一些C库函数实现

memcpy

void* my_memcpy(void *dest, void *src, int n) {
    if (dest == NULL || src == NULL || n <= 0)
        return NULL;
    char* pdest = (char*) dest;
    char* psrc = (char*) src;
    if (pdest > psrc && pdest < psrc + n) {
        pdest = pdest + n - 1;
        psrc = psrc + n - 1;
        while (n--) {
            *pdest-- = *psrc--;
        }
    }
    else {
        while (n--) {
            *pdest++ = *psrc++;
        }
    }
    return dest;
}

strcpy

char* my_strcpy(char* strDest, const char* strSrc) {
    if (strDest == NULL || strSrc == NULL)
        return NULL;
    char* p = strDest;
    while (*strSrc != '') {
        *strDest = *strSrc;
        strDest++;
        strSrc++;
    }
    return p;
}

atoi

int my_atoi(const char* str) {
    if (str == NULL)
        return 0;
    int res = 0, sign = 1;
    while (isspace(*str))
        str++;
    if (*str == '-') {
        sign = -1;
        str++;
    }
    else if (*str == '+') {
        str++;
    }
    while (*str >= '0' && *str <= '9') {
        res = res * 10 + (*str - '0');
        str++;
    }
    res *= sign;
    return res;
}