Hi,
On 11/6/22 14:07, aitor wrote:
I started documenting libudev-compat in my website as part of the documentation
about vdev:
https://www.gnuinos.org/libudev-compat/
<https://www.gnuinos.org/libudev-compat/>
This documentation is a work in progress.
Jude Nelson was wondering whether or not is possible to use `sendfile(2)`:
*ssize_t sendfile(int */out_fd/*, int */in_fd/*, off_t **/offset/*, size_t
*/count/*); *
in the function:
static int udev_monitor_fs_push_event( int fd, struct udev_monitor* monitor
);
of the file:
https://github.com/jcnelson/vdev/blob/master/libudev-compat/libudev-fs.c
<https://github.com/jcnelson/vdev/blob/master/libudev-compat/libudev-fs.c>
If you read his comment between the lines 759-771:
// send the contents of a file containing a serialized packet to the libudev
client:
// * read the contents
// * send it along to the receiving struct udev_monitor
// NOTE: The file format is expected to be the same as a uevent packet:
// * all newlines (\n) will be converted to null (\0), since that's how
// the kernel sends it.
// * the buffer is expected to be at most 8192 bytes long.
// return 0 on success
// return -errno on failure
// return -EMSGSIZE if the file is too big
// return -EBADMSG if the file is invalid
// return -EAGAIN if we'd block on send
// TODO: can we use sendfile(2)?
The use of `sendfile` would imply to convert the function below (used by
`udev_monitor_fs_push_event`):
// send udev_device along
// TODO: sendfile(2)?
rc = udev_monitor_send_device( monitor, NULL, dev );
if( rc < 0 ) {
log_error("udev_monitor_send_device rc = %d", rc );
}
else {
rc = 0;
}
into something like this:
off_t offset = 0;
// send file descriptor
rc = sendfile( monitor->sock_fs, fd, &offset, BUFSIZ );
if( rc < 0 ) {
rc = -errno;
log_error("udev_monitor_send_device rc = %d", rc );
}
else {
rc = 0;
}
Other modifications would be required in the receiver side, of course.
According to the description in the Linux Man Page:
https://man7.org/linux/man-pages/man2/sendfile.2.html
<https://man7.org/linux/man-pages/man2/sendfile.2.html>
*sendfile*() copies data between one file descriptor and another.
Because this copying is done within the kernel,*sendfile*() is
more efficient than the combination ofread(2)
<https://man7.org/linux/man-pages/man2/read.2.html> andwrite(2)
<https://man7.org/linux/man-pages/man2/write.2.html>,
which would require transferring data to and from user space.
Well, I've been trying to make such improvement in libudev-compat, and
appearently it's working fine.
So, I'll update my git repository soon.
... And again: if you catch some inaccuracy or glaring error in my
documentation,
please let me know and I'll update it.
Thanks in advance,
Aitor.
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