What are Makefile.am and Makefile.in? - makefile

Question

These two files are mostly seen in open source projects.
What are they for, and how do they work?

Answer 1

Makefile.am is a programmer-defined file and is used by automake to generate the Makefile.in file (the .am stands for automake).
The configure script typically seen in source tarballs will use the Makefile.in to generate a Makefile.
The configure script itself is generated from a programmer-defined file named either configure.ac or configure.in (deprecated). I prefer .ac (for autoconf) since it differentiates it from the generated Makefile.in files and that way I can have rules such as make dist-clean which runs rm -f *.in. Since it is a generated file, it is not typically stored in a revision system such as Git, SVN, Mercurial or CVS, rather the .ac file would be.
Read more on GNU Autotools.
Read about make and Makefile first, then learn about automake, autoconf, libtool, etc.

Answer 2

Simple example
Shamelessly adapted from: http://www.gnu.org/software/automake/manual/html_node/Creating-amhello.html and tested on Ubuntu 14.04 Automake 1.14.1.
Makefile.am
SUBDIRS = src
dist_doc_DATA = README.md
README.md
Some doc.
configure.ac
AC_INIT([automake_hello_world], [1.0], [bug-automake#gnu.org])
AM_INIT_AUTOMAKE([-Wall -Werror foreign])
AC_PROG_CC
AC_CONFIG_HEADERS([config.h])
AC_CONFIG_FILES([
Makefile
src/Makefile
])
AC_OUTPUT
src/Makefile.am
bin_PROGRAMS = autotools_hello_world
autotools_hello_world_SOURCES = main.c
src/main.c
#include <config.h>
#include <stdio.h>
int main (void) {
puts ("Hello world from " PACKAGE_STRING);
return 0;
}
Usage
autoreconf --install
mkdir build
cd build
../configure
make
sudo make install
autotools_hello_world
sudo make uninstall
This outputs:
Hello world from automake_hello_world 1.0
Notes
autoreconf --install generates several template files which should be tracked by Git, including Makefile.in. It only needs to be run the first time.
make install installs:
the binary to /usr/local/bin
README.md to /usr/local/share/doc/automake_hello_world
On GitHub for you to try it out.

Answer 3

DEVELOPER runs autoconf and automake:
autoconf -- creates shippable configure script
(which the installer will later run to make the Makefile)
‘autoconf’ is a macro processor.
It converts configure.ac, which is a shell script using macro instructions, into configure, a full-fledged shell script.
automake - creates shippable Makefile.in data file
(which configure will later read to make the Makefile)
Automake helps with creating portable and GNU-standard compliant Makefiles.
‘automake’ creates complex Makefile.ins from simple Makefile.ams
INSTALLER runs configure, make and sudo make install:
./configure # Creates Makefile (from Makefile.in).
make # Creates the application (from the Makefile just created).
sudo make install # Installs the application
# Often, by default its files are installed into /usr/local
INPUT/OUTPUT MAP
Notation below is roughly: inputs --> programs --> outputs
DEVELOPER runs these:
configure.ac -> autoconf -> configure (script) --- (*.ac = autoconf)
configure.in --> autoconf -> configure (script) --- (configure.in depreciated. Use configure.ac)
Makefile.am -> automake -> Makefile.in ----------- (*.am = automake)
INSTALLER runs these:
Makefile.in -> configure -> Makefile (*.in = input file)
Makefile -> make ----------> (puts new software in your downloads or temporary directory)
Makefile -> make install -> (puts new software in system directories)
"autoconf is an extensible package of M4 macros that produce shell scripts to automatically configure software source code packages. These scripts can adapt the packages to many kinds of UNIX-like systems without manual user intervention. Autoconf creates a configuration script for a package from a template file that lists the operating system features that the package can use, in the form of M4 macro calls."
"automake is a tool for automatically generating Makefile.in files compliant with the GNU Coding Standards. Automake requires the use of Autoconf."
Manuals:
GNU AutoTools (The definitive manual on this stuff)
m4 (used by autoconf)
autoconf
automake
Free online tutorials:
Using GNU Autotools
Example:
The main configure.ac used to build LibreOffice is over 12k lines of code, (but there are also 57 other configure.ac files in subfolders.)
From this my generated configure is over 41k lines of code.
And while the Makefile.in and Makefile are both only 493 lines of code. (But, there are also 768 more Makefile.in's in subfolders.)

Answer 4

reference :
Makefile.am -- a user input file to automake
configure.in -- a user input file to autoconf
autoconf generates configure from configure.in
automake gererates Makefile.in from Makefile.am
configure generates Makefile from Makefile.in
For ex:
$]
configure.in Makefile.in
$] sudo autoconf
configure configure.in Makefile.in ...
$] sudo ./configure
Makefile Makefile.in