C language strncpy \ analog strncat \ strncmp implementation

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
//strncpy:从str2中复制前n个字符串到str1
void my_cpy(char * str1, const char * str2, int n){
	int i;
	for (i = 0; i < n; ++i){
		str1[i] = str2[i];
	}
	puts(str1);
}
int main(){
	char str2[20] = "abcdefgh" ;
	char str1[20]="1234";
	my_cpy(str1, str2, 3);
	system("pause");
	return 0;
}

//strncat:将str2中的前n个字符拼接到str1的后面
void my_cat(char *str1, const char *str2, int n){
	int i;
	int ret = strlen(str1);
	for (i = 0; i < n; ++i){
		str1[ret + i] = str2[i];
	}
	puts(str1);
}
int main(){
	char str2[20] = "abcdefgh";
	char str1[20]="1234";
	my_cat(str1, str2, 3);
	system("pause");
	return 0;
}

//strncmp:将str1与str2里面前n个字符串进行比较 
//大于返回1，小于返回-1，等于返回0
int my_cmp(const char * str1, const char * str2, int n){
	int i, j;
	for (i = 0; i < n; ++i){
		for (j = 0; j < n; ++j){
			}
		if (str1[i] == str2[j]){
				return 0;
		}
		else if (str1[i]>str2[j]){
			return 1;
		}
		else
			return -1;
	}
}
int main(){
	char str2[20] = "125defgh";
	char str1[20]="1234";
	int ret=my_cmp(str1, str2, 3);
	printf("%d\n", ret);
	system("pause");
	return 0;
}

Follow the blog will also write about simulation implement other string manipulation functions, interested can look at me,
another beginner, I hope we can come to criticism