Difference Between Makefile.am and Makefile.in

Introduction to Autotools

Autotools is a suite of programming tools widely used in the Unix/Linux ecosystem to automate the process of building and distributing software. The core components of Autotools are the Autoconf and Automake tools, which generate configuration scripts and Makefiles to handle the build process across different platforms.

In Autotools, two important files play a significant role in the build process: Makefile.am and Makefile.in. In this article, we will explore the differences between these two files and their respective roles in the build process.

Understanding Makefile.am

The Makefile.am file is a key component of the Automake tool, which is responsible for generating the Makefile.in from Makefile.am. The Makefile.am is authored by the developer and defines the rules for building the software.

Syntax of Makefile.am

The Makefile.am file uses a straightforward and concise syntax. Each line contains a rule to build a particular target, along with its dependencies and commands. Here’s an example of a simple Makefile.am:

bin_PROGRAMS = my_program
my_program_SOURCES = main.c utils.c

In this example, we define a variable bin_PROGRAMS, which specifies the name of the binary executable to be generated (in this case, my_program). We also define a variable my_program_SOURCES, listing the source files required to build the binary.

Understanding Makefile.in

The Makefile.in is a template file generated by the Autoconf tool. It acts as an intermediate step in the build process and serves as a platform-independent template for generating the final Makefile.

Role of Makefile.in

The Makefile.in is created by replacing certain placeholders (such as @VARIABLE@) with values specific to the build environment and configuration options. These placeholders are determined during the configuration process, which is usually initiated by running the ./configure script generated by Autoconf.

Once the Makefile.in is configured, it becomes the platform-specific Makefile containing all the necessary rules and dependencies to build the software on the target platform.

Syntax of Makefile.in

Unlike Makefile.am, the Makefile.in is not authored directly by the developer. Instead, it is generated by the Autoconf tool during the configuration process.

After configuration, the Makefile.in may contain various expanded variables and platform-specific settings. The actual content of the Makefile.in is specific to each project and its configuration options.

Differences Between Makefile.am and Makefile.in

Now that we have a basic understanding of Makefile.am and Makefile.in, let’s summarize the main differences between these two files:

1. Authorship: Makefile.am is authored by the developer, whereas Makefile.in is generated by the Autoconf tool.
2. Content: Makefile.am contains rules and dependencies that are consistent across all platforms. It serves as a blueprint for the build process. On the other hand, Makefile.in is a template containing placeholders that will be replaced with platform-specific values during configuration.
3. File Generation: Makefile.am is processed by the Automake tool to generate the Makefile.in. The Makefile.in is then further processed by Autoconf during configuration to generate the platform-specific Makefile.
4. Usage: Developers work with Makefile.am, providing build rules and information about source files. End-users or package maintainers typically use Makefile.in as part of the build process after running the ./configure script.

Additional Notes on Autotools Build Process

Before concluding, it’s worth mentioning a few additional points about the Autotools build process and the role of configure.ac.

The Role of configure.ac

In addition to Makefile.am and Makefile.in, the configure.ac file is a critical component of the Autotools build process. It is authored by the developer and contains Autoconf macros and checks to determine the build environment and system capabilities. The configure.ac file is processed by Autoconf to generate the configure script, which is used to configure the software for the target platform.

The Configuration Process

The configuration process is a crucial step in the Autotools build process. When users or package maintainers obtain the source code for a project, they typically run the ./configure script, which is generated from the configure.ac file.

During the configuration process, ./configure examines the build environment and system capabilities. It replaces the placeholders in Makefile.in with appropriate values specific to the target platform. This creates a fully-configured Makefile tailored for the user’s system.

The Build Process

After the configuration process is complete, users or package maintainers can use the generated Makefile to build the software by running the make command. The Makefile contains all the necessary rules and dependencies to compile the source files, generate object files, and link the final executable or library.

Cross-Compiling with Autotools

Autotools also supports cross-compilation, which allows developers to build software for a target platform that is different from the platform on which the build process is running. Cross-compiling is commonly used in embedded systems development or when building software for specialized platforms.

To perform cross-compilation with Autotools, developers need to provide appropriate --host, --build, and --target options to the ./configure script, specifying the target platform and build environment.

Conclusion

The Autotools suite, consisting of Autoconf and Automake, provides a robust and flexible build system for Unix/Linux software projects. Makefile.am and Makefile.in are integral parts of this build system, where Makefile.am serves as the input authored by developers, and Makefile.in is a generated template that gets configured for the target platform.

The combination of Autoconf, Automake, and configure.ac allows developers to create platform-independent build configurations and ensure that the software can be easily built and configured on various systems.

By understanding the roles and differences between Makefile.am and Makefile.in, as well as the configuration and build process with Autotools, developers can efficiently manage the build process and produce software that can be seamlessly deployed on different platforms.