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MAKE_DEV(9) Kernel Developer's Manual MAKE_DEV(9)


make_dev, make_dev_cred, make_dev_credf, make_dev_p, make_dev_s, make_dev_alias, make_dev_alias_p, destroy_dev, destroy_dev_sched, destroy_dev_sched_cb, destroy_dev_drain, dev_dependsmanage cdev's and DEVFS registration for devices


#include <sys/param.h>
#include <sys/conf.h>

make_dev_args_init(struct make_dev_args *args);

make_dev_s(struct make_dev_args *args, struct cdev **cdev, const char *fmt, ...);

make_dev_alias_p(int flags, struct cdev **cdev, struct cdev *pdev, const char *fmt, ...);

destroy_dev(struct cdev *dev);

destroy_dev_sched(struct cdev *dev);

destroy_dev_sched_cb(struct cdev *dev, void (*cb)(void *), void *arg);

destroy_dev_drain(struct cdevsw *csw);

dev_depends(struct cdev *pdev, struct cdev *cdev);

struct cdev *
make_dev(struct cdevsw *cdevsw, int unit, uid_t uid, gid_t gid, int perms, const char *fmt, ...);

struct cdev *
make_dev_cred(struct cdevsw *cdevsw, int unit, struct ucred *cr, uid_t uid, gid_t gid, int perms, const char *fmt, ...);

struct cdev *
make_dev_credf(int flags, struct cdevsw *cdevsw, int unit, struct ucred *cr, uid_t uid, gid_t gid, int perms, const char *fmt, ...);

make_dev_p(int flags, struct cdev **cdev, struct cdevsw *devsw, struct ucred *cr, uid_t uid, gid_t gid, int mode, const char *fmt, ...);

struct cdev *
make_dev_alias(struct cdev *pdev, const char *fmt, ...);


The () function creates a cdev structure for a new device, which is returned into the cdev argument. It also notifies devfs(5) of the presence of the new device, that causes corresponding nodes to be created. Besides this, a devctl(4) notification is sent. The function takes the structure struct make_dev_args args, which specifies the parameters for the device creation:

struct make_dev_args {
	size_t		 mda_size;
	int		 mda_flags;
	struct cdevsw	*mda_devsw;
	struct ucred	*mda_cr;
	uid_t		 mda_uid;
	gid_t		 mda_gid;
	int		 mda_mode;
	int		 mda_unit;
	void		*mda_si_drv1;
	void		*mda_si_drv2;
Before use and filling with the desired values, the structure must be initialized by the () function, which ensures that future kernel interface expansion does not affect driver source code or binary interface.

The created device will be owned by args.mda_uid, with the group ownership as args.mda_gid. The name is the expansion of fmt and following arguments as printf(9) would print it. The name determines its path under /dev or other devfs(5) mount point and may contain slash ‘/’ characters to denote subdirectories. The permissions of the file specified in args.mda_mode are defined in <sys/stat.h>:

#define S_IRWXU 0000700    /* RWX mask for owner */
#define S_IRUSR 0000400    /* R for owner */
#define S_IWUSR 0000200    /* W for owner */
#define S_IXUSR 0000100    /* X for owner */

#define S_IRWXG 0000070    /* RWX mask for group */
#define S_IRGRP 0000040    /* R for group */
#define S_IWGRP 0000020    /* W for group */
#define S_IXGRP 0000010    /* X for group */

#define S_IRWXO 0000007    /* RWX mask for other */
#define S_IROTH 0000004    /* R for other */
#define S_IWOTH 0000002    /* W for other */
#define S_IXOTH 0000001    /* X for other */

#define S_ISUID 0004000    /* set user id on execution */
#define S_ISGID 0002000    /* set group id on execution */
#define S_ISVTX 0001000    /* sticky bit */
#define S_ISTXT 0001000

The args.mda_cr argument specifies credentials that will be stored in the si_cred member of the initialized struct cdev.

The args.mda_flags argument alters the operation of () The following values are currently accepted:

reference the created device
do not sleep, the call may fail
allow the function to sleep to satisfy malloc
created device will be never destroyed
return an error if the device name is invalid or already exists

Only MAKEDEV_NOWAIT, MAKEDEV_WAITOK and MAKEDEV_CHECKNAME values are accepted for the () function.

The MAKEDEV_WAITOK flag is assumed if none of MAKEDEV_WAITOK, MAKEDEV_NOWAIT is specified.

The dev_clone(9) event handler shall specify MAKEDEV_REF flag when creating a device in response to lookup, to avoid race where the device created is destroyed immediately after devfs_lookup(9) drops his reference to cdev.

