/proc/kmsg blocks with O_NONBLOCK, read() should return 0 with EAGAIN

/proc/kmsg blocks with O_NONBLOCK, read() should return 0 with EAGAIN

[Graham Swallow]

/proc/kmsg  blocks with O_NONBLOCK, read() should return 0 with EAGAIN

	fd = open( "/proc/kmsg", O_RDONLY|O_NONBLOCK );
	...
	poll() -- says read it

	len = read(fd,buff,len); // first read is fine
	...
	len = read(fd,buff,len); // second read blocks

I call read() a second time, because I'm feeling lucky,
(this is library code, which also reads from sockets).

I expected it to return 0 with errno==EAGAIN.
Instead it waits:  do_syslog(2,buf,count); 

I'm not on this list, so please raise a ticket, linux-2.4.23,

   thanks
--
   Graham
   Information-Cascade (at) ntlworld.com

Re: /proc/kmsg blocks with O_NONBLOCK, read() should return 0 with EAGAIN

[Andrew Morton]

Graham Swallow <Information-Cascade@ntlworld.com> wrote:
>
> /proc/kmsg  blocks with O_NONBLOCK, read() should return 0 with EAGAIN
> 
> 	fd = open( "/proc/kmsg", O_RDONLY|O_NONBLOCK );
> 	...
> 	poll() -- says read it
> 
> 	len = read(fd,buff,len); // first read is fine
> 	...
> 	len = read(fd,buff,len); // second read blocks
> 
> I call read() a second time, because I'm feeling lucky,
> (this is library code, which also reads from sockets).
> 
> I expected it to return 0 with errno==EAGAIN.
> Instead it waits:  do_syslog(2,buf,count); 
> 

Does this work?


diff -puN fs/proc/kmsg.c~kmsg-nonblock fs/proc/kmsg.c
--- 24/fs/proc/kmsg.c~kmsg-nonblock	Mon Feb 23 16:00:48 2004
+++ 24-akpm/fs/proc/kmsg.c	Mon Feb 23 16:01:12 2004
@@ -32,6 +32,8 @@ static int kmsg_release(struct inode * i
 static ssize_t kmsg_read(struct file * file, char * buf,
 			 size_t count, loff_t *ppos)
 {
+	if ((file->f_flags & O_NONBLOCK) && !do_syslog(9, 0, 0))
+		return -EAGAIN;
 	return do_syslog(2,buf,count);
 }
 

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