1 .\" $NetBSD: ioctl.9,v 1.26 2008/11/12 12:35:54 ad Exp $
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39 .Nd "how to implement a new ioctl call to access device drivers"
49 call is made, the kernel dispatches it to the device driver
50 which can then interpret the request number and data in a specialized
52 Ioctls are defined as:
54 #define MYDEVIOCTL fun(g, n, t)
57 where the different symbols correspond to:
58 .Bl -tag -width ".Dv MYDEVIOCTL"
60 The name which will later be given in the
62 system call as second argument, e.g.,
64 ioctl(fd, MYDEVIOCTL, ...)
67 A macro which can be one of:
68 .Bl -tag -width ".Fn _IOWR"
70 The call is a simple message to the kernel by itself.
71 It does not copy anything into the kernel, nor does it want anything back.
73 The call only reads parameters from the kernel and does not
76 The call only writes parameters to the kernel, but does not want anything
79 The call writes data to the kernel and wants information back.
82 We always consider reading or writing to the kernel, from the user perspective.
84 This integer describes to which subsystem the ioctl applies.
85 Here are some examples:
87 .Bl -tag -width xxxxx -compact
105 generic file-descriptor
144 random number generator
157 This number uniquely identifies the ioctl within the group.
158 That said, two subsystems may share the same
160 but there may be only one
164 This is an unsigned 8 bit number.
166 This specifies the type of the passed parameter.
167 This one gets internally transformed to the size of the parameter, so
168 for example, if you want to pass a structure, then you have to specify that
169 structure and not a pointer to it or sizeof(struct MYDEV).
172 In order for the new ioctl to be visible to the system, it is installed
174 .In sys/ioctl.h or one of the files that are reached from
177 A distinction must be made at this point.
182 should return either 0 for success
183 or a defined error code, as described in
185 At the libc level though a conversion takes place, so that eventually
187 returns either 0 for success or -1 for failure, in which case the
189 variable is set accordingly.
191 The use of magic numbers such as -1, to indicate that a given ioctl
192 code was not handled, is strongly discouraged.
193 The value -1 is bound to the
195 pseudo-error, which is returned inside kernel to modify return to process.
197 Let's suppose that we want to pass an integer value to the kernel.
198 From the user point of view, this is like writing to the kernel.
199 So we define the ioctl as:
200 .Bd -literal -offset indent
201 #define MYDEVIOCTL _IOW('i', 25, int)
206 routine of the driver, it can be then accessed like:
207 .Bd -literal -offset indent
209 mydev_ioctl(struct dev_ioctl_args *ap)
217 kprintf("Value passed from userspace: %d\\n", *a);
218 return (0); /* Success */
221 /* Handle other ioctls here */
224 /* Inappropriate ioctl for device */
234 .Bd -literal -offset indent
236 if (ioctl(fd, MYDEVIOCTL, \*[Am]a) == -1) {