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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