make_dev.9

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.Dd January 19, 2025
.Dt MAKE_DEV 9
.Os
.Sh NAME
.Nm make_dev ,
.Nm make_dev_cred ,
.Nm make_dev_credf ,
.Nm make_dev_p ,
.Nm make_dev_s ,
.Nm make_dev_alias ,
.Nm make_dev_alias_p ,
.Nm destroy_dev ,
.Nm destroy_dev_sched ,
.Nm destroy_dev_sched_cb ,
.Nm destroy_dev_drain ,
.Nm dev_depends
.Nd create and destroy character devices including devfs registration
.Sh SYNOPSIS
.In sys/param.h
.In sys/conf.h
.Ft void
.Fn make_dev_args_init "struct make_dev_args *args"
.Ft int
.Fn make_dev_s "struct make_dev_args *args" "struct cdev **cdev" "const char *fmt" ...
.Ft int
.Fn make_dev_alias_p "int flags" "struct cdev **cdev" "struct cdev *pdev" "const char *fmt" ...
.Ft void
.Fn destroy_dev "struct cdev *dev"
.Ft void
.Fn destroy_dev_sched "struct cdev *dev"
.Ft void
.Fn destroy_dev_sched_cb "struct cdev *dev" "void (*cb)(void *)" "void *arg"
.Ft void
.Fn destroy_dev_drain "struct cdevsw *csw"
.Ft void
.Fn dev_depends "struct cdev *pdev" "struct cdev *cdev"
.Pp
LEGACY INTERFACES
.Ft struct cdev *
.Fn make_dev "struct cdevsw *cdevsw" "int unit" "uid_t uid" "gid_t gid" "int perms" "const char *fmt" ...
.Ft struct cdev *
.Fn make_dev_cred "struct cdevsw *cdevsw" "int unit" "struct ucred *cr" "uid_t uid" "gid_t gid" "int perms" "const char *fmt" ...
.Ft struct cdev *
.Fn make_dev_credf "int flags" "struct cdevsw *cdevsw" "int unit" "struct ucred *cr" "uid_t uid" "gid_t gid" "int perms" "const char *fmt" ...
.Ft int
.Fn 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" ...
.Ft struct cdev *
.Fn make_dev_alias "struct cdev *pdev" "const char *fmt" ...
.Sh DESCRIPTION
The
.Fn make_dev_s
function creates a
.Fa cdev
structure for a new device, which is returned into the
.Fa cdev
argument.
It also notifies
.Xr devfs 4
of the presence of the new device, that causes corresponding nodes
to be created.
Besides this, a
.Xr devctl 4
notification is sent.
The function takes the structure
.Va struct make_dev_args args ,
which specifies the parameters for the device creation:
.Pp
.Bd -literal -offset indent -compact
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;
};
.Ed
.Pp
Before use and filling with the desired values, the structure must be
initialized by the
.Fn make_dev_args_init
function, which ensures that future kernel interface expansion does
not affect driver source code or binary interface.
.Pp
The created device will be owned by
.Va args.mda_uid ,
with the group ownership as
.Va args.mda_gid .
The name is the expansion of
.Va fmt
and following arguments as
.Xr printf 9
would print it.
The name determines its path under
.Pa /dev
or other
.Xr devfs 4
mount point and may contain slash
.Ql /
characters to denote subdirectories.
The permissions of the file specified in
.Va args.mda_mode
are defined in
.In sys/stat.h :
.Pp
.Bd -literal -offset indent -compact
#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 */
#ifndef _POSIX_SOURCE
#define S_ISTXT 0001000
#endif
.Ed
.Pp
The
.Va args.mda_cr
argument specifies credentials that will be stored in the
.Fa si_cred
member of the initialized
.Fa struct cdev .
.Pp
The
.Va args.mda_flags
argument alters the operation of
.Fn make_dev_s .
