open - gain access to a device
int prefixopen(dev_t *devp, int flag, int otyp,
int prefixopen(queue_t *q, dev_t *devp, int oflag, int sflag,
Architecture independent level 1 (DDI/DKI). This entry point is required, but it
can be nulldev(9F).
Pointer to a device number.
A bit field passed from the user program open(2)
system call that instructs the driver on how to open the file. Valid settings
Open the device with exclusive access; fail all other
attempts to open the device.
Open the device and return immediately. Do not block the
open even if something is wrong.
Open the device with read-only permission, If ORed
with FWRITE, allow both read and write access.
Open a device with write-only permission. If ORed
with FREAD, allow both read and write access.
Parameter supplied for driver to determine how many times
a device was opened and for what reasons. For OTYP_BLK
, the open()
function can be called many times, but the
function is called only when the last reference to a device
is removed. If the device is accessed through file descriptors, it is done by
a call to close(2)
. If the device is accessed through
memory mapping, it is done by a call to munmap(2)
, there is exactly one close(9E)
operation that is called. This permits software drivers to exist
above hardware drivers and removes any ambiguity from the hardware driver
regarding how a device is used.
Open occurred through block interface for the
Open occurred through the raw/character interface for the
Open a layered process. This flag is used when one driver
calls another driver's open()
function. The calling
driver ensures that there is one-layered close for each layered open. This
flag applies to both block and character devices.
Pointer to the user credential structure.
A pointer to the read queue.
Pointer to a device number. For STREAMS modules,
devp always points to the device number associated with the driver at
the end (tail) of the stream.
Valid oflag values are FEXCL,
FNDELAY, FREAD, and FWRITEL — the same as those
listed above for flag. For STREAMS modules, oflag is
always set to 0.
Valid values are as follows:
Indicates that the open() function is called
through the clone driver. The driver should return a unique device
Modules should be called with sflag set to this
value. Modules should return an error if they are called with sflag set
to a different value. Drivers should return an error if they are called with
sflag set to this value.
Indicates a driver is opened directly, without calling
the clone driver.
Pointer to the user credential structure.
The driver's open() function is called by the kernel during an
open(2) or a mount(2) on the special file for the device. A
device can be opened simultaneously by multiple processes and the
open() driver operation is called for each open. Note that a device is
referenced once its associated open(9E) function is entered, and thus
open(9E) operations which have not yet completed will prevent
close(9E) from being called. The function should verify that the minor
number component of *devp is valid, that the type of access requested
by otyp and flag is appropriate for the device, and, if
required, check permissions using the user credentials pointed to by
When exclusive access is requested by including the FEXCL
flag in flag or oflag, but the caller cannot be granted
exclusive access to the device because it is already open, then the device
driver should conventionally return EBUSY. If instead, exclusive
opens are not supported, then the driver should return ENOTSUP or
The kernel provides open() close() exclusion
guarantees to the driver at *devp, otyp granularity. This
delays new open() calls to the driver while a last-reference
close() call is executing. If the driver has indicated that an
EINTR returns safe via the D_OPEN_RETURNS_EINTR
cb_ops(9S) cb_flag, a delayed open() may be interrupted
by a signal that results in an EINTR return.
Last-reference accounting and open() close()
exclusion typically simplify driver writing. In some cases, however, they
might be an impediment for certain types of drivers. To overcome any
impediment, the driver can change minor numbers in open(9E), as
described below, or implement multiple minor nodes for the same device. Both
techniques give the driver control over when close() calls occur and
whether additional open() calls will be delayed while close()
The open() function is passed a pointer to a device number
so that the driver can change the minor number. This allows drivers to
dynamically create minor instances of the device. An example of this might
be a pseudo-terminal driver that creates a new pseudo-terminal whenever it
is opened. A driver that chooses the minor number dynamically, normally
creates only one minor device node in attach(9E) with
ddi_create_minor_node(9F). It then changes the minor number component
of *devp using makedevice(9F) and getmajor(9F). The
driver needs to keep track of available minor numbers internally. A driver
that dynamically creates minor numbers might want to avoid returning the
original minor number since returning the original minor will result in
postponed dynamic opens when original minor close() call occurs.
*devp = makedevice(getmajor(*devp), new_minor);
The open() function should return 0 for success, or the
appropriate error number.
close(2), exit(2), mmap(2), mount(2),
munmap(2), open(2), Intro(9E), attach(9E),
close(9E), ddi_create_minor_node(9F), getmajor(9F),
getminor(9F), makedevice(9F), nulldev(9F),
Writing Device Drivers
STREAMS Programming Guide
Do not attempt to change the major number.
When a driver modifies the device number passed in, it must not
change the major number portion of the device number. Unless CLONEOPEN is
specified, the modified device number must map to the same driver instance
indicated by the driver's getinfo(9e) implementation. In other words,
cloning across different drivers is not supported. Cloning across different
instances of the same driver in only permitted if the driver specified in
CLONE_DEV in ddi_create_minor_node(9F) is not supported.