strcpy,strlen,strcmp实现

1.strcpy的实现

char * my_strcpy(char * s1, char * s2)
{
    assert(s1 != NULL&&s2 != NULL);
    char *res = s1;
    while ((*(res++) = *(s2++))!=‘\0‘);
    return res;
}

2.strlen的实现

int my_strlen(char * s)
{
    assert(s != NULL);

    int num = 0;
    while (*(s++))
        ++num;
    return num;
}

3.strcmp的实现

int my_strcmp(char * s1, char * s2)
{
    assert(s1 != NULL&&s2 != NULL);

    while (*s1 == *s2 && ‘\0‘ != *s1 && ‘\0‘ != *s2)
    {
        ++s1;
        ++s2;
    }

    if (*s1 == *s2)
        return 0;
    else if (*s1 > *s2)
        return 1;
    else
        return -1;
}