The following values are currently accepted:
.Pp
.Bl -tag -width "MAKEDEV_CHECKNAME" -compact -offset indent
.It Dv MAKEDEV_REF
reference the created device
.It Dv MAKEDEV_NOWAIT
do not sleep, the call may fail
.It Dv MAKEDEV_WAITOK
allow the function to sleep to satisfy malloc
.It Dv MAKEDEV_ETERNAL
created device will be never destroyed
.It Dv MAKEDEV_CHECKNAME
return an error if the device name is invalid or already exists
.El
.Pp
Only
.Dv MAKEDEV_NOWAIT ,
.Dv MAKEDEV_WAITOK
and
.Dv MAKEDEV_CHECKNAME
values are accepted for the
.Fn make_dev_alias_p
function.
.Pp
The
.Dv MAKEDEV_WAITOK
flag is assumed if none of
.Dv MAKEDEV_WAITOK ,
.Dv MAKEDEV_NOWAIT
is specified.
.Pp
The
.Xr dev_clone 9
event handler shall specify the
.Dv MAKEDEV_REF
flag when creating a device in response to lookup, to avoid a race where
the created device is immediately destroyed after
.Fn devfs_lookup
drops its reference to
.Fa cdev .
.Pp
The
.Dv 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
.Fn destroy_dev
is never called on the returned cdev.
For the convenience, use the
.Dv MAKEDEV_ETERNAL_KLD
flag for the code that can be compiled into kernel or loaded
(and unloaded) as loadable module.
.Pp
A panic will occur if the
.Dv MAKEDEV_CHECKNAME
flag is not specified
and the device name is invalid or already exists.
.Pp
The
.Fn make_dev_p
use of the form:
.Bd -literal -offset indent
struct cdev *dev;
int res;
res = make_dev_p(flags, &dev, cdevsw, cred, uid, gid, perms, name);
.Ed
.Pp
is equivalent to the code:
.Bd -literal -offset indent
struct cdev *dev;
struct make_dev_args args;
int res;

make_dev_args_init(&args);
args.mda_flags = flags;
args.mda_devsw = cdevsw;
args.mda_cr = cred;
args.mda_uid = uid;
args.mda_gid = gid;
args.mda_mode = perms;
res = make_dev_s(&args, &dev, name);
.Ed
.Pp
Similarly, the
.Fn make_dev_credf
function call is equivalent to:
.Bd -literal -offset indent
(void) make_dev_s(&args, &dev, name);
.Ed
.Pp
In other words,
.Fn make_dev_credf
does not allow the caller to obtain the return value, and in
kernels compiled with the
.Va INVARIANTS
options, the function asserts that the device creation succeeded.
.Pp
The
.Fn make_dev_cred
function is equivalent to the call:
.Bd -literal -offset indent
make_dev_credf(0, cdevsw, unit, cr, uid, gid, perms, fmt, ...);
.Ed
.Pp
The
.Fn make_dev
function call is the same as:
.Bd -literal -offset indent
make_dev_credf(0, cdevsw, unit, NULL, uid, gid, perms, fmt, ...);
.Ed
.Pp
The
.Fn make_dev_alias_p
function takes the returned
.Ft cdev
from
.Fn make_dev
and makes another (aliased) name for this device.
It is an error to call
.Fn make_dev_alias_p
prior to calling
.Fn make_dev .
.Pp
The
.Fn make_dev_alias
function is similar to
.Fn make_dev_alias_p
but it returns the resulting aliasing
.Ft *cdev
and may not return an error.
.Pp
The
.Fa cdev
returned by
.Fn make_dev_s
and
.Fn make_dev_alias_p
has two fields,
.Fa si_drv1
and
.Fa si_drv2 ,
that are available to store state.
Both fields are of type
.Ft void * ,
and can be initialized simultaneously with the
.Va cdev
allocation by filling
.Va args.mda_si_drv1
and
.Va args.mda_si_drv2
members of the
.Fn make_dev_s
argument structure, or filled after the
.Va cdev
is allocated, if using legacy interfaces.
