2.3 Structure of a C Program

                                  

     Programming in C    

 Structure of a C Program

The structure of a C program defines the organization and sequence in which different components of a program are written. A well-structured C program improves readability, maintainability, and error detection. The general structure of a C program consists of preprocessor directives, global declarations, the main() function, and other user-defined functions.

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General Structure of a C Program

Preprocessor directives
Global declarations

int main(void)
{
    Local definitions
    Statements
    return 0;
}

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1. Preprocessor Directives

Preprocessor directives are instructions given to the preprocessor before the actual compilation starts. These directives tell the preprocessor to:

• Include header files

• Perform macro substitutions

• Prepare the source code for translation into machine language

Examples include #include, #define, etc.

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2. Global Declarations

Global declarations declare variables and functions that are visible to all parts of the program. These declarations are placed outside all functions.

The basic idea behind global declarations is that they can be accessed by any function in the program. They are mainly used when data needs to be shared among multiple functions.

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3. main() Function

The main() function is the entry point of every C program. Program execution always begins from the main() function.

int main(void)
{
    ...
    return 0;
}

The return 0; statement indicates the successful termination of the program.

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4. Local Definitions

All functions, including main(), are divided into two sections:

• Local definitions

• Statements

Local definitions appear at the beginning of a function and describe the data objects used inside that function. These data objects are visible only within the function in which they are declared, unlike global declarations.

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5. Statement Section

The statement section consists of executable instructions that tell the computer what actions to perform. These statements control program logic, calculations, input/output, and function calls.

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6. Declaration vs Definition

The difference between a declaration and a definition is very important in C.

• A declaration announces the name, type, and properties of a data object or function. Its main purpose is type checking.

• A definition actually allocates memory for a variable or specifies the body of a function.

Declaring variables and functions allows the compiler to detect type errors at compile time, which is safer than detecting them during program execution.

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Conclusion

Thus, a C program is organized into clearly defined sections such as preprocessor directives, global declarations, main() function, local definitions, and statements. Understanding this structure is essential for writing correct, efficient, and error-free C programs.

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📌 Exam Tip

👉 Draw the structure diagram and bold keywords like Preprocessor directives, Global declarations, main(), Local definitions, and Statements for extra marks.

📖 Reference

The content for this subject is prepared by referring to the standard textbook
“Computer Fundamentals and Programming in C” by Pradip Dey and Manas Ghosh,
Second Edition, Oxford University Press (2018).
The explanations are exam-oriented and strictly aligned with the concepts presented in the reference book.

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