#linux src compilation

There are lot of confusions for usual configure, make, make install sequence to get a program running in Linux. Unfortunately, most developers today have never used a compiler to install an application and used the GUI application builders.

The make utility is designed to decrease a programmer’s need to remember line of commands. I guess that is actually the good way of saying, it decreases a programmer’s need to document. In any case, the idea is that if you establish a set of rules to create a program in a format make understands, you don’t have to remember them again. The make utility has a set of built-in rules so you only need to tell it what new things it needs to know to build your particular utility. For example, if you typed in make program, make would first look for some new rules from you. If you didn’t supply it any then it would look at its built-in rules. One of those built-in rules tells make that it can run the linker (ld) on a program name ending in .o to produce the executable program.

So, make would look for a file named program.o. But, it wouldn’t stop there. Even if it found the .o file, it has some other rules that tell it to make sure the .o file is up to date. In other words, newer than the source program. The most common source program on Linux systems is written in C and its file name ends in .c.

If make finds the .c file (program.c in our example) as well as the .o file, it would check their timestamps to make sure the .o was newer. If it was not newer or did not exist, it would use another built-in rule to build a new .o from the .c (using the C compiler). This same type of situation exists for other programming languages. The end result, in any case, is that when make is done, assuming it can find the right pieces, the executable program will be built and up to date.

The old UNIX joke, by the way, is what early versions of make said when it could not find the necessary files. In the example above, if there was no program.o, program.c or any other source format, the program would have said: make: don’t know how to make program. Stop.

Getting back to the task at hand, the default file for additional rules in Makefile in the current directory. If you have some source files for a program and there is a Makefile file there, take a look. It is just text. The lines that have a word followed by a colon are targets. That is, these are words you can type following the make command name to do various things. If you just type make with no target, the first target will be executed.

What you will likely see at the beginning of most Makefile files are what look like some assignment statements. That is, lines with a couple of fields with an equal sign between them. Surprise, that is what they are. They set internal variables in make. Common things to set are the location of the C compiler (yes, there is a default), version numbers of the program and such.

This now beings up back to configure. On different systems, the C compiler might be in a different place, you might be using ZSH instead of BASH as your shell, the program might need to know your host name, it might use a dbm library and need to know if the system had gdbm or ndbm and a whole bunch of other things. You used to do this configuring by editing Makefile. Another pain for the programmer and it also meant that any time you wanted to install software on a new system you needed to do a complete inventory of what was where.

As more and more software became available and more and more POSIX-compliant platforms appeared, this got harder and harder. This is where configure comes in. It is a shell script (generally written by GNU Autoconf) that goes up and looks for software and even tries various things to see what works. It then takes its instructions from Makefile.in and builds Makefile (and possibly some other files) that work on the current system.

Background work done, let me put the pieces together.

You run configure (you usually have to type ./configure as most people don’t have the current directory in their search path). This builds a new Makefile. Type make This builds the program. That is, make would be executed, it would look for the first target in Makefile and do what the instructions said. The expected end result would be to build an executable program. Now, as root, type make install. This again invokes make, make finds the target install in Makefile and files the directions to install the program.