设备文件应该代表物理设备。大多数物理设备既用于输出也用于输入，因此必须存在某种机制，让内核中的设备驱动程序能够从进程获取要发送到设备的输出。 这是通过打开设备文件进行输出并写入来实现的，就像写入文件一样。在以下示例中，这是通过以下方式实现的device_write.
这并不总是足够的。 想象一下，你有一个连接到调制解调器的串行端口（即使你有一个内置调制解调器，从 CPU 的角度来看，它仍然被实现为连接到调制解调器的串行端口，所以你不必过度发挥你的想象力）。 自然的做法是使用设备文件向调制解调器写入内容（调制解调器命令或要通过电话线发送的数据）并从调制解调器读取内容（命令的响应或通过电话线接收的数据）。 但是，这留下了一个悬而未决的问题，即当你需要与串行端口本身通信时该怎么办，例如发送数据发送和接收的速率。
Unix 中的答案是使用一个名为ioctl（输入输出控制的缩写）的特殊函数。 每个设备都可以有自己的ioctl命令，可以进行读ioctl（将信息从进程发送到内核），写ioctl（将信息返回到进程），[1] 两者或都不进行。ioctl该函数使用三个参数调用：相应设备文件的文件描述符、ioctl 编号和一个参数，该参数的类型为 long，因此你可以使用强制类型转换来传递任何内容。 [2]
ioctl 编号编码了主设备号、ioctl 的类型、命令和参数的类型。 此 ioctl 编号通常由宏调用创建（_IO, _IOR, _IOW或_IOWR--- 取决于类型）在头文件中。 然后，程序和内核模块都应包含此头文件，以便程序可以使用ioctl（以便它们可以生成适当的ioctl），而内核模块可以理解它。 在下面的示例中，头文件是chardev.h而使用它的程序是ioctl.c.
如果你想在自己的内核模块中使用ioctl，最好接收官方的ioctl分配，这样如果你不小心获得了别人的ioctl，或者他们获得了你的，你就会知道有问题。 有关更多信息，请查阅内核源代码树，网址为Documentation/ioctl-number.txt.
示例 7-1. chardev.c
/*
* chardev.c - Create an input/output character device
*/
#include <linux/kernel.h> /* We're doing kernel work */
#include <linux/module.h> /* Specifically, a module */
#include <linux/fs.h>
#include <asm/uaccess.h> /* for get_user and put_user */
#include "chardev.h"
#define SUCCESS 0
#define DEVICE_NAME "char_dev"
#define BUF_LEN 80
/*
* Is the device open right now? Used to prevent
* concurent access into the same device
*/
static int Device_Open = 0;
/*
* The message the device will give when asked
*/
static char Message[BUF_LEN];
/*
* How far did the process reading the message get?
* Useful if the message is larger than the size of the
* buffer we get to fill in device_read.
*/
static char *Message_Ptr;
/*
* This is called whenever a process attempts to open the device file
*/
static int device_open(struct inode *inode, struct file *file)
{
#ifdef DEBUG
printk(KERN_INFO "device_open(%p)\n", file);
#endif
/*
* We don't want to talk to two processes at the same time
*/
if (Device_Open)
return -EBUSY;
Device_Open++;
/*
* Initialize the message
*/
Message_Ptr = Message;
try_module_get(THIS_MODULE);
return SUCCESS;
}
static int device_release(struct inode *inode, struct file *file)
{
#ifdef DEBUG
printk(KERN_INFO "device_release(%p,%p)\n", inode, file);
#endif
/*
* We're now ready for our next caller
*/
Device_Open--;
module_put(THIS_MODULE);
return SUCCESS;
}
/*
* This function is called whenever a process which has already opened the
* device file attempts to read from it.
*/
static ssize_t device_read(struct file *file, /* see include/linux/fs.h */
char __user * buffer, /* buffer to be
* filled with data */
size_t length, /* length of the buffer */
loff_t * offset)
{
/*
* Number of bytes actually written to the buffer
*/
int bytes_read = 0;
#ifdef DEBUG
printk(KERN_INFO "device_read(%p,%p,%d)\n", file, buffer, length);
#endif
/*
* If we're at the end of the message, return 0
* (which signifies end of file)
*/
if (*Message_Ptr == 0)
return 0;
/*
* Actually put the data into the buffer
*/
while (length && *Message_Ptr) {
/*
* Because the buffer is in the user data segment,
* not the kernel data segment, assignment wouldn't
* work. Instead, we have to use put_user which
* copies data from the kernel data segment to the
* user data segment.
