系统级IO

#include<cstddef>

// unixIO
ssize_t read(int fd, void *buf, size_t n);      //若成功则为读的字节数，若EOF则为0，若出错则为-1
ssize_t write(int fd, const void *buf, size_t n);

//Robust IO

//不带缓存版本
ssize_t rio_readn(int fd, void *userbuf, size_t n)
{
    size_t nleft = n;
    ssize_t nread;
    char *bufp = (char*)userbuf;


    while(nleft>0)
    {
        if((nread = read(fd, bufp, nleft))<0)
        {
            if(errno == EINTR)      //被中断，重启函数
                nread = 0;
            else
                return -1;
        }
        else if(nread == 0)
            break;

        nleft -= nread;
        bufp += nread;
    }

    return (n - nleft);

}

ssize_t rio_writen(int fd, void *userbuf, size_t n)
{
    size_t nleft = n;
    ssize_t nwrite;
    char *bufp = (char*)userbuf;


    while(nleft > 0)
    {
        if((nwrite = write(fd, bufp, nleft))<0)
        {
            if(errno == EINTR)
                nwrite = 0;
            else
                return -1;
        }


        nleft -= nwrite;
        bufp += nwrite;
    }

    return n;

}
//带缓存版本
static const int RIO_BUFSIZE = 8192;

struct rio_t
{
    int     rio_fd;             //文件描述符
    int     rio_cnt;            //未读字节数
    char    *rio_bufptr;        //下一个未读字节
    char    rio_buf[RIO_BUFSIZE];//内部缓冲
};

void rio_readinitb(rio_t *rp, int fd)
{
    rp->rio_fd = fd;
    rp->rio_cnt = 0;
    rp->rio_bufptr = rp->rio_buf;
}

ssize_t rio_read(rio_t *rp, void *userbuf, size_t n)
{


    while(rp->rio_cnt <= 0)
    {
        rp->rio_cnt = read(rp->rio_fd, rp->rio_buf, sizeof(rp->rio_buf));

        if(rp->rio_cnt < 0)
        {
            if(errno != EINTR)
                return -1;
        }
        else if (rp->rio_cnt == 0)
            return 0;
        else
            rp->rio_bufptr = rp->rio_buf;
    }



    size_t cnt = n > rp->rio_cnt ? rp->rio_cnt : n;


    memcpy(userbuf, rp->rio_bufptr, cnt);

    rp->rio_bufptr += cnt;
    rp->rio_cnt -= cnt;

    return cnt;
}
//读文本行，复制到内存位置userbuf，最后在结尾添NULL（0）
ssize_t rio_readlineb(rio_t *rp, void *userbuf, size_t maxlen)
{
    ssize_t readn;
    char c;
    char *buf = (char*)userbuf;
    int i;
    for(i = 1; i < maxlen; ++i)
    {
        readn = rio_read(rp, &c, 1);
        if(readn < 0)
            return -1;
        else if (readn == 0)
        {
            if(i == 1)
                return 0;
            else
                break;
        }
        else
        {
            *buf++ = c;
            if(c == '\n')
            {
                ++i;
                break;
            }
        }



    }

    *buf = 0;
    return i-1;

}
//读二进制文件和文本行
ssize_t rio_readnb(rio_t *rp, void *userbuf, size_t n)
{
    size_t left = n;
    ssize_t readn;
    char *buf = (char*)userbuf;
    while(left > 0)
    {
        readn = rio_read(rp, buf, left);

        if(readn == 0)
            break;
        else if (readn < 0)
            return -1;
        else
        {

        }

        left -= readn;
        buf += readn;
    }
    return (n - left);
}