In the latter case, the driver should handle the race of
accessing uninitialized
.Va si_drv1
and
.Va si_drv2
itself.
These are designed to replace the
.Fa unit
argument to
.Fn make_dev ,
which can be obtained with
.Fn dev2unit .
.Pp
The
.Fn destroy_dev
function takes the returned
.Fa cdev
from
.Fn make_dev
and destroys the registration for that device.
The notification is sent to
.Xr devctl 4
about the destruction event.
Do not call
.Fn destroy_dev
on devices that were created with
.Fn make_dev_alias .
.Pp
The
.Fn dev_depends
function establishes a parent-child relationship between two devices.
The net effect is that a
.Fn 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.
.Pp
The
.Fn destroy_dev_sched_cb
function schedules execution of the
.Fn destroy_dev
for the specified
.Fa cdev
in the safe context.
After
.Fn destroy_dev
is finished, and if the supplied
.Fa cb
is not
.Dv NULL ,
the callback
.Fa cb
is called, with argument
.Fa arg .
The
.Fn destroy_dev_sched
function is the same as:
.Bd -literal -offset indent
destroy_dev_sched_cb(cdev, NULL, NULL);
.Ed
.Pp
The
.Fn d_close
driver method cannot call
.Fn destroy_dev
directly.
Doing so causes deadlock when
.Fn destroy_dev
waits for all threads to leave the driver methods.
Also, because
.Fn destroy_dev
sleeps, no non-sleepable locks may be held over the call.
The
.Fn destroy_dev_sched
family of functions overcome these issues.
.Pp
The device driver may call the
.Fn destroy_dev_drain
function to wait until all devices that have supplied
.Fa csw
as cdevsw, are destroyed.
This is useful when driver knows that
.Fn destroy_dev_sched
is called for all instantiated devices, but need to postpone module
unload until
.Fn destroy_dev
is actually finished for all of them.
.Sh RETURN VALUES
If successful,
.Fn make_dev_s
and
.Fn make_dev_p
will return 0, otherwise they will return an error.
If successful,
.Fn make_dev_credf
will return a valid
.Fa cdev
pointer, otherwise it will return
.Dv NULL .
.Sh ERRORS
The
.Fn make_dev_s ,
.Fn make_dev_p
and
.Fn make_dev_alias_p
calls will fail and the device will be not registered if:
.Bl -tag -width Er
.It Bq Er ENOMEM
The
.Dv MAKEDEV_NOWAIT
flag was specified and a memory allocation request could not be satisfied.
.It Bq Er ENAMETOOLONG
The
.Dv MAKEDEV_CHECKNAME
flag was specified and the provided device name is longer than
.Dv SPECNAMELEN .
.It Bq Er EINVAL
The
.Dv MAKEDEV_CHECKNAME
flag was specified and the provided device name is empty, contains a
.Qq \&.
or
.Qq ..
path component or ends with
.Ql / .
.It Bq Er EINVAL
The
.Dv MAKEDEV_CHECKNAME
flag was specified and the provided device name contains invalid characters.
.It Bq Er EEXIST
The
.Dv MAKEDEV_CHECKNAME
flag was specified and the provided device name already exists.
.El
.Sh SEE ALSO
.Xr devctl 4 ,
.Xr devfs 4 ,
.Xr dev_clone 9
.Sh HISTORY
The
.Fn make_dev
and
.Fn destroy_dev
functions first appeared in
.Fx 4.0 .
The function
.Fn make_dev_alias
first appeared in
.Fx 4.1 .
The function
.Fn dev_depends
first appeared in
.Fx 5.0 .
The functions
.Fn make_dev_credf ,
.Fn destroy_dev_sched ,
.Fn destroy_dev_sched_cb
first appeared in
.Fx 7.0 .
The function
.Fn make_dev_p
first appeared in
.Fx 8.2 .
The function
.Fn make_dev_s
first appeared in
.Fx 11.0 .