UNIX 網路程式設計學習（7）--readn，writen,readline的實現

readn函式：

//lib/readn.c

/* include readn */
#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) {
			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 */

ssize_t
Readn(int fd, void *ptr, size_t nbytes)
{
	ssize_t		n;

	if ( (n = readn(fd, ptr, nbytes)) < 0)
		err_sys("readn error");
	return(n);
}

writen函式：

//lib/writen.c

/* include writen */
#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);
}
/* end writen */

void
Writen(int fd, void *ptr, size_t nbytes)
{
	if (writen(fd, ptr, nbytes) != nbytes)
		err_sys("writen error");
}

readline函式：

lib/readline.c

//效率提高了，比每次呼叫read讀一個位元組效率要高。

/* include readline */
#include	"unp.h"

static int	read_cnt;//剛開始可以置為一個負值（我的理解）
static char	*read_ptr;
static char	read_buf[MAXLINE];

static ssize_t
my_read(int fd, char *ptr)//每次最多讀取MAXLINE個字元，呼叫一次，每次只返回一個字元
{

	if (read_cnt <= 0) {
again:
		if ( (read_cnt = read(fd, read_buf, sizeof(read_buf))) < 0) {//如果讀取成功，返回read_cnt=讀取的字元			if (errno == EINTR)
				goto again;
			return(-1);
		} else if (read_cnt == 0)
			return(0);
		read_ptr = read_buf;
	}

	read_cnt--;//每次遞減1，直到<0讀完，才執行上面if的命令。
	*ptr = *read_ptr++;//每次讀取一個字元，轉移一個字元
	return(1);
}

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 = my_read(fd, &c)) == 1) {
			*ptr++ = c;
			if (c == '\n')
				break;	/* newline is stored, like fgets() */
		} else if (rc == 0) {
			*ptr = 0;
			return(n - 1);	/* EOF, n - 1 bytes were read */
		} else
			return(-1);		/* error, errno set by read() */
	}

	*ptr = 0;	/* null terminate like fgets() */
	return(n);
}

ssize_t
readlinebuf(void **vptrptr)
{
	if (read_cnt)
		*vptrptr = read_ptr;
	return(read_cnt);
}
/* end readline */

ssize_t
Readline(int fd, void *ptr, size_t maxlen)
{
	ssize_t		n;

	if ( (n = readline(fd, ptr, maxlen)) < 0)
		err_sys("readline error");
	return(n);
}