The C compilation process

When you develop a C++ program, the next step is to compile the program before running it. The compilation is the process which converts the program written in human readable language like C, C++ etc into a machine code, directly understood by the Central Processing Unit. For example, if you have a C++ source code file named prog.cpp and you execute the compile command,

g++ -Wall -ansi -o prog prog.cpp

There are 4 main stages involved in creating an executable file from the source file.

1. The C++ the preprocessor takes a C++ source code file and deals with the headers(#include), macros(#define) and other preprocessor directives.
2. The expanded C++ source code file produced by the C++ preprocessor is compiled into the assembly language for the platform.
3. The assembler code generated by the compiler is assembled into the object code for the platform.
4. The object code file produced by the assembler is linked together

with the object code files for any library functions used to produce
either a library or an executable file.

Preprocessing

The preprocessor handles the preprocessor directives, like #include and #define. It is agnostic of the syntax of C++, which is why it must be used with care.

It works on one C++ source file at a time by replacing #include directives with the content of the respective files (which is usually just declarations), doing replacement of macros (#define), and selecting different portions of text depending of #if, #ifdef and #ifndef directives.

The preprocessor works on a stream of preprocessing tokens. Macro substitution is defined as replacing tokens with other tokens (the operator ## enables merging two tokens when it make sense).

After all this, the preprocessor produces a single output that is a stream of tokens resulting from the transformations described above. It also adds some special markers that tell the compiler where each line came from so that it can use those to produce sensible error messages.

Some errors can be produced at this stage with clever use of the #if and #error directives.

By using below compiler flag, we can stop the process at preprocessing stage.

g++ -E prog.cpp

Compilation

The compilation step is performed on each output of the preprocessor. The compiler parses the pure C++ source code (now without any preprocessor directives) and converts it into assembly code. Then invokes underlying back-end(assembler in toolchain) that assembles that code into machine code producing actual binary file in some format(ELF, COFF, a.out, …). This object file contains the compiled code (in binary form) of the symbols defined in the input. Symbols in object files are referred to by name.

Object files can refer to symbols that are not defined. This is the case when you use a declaration, and don’t provide a definition for it. The compiler doesn’t mind this, and will happily produce the object file as long as the source code is well-formed.

Compilers usually let you stop compilation at this point. This is very useful because with it you can compile each source code file separately. The advantage this provides is that you don’t need to recompile everything if you only change a single file.

The produced object files can be put in special archives called static libraries, for easier reusing later on.

It’s at this stage that “regular” compiler errors, like syntax errors or failed overload resolution errors, are reported.

In order to stop the process after the compile step, we can use the -S option:

g++ -Wall -ansi -S prog.cpp

Assembling

The assembler creates object code. On a UNIX system you may see files with a .o suffix (.OBJ on MSDOS) to indicate object code files. In this phase the assembler converts those object files from assembly code into machine level instructions and the file created is a relocatable object code. Hence, the compilation phase generates the relocatable object program and this program can be used in different places without having to compile again.

To stop the process after the assembly step, you can use the -c option:

g++ -Wall -ansi -c prog.cpp

Linking

The linker is what produces the final compilation output from the object files the assembler produced. This output can be either a shared (or dynamic) library (and while the name is similar, they don’t have much in common with static libraries mentioned earlier) or an executable.

It links all the object files by replacing the references to undefined symbols with the correct addresses. Each of these symbols can be defined in other object files or in libraries. If they are defined in libraries other than the standard library, you need to tell the linker about them.

At this stage the most common errors are missing definitions or duplicate definitions. The former means that either the definitions don’t exist (i.e. they are not written), or that the object files or libraries where they reside were not given to the linker. The latter is obvious: the same symbol was defined in two different object files or libraries.