[Adult Learning Log] C Language - Week 2 Review

[C Language Week 2] Comments, Preprocessor, Variables, and More

○ Key Takeaways from Week 2

• Hands-on practice: Simple calculator, monthly salary calculator
• Learned that most C programs define functions inside the main function
• Learned three ways to write comments in C
• Understood what a preprocessor is, why it’s used, and the meaning of directives like #include <stdio.h>
• Learned the structure of functions and the significance of return values
• Covered variable declarations, identifier naming rules, and standard data types
• Practiced using printf(), scanf(), and scanf_s() functions

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○ Typical Program Structure

Most programs follow this pattern: Input → Process → Output

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○ Comments in C

C supports the following types of comments:

• Single-line comment:

  /* This is a single-line comment */

• Multi-line comment:

  /* This is a
     multi-line
     comment */

• Since C99:

  // Comment from here to end of the line

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○ Preprocessor

Preprocessing refers to tasks performed before compilation starts.

Example:

#include <stdio.h>

• #include: directive to include external files
• stdio.h: standard input/output header file
  • stdio = Standard Input Output
  • .h = Header file

This allows access to functions like scanf() and printf().

Why use header files?

• Organize frequently used code (functions, constants, structures, etc.)
• Reduce duplication, improve modularity and maintenance
• Directives like #include allow reusable code blocks

Example 2:

#define _CRT_SECURE_NO_WARNINGS  // Used in Visual Studio to suppress scanf warnings

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○ Structure of a Function

Example:

int main(void) {
    return 0;
}

• int: return type (integer)
• main: function name (entry point)
• void: no input parameters
• { }: function body
• return 0;: signals successful termination

return 0 = Success

return 1 or other values = Error or abnormal termination

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○ Variables

Declaration:

int x;        // Declares integer variable x
float radius; // Declares float variable

Standard Data Types:

• Integer types: short, int, long long
• Floating-point types: float, double, long double
• Character type: char (read as “character”)

Identifier Rules:

• Must consist of letters, digits, and underscores (_)
• Cannot contain spaces
• First character must be a letter or _ (not a digit)
• Case-sensitive
• Cannot use reserved keywords (e.g., int, float)

Good names describe purpose: year, bank_account, BankAccount

Poor names: i, j, k (unclear meaning)

Initialization:

int x = 10;
int y = 20;
int sum = 0;

Multiple variables of the same type can be initialized in one line:

int width = 100, height = 200;  // Recommended
int width, height = 200;        // Not recommended (only height is initialized)

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○ printf() and scanf()

printf() Example:

int x = 10;
printf("%d", x);  // %d = decimal integer

• %d tells printf to print an integer in decimal format

scanf() Example:

float radius;
printf("Enter radius: ");
scanf("%f", &radius);

• & (ampersand) passes the address of the variable so scanf() knows where to store the input

○ Omitting &radius (e.g., scanf("%f", radius);) will cause an error.

• Use %lf to read a double value (lf = long float)

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○ scanf() vs scanf_s() and Security

In Visual Studio, using scanf() often triggers a warning recommending scanf_s().

Why?

To prevent buffer overflow, which is a common vulnerability.

• scanf() without input size limits can cause excessive input to overwrite memory
• This can allow attackers to execute arbitrary code (a classic exploit)

This technique is known as pwnable hacking

• Many systems (Windows, Adobe, UNIX tools) have been exploited in the past due to such vulnerabilities
• scanf_s() is a Microsoft-specific secure function that forces the developer to specify buffer sizes
• Not portable to Linux/GCC environments → safer alternatives like fgets() are often used

To disable the warning in Visual Studio:

#define _CRT_SECURE_NO_WARNINGS

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○ Practice Exercises

1. Input two numbers and print the sum

1. Calculate monthly salary based on annual salary