UNIX网络编程卷1（第三版）readn，writen和readline函数

#include    "unp.h"
ssize_t                     /* Read "n" bytes from a descriptor. */
readn(int fd, void *vptr, size_t n)
{
    size_t  nleft;
    ssize_t nread;
    char    *ptr;
    ptr = vptr;
    nleft = n;
    while (nleft > 0) {
        if ( (nread = read(fd, ptr, nleft)) < 0) {//循环读取
            if (errno == EINTR)
                nread = 0;      /* and call read() again */
            else
                return(-1);
        } else if (nread == 0)
            break;              /* EOF */
        nleft -= nread;
        ptr   += nread;
    }
    return(n - nleft);      /* return >= 0 */
}
/* end readn */

#include    "unp.h"
ssize_t                     /* Write "n" bytes to a descriptor. */
writen(int fd, const void *vptr, size_t n)
{
    size_t      nleft;
    ssize_t     nwritten;
    const char  *ptr;
    ptr = vptr;
    nleft = n;
    while (nleft > 0) {
        if ( (nwritten = write(fd, ptr, nleft)) <= 0) {
            if (nwritten < 0 && errno == EINTR)
                nwritten = 0;       /* and call write() again */
            else
                return(-1);         /* error */
        }
        nleft -= nwritten;
        ptr   += nwritten;
    }
    return(n);
}

//较慢的一个版本，需要每次都调用系统的read函数
//作者还写了一个较快的版本，这里就不贴出代码了，主要思想是定义一个my_read一次读取进来，但每次都只返回给readline一个字符
ssize_t
readline(int fd, void *vptr, size_t maxlen)
{
    ssize_t n, rc;
    char    c, *ptr;
    ptr = vptr;
    for (n = 1; n < maxlen; n++) {
        if ( (rc = read(fd, &c, 1)) == 1) {
            *ptr++ = c;
            if (c == '\n')
                break;
        } else if (rc == 0) {
            if (n == 1)
                return(0);  /* EOF, no data read */
            else
                break;      /* EOF, some data was read */
        } else
            return(-1); /* error */
    }
    *ptr = 0;
    return(n);
}
/* end readline */