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MAKE_DEV(9) Kernel Developer's Manual MAKE_DEV(9) NAME 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_depends -- create and destroy character devices including devfs registration SYNOPSIS #include <sys/param.h> #include <sys/conf.h> void make_dev_args_init(struct make_dev_args *args); int make_dev_s(struct make_dev_args *args, struct cdev **cdev, const char *fmt, ...); int make_dev_alias_p(int flags, struct cdev **cdev, struct cdev *pdev, const char *fmt, ...); void destroy_dev(struct cdev *dev); void destroy_dev_sched(struct cdev *dev); void destroy_dev_sched_cb(struct cdev *dev, void (*cb)(void *), void *arg); void destroy_dev_drain(struct cdevsw *csw); void dev_depends(struct cdev *pdev, struct cdev *cdev); LEGACY INTERFACES 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, ...); int 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, ...); DESCRIPTION The make_dev_s() function creates a cdev structure for a new device, which is returned into the cdev argument. It also notifies devfs(4) of the pres- ence of the new device, that causes corresponding nodes to be created. Be- sides 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 ini- tialized by the make_dev_args_init() function, which ensures that future kernel interface expansion does not affect driver source code or binary in- terface. 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(4) 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 */ #ifndef _POSIX_SOURCE #define S_ISTXT 0001000 #endif 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 make_dev_s(). The fol- lowing values are currently accepted: MAKEDEV_REF reference the created device MAKEDEV_NOWAIT do not sleep, the call may fail MAKEDEV_WAITOK allow the function to sleep to satisfy malloc MAKEDEV_ETERNAL created device will be never destroyed MAKEDEV_CHECKNAME return an error if the device name is invalid or already exists Only MAKEDEV_NOWAIT, MAKEDEV_WAITOK and MAKEDEV_CHECKNAME values are ac- cepted for the make_dev_alias_p() function. The MAKEDEV_WAITOK flag is assumed if none of MAKEDEV_WAITOK, MAKEDEV_NOWAIT is specified. The dev_clone(9) event handler shall specify the MAKEDEV_REF flag when cre- ating a device in response to lookup, to avoid a race where the created de- vice is immediately destroyed after devfs_lookup() drops its 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 responsibil- ity of the driver author to make sure that destroy_dev() 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 make_dev_p() 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; 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); Similarly, the make_dev_credf() 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 make_dev_cred() function is equivalent to the call: make_dev_credf(0, cdevsw, unit, cr, uid, gid, perms, fmt, ...); The make_dev() function call is the same as: make_dev_credf(0, cdevsw, unit, NULL, uid, gid, perms, fmt, ...); The make_dev_alias_p() 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 make_dev_alias() function is similar to make_dev_alias_p() but it re- turns the resulting aliasing *cdev and may not return an error. The cdev returned by make_dev_s() 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 de- signed to replace the unit argument to make_dev(), which can be obtained with dev2unit(). The destroy_dev() function takes the returned cdev from make_dev() and de- stroys the registration for that device. The notification is sent to de- vctl(4) about the destruction event. Do not call destroy_dev() on devices that were created with make_dev_alias(). The dev_depends() 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 destroy_dev_sched_cb() function schedules execution of the de- stroy_dev() for the specified cdev in the safe context. After de- stroy_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); Neither the d_close() driver method, nor a devfs_cdevpriv(9) dtr method can destroy_dev() directly. Doing so causes deadlock when destroy_dev() waits for all threads to leave the driver methods and finish executing any per- open destructors. Also, because destroy_dev() sleeps, no non-sleepable locks may be held over the call. The destroy_dev_sched() family of func- tions overcome these issues. The device driver may call the destroy_dev_drain() function to wait until all devices that have supplied csw as cdevsw, are destroyed. This is use- ful when driver knows that destroy_dev_sched() is called for all instanti- ated devices, but need to postpone module unload until destroy_dev() is ac- tually finished for all of them. RETURN VALUES 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. ERRORS The make_dev_s(), make_dev_p() and make_dev_alias_p() calls will fail and the device will be not registered if: [ENOMEM] The MAKEDEV_NOWAIT flag was specified and a memory allo- cation request could not be satisfied. [ENAMETOOLONG] The MAKEDEV_CHECKNAME flag was specified and the pro- vided device name is longer than SPECNAMELEN. [EINVAL] The MAKEDEV_CHECKNAME flag was specified and the pro- vided device name is empty, contains a "." or ".." path component or ends with `/'. [EINVAL] The MAKEDEV_CHECKNAME flag was specified and the pro- vided device name contains invalid characters. [EEXIST] The MAKEDEV_CHECKNAME flag was specified and the pro- vided device name already exists. SEE ALSO devctl(4), devfs(4), dev_clone(9) HISTORY 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 ap- peared 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. FreeBSD ports 15.quarterly November 4, 2025 MAKE_DEV(9)
NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | HISTORY
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