions or comments, don’t hesitate to share them with us in the comments below!

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