Step-by-Step Guide to the Compilation Process in C

The compilation process in C involves several steps that convert human-readable C code into machine-readable code. The process is broken down into the following stages:

1. Preprocessing

The first step of the compilation process is preprocessing, which involves preparing the C source code for compilation by performing various operations like file inclusion, macro substitution, and conditional compilation.

Tasks performed during preprocessing:

• File Inclusion: #include directives are processed, including external header files into the source code.
• Macro Expansion: #define macros are replaced with their definitions.
• Conditional Compilation: Code between #if, #ifdef, #else, #endif directives is evaluated and either included or excluded.
• Comments Removal: All comments in the code are removed.

Example:

#include
#define MAX 100

int main() {
printf(“Max value is %d”, MAX);
return 0;
}
After preprocessing, the code will look like:

int main() {
printf(“Max value is %d”, 100);
return 0;
}

The result of preprocessing is called the preprocessed source code.

2. Compilation (Code Generation)

During this stage, the preprocessed C source code is compiled into assembly code specific to the target architecture. The compiler translates the high-level C code into a lower-level language that the machine can understand.

• The compiler performs syntax checking, optimizations, and translates the preprocessed code into assembly language.
• Syntax errors are checked here, and if found, the process is halted, and errors are reported.

Example: The code might be transformed into assembly language:

mov eax, 100

call printf

3. Assembly (Assembly Code to Object Code)

In this step, the assembly code produced by the compiler is translated into machine code (binary code) by the assembler. The result is an object file (with .o or .obj extension).

The assembler converts human-readable assembly instructions into machine instructions that can be executed by the processor.

• It also handles symbol resolution (mapping variables and functions to memory addresses).
• The object file contains machine code and other metadata but cannot be executed directly.

Example: The assembly code will be converted into machine code like:

01010100 01101001 11001000

4. Linking

The final step is linking, where one or more object files generated by the assembler are combined to form the executable program. The linker also handles the integration of external libraries and ensures that function calls and variables are properly connected.

There are two types of linking:

• Static Linking: The linker includes all necessary library functions and object files into the final executable during the compilation process. The resulting executable contains everything needed to run the program.
• Dynamic Linking: The linker resolves external function calls to libraries at runtime (when the program is executed). Only the references to external libraries are included in the executable, and the actual libraries are loaded dynamically during execution.

The linker resolves symbols and addresses for function calls, variables, and external libraries. The output of this stage is the final executable file (usually .exe on Windows or no extension on Unix-based systems).

5. Execution

After linking, the final executable can be run. At this point, the program has been fully translated into machine code and can be executed by the operating system.

Steps in summary:

1. Preprocessing: Handle macros, file inclusions, and conditional code.
2. Compilation: Convert preprocessed code into assembly.
3. Assembly: Translate assembly code into object code (machine code).
4. Linking: Combine object files and external libraries to create an executable.
5. Execution: Run the final executable on the system.

Diagram of Compilation Process:

Source Code (.c)  –> Preprocessor –> Preprocessed Code

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    v

Compiler –> Assembly Code (.s)

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    v

Assembler –> Object Code (.o)

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    v

Linker –> Executable File (.exe or no extension)

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    v

Program Execution