Module 11: Strings & Character Algorithms

1. Null Termination 1 / 4

C-Strings as Null-Terminated Character Arrays

Why the '\0' byte is the sentinel for every string algorithm

C has no built-in "string" primitive type. A string in C is simply an array of char bytes ending with a special sentinel byte called the Null Terminator ('\0', ASCII 0).

Index	0	1	2	3	4	5
Character	`'A'`	`'p'`	`'p'`	`'l'`	`'e'`	`'\0'` (Sentinel)
ASCII	65	112	112	108	101	0

⚠️

Remember Array Sizing for Null Terminator

To store an N-character word, the array size MUST be at least N + 1 bytes to accommodate the trailing '\0'. Declaring char s[5] = "Apple"; is an error because it lacks room for '\0'!

2. Stack vs .rodata 2 / 4

Stack Array vs Read-Only Literal in .rodata

char str[] = "apple" vs char *str = "apple"

string_mutability.c

// 1. Mutable Character Array on Stack:
char stack_str[] = "Apple";
stack_str[0] = 'a'; // VALID: Modifies bytes copied into local stack frame

// 2. Pointer to String Literal in Read-Only Data (.rodata text segment):
char *literal_str = "Apple";
// literal_str[0] = 'a'; // SEGMENTATION FAULT! Hardware MMU traps write to read-only page

3. Custom Implementations 3 / 4

Custom Implementations of strlen, strcpy & strcmp

Under the hood of standard library string algorithms

custom_string_lib.c

size_t my_strlen(const char *s) {
    const char *p = s;
    while (*p != '\\0') p++;
    return p - s;
}

char* my_strcpy(char *dest, const char *src) {
    char *ret = dest;
    while ((*dest++ = *src++));
    return ret;
}

4. Safe Strings 4 / 4

Modern Safe String Formatting: snprintf

Guaranteed null-termination and boundary truncation

safe_snprintf.c

char buffer[64];
int id = 1042;
const char *user = "Alexander";

// snprintf guarantees max bytes written will never exceed sizeof(buffer):
snprintf(buffer, sizeof(buffer), "User_%s_ID_%d", user, id);
printf("Result: %s\\n", buffer);

Module 11: Strings & Character Algorithms (<string.h>)

C-Strings as Null-Terminated Character Arrays

Stack Array vs Read-Only Literal in .rodata

Custom Implementations of strlen, strcpy & strcmp

Modern Safe String Formatting: snprintf

1. Essential <ctype.h> Classification Functions

2. Practice Exercises & Solutions