*/
put_user(*(Message_Ptr++), buffer++);
length--;
bytes_read++;
}
#ifdef DEBUG
printk(KERN_INFO "Read %d bytes, %d left\n", bytes_read, length);
#endif
/*
* Read functions are supposed to return the number
* of bytes actually inserted into the buffer
*/
return bytes_read;
}
/*
* This function is called when somebody tries to
* write into our device file.
*/
static ssize_t
device_write(struct file *file,
const char __user * buffer, size_t length, loff_t * offset)
{
int i;
#ifdef DEBUG
printk(KERN_INFO "device_write(%p,%s,%d)", file, buffer, length);
#endif
for (i = 0; i < length && i < BUF_LEN; i++)
get_user(Message[i], buffer + i);
Message_Ptr = Message;
/*
* Again, return the number of input characters used
*/
return i;
}
/*
* This function is called whenever a process tries to do an ioctl on our
* device file. We get two extra parameters (additional to the inode and file
* structures, which all device functions get): the number of the ioctl called
* and the parameter given to the ioctl function.
*
* If the ioctl is write or read/write (meaning output is returned to the
* calling process), the ioctl call returns the output of this function.
*
*/
int device_ioctl(struct inode *inode, /* see include/linux/fs.h */
struct file *file, /* ditto */
unsigned int ioctl_num, /* number and param for ioctl */
unsigned long ioctl_param)
{
int i;
char *temp;
char ch;
/*
* Switch according to the ioctl called
*/
switch (ioctl_num) {
case IOCTL_SET_MSG:
/*
* Receive a pointer to a message (in user space) and set that
* to be the device's message. Get the parameter given to
* ioctl by the process.
*/
temp = (char *)ioctl_param;
/*
* Find the length of the message
*/
get_user(ch, temp);
for (i = 0; ch && i < BUF_LEN; i++, temp++)
get_user(ch, temp);
device_write(file, (char *)ioctl_param, i, 0);
break;
case IOCTL_GET_MSG:
/*
* Give the current message to the calling process -
* the parameter we got is a pointer, fill it.
*/
i = device_read(file, (char *)ioctl_param, 99, 0);
/*
* Put a zero at the end of the buffer, so it will be
* properly terminated
*/
put_user('\0', (char *)ioctl_param + i);
break;
case IOCTL_GET_NTH_BYTE:
/*
* This ioctl is both input (ioctl_param) and
* output (the return value of this function)
*/
return Message[ioctl_param];
break;
}
return SUCCESS;
}
/* Module Declarations */
/*
* This structure will hold the functions to be called
* when a process does something to the device we
* created. Since a pointer to this structure is kept in
* the devices table, it can't be local to
* init_module. NULL is for unimplemented functions.
*/
struct file_operations Fops = {
.read = device_read,
.write = device_write,
.ioctl = device_ioctl,
.open = device_open,
.release = device_release, /* a.k.a. close */
};
/*
* Initialize the module - Register the character device
*/
int init_module()
{
int ret_val;
/*
* Register the character device (atleast try)
*/
ret_val = register_chrdev(MAJOR_NUM, DEVICE_NAME, &Fops);
/*
* Negative values signify an error
*/
if (ret_val < 0) {
printk(KERN_ALERT "%s failed with %d\n",
"Sorry, registering the character device ", ret_val);
return ret_val;
}
printk(KERN_INFO "%s The major device number is %d.\n",
"Registeration is a success", MAJOR_NUM);
printk(KERN_INFO "If you want to talk to the device driver,\n");
printk(KERN_INFO "you'll have to create a device file. \n");
printk(KERN_INFO "We suggest you use:\n");
printk(KERN_INFO "mknod %s c %d 0\n", DEVICE_FILE_NAME, MAJOR_NUM);
printk(KERN_INFO "The device file name is important, because\n");
printk(KERN_INFO "the ioctl program assumes that's the\n");
printk(KERN_INFO "file you'll use.\n");
return 0;
}
/*
* Cleanup - unregister the appropriate file from /proc
