[C] 實作strcpy, strncpy, strcmp

strcpy:

char* strcpy(char* des, const char* src){
    if(des == NULL || src == NULL)
        return NULL;
    while(*src != '\0'){
        *des = *src;
        des++;
        src++;
    }
    *des = '\0';
    return des;
}

strncpy:

char* strncpy(char* des, const char* src, unsigned count){
    if(des == NULL || src == NULL)
        return NULL;
    while(count > 0){
        *des = *src;
        des++;
        src++;
        count--;
    }
    *des = '\0';
    return des;
}

strcmp:

int strcmp(const char* a, const char* b){
    if(a == NULL || b == NULL)
        return 0;
    while(*a == *b){
        if(*a == '\0')
            return 0;
        a++;
        b++;
    }
    return *a - *b;
}