Custom String Functions

Implementations of common string operatiosn without standard library functions.

#include <stdio.h>
#include <stddef.h>

// Calculate string length
size_t custom_strlen(const char *str) {
    size_t len = 0;
    while (str[len] != '\0') {
        len++;
    }
    return len;
}

// Compare two strings
int custom_strcmp(const char *a, const char *b) {
    while (*a && *b && *a == *b) {
        a++;
        b++;
    }
    return *(const unsigned char*)a - *(const unsigned char*)b;
}

// Copy string from source to destination
char *custom_strcpy(char *dest, const char *src) {
    char *start = dest;
    while ((*dest++ = *src++)) {}
    return start;
}

// Concatenate source string to destination
char *custom_strcat(char *dest, const char *src) {
    char *ptr = dest + custom_strlen(dest);
    while ((*ptr++ = *src++)) {}
    return dest;
}

// Find substring in a string
char *custom_strstr(const char *haystack, const char *needle) {
    if (!*needle) return (char*)haystack;
    for (; *haystack; haystack++) {
        const char *h = haystack;
        const char *n = needle;
        while (*h && *n && *h == *n) {
            h++;
            n++;
        }
        if (!*n) return (char*)haystack;
    }
    return NULL;
}

int main() {
    char text1[50] = "Hello World";
    char text2[] = "Programming";
    
    printf("Length of text1: %zu\n", custom_strlen(text1));
    
    int cmp = custom_strcmp(text1, text2);
    if (cmp > 0) printf("'%s' is greater\n", text1);
    else if (cmp < 0) printf("'%s' is less\n", text1);
    else printf("Strings are equal\n");
    
    custom_strcpy(text1, text2);
    printf("After copy: text1='%s', text2='%s'\n", text1, text2);
    
    char *found = custom_strstr(text1, "gram");
    if (found) printf("Substring found: %s\n", found);
    
    custom_strcat(text1, " is fun!");
    printf("After concatenation: text1='%s'\n", text1);
    
    return 0;
}

Array Pointers and 2D Arrays

An array pointer holds the address of an entire array. When incremented, it moves by the size of the array.

#include <stdio.h>

int main() {
    int grid[3][4] = {{1,2,3,4}, {5,6,7,8}, {9,10,11,12}};
    
    printf("Address of grid[0][0]: %p\n", (void*)&grid[0][0]);
    printf("Value of grid[0]: %p\n", (void*)grid[0]);
    printf("Value of grid: %p\n", (void*)grid);
    
    printf("After incrementing by 1:\n");
    printf("&grid[0][0]+1: %p\n", (void*)(&grid[0][0]+1));
    printf("grid[0]+1: %p\n", (void*)(grid[0]+1));
    printf("grid+1: %p\n", (void*)(grid+1));
    
    int (*ptr)[4] = grid; // Pointer to an array of 4 integers
    
    for (int i = 0; i < 3; i++) {
        for (int j = 0; j < 4; j++) {
            printf("%d\t", *(*(ptr + i) + j));
        }
        printf("\n");
    }
    
    return 0;
}

Pointer Arrays

A pointer array is an array where each element is a pointer.

Definition: data_type *array_name[size];

Functions Returning Pointers

These functions return a memory address. The returned address should point to static, global, dynamically allocated, or caller-provided memory.

Definition: return_type *function_name(parameters);

Example:

#include <stdio.h>
#include <stdlib.h>

#define BUFFER_SIZE 100

char *create_message(const char *name) {
    static char buffer[BUFFER_SIZE];
    snprintf(buffer, BUFFER_SIZE, "Greetings, %s!", name);
    return buffer;
}

int main() {
    char *msg = create_message("Developer");
    printf("%s\n", msg);
    return 0;
}

Function Pointers and Callbacks

A function pointer stores the address of a function. It enables callbacks, where a function is past as an argument.

Definition: return_type (*pointer_name)(parameter_types);

Example:

#include <stdio.h>

int add(int a, int b) { return a + b; }
int subtract(int a, int b) { return a - b; }
int multiply(int a, int b) { return a * b; }

void calculate(int x, int y, int (*op_func)(int, int)) {
    printf("Result: %d\n", op_func(x, y));
}

int main() {
    int a = 20, b = 8;
    
    calculate(a, b, add);
    calculate(a, b, subtract);
    calculate(a, b, multiply);
    
    return 0;
}