strcpy,strcmp,strcat

1.strcpy

const char *strcpy(char *strDest, const char *strSrc)
{
    if ((strDest == NULL) || (strSrc == NULL))
    {
        return NULL;
    } 
    else if(strDest == strSrc)
    {
        return strDest;
    }
    else
    {
        const char *tmp = strSrc;while((strlen(strSrc)))
        {
            *strDest++ = *strSrc++;
        }
        *strDest = '\0';
        return tmp;
    }
}

2.strcmp

int strcmp(const char *str1, const char *str2)
{
    assert(str1 && str2);
    while (*str1 && *str1++ == *str2++);
    return *str1 - *str2;
}

3.strcat

char *_cdecl strcat(char *dst, const char *src) 
{ 
　　char *cp = dst; 
　　while(*cp) 
　　　　cp++; 
　　while(*cp++ = *src++); 
　　return dst; 
}