fcntl設定檔案描述符

How would I put my socket in non-blocking mode?
From: Andrew Gierth (andrew@erlenstar.demon.co.uk
):

 

Technically,
fcntl(soc, F_SETFL, O_NONBLOCK) is incorrect since it clobbers all
other file flags. Generally one gets away with it since the other flags
(O_APPEND for example) don't really apply much to sockets. In a
similarly rough vein, you would use fcntl(soc, F_SETFL, 0) to go back
to blocking mode.

To do it right, use F_GETFL to get the current flags, set or clear the O_NONBLOCK flag, then use F_SETFL to set the flags.

And yes, the flag can be changed either way at will.

 

From: Michael Lampkin
Added on: 2002-06-01 00:53:57

Since this is a common question... the follow is sample code showing setting and un-setting for non-blocking on a socket.

Code:

#include "apue.h"<br />#include <fcntl.h><br />void<br />set_fl(int fd, int flags) /* flags are file status flags to turn on */<br />{<br /> int val;<br /> if ((val = fcntl(fd, F_GETFL, 0)) < 0)<br /> err_sys("fcntl F_GETFL error");<br /> val |= flags; /* turn on flags */<br /> if (fcntl(fd, F_SETFL, val) < 0)<br /> err_sys("fcntl F_SETFL error");<br />}<br />

 

int flags;<br />/* Set socket to non-blocking */<br />if ((flags = fcntl(sock_descriptor, F_GETFL, 0)) < 0)<br />{<br /> /* Handle error */<br />}<br />if (fcntl(socket_descriptor, F_SETFL, flags | O_NONBLOCK) < 0)<br />{<br /> /* Handle error */<br />}<br />/* Set socket to blocking */<br />if ((flags = fcntl(sock_descriptor, F_GETFL, 0)) < 0)<br />{<br /> /* Handle error */<br />}<br />if (fcntl(socket_descriptor, F_SETFL, flags & (~O_NONBLOCK)) < 0)<br />{<br /> /* Handle error */<br />} 

 

UNIX環境進階編程會講得詳細一些。詳見《UNIX環境進階編程》 3.14. Fcntl Function