Shulou(Shulou.com)06/03 Report--

Those who have written code and done development under Linux must have heard of Makefile.

Yes, make file, not make love. If you see the latter, you can only say: comrade, there is a problem with your three views, you need to format your hard disk ~

Linux development programs, no IDE(integrated development environment), no VC++6.0,

Only Makefile and cold dark shell window, cold night, test every engineer tired heart

Makefile syntax is complex and difficult to maintain. For a small project is OK, for large projects and open source projects, it is now popular to use some tools to automatically generate Makefiles, which can greatly reduce the burden on software developers.

Common tools include GNU Autotools, CMake, QMake, SCons, Ant, etc. Today we talk about GNU Autotools, a tool widely used in open source software and large projects.

Autotools is mainly composed of Autoconf, automake, libtool and other software packages, we can call Autotools three musketeers.

Using these tools can help us generate Makefiles automatically, but the command process is complex, and a large number of configuration files and scripts will be generated in the middle. Many people often find it very troublesome and do not understand the complicated relationship inside. Today, with the development process of these tools, I will explain the relationship inside to everyone.

I. Manual makefile era

In the early days we developed software in Unix and Linux environments, and makefiles were handwritten. With makefile, we can compile our source code directly using make.

Later, with more and more Unix versions, the differences between the branches are getting bigger and bigger, and the makefiles we write may sometimes fail to compile on other Unix platforms, such as library problems. Different operating system versions may have different paths to libraries and header files. What should I do?

Later we solved this problem by writing a configuration script configure: configure the makefile for your current Unix platform with a script that compiles happily on your current platform.

The Autoconf era

Later, the Linux operating system came out, and subsequent versions were more and more complicated, such as Redhat series, debian series, etc. The differences are getting bigger and bigger, even including the interface of the operating system. At this time, let alone the makefile can not compile correctly, even if we write the application, even if compiled correctly, it may not run on other platforms. This problem, as we all know, was followed by the POSIX API standard, a portable operating system interface. That is, no matter what version of Unix or Linux you release, the interface of OS should follow this standard, so that the application programs we write can run smoothly on Linux, Unix and other series of operating systems.

As for makefiles, in order to adapt to more versions of the operating system, code can only be added continuously, which leads to the configuration script becoming more and more ambitious, resulting in developers can no longer stand it, and maintenance costs are getting higher and higher.

In 1991, David Mackenzie developed Autoconf to automate the generation of configure scripts. Because for most users, the difference between different versions, the version of the library, the details of the underlying, the ghost is too lazy to care. All they care about is where the library files are, where the header files are, and where the final installation path of the software is. Therefore, the emergence of this tool has greatly liberated the developer's time and brought the gospel to the majority of programmers.

Users only need to define a few macros, indicating the configuration options we care about, save them in the configure.ac file, and then use the Autoconf tool to automatically generate the configure script for us!

Autoconf tool is really more intimate than aunt period, in order to reduce the burden of programmers, configure.ac file does not need us to hand-write. Autoconf toolkit has an autoscan tool, can automatically scan our project, generate a configure.scan file, which automatically added a lot of macros, save us to add manually.

We just need to rename the configure.scan file to configure.ac and tinker with it.

Autoconf tool greatly reduces the burden of programmers, mom, no longer have to worry about coming home late for dinner

Third, the age of automake

However, as the project gets bigger, makefiles get more complex, especially for large projects, and handwriting becomes more difficult.

The automake tool has come out at this time!

For developers, all we care about is what executable files the project generates, what source files need to be compiled, and how? The details of the underlying link, the ghost is too lazy to care. Programmer overtime is enough, the heart is tired enough, we refuse to makefile abuse and oppression!

Using automake tool, we only need to manually write a makefile.am file, which defines the target we want to generate, the source file that needs to be compiled, and then use automake tool to help us automatically generate makefile!

However, the makefile at this time is a general makefile, which defines the general rules for compiling and linking programs and is stored in makefile.in.

If you want to compile successfully on a specific platform, you need to configure our previous script configure to generate our project Makefile.

Autoconf defines a series of macros for us to use, allowing us to set some of the required configuration options, to configure our makefile.

Later, after Automake tool appeared, some macros were customized and extended. For example, Autoconf used to be used alone, but now it needs to be used with automake by adding the AM_INIT_AUTOMAKE macro to the configure.ac file.

This macro is defined in the automake toolkit. When we run autoconf, we get an error because we can't find the macro definition. What to do, what to do?

Later, under the same directory as configure.ac, define a file in aclocal.m4 format, which stores some user-defined macros, or some macros of automake, so that when autoconf runs, you can find macro definitions in this file.

Laziness is the greatest driving force for the progress of human society. Later, to reduce the workload even further, aclocal came along that automatically placed automake, autoconf, and all user-defined macros in the aclocal.m4 file.

Why save it in aclocal.m4? I don't know... m4, macro macro after 4 letters, abbreviation is m4.

Text file, suffix name can be defined arbitrarily, for example, my surname Wang, suffix name defined as.wang is also possible. Let's not dwell on these details, our journey is to build makefiles.

Autoconf toolkit also has an autoheader tool, used to configure.ac inside the macro configuration into our C language format #define form, and saved in config.h.in file, when we run./ When configure generates makefile, it will also convert config.h.in to config.h file by the way, so that in our program, if you want to use these macros, you can directly #include "config.h".

For example, the software version number VERSION macro defined in the header file can be used directly in the program, printing our software version number in the program.

IV. The libtool era

As Unix and Linux become more and more different, the management of dynamic shared libraries becomes more and more different. For example, some shared libraries use the.so format, some are.a, and some are.o. Dynamic libraries are managed the same way at runtime, with some operating systems supporting dynamic loading and others not. This poses a challenge to our Makefile. What should I do? The libtool was created to solve this problem. It abstracts the generated dynamic library and generates a unified form of.la, which can support more than a dozen different platforms.

For a detailed tutorial, please refer to the video tutorial I published: Makefile Engineering Practice Season 2-Using autotools to automatically build Makefiles for projects.

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