What Does Compilation Mean in C Programming?

Programming – the ability to code ideas into existence is no less than a superpower. Programming languages can vary from simple to complex and from low-level to high-level. However, compilation of the written code is a common process in most high-level languages.

One of the most extensively used programming languages globally is the C language. It is a machine-independent language introduced way back in 1972. Since its introduction, C has been used for coding everything from complex games and applications to fully-featured operating systems like Microsoft Windows.

However, without the compiler of ‘C’ language, the high-level code developers wrote to create these products could not be assembled into machine code. So, it is safe to say that these products would not exist without compilation! Before we delve deeper into the compilation process for C, let us understand the term in detail.

What does Compilation mean?

Most programming or ‘coding’ is done in languages that usually mimic one or the other natural languages of the world, most commonly – English.

These are also called high-level programming languages. However, these languages are incomprehensible to computers, as they only understand the language of bits, Ones and Zeroes, otherwise known as the assembly or machine language. 

So what magic converts our human coding into machine language? That’s where the Compiler comes in! Most programming languages have compilers built-in. They check the written code for syntax, logical, or structural errors and only convert it to machine language if the code is error-free. This process is called Compilation.

The compilation process consists of four significant steps:

1. Preprocessing
2. Compiling
3. Assembling
4. Linking

Let us look at each of them one by one.

1. Preprocessing

The code that a human writes is called the source code. The source code file is saved with a ‘.c’ extension. During the first phase of compilation, the source code comes to the pre-processor, which performs three essential functions on it:

1. Cleaning the code
2. Passing directives
3. Expansion of code

One of the best practices of coding is for developers to add comments to their code so that it is easy to understand by other programmers. While these comments are helpful for humans, they are not understood by machines.

During the first phase, the pre-processor takes the source code file as input and cleans the code, which simply means it removes all unnecessary elements such as comments. It makes the code easily decipherable by the machines.

The next action taken by the preprocessor is to pass preprocessor directives. These are simply lines of code within the source code file that begin with the character ‘#.’ Some examples of preprocessor directives are as follows:

• #include
• #define
• #undef
• #if
• #ifelse

The preprocessor interprets the directives and takes appropriate action. For example, if the directive #include<stdio.h> is found in the program, the preprocessor interprets the directive and substitutes it with the contents of the <stdio.h> file.

Lastly, the preprocessor expands the code in the source code file, and this expanded code is then passed to the compiler.

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2. Compiling

As the name suggests, the compiler compiles the expanded code sent by the preprocessor into assembly language. This language is specific to the target processor and is also human-readable.

3. Assembling 

The assembler converts the compiled code into pure binary code or assembly code (Ones and Zeroes). The file that the assembler creates is called the object file.

It has the same file name as the source file, but its extension is ‘.obj’ in DOS and ‘.o’ in UNIX. For example, if the source file is named ‘Hello World.c’, the object file would be named ‘Hello World.obj’.

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4. Linking

The ‘C’ programming language has a library of pre-compiled functions. All programs coded in the ‘C’ language use one or more of these library functions. The assembled code of these library files is stored with ‘.lib’ (or ‘.a’) extension.

During the linking process of compilation, the assembly code of the source file is combined with that of the used library files to have seamless execution of the program.

In some cases, where a program uses multiple source files, one can call a function within another. In such cases, the linking phase becomes the most crucial phase of compilation as it links the assembly code of both files.

The resulting file is a fully-compiled one, ready for execution. It always has the same file name as the source file but can have an extension of ‘.exe’ in DOS or ‘a.out’ in UNIX.

Let us look at compilation in action via an example of a simple program that uses a preprocessor directive and ‘printf’ function.

#include <stdio.h>  

int main()  

{  

    printf(“Hello CodeQuotient”)  

    return 0;  

}  

To understand things better, here is a flow of the compilation of this program:

C program (Hello CodeQuotient.C) > Preprocessing > Expanded Source file > Compiling > Assembly Code file > Assembling > Object file (Hello CodeQuotient.obj) > Linking > Executable file (Hello CodeQuotient.exe) 

After successful compilation, the file is ready for execution. However, the compilation would fail for the program above, as our syntax has an error. The semicolon is missing after the ‘printf’ function is called.

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