Switch a fd between blocking and non-blocking mode?

Switch a fd between blocking and non-blocking mode?

[Jeffrey Walton]

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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Re: Switch a fd between blocking and non-blocking mode?

[Konstantin Andreev]

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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Re: Switch a fd between blocking and non-blocking mode?

[Jeffrey Walton]

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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