A quick guide to compiling C/C++ code and writing Makefiles (Part 1: Compiling)

This is the first of a 3 series post. This post will be about compiling. The second post will be about Makefiles. The third post will be about compiling libraries.

C/C++ coding is difficult enough as it is to code and debug without having to spend more time figuring out how write a good Makefile to compile the damn code and fix the Makefile bugs! In this post I will talk about compiling using the command line tools. But usually you don't write these commands manually on the console to build a project and be able to run the application. You write the commands in the Makefile and then you just build the project using the Makefile.

A Makefile is used to store the commands needed to build or clean a project. The user may download your project and just has to write the make command on the console to build the project to create the executable binary file that will run on their machine. In Windows you get given binaries like virus.exe with the .exe executable extension that just works on pretty much all Windows computers as long as there are no architectural requirements (64bit versions of software will not run on 32bit systems). But this is not the case for Unix/Linux. With all the different distros available and their internal complexities, it is not likely that a binary executable built on FreeBSD will also run on Ubuntu. Also there is no quick way to state a file is executable because .exe is not used to denote an executable file in the Linux world. So, for Linux mainly, the source code is often packaged so you can dowload and extract it, and you are expected to compile the code using the make command to create the executable for your machine in order to run it.

Which tool you compile your code with varies by operating system and there is a choice of compilers for each system anyway. On Windows I use MingW and on Linux I use GCC. There are often different compilers for C or C++ code too. It should be noted that C++ compilers are capable of compiling C code but sometimes complain about things that are not acceptable anymore in C++. I recommend compiling C code with a C compiler and C++ code with a C++ compiler. You can mix C and C++ code, but if you do, be prepared to edit your C code.

Anyway, compiling. The C compiler tool in Linux is gcc and the C++ compiler is g++. If you use MinGW to build a project on Windows, it will know what you are trying to compile your code with, so it's cool to carry on using gcc and g++. There are two sequential steps to compiling.

1. (dependent on the project) Make sure you have linked in any required library files (.a/.so) and their header files (.h).
2. Compile the source code (.c/.cpp) files into object(.o) files.
3. Combine the object files into one executable file.

Let's start with step two first because I will write another article about compiling and linking libraries at a later time. The following command will compile the code into object files for a hypothetical project consisting of two source files and the header file associated with the important program: exampleProg.c, reusableCmd.c and reusableCmd.h.

gcc -I . -c exampleProg.c reusableCmd.c

This will create the files exampleProg.o and reusableCmd.o. The flag -I . means find the header files required to compile this program are in the current directory (.). If we needed to include other header files in another directory, we would add an additional I flag like so: -I ./../dir2/libX.

Then we have to compile those files into an executable file. On Linux we don't give it an extension usually, but on Windows we specify it ends with .exe. To compile our files on Linux we do:

gcc -I . -o exampleProg exampleProg.o reusableCmd.o

And there you have it. The code will produce the executable file exampleProg, which is ready to run. The C++ compiler works in much the same way too. It is best to add some additional flags to optimize the executable file or improve debugging. Here are some flags I use a lot in my projects:

-Wall = Show all code warnings
-lm = Load the Math library if you #include <math .h%gt;- you need to add this flag explicitly if you include this header!
-funroll-loops = optimize loops
-O3 = Use maximum processor and memory optimizations
-fomit-frame-pointer = Omit some of the debugging pointers included in the executable - use this only once your code works perfectly
-msse4.2 = Use SSE4.2 machine instructions where available to improve performance - this is especially useful on modern Intel processors.
-std=c99 = This sets the expected standard of the compilation. You can also use an even newer standard such as c11 if your compiler supports it.


In the next post I will talk about Makefiles.