open − gain access to a device


Block and Character
#include <sys/types.h>
#include <sys/file.h>
#include <sys/errno.h>
#include <sys/open.h>
#include <sys/cred.h>
#include <sys/ddi.h>
#include <sys/sunddi.h>

int prefixopen(dev_t *devp, int flag, int otyp, cred_t *cred_p);

#include <sys/file.h>
#include <sys/stream.h>
#include <sys/ddi.h>
#include <sys/sunddi.h>

int prefixopen(queue_t *q, dev_t *devp, int oflag, int sflag, cred_t *cred_p);


Architecture independent level 1 (DDI/DKI). This entry point is required, but it can be nulldev(9F)


Block and Character


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 are:


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, then allow both read and write access)


Open a device with write-only permission (if ORed with FREAD, then allow both read and write access)


Parameter supplied so that the driver can determine how many times a device was opened and for what reasons. For OTYP_BLK and OTYP_CHR, the open() routine may be called many times, but the close(9E) routine is called only when the last reference to a device is removed. If the device is accessed through file descriptors, this is by a call to close(2) or exit(2) If the device is accessed through memory mapping, this is by a call to munmap(2) or exit(2) For OTYP_LYR, there is exactly one close(9E) for each open() 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 device


Open occurred through the raw/character interface for the device


Open a layered process. This flag is used when one driver calls another driver’s open() or close(9E) routine. The calling driver will make sure 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() routine is called through the clone driver. The driver should return a unique device number.


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() routine is called by the kernel during an open(2) or a mount(2) on the special file for the device. The routine 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 cred_p.

The open() routine 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) 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:

*devp = makedevice(getmajor(*devp), new_minor);


The open() routine 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.