与设备交互¶
设备文件一般表示物理设备。大多数物理设备既用于输出也用于输入，因此内核中的设备驱动程序必须有某种机制来获取要从进程发送到设备的输出。这是通过打开要输出的设备文件并写入它来完成的，就像写入文件一样。在下面的示例中，这是通过device_write实现的。
假设您有一个连接到调制解调器的串行端口(即使您有一个内部调制解调器，从CPU的角度来看，它仍然是作为连接到调制解调器的串行端口实现的，因此您不必太费想象力)。自然要做的事情是使用设备文件向调制解调器写入内容(调制解调器命令或通过电话线发送的数据)，并从调制解调器读取内容(命令响应或通过电话线接收的数据)。然而，这留下了一个问题，当您需要与串行端口本身进行通信时该怎么办，例如发送发送和接收数据的速率。
在Unix中，答案是使用一个称为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，可以读ioctl命令(将信息从进程发送到内核)，写ioctl命令(将信息返回给进程)。调用ioctl函数时带三个参数：指定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，因此您可以使用强制转换来使用它来传递任何内容。
ioctl号编码主设备号、ioctl类型、命令和参数类型。这个ioctl号通常由头文件中的宏调用(_IO，_IOR，_IOW或_IOWR——取决于类型)创建。这个头文件应该被使用ioctl的程序#include(这样它们就可以生成合适的ioctl)和内核模块#include(这样它就可以理解它)。在下面的例子中，头文件是chardev.h，使用它的程序是ioctl.c。
如果您想在自己的内核模块中使用ioctl，最好接收一个正式的ioctl值，这样，如果您不小心用了别人的ioctl，或者别人用了您的ioctl，您就会知道出了问题。要了解更多信息，请查阅内核源代码树的“Documentation/ioctl-number.txt”。
chardev.c
/* chardev.c
*
* Create an input/output character device
*/
/* Copyright (C) 1998-99 by Ori Pomerantz */
/* The necessary header files */
/* Standard in kernel modules */
#include <linux/kernel.h> /* We're doing kernel work */
#include <linux/module.h> /* Specifically, a module */
/* Deal with CONFIG_MODVERSIONS */
#if CONFIG_MODVERSIONS==1
#define MODVERSIONS
#include <linux/modversions.h>
#endif
/* For character devices */
/* The character device definitions are here */
#include <linux/fs.h>
/* A wrapper which does next to nothing at
* at present, but may help for compatibility
* with future versions of Linux */
#include <linux/wrapper.h>
/* Our own ioctl numbers */
#include "chardev.h"
/* In 2.2.3 /usr/include/linux/version.h includes a
* macro for this, but 2.0.35 doesn't - so I add it
* here if necessary. */
#ifndef KERNEL_VERSION
#define KERNEL_VERSION(a,b,c) ((a)*65536+(b)*256+(c))
#endif
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
#include <asm/uaccess.h> /* for get_user and put_user */
#endif
#define SUCCESS 0
/* Device Declarations ******************************** */
/* The name for our device, as it will appear in
* /proc/devices */
#define DEVICE_NAME "char_dev"
/* The maximum length of the message for the device */
#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 function is called whenever a process attempts
* to open the device file */
static int device_open(struct inode *inode,
struct file *file)
{
#ifdef DEBUG
printk ("device_open(%p)\n", file);
#endif
/* We don't want to talk to two processes at the
* same time */
if (Device_Open)
return -EBUSY;
/* If this was a process, we would have had to be
* more careful here, because one process might have
* checked Device_Open right before the other one
* tried to increment it. However, we're in the
* kernel, so we're protected against context switches.
*
* This is NOT the right attitude to take, because we
* might be running on an SMP box, but we'll deal with
* SMP in a later chapter.