*/
void cleanup_module()
{
int ret;
/*
* Unregister the device
*/
ret = unregister_chrdev(MAJOR_NUM, DEVICE_NAME);
/*
* If there's an error, report it
*/
if (ret < 0)
printk(KERN_ALERT "Error: unregister_chrdev: %d\n", ret);
} |
示例 7-2. chardev.h
/* * chardev.h - the header file with the ioctl definitions. * * The declarations here have to be in a header file, because * they need to be known both to the kernel module * (in chardev.c) and the process calling ioctl (ioctl.c) */ #ifndef CHARDEV_H #define CHARDEV_H #include <linux/ioctl.h> /* * The major device number. We can't rely on dynamic * registration any more, because ioctls need to know * it. */ #define MAJOR_NUM 100 /* * Set the message of the device driver */ #define IOCTL_SET_MSG _IOR(MAJOR_NUM, 0, char *) /* * _IOR means that we're creating an ioctl command * number for passing information from a user process * to the kernel module. * * The first arguments, MAJOR_NUM, is the major device * number we're using. * * The second argument is the number of the command * (there could be several with different meanings). * * The third argument is the type we want to get from * the process to the kernel. */ /* * Get the message of the device driver */ #define IOCTL_GET_MSG _IOR(MAJOR_NUM, 1, char *) /* * This IOCTL is used for output, to get the message * of the device driver. However, we still need the * buffer to place the message in to be input, * as it is allocated by the process. */ /* * Get the n'th byte of the message */ #define IOCTL_GET_NTH_BYTE _IOWR(MAJOR_NUM, 2, int) /* * The IOCTL is used for both input and output. It * receives from the user a number, n, and returns * Message[n]. */ /* * The name of the device file */ #define DEVICE_FILE_NAME "char_dev" #endif |
示例 7-3. ioctl.c
/*
* ioctl.c - the process to use ioctl's to control the kernel module
*
* Until now we could have used cat for input and output. But now
* we need to do ioctl's, which require writing our own process.
*/
/*
* device specifics, such as ioctl numbers and the
* major device file.
*/
#include "chardev.h"
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h> /* open */
#include <unistd.h> /* exit */
#include <sys/ioctl.h> /* ioctl */
/*
* Functions for the ioctl calls
*/
ioctl_set_msg(int file_desc, char *message)
{
int ret_val;
ret_val = ioctl(file_desc, IOCTL_SET_MSG, message);
if (ret_val < 0) {
printf("ioctl_set_msg failed:%d\n", ret_val);
exit(-1);
}
}
ioctl_get_msg(int file_desc)
{
int ret_val;
char message[100];
/*
* Warning - this is dangerous because we don't tell
* the kernel how far it's allowed to write, so it
* might overflow the buffer. In a real production
* program, we would have used two ioctls - one to tell
* the kernel the buffer length and another to give
* it the buffer to fill
*/
ret_val = ioctl(file_desc, IOCTL_GET_MSG, message);
if (ret_val < 0) {
printf("ioctl_get_msg failed:%d\n", ret_val);
exit(-1);
}
printf("get_msg message:%s\n", message);
}
ioctl_get_nth_byte(int file_desc)
{
int i;
char c;
printf("get_nth_byte message:");
i = 0;
do {
c = ioctl(file_desc, IOCTL_GET_NTH_BYTE, i++);
if (c < 0) {
printf
("ioctl_get_nth_byte failed at the %d'th byte:\n",
i);
exit(-1);
}
putchar(c);
} while (c != 0);
putchar('\n');
}
/*
* Main - Call the ioctl functions
*/
main()
{
int file_desc, ret_val;
char *msg = "Message passed by ioctl\n";
file_desc = open(DEVICE_FILE_NAME, 0);
if (file_desc < 0) {
printf("Can't open device file: %s\n", DEVICE_FILE_NAME);
exit(-1);
}
ioctl_get_nth_byte(file_desc);
ioctl_get_msg(file_desc);
ioctl_set_msg(file_desc, msg);
close(file_desc);
} |
| [1] | 请注意，在这里，读和写的作用再次反转，所以在ioctl中，读是将信息发送到内核，而写是从内核接收信息。 |
| [2] | 这并不完全准确。 你无法通过 ioctl 传递结构，例如，但你可以传递指向结构的指针。 |