[simulate strncpy], [simulate strncat], [simulate strncmp]
All three functions are implemented in bytes
[strncpy]
char* strncpy(char* destination,const char* source,size_t num);
Copy num characters from the source character to the target space. If the length of the source string is less than num, add 0 to the back of the target after the source string is tested, until num characters.

char* my_strncpy(char* dest, const char*src, size_t num)
{
    char* ret = dest;
    assert(dest);//Detect whether dest exists, not NULL
    assert(src);
    while (num)
    {
        *dest++ = *src++;//One byte one byte copy
        num--;
        if ((*src == '\0') && (num <= 0))//If the source string bytes are less than num, add enough num after the target string
        {
            *dest++ = '\0';
            return ret;//Return target string
        }
    }
    return ret;
}

[strncat]
char* strncat(char* destination,const char* source,size_t num);
Splice the num characters of the source string to the target string. When splicing the string, it will add '\ 0' to you by default.

char* my_strncat(char* dest, const char* src, size_t num)
{
    char* ret = dest;
    assert(dest);//Detect whether dest exists, not NULL
    assert(src);
    while (*dest)
    {
        dest++;//Find the end of the target string first
    }
    while (num)
    {
        *dest++ = *src++;//Start splicing one by one from the end back
        num--;
    }
    *dest++ = '\0';//After splicing, add '\ 0' after the target string
    return ret;//Return target string
}

[strncmp]
int strncmp(const char* str1,const char* str2,size_t num);
Compare two strings by byte, and find that there are different characters, or one string ends or num characters are all compared.

int my_strncmp(const char* str1, const char* str2, size_t num)
{
    assert(str1);//Detect whether dest exists, not NULLw
    assert(str2);
    int ret = 0;
    while (num)//Compare num times
    {
        ret = *(unsigned char*)str1 - *(unsigned char*)str2;
        str1++;
        str2++;
        if (ret < 0)
            ret = -1;
        else if (ret>0)
            ret = 1;
        if (!*str1 || !*str2)
            ret = 0;
    }
    return ret;
}

The Works of Liezi:

//As you can see, this function is to find out the string starting with A4 in six strings
int test()
{
    char str[][6] = { "A499", "BF77", "3456", "A401", "A4HJ", "B612" };
    int n;
    puts("Looking for A4 ...\n");
    for (n = 0; n < 6; n++)
    {
        if (strncmp(str[n], "A4xx", 2) == 0)//Print when you find it
            printf("%s\n", str[n]);
    }
    return 0;
}

If there is something wrong, you can comment and tell me, hope to guide!