c语言中strncat函数

#include <stdio.h>

char *strncat(char *s1, const char *s2, size_t n)  // 这里的cat指的是：concatenate 
{
    char *tmp = s1;
    
    while(*s1)
        s1++;
    
    while(n--)
        if(!(*s1++ = *s2++))
            break;
    
    *s1 = ‘\0‘;  //????
    return tmp;
}

int main(void)
{
    char str1[128] = "abcdefg";
    char str2[128] = "1234567";
    
    printf("result: %s\n", strncat(str1, str2, 3));
    
    return 0;
}

#include <stdio.h>
#include <string.h>  //strncat函数的头文件

int main(void)
{
    char str1[128] = "abcdefg";
    char str2[128] = "1234567";
    
    printf("result: %s\n", strncat(str1, str2, 5));
    return 0;
}