C语言实现strcat / strlen / strcmp / strcpy

主要考虑两点：

1. 返回值对使用的便利性。
2. 边界，null的判断。

---

strcat

char *m_strcat(char *des, const char *src)
{
    assert((des != NULL) && (src != NULL));
    char *add = des;
    while (*des != '\0')
        ++des;
    while (*des++ = *src++)
        ;
    return add;
}

strlen

int m_strlen(const char *str)
{
    assert(str != NULL);
    int len = 0;
    while (*str != '\0')
    {
        ++str;
        ++len;
    }
    return len;
}

strcmp

int m_strcmp(const char *des, const char *src)
{
    // return 0 des==src , + des>src , - des<src
    assert((des != NULL) && (src != NULL));
    while ((*des) == (*src))
    {
        if (*des == '\0')
            return 0;
        ++des;
        ++src;
    }
    return *des - *src;
}

strcpy

char *m_strcpy(char *des, const char *src)
{
    assert((des != NULL) && (src != NULL));
    char *add = des;
    while ((*des++ = *src++) != '\0')
        ;
    return add;
}