How do you define string functions in the C programming language?

In C language, string functions can be defined in two ways:

1. Use character arrays as parameters and return values.

#include <stdio.h>

// 字符串长度
int my_strlen(char str[]) {
    int len = 0;
    while (str[len] != '\0') {
        len++;
    }
    return len;
}

// 字符串复制
void my_strcpy(char dest[], char src[]) {
    int i = 0;
    while (src[i] != '\0') {
        dest[i] = src[i];
        i++;
    }
    dest[i] = '\0';
}

// 字符串连接
void my_strcat(char dest[], char src[]) {
    int dest_len = my_strlen(dest);
    int i, j = 0;
    for (i = dest_len; src[j] != '\0'; i++, j++) {
        dest[i] = src[j];
    }
    dest[i] = '\0';
}

int main() {
    char str1[100] = "Hello";
    char str2[100] = "World";

    printf("str1 length: %d\n", my_strlen(str1));
    
    my_strcpy(str1, str2);
    printf("str1 after copy: %s\n", str1);
    
    my_strcat(str1, str2);
    printf("str1 after concatenation: %s\n", str1);

    return 0;
}

1. Passing pointers as arguments and return values.

#include <stdio.h>

// 字符串长度
int my_strlen(char *str) {
    int len = 0;
    while (*str != '\0') {
        len++;
        str++;
    }
    return len;
}

// 字符串复制
char* my_strcpy(char *dest, char *src) {
    char *p = dest;
    while (*src != '\0') {
        *dest = *src;
        dest++;
        src++;
    }
    *dest = '\0';
    return p;
}

// 字符串连接
char* my_strcat(char *dest, char *src) {
    char *p = dest;
    while(*dest != '\0') {
        dest++;
    }
    while (*src != '\0') {
        *dest = *src;
        dest++;
        src++;
    }
    *dest = '\0';
    return p;
}

int main() {
    char str1[100] = "Hello";
    char str2[100] = "World";

    printf("str1 length: %d\n", my_strlen(str1));
    
    my_strcpy(str1, str2);
    printf("str1 after copy: %s\n", str1);
    
    my_strcat(str1, str2);
    printf("str1 after concatenation: %s\n", str1);

    return 0;
}

Both using character arrays and pointers can achieve the same function. However, it is important to be more cautious when using pointers to prevent pointer operations from going beyond the string’s range.