You should be aware that that you should avoid file I/O when possible. The main idea is to go "one level deeper" and call VFS level functions instead of the syscall handler directly:
Includes:
#include <linux/fs.h>
#include <asm/segment.h>
#include <asm/uaccess.h>
#include <linux/buffer_head.h>
Opening a file (similar to open):
struct file* file_open(const char* path, int flags, int rights) {
struct file* filp = NULL;
mm_segment_t oldfs;
int err = 0;
oldfs = get_fs();
set_fs(get_ds());
filp = filp_open(path, flags, rights);
set_fs(oldfs);
if(IS_ERR(filp)) {
err = PTR_ERR(filp);
return NULL;
}
return filp;
}
Close a file (similar to close):
void file_close(struct file* file) {
filp_close(file, NULL);
}
Reading data from a file (similar to pread):
int file_read(struct file* file, unsigned long long offset, unsigned char* data, unsigned int size) {
mm_segment_t oldfs;
int ret;
oldfs = get_fs();
set_fs(get_ds());
ret = vfs_read(file, data, size, &offset);
set_fs(oldfs);
return ret;
}
Writing data to a file (similar to pwrite):
int file_write(struct file* file, unsigned long long offset, unsigned char* data, unsigned int size) {
mm_segment_t oldfs;
int ret;
oldfs = get_fs();
set_fs(get_ds());
ret = vfs_write(file, data, size, &offset);
set_fs(oldfs);
return ret;
}
Syncing changes a file (similar to fsync):
int file_sync(struct file* file) {
vfs_fsync(file, 0);
return 0;
}
[Edit] Originally, I proposed using file_fsync, which is gone in newer kernel versions. Thanks to the poor guy suggesting the change, but whose change was rejected. The edit was rejected before I could review it.
在VFS的支援下,使用者态程序讀寫任何類型的檔案系統都可以使用read和write着兩個系統調用,但是在linux核心中沒有這樣的系統調用我們如何操作檔案呢?我們知道read和write在進入核心态之後,實際執行的是sys_read和sys_write,但是檢視核心源代碼,發現這些操作檔案的函數都沒有導出(使用EXPORT_SYMBOL導出),也就是說在核心子產品中是不能使用的,那如何是好?
通過檢視sys_open的源碼我們發現,其主要使用了do_filp_open()函數,該函數在fs/namei.c中,而在改檔案中,filp_open函數也是調用了do_filp_open函數,并且接口和sys_open函數極為相似,調用參數也和sys_open一樣,并且使用EXPORT_SYMBOL導出了,是以我們猜想該函數可以打開檔案,功能和open一樣。使用同樣的查找方法,我們找出了一組在核心中操作檔案的函數,如下:
功能 | 函數原型 |
打開檔案 | struct file *filp_open(const char *filename,int flags, int mode) |
讀取檔案 | ssize_t vfs_read(struct file *file,char __user *buf, size_t count, loff_t *pos) |
寫檔案 | ssize_t vfs_write(struct file *file,const char __user *buf,size_t count, loff_t *pos) |
關閉檔案 | int filp_close(struct file *filp, fl_owner_t id) |
我們注意到在vfs_read和vfs_write函數中,其參數buf指向的使用者空間的記憶體位址,如果我們直接使用核心空間的指針,則會傳回-EFALUT。是以我們需要使用
set_fs()和get_fs()宏來改變核心對記憶體位址檢查的處理方式,是以在核心空間對檔案的讀寫流程為:
- mm_segment_tfs = get_fs();
- set_fs(KERNEL_FS);
- //vfs_write();
- vfs_read();
- set_fs(fs);
下面為一個在核心中對檔案操作的例子:
- #include <linux/module.h>
- #include <linux/init.h>
- #include <linux/fs.h>
- #include <linux/uaccess.h>
- static charbuf[] ="你好";
- static charbuf1[10];
- int __inithello_init(void)
- {
- struct file *fp;
- mm_segment_t fs;
- loff_t pos;
- printk("hello enter/n");
- fp =filp_open("/home/niutao/kernel_file",O_RDWR | O_CREAT,0644);
- if (IS_ERR(fp)){
- printk("create file error/n");
- return -1;
- }
- fs =get_fs();
- set_fs(KERNEL_DS);
- pos =0;
- vfs_write(fp,buf, sizeof(buf), &pos);
- pos =0;
- vfs_read(fp,buf1, sizeof(buf), &pos);
- printk("read: %s/n",buf1);
- filp_close(fp,NULL);
- set_fs(fs);
- return 0;
- }
- void __exithello_exit(void)
- {
- printk("hello exit/n");
- }
- module_init(hello_init);
- module_exit(hello_exit);
- MODULE_LICENSE("GPL");