*/
Device_Open++;
/* Initialize the message */
Message_Ptr = Message;
MOD_INC_USE_COUNT;
return SUCCESS;
}
/* This function is called when a process closes the
* device file. It doesn't have a return value because
* it cannot fail. Regardless of what else happens, you
* should always be able to close a device (in 2.0, a 2.2
* device file could be impossible to close). */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
static int device_release(struct inode *inode,
struct file *file)
#else
static void device_release(struct inode *inode,
struct file *file)
#endif
{
#ifdef DEBUG
printk ("device_release(%p,%p)\n", inode, file);
#endif
/* We're now ready for our next caller */
Device_Open --;
MOD_DEC_USE_COUNT;
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
return 0;
#endif
}
/* This function is called whenever a process which
* has already opened the device file attempts to
* read from it. */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
static ssize_t device_read(
struct file *file,
char *buffer, /* The buffer to fill with the data */
size_t length, /* The length of the buffer */
loff_t *offset) /* offset to the file */
#else
static int device_read(
struct inode *inode,
struct file *file,
char *buffer, /* The buffer to fill with the data */
int length) /* The length of the buffer
* (mustn't write beyond that!) */
#endif
{
/* Number of bytes actually written to the buffer */
int bytes_read = 0;
#ifdef DEBUG
printk("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 ("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. */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
static ssize_t device_write(struct file *file,
const char *buffer,
size_t length,
loff_t *offset)
#else
static int device_write(struct inode *inode,
struct file *file,
const char *buffer,
int length)
#endif
{
int i;
#ifdef DEBUG
printk ("device_write(%p,%s,%d)",
file, buffer, length);
#endif
for(i=0; i<length && i<BUF_LEN; i++)
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
get_user(Message[i], buffer+i);
#else
Message[i] = get_user(buffer+i);
#endif
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,
struct file *file,
unsigned int ioctl_num,/* The number of the ioctl */
unsigned long ioctl_param) /* The parameter to it */
{
int i;
char *temp;
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
char ch;
#endif
/* 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 */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
get_user(ch, temp);
for (i=0; ch && i<BUF_LEN; i++, temp++)
get_user(ch, temp);
#else
for (i=0; get_user(temp) && i<BUF_LEN; i++, temp++)
;
#endif
/* Don't reinvent the wheel - call device_write */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
device_write(file, (char *) ioctl_param, i, 0);
#else
device_write(inode, file, (char *) ioctl_param, i);
#endif
break;
case IOCTL_GET_MSG:
/* Give the current message to the calling
* process - the parameter we got is a pointer,
* fill it. */
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
i = device_read(file, (char *) ioctl_param, 99, 0);
#else
i = device_read(inode, file, (char *) ioctl_param,
99);
#endif
/* Warning - we assume here the buffer length is
* 100. If it's less than that we might overflow
* the buffer, causing the process to core dump.
*
* The reason we only allow up to 99 characters is
* that the NULL which terminates the string also
* needs room. */
/* 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 = {
NULL, /* seek */
device_read,
device_write,
NULL, /* readdir */
NULL, /* select */
device_ioctl, /* ioctl */
NULL, /* mmap */
device_open,
#if LINUX_VERSION_CODE >= KERNEL_VERSION(2,2,0)
NULL, /* flush */
#endif
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 = module_register_chrdev(MAJOR_NUM,
DEVICE_NAME,
&Fops);
/* Negative values signify an error */
if (ret_val < 0) {
printk ("%s failed with %d\n",
"Sorry, registering the character device ",
ret_val);
return ret_val;
}
printk ("%s The major device number is %d.\n",
"Registeration is a success",
MAJOR_NUM);
printk ("If you want to talk to the device driver,\n");
printk ("you'll have to create a device file. \n");
printk ("We suggest you use:\n");
printk ("mknod %s c %d 0\n", DEVICE_FILE_NAME,
MAJOR_NUM);
printk ("The device file name is important, because\n");
printk ("the ioctl program assumes that's the\n");
printk ("file you'll use.\n");
return 0;
}
/* Cleanup - unregister the appropriate file from /proc */
void cleanup_module()
{
int ret;
/* Unregister the device */
ret = module_unregister_chrdev(MAJOR_NUM, DEVICE_NAME);
/* If there's an error, report it */
if (ret < 0)
printk("Error in module_unregister_chrdev: %d\n", ret);
}
ex 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
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.
*/
/* Copyright (C) 1998 by Ori Pomerantz */
/* device specifics, such as ioctl numbers and the
* major device file. */
#include "chardev.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;
while (c != 0) {
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);
}
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);
}