The MAKEDEV_ETERNAL flag allows the kernel to not acquire some locks when translating system calls into the cdevsw methods calls. It is responsibility of the driver author to make sure that () is never called on the returned cdev. For the convenience, use the MAKEDEV_ETERNAL_KLD flag for the code that can be compiled into kernel or loaded (and unloaded) as loadable module.

A panic will occur if the MAKEDEV_CHECKNAME flag is not specified and the device name is invalid or already exists.

The () use of the form

struct cdev *dev;
int res;
res = make_dev_p(flags, &dev, cdevsw, cred, uid, gid, perms, name);
is equivalent to the code
struct cdev *dev;
struct make_dev_args args;
int res;

args.mda_flags = flags;
args.mda_devsw = cdevsw;
args.mda_cred = cred;
args.mda_uid = uid;
args.mda_gid = gid;
args.mda_mode = perms;
res = make_dev_s(&args, &dev, name);

Similarly, the () function call is equivalent to

	(void) make_dev_s(&args, &dev, name);
In other words, make_dev_credf() does not allow the caller to obtain the return value, and in kernels compiled with the INVARIANTS options, the function asserts that the device creation succeeded.

The () function is equivalent to the call

make_dev_credf(0, cdevsw, unit, cr, uid, gid, perms, fmt, ...);

The () function call is the same as

make_dev_credf(0, cdevsw, unit, NULL, uid, gid, perms, fmt, ...);

The () function takes the returned cdev from make_dev() and makes another (aliased) name for this device. It is an error to call make_dev_alias_p() prior to calling make_dev().

The () function is similar to make_dev_alias() but it returns the resulting aliasing *cdev and may not return an error.

The cdev returned by () and make_dev_alias_p() has two fields, si_drv1 and si_drv2, that are available to store state. Both fields are of type void *, and can be initialized simultaneously with the cdev allocation by filling args.mda_si_drv1 and args.mda_si_drv2 members of the make_dev_s() argument structure, or filled after the cdev is allocated, if using legacy interfaces. In the latter case, the driver should handle the race of accessing uninitialized si_drv1 and si_drv2 itself. These are designed to replace the unit argument to make_dev(), which can be obtained with ().

The () function takes the returned cdev from make_dev() and destroys the registration for that device. The notification is sent to devctl(4) about the destruction event. Do not call destroy_dev() on devices that were created with make_dev_alias().

The () function establishes a parent-child relationship between two devices. The net effect is that a destroy_dev() of the parent device will also result in the destruction of the child device(s), if any exist. A device may simultaneously be a parent and a child, so it is possible to build a complete hierarchy.

The () function schedules execution of the destroy_dev() for the specified cdev in the safe context. After destroy_dev() is finished, and if the supplied cb is not NULL, the callback cb is called, with argument arg. The destroy_dev_sched() function is the same as

destroy_dev_sched_cb(cdev, NULL, NULL);

The () driver method cannot call destroy_dev() directly. Doing so causes deadlock when destroy_dev() waits for all threads to leave the driver methods. Also, because destroy_dev() sleeps, no non-sleepable locks may be held over the call. The destroy_dev_sched() family of functions overcome these issues.

The device driver may call the () function to wait until all devices that have supplied csw as cdevsw, are destroyed. This is useful when driver knows that () is called for all instantiated devices, but need to postpone module unload until destroy_dev() is actually finished for all of them.


If successful, make_dev_s() and make_dev_p() will return 0, otherwise they will return an error. If successful, make_dev_credf() will return a valid cdev pointer, otherwise it will return NULL.


The make_dev_s(), make_dev_p() and make_dev_alias_p() calls will fail and the device will be not registered if:

The MAKEDEV_NOWAIT flag was specified and a memory allocation request could not be satisfied.
The MAKEDEV_CHECKNAME flag was specified and the provided device name is longer than SPECNAMELEN.
The MAKEDEV_CHECKNAME flag was specified and the provided device name is empty, contains a "." or ".." path component or ends with ‘/’.
The MAKEDEV_CHECKNAME flag was specified and the provided device name contains invalid characters.
The MAKEDEV_CHECKNAME flag was specified and the provided device name already exists.


devctl(4), devfs(5), dev_clone(9)


The make_dev() and destroy_dev() functions first appeared in FreeBSD 4.0. The function make_dev_alias() first appeared in FreeBSD 4.1. The function dev_depends() first appeared in FreeBSD 5.0. The functions make_dev_credf(), destroy_dev_sched(), destroy_dev_sched_cb() first appeared in FreeBSD 7.0. The function make_dev_p() first appeared in FreeBSD 8.2. The function make_dev_s() first appeared in FreeBSD 11.0.

March 2, 2016 Debian