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C_06 Execution Process of a C Program | C Programming Tutorials
Jenny's Lectures CS IT
Overview
This video explains the step-by-step process of how a C program is executed, from writing the source code to getting the final output. It details the roles of the preprocessor, compiler, assembler, linker, and loader in transforming human-readable code into machine-executable instructions. The explanation follows the translation, linking, and loading model, highlighting the intermediate files generated at each stage and the importance of the main function as the program's entry point.
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Chapters
- C program execution involves several stages: translation, linking, and loading.
- Understanding these stages helps in comprehending how source code becomes an executable program.
- The process begins with writing the C source code, saved with a .c extension.
Knowing the execution process helps diagnose errors and understand the underlying mechanisms that turn code into a running application.
Writing a C program and saving it as 'first.c'.
- The preprocessor handles lines starting with '#', known as preprocessor directives.
- It includes header files (like stdio.h) which contain declarations for predefined functions (e.g., printf, scanf).
- The preprocessor expands macros and replaces header file content with actual declarations, creating an 'expanded source code' (or intermediate code) with a .i or .e extension.
- It removes all '#' directives, making the code understandable to the compiler.
The preprocessor prepares the source code by incorporating necessary definitions and expanding macros, ensuring the compiler can correctly interpret function calls and other directives.
Replacing a macro like '#define MAX 100' with '100' throughout the code.
- The compiler checks the expanded source code for syntax errors.
- If errors are found, the source code must be corrected and the process repeated.
- If no syntax errors exist, the compiler generates an assembly file (.asm) containing low-level instructions.
- The assembler then converts this assembly code into machine-readable object code (.obj or .o).
This stage translates high-level C code into machine-specific instructions, identifying and flagging syntax errors early in the development cycle.
Generating 'first.asm' from the expanded source code, and then 'first.obj' from 'first.asm'.
- The linker combines the generated object code with necessary code from system libraries.
- System libraries contain the definitions (actual implementations) of predefined functions used in the program.
- If multiple object files exist, the linker merges them into a single executable file.
- The output of the linker is an executable file (e.g., .exe).
Linking ensures that all necessary components, including function definitions from libraries, are present, creating a complete program ready for execution.
Linking the 'first.obj' file with the C standard library to include the definitions for functions like printf and scanf.
- The loader is responsible for loading the executable file from secondary storage (like a hard drive) into the main memory (RAM).
- The CPU can only execute programs that are loaded into RAM.
- Once loaded, the program begins execution, typically starting from the 'main' function.
- Runtime errors or logical errors are typically identified during this execution phase.
Loading makes the program accessible to the CPU for execution, and the execution phase is where the program's logic is actually performed and outputs are generated.
The loader placing the contents of 'first.exe' into RAM so the CPU can run it.
- Execution of a C program always begins at the 'main' function.
- The control flow starts with the first statement inside the 'main' function.
- A C program can have only one 'main' function.
- Errors detected during runtime require returning to the source code for correction.
The 'main' function serves as the mandatory entry point, defining the starting point for the program's operational sequence.
The program's execution pointer immediately jumping to the first line of code within the 'main()' block.
Key takeaways
- The journey from C source code to an executable program involves distinct phases: preprocessing, compilation, assembly, linking, and loading.
- The preprocessor handles directives and header files, expanding the source code before compilation.
- The compiler translates source code into assembly, and the assembler converts assembly into object code.
- The linker resolves external references by combining object code with library definitions to create an executable.
- The loader places the executable into memory, allowing the CPU to run the program.
- The 'main' function is the mandatory starting point for C program execution.
- Errors can occur at various stages (syntax during compilation, runtime during execution), often requiring a return to the source code for fixes.
Key terms
Source CodePreprocessorPreprocessor DirectivesHeader FilesCompilerAssembly CodeAssemblerObject CodeLinkerSystem LibrariesExecutable FileLoaderMain Function
Test your understanding
- What is the primary role of the preprocessor in the C execution model?
- How does the compiler differentiate between syntax errors and logical errors?
- What is the purpose of the linker, and why is it necessary?
- Explain the function of the loader in preparing a program for execution.
- Why is the 'main' function considered the entry point for a C program?