* Switch a fd between blocking and non-blocking mode?
@ 2019-11-11 23:21 Jeffrey Walton
2019-11-13 9:58 ` Konstantin Andreev
0 siblings, 1 reply; 3+ messages in thread
From: Jeffrey Walton @ 2019-11-11 23:21 UTC (permalink / raw)
To: kernelnewbies
Hi Everyone,
I have one more question related to my problem of my program losing
data. I want to ensure I'm not using an anti-pattern that's causing
the problem.
I have a worker thread that blocks on read(2). When the read() occurs,
the fd is switch from blocking to non-blocking. Additional reads are
performed until EAGAIN or EWOULDBLOCK. Then the fd is switched back to
blocking mode.
Does switching a socket between blocking and non-blocking mode cause
the kernel to reset or delete queued data (that has not been read by
the application yet)?
Thanks in advance.
Jeff
----------
Here are the functions that switch between blocking and non-blocking
mode. There's not much to them.
void make_blocking_fd(int fd)
{
const int old = fcntl(fd, F_GETFL, 0);
fcntl(fd, F_SETFL, old & ~(int)O_NONBLOCK);
}
void make_nonblocking_fd(int fd)
{
const int old = fcntl(fd, F_GETFL, 0);
fcntl(fd, F_SETFL, old | (int)O_NONBLOCK);
}
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^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Switch a fd between blocking and non-blocking mode?
2019-11-11 23:21 Switch a fd between blocking and non-blocking mode? Jeffrey Walton
@ 2019-11-13 9:58 ` Konstantin Andreev
2019-11-14 2:20 ` Jeffrey Walton
0 siblings, 1 reply; 3+ messages in thread
From: Konstantin Andreev @ 2019-11-13 9:58 UTC (permalink / raw)
To: noloader, kernelnewbies
Hi, Jeffrey.
I have looked into the sources of the kernel 4.19.13.
After the series of checks the fcntl(F_SETFL) just stores flags into the file descriptor, and that's all:
fs/fcntl.c:
| int setfl(int fd, struct file * filp, unsigned long arg)
| {
| int error = 0;
| ...
| if (filp->f_op->check_flags)
| error = filp->f_op->check_flags(arg);
|
| if (!error && filp->f_op->setfl)
| error = filp->f_op->setfl(filp, arg);
|
| if (error)
| return error;
| ...
| /* #define SETFL_MASK (O_APPEND | O_NONBLOCK | O_NDELAY | O_DIRECT | O_NOATIME) */
| filp->f_flags = (arg & SETFL_MASK) | (filp->f_flags & ~SETFL_MASK);
| ...
| return error;
| }
You can see two filesystem-specific hooks above, ->check_flags() and ->setfl().
But, to my surprise, ->setfl() is not used by any filesystem in the kernel tree. ->check_flags() is used by nfs only, and just checks the flags, as it's name suggests.
Regards, Konstantin Andreev
Jeffrey Walton, 12 Nov 2019 02:21 MSK:
>
> I have one more question related to my problem of my program losing data. I want to ensure I'm not using an anti-pattern that's causing the problem.
>
> I have a worker thread that blocks on read(2). When the read() occurs, the fd is switch from blocking to non-blocking. Additional reads are performed until EAGAIN or EWOULDBLOCK. Then the fd is switched back to blocking mode.
>
> Does switching a socket between blocking and non-blocking mode cause the kernel to reset or delete queued data (that has not been read by the application yet)?
> ----------
>
> Here are the functions that switch between blocking and non-blocking mode. There's not much to them.
>
> void make_blocking_fd(int fd)
> {
> const int old = fcntl(fd, F_GETFL, 0);
> fcntl(fd, F_SETFL, old & ~(int)O_NONBLOCK);
> }
>
> void make_nonblocking_fd(int fd)
> {
> const int old = fcntl(fd, F_GETFL, 0);
> fcntl(fd, F_SETFL, old | (int)O_NONBLOCK);
> }
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^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: Switch a fd between blocking and non-blocking mode?
2019-11-13 9:58 ` Konstantin Andreev
@ 2019-11-14 2:20 ` Jeffrey Walton
0 siblings, 0 replies; 3+ messages in thread
From: Jeffrey Walton @ 2019-11-14 2:20 UTC (permalink / raw)
To: Konstantin Andreev; +Cc: kernelnewbies
On Wed, Nov 13, 2019 at 4:59 AM Konstantin Andreev <andreev@swemel.ru> wrote:
>
> I have looked into the sources of the kernel 4.19.13.
>
> After the series of checks the fcntl(F_SETFL) just stores flags into the file descriptor, and that's all:
Ack, thanks. I did not think the kernel discarded the data when
switching the mode. (a tcflush or tcdrain may do it, but I did not
call them).
I'm almost certain there's a bug in the firmware on the modem I am using.
Jeff
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2019-11-11 23:21 Switch a fd between blocking and non-blocking mode? Jeffrey Walton
2019-11-13 9:58 ` Konstantin Andreev
2019-11-14 2:20 ` Jeffrey Walton
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