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Subjectwatch_devices(2) manpage
Date
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.\" Copyright (c) 2019 David Howells <dhowells@redhat.com>
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.TH WATCH_DEVICES 2 2019-08-29 "Linux" "Linux Programmer's Manual"
.SH NAME
watch_devices \- Watch for global device notifications
.SH SYNOPSIS
.nf
.B #include <linux/watch_queue.h>
.br
.B #include <unistd.h>
.br
.BI "int watch_devices(int " watch_fd ", int " watch_id ", unsigned int " flags );
.fi
.PP
.IR Note :
There are no glibc wrappers for these system calls.
.SH DESCRIPTION
.PP
.BR watch_devices ()
attaches a watch on the global device notification source to a previously
opened and configured watch queue. See
.BR watch_queue (7)
for more information on how to set up and use those.
.PP
The global device notification source is provided with events from a number of
sources, including block device errors and USB events. Each notification type
has a specific format.

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.SS Block Device Notifications
Events on block devices, such as I/O errors are posted to any watching queues.
The message format is:
.PP
.in +4n
.EX
struct block_notification {
struct watch_notification watch;
__u64 dev;
__u64 sector;
};
.EE
.in
.PP
The
.I watch.type
field will be set to
.BR WATCH_TYPE_BLOCK_NOTIFY ,
the
.I watch.subtype
field will contain a constant that indicates the particular event that occurred
and the watch_id passed to watch_devices() will be placed in
.I watch.info
in the ID field.
.PP
.I dev
will contain the major and minor device numbers in
.B dev_t
form and
.I sector
will contain the first sector the notification pertains to.
.PP
The following events are defined:
.PP
.in +4n
.TS
lB l.
NOTIFY_BLOCK_ERROR_TIMEOUT
NOTIFY_BLOCK_ERROR_NO_SPACE
NOTIFY_BLOCK_ERROR_RECOVERABLE_TRANSPORT
NOTIFY_BLOCK_ERROR_CRITICAL_TARGET
NOTIFY_BLOCK_ERROR_CRITICAL_NEXUS
NOTIFY_BLOCK_ERROR_CRITICAL_MEDIUM
NOTIFY_BLOCK_ERROR_PROTECTION
NOTIFY_BLOCK_ERROR_KERNEL_RESOURCE
NOTIFY_BLOCK_ERROR_DEVICE_RESOURCE
NOTIFY_BLOCK_ERROR_IO
.TE
.in
.PP
All of which indicate error conditions.

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.SS USB Device Notifications
Events on USB devices, such as I/O errors are posted to any watching queues.
The message format is:
.PP
.in +4n
.EX
struct usb_notification {
struct watch_notification watch;
__u32 error;
__u32 reserved;
__u8 name_len;
__u8 name[0];
};
.EE
.in
.PP
The
.I watch.type
field will be set to
.BR WATCH_TYPE_USB_NOTIFY ,
the
.I watch.subtype
field will contain a constant that indicates the particular event that occurred
and the watch_id passed to watch_devices() will be placed in
.I watch.info
in the ID field.
.PP
.IR name " and " name_len
indicates the textual name of the USB device that originated the notification.
The name will be truncated to
.B USB_NOTIFICATION_MAX_NAME_LEN
if it is longer than that.
.PP
The following subtypes are currently defined:
.TP
.B NOTIFY_USB_DEVICE_ADD
A new USB device has been plugged in.
.TP
.B NOTIFY_USB_DEVICE_REMOVE
A USB device has been unplugged.
.TP
.B NOTIFY_USB_BUS_ADD
A new USB bus is now available.
.TP
.B NOTIFY_USB_BUS_REMOVE
A USB bus has been removed.
.TP
.B NOTIFY_USB_DEVICE_RESET
A USB device has been reset.
.TP
.B NOTIFY_USB_DEVICE_ERROR
A USB device has generated an error; a suitable error code will have been
placed in
.IR error .


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.SH RETURN VALUE
On success, the function returns 0. On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
The following errors may be returned:
.TP
.B EBADF
.I watch_fd
is an invalid file descriptor.
.TP
.B EBADSLT
The watch does not exist and so cannot be removed.
.TP
.B EBUSY
The source is already attached to the watch device instance specified by
.I watch_fd
and so cannot be added.
.TP
.B EINVAL
.I watch_fd
does not refer to a watch_queue device file.
.TP
.B EINVAL
.IR watch_fd " or " watch_id
is out of range.
.TP
.B EINVAL
Unsupported
.I flags
set.
.TP
.B ENOMEM
Insufficient memory available to allocate a watch record.
.TP
.B EPERM
The caller does not have the required privileges.
.SH CONFORMING TO
These functions are Linux-specific and should not be used in programs intended
to be portable.
.SH VERSIONS
The notification queue driver first appeared in v??? of the Linux kernel.
.SH NOTES
Glibc does not (yet) provide a wrapper for the
.BR watch_devices "()"
system call; call it using
.BR syscall (2).
.SH SEE ALSO
.BR watch_queue (7)

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