strstr、strcmp、strlen、strcpy

const char* strstr(const char *str, const char* substr)
{
    int i, j, temp;

    for (i = 0; str[i] != '\0'; i++)
    {
        j = 0;
        temp = i; //记录当前主字符串位置
        while (str[i++]==substr[j++])
        {
            if (substr[j] == '\0')
            {
                return &str[temp];
            }
            else if (str[i]=='\0')
            {
                return NULL; 
            }
        }
    }

    return NULL;
}

int strcmp(const char* str1, const char* str2)
{
    while (*str1==*str2)
    {
        if (*str1 == '\0')
        {
            return 0;
        }

        ++str1; ++str2;
    }

    return *str1 - *str2;
}

unsigned int strlen(const char* str)
{
    const char* res = str;
    while (*res++); //res跳出循环后,仍执行++
    return (res - str - 1);
}

 

char* strcpy(char *dst,const char* src)
{
    assert(0==dst||0==src);

    char* tmp = dst;

    while (*src!='0')
    {
        *dst++ = *src++;
    }
    *dst = '\0';

    return tmp;
}

strcpy为较安全的做法，且构造成易于实现链式表达式的形式。