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发信人: AA (振奋，成长中的AA), 信区: Linux
标题: Linux内核编程(和设备文件对话)(zz)
发信站: 哈工大紫丁香 (2002年06月27日20:00:18 星期四), 站内信件

著者：Ori Pomerantz
翻译：徐辉

5．和设备文件对话（写和IOCTLS）
设备文件是用来代表物理设备的。多数物理设备是用来进行输出或输入的，所以必
须由某种机制使得内核中的设备驱动从进程中得到输出送给设备。这可以通过打开
输出设备文件并且写入做到，就想写入一个普通文件。在下面的例子里，这由
device_write实现。
这不是总能奏效的。设想你与一个连向modem的串口（技是你有一个内猫，从CPU看
来它也是作为一个串口实现，所以你不需要认为这个设想太困难）。最自然要做的
事情就是使用设备文件把内容写到modem上（无论用modem命令还是电话线）或者从
modem读信息（同样可以从modem命令回答或者通过电话线）。但是这留下的问题是
当你需要和串口本身对话的时候需要怎样做？比如发送数据发送和接收的速率。

回答是Unix使用一个叫做ioctl(input output control的简写)的特殊函数。每个
设备都有自己的ioctl命令，这个命令可以是ioctl读的，也可以是写的，也可以是
两者都是或都不是。Ioctl函数由三个参数调用：适当设备的描述子，ioctl数，和
一个长整型参数，可以赋予一个角色用来传递任何东西。
Ioctl数对设备主码、ioctl类型、编码、和参数的类型进行编码。Ioctl数通常在
头文件由一个宏调用（_IO，_IOR，_IOW或_IOWR——决定于类型）。这个头文件必
须包含在使用ioctl（所以它们可以产生正确的ioctl’s）程序和内核模块（所以
它可以理解）中。在下面的例子里，这个头文件是chardev.h，使用它的程序是
ioctl.c。
如果你希望在你自己的内核模块中使用ioctl’s，最好去接受一分正式的ioctl职
位，这样你就可以得到别人的ioctl’s，或者他们得到你，你就可以知道哪里出了
错误。如果想得到更多的信息，到’documentation/ioctl-number.txt’中查看内
核源文件树。
ex 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 /* Were doing kernel work */
#include /* Specifically, a module */

/* Deal with CONFIG_MODVERSIONS */
#if CONFIG_MODVERSIONS==1
#define MODVERSIONS
#include
#endif

/* For character devices */

/* The character device definitions are here */
#include

/* A wrapper which does next to nothing at
* at present, but may help for compatibility
* with future versions of Linux */
#include

/* 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 doesnt - 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 /* 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 dont 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, were in the
* kernel, so were protected against context switches.
*
* This is NOT the right attitude to take, because we
* might be running on an SMP box, but well 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 doesnt 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

/* Were 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
* (mustnt 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 were 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 wouldnt
* 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
#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 devices 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 && ibr temp++) i++,> get_user(ch, temp);
#else
for (i=0; get_user(temp) && ibr temp++) i++,> ;
#endif

/* Dont 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 its 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(\, (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 cant 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 ("youll 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 thats the\n");
printk ("file youll 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 theres 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

/* The major device number. We cant 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 were 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 were 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 nth 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

ex ioctl.c

/* ioctl.c - the process to use ioctls to control the
* kernel module
*
* Until now we could have used cat for input and
* output. But now we need to do ioctls, 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 /* open */
#include /* exit */
#include /* 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 dont tell
* the kernel how far its 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 %dth 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 ("Cant 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);
}

--
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