Linux操作系統(tǒng)內(nèi)核和設(shè)備文件對(duì)話
設(shè)備文件是用來(lái)代表物理設(shè)備的。多數(shù)物理設(shè)備是用來(lái)進(jìn)行輸出或輸入的，所以必須由某種機(jī)制使得內(nèi)核中的設(shè)備驅(qū)動(dòng)從進(jìn)程中得到輸出送給設(shè)備。這可以通過(guò)打開(kāi)輸出設(shè)備文件并且寫(xiě)入做到，就想寫(xiě)入一個(gè)普通文件。在下面的例子里，這由device_write實(shí)現(xiàn)。
這不是總能奏效的。設(shè)想你與一個(gè)連向modem的串口（技是你有一個(gè)內(nèi)貓，從CPU看來(lái)它也是作為一個(gè)串口實(shí)現(xiàn)，所以你不需要認(rèn)為這個(gè)設(shè)想太困難）。最自然要做的事情就是使用設(shè)備文件把內(nèi)容寫(xiě)到modem上（無(wú)論用modem命令還是電話線）或者從modem讀信息（同樣可以從modem命令回答或者通過(guò)電話線）。但是這留下的問(wèn)題是當(dāng)你需要和串口本身對(duì)話的時(shí)候需要怎樣做？比如發(fā)送數(shù)據(jù)發(fā)送和接收的速率。
回答是Unix使用一個(gè)叫做ioctl(input output control的簡(jiǎn)寫(xiě))的特殊函數(shù)。每個(gè)設(shè)備都有自己的ioctl命令，這個(gè)命令可以是ioctl讀的，也可以是寫(xiě)的，也可以是兩者都是或都不是。Ioctl函數(shù)由三個(gè)參數(shù)調(diào)用：適當(dāng)設(shè)備的描述子，ioctl數(shù)，和一個(gè)長(zhǎng)整型參數(shù)，可以賦予一個(gè)角色用來(lái)傳遞任何東西。
Ioctl數(shù)對(duì)設(shè)備主碼、ioctl類(lèi)型、編碼、和參數(shù)的類(lèi)型進(jìn)行編碼。Ioctl數(shù)通常在頭文件由一個(gè)宏調(diào)用（_IO，_IOR，_IOW或_IOWR——決定于類(lèi)型）。這個(gè)頭文件必須包含在使用ioctl（所以它們可以產(chǎn)生正確的ioctl's）程序和內(nèi)核模塊（所以它可以理解）中。在下面的例子里，這個(gè)頭文件是chardev.h，使用它的程序是ioctl.c。
如果你希望在你自己的內(nèi)核模塊中使用ioctl's，最好去接受一分正式的ioctl職位，這樣你就可以得到別人的ioctl's，或者他們得到你，你就可以知道哪里出了錯(cuò)誤。如果想得到更多的信息，到'documentation/ioctl-number.txt'中查看內(nèi)核源文件樹(shù)。
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
#p#/* 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
#p#{
/* 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, buffer+i);
#else
Message = 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 */
#p#{
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;
}
#p#/* 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.
#p#*
* 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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