Compilation stages in C

The image shown below can brief you out the different stages and file extensions during the compilation of a C program.

Stages of Compilation in C

Preprocessing:

A preprocessor produces pure C code. The actions performed for this are – removal of comments, file inclusion, macro replacement, pragma processing and conditional compilation.

The input to a pre-processor is a .c file and the output is a .i file.

Preprocessing

Preprocessing using gcc is done using the following command on the terminal.

 gcc – E test.c -o test.i  

Translation:

Translator converts the pure C code into assembly level code i.e., from high-level code to low-level code. This is the level of compilation where all the syntaxes are checked. The .i file generated after pre-processing is given as input to the translator, which generates .s file.

Translation

Translation using gcc is done using the following command on the terminal.

 gcc – S test.i -o test.s  

Assembling:

An assembler produces target-based code from the pure C code. This is the main stage where the original C code written by the programmer becomes hardware specific, by generating respective micro-instructions for a given hardware.

The input to an assembler is a .i file generated after translation and the output file comes with .o extension.

Assemblilng

Assembling using gcc is done using the following command on the terminal.

 gcc – c test.i -o test.o  

Linking:

When you are working with multiple source files, the respective objects files are generated and are compiled together to produce a combined application. The linker links all the dependencies of a project and generates single application that can be finally executed. The input to the linker is .o file and the output file is an executable file with no extension.

Linking

Linking using gcc is done using the following command on the terminal.

 gcc – c test1.c test2.c test3.c -o test  

This command “links” the source code files test1.c, test2.c and test3.c altogether and generates single executable file test, which is the targeted application.
If you have single file, then the following command is to be used.

 gcc test.o -o test  

where 'test' is the executable file generated.
Then, the executable file can be executed as follows:

 ./test 

If you want to skip all the four stages of compilation, you can directly get the executable file from the source file itself, as follows:

 gcc test.c

With this command, the operating system produces the default executable file a.out, which can be executed as follows:

 ./a.out

However, there are two types of linking – Static and Dynamic linking, which will be discussed in later sessions.