strcmp(const char *str1, const char *str2): Compares two strings lexicographically. Returns 0 if strings are equal, a negative value if str1 is less than str2, and a positive value if str1 is greater than str2.

char s1[] = "apple";
char s2[] = "banana";
int cmp = strcmp(s1, s2); // cmp will be < 0

strstr(const char *haystack, const char *needle): Finds the first occurrence of the substring needle in the string haystack. Returns a pointer to the beginning of the located substring, or NULL if the substring is not found.

char text[] = "Hello World";
char sub[] = "World";
char *ptr = strstr(text, sub); // ptr points to 'W' in "World"

Chapter 3: Functions

Call by Value vs. Call by Reference

Feature	Call by Value	Call by Reference
Mechanism	A copy of the actual argument's value is passed to the formal parameter.	The address of the actual argument is passed to the formal parameter.
Data Modification	Changes made to the formal parameter inside the function do NOT affect the original actual argument.	Changes made to the formal parameter (via pointer dereferencing) DO affect the original actual argument.
Memory	A separate memory location is created for the formal parameter.	No separate memory is created for the formal parameter; it uses the same memory location as the actual argument.
Arguments	Actual arguments are variables or expressions.	Actual arguments are addresses of variables (using `&`).
Usage	When you want to protect the original value from modification.	When you need to modify the original value of the actual argument from within the function.

Code: Factorial using Recursion

#include <stdio.h>

long long factorial(int n) {
    if (n == 0 || n == 1) {
        return 1;
    } else {
        return n * factorial(n - 1); // Recursive call
    }
}

int main() {
    int num;
    printf("Enter a non-negative integer: ");
    scanf("%d", &num);
    if (num < 0) {
        printf("Factorial is not defined for negative numbers.\n");
    } else {
        printf("Factorial of %d = %lld\n", num, factorial(num));
    }
    return 0;
}

Chapter 4: Structures and Unions

Differences: `structure` vs. `union`

Feature	Structure	Union
Keyword	`struct`	`union`
Memory Allocation	Each member gets its own separate memory location. Total size is sum of sizes of all members (plus padding).	All members share the same memory location. Total size is equal to the size of the largest member.
Data Access	All members can hold values simultaneously.	Only one member can hold a value at any given time. Assigning a value to one member overwrites the value of previous member.
Purpose</