GCC Compilation Process

GCC compiles a C/C++ program into executable through a sequence of four steps, as depicted in the diagram above. To illustrate, let's consider the command g++ -o hello hello.cpp¹ and break down its execution:

1. Pre-processing: This initial step involves the GNU C Preprocessor (cpp), which handles tasks such as including headers (#include) and expanding macros (#define).

   cpp hello.cpp > hello.i
   

   The resultant intermediate file hello.i contains the expanded source code.

2. Compilation: The compiler takes the the pre-processed source code and translates it into assembly code specific to the target processor.

   g++ -S hello.i
   

   Using the -S flag instrructs the compiler to generate assembly code, instead of object code. The resultant assembly file is hello.s.

3. Assembly: The assembler (as) converts the assembly code into machine code, stored in an object file named hello.o.

   as -o hello.o hello.s
   

4. Linking: Finally, the linker (ld) combines the object code with any required library code to produce the executable file hello.

   ld -o hello hello.o ...libraries...
   

   If you're still unsure how to link object files using ld, you can run the compilation command with verbose mode enabled to get a detailed view of the process as shown below. Additionaly, you can refer to this stackoverflow post for further insights.

Verbose Mode (-v)

To observe a detailed of the compilation process, you can enable the verbose mode by adding the -v option along with the compilation command. For instance:

g++ -v -o hello hello.cpp

Resources

1. https://www3.ntu.edu.sg/home/ehchua/programming/cpp/gcc_make.html
2. https://askubuntu.com/questions/156392/what-is-the-equivalent-of-an-exe-file
3. https://stackoverflow.com/questions/14163208/how-to-link-c-object-files-with-ld