open.c.bak

一份精简的linux内核源代码

/*
 *  linux/fs/open.c
 *
 *  (C) 1991  Linus Torvalds
 */

#include <string.h>
#include <errno.h>
#include <fcntl.h>
#include <sys/types.h>
#include <utime.h>
#include <sys/stat.h>

#include <linux/sched.h>
#include <linux/tty.h>
#include <linux/kernel.h>
#include <asm/segment.h>

int sys_ustat(int dev, struct ustat * ubuf)/*取文件系统信息 */
{
	return -ENOSYS;
}

int sys_utime(char * filename, struct utimbuf * times)/*设置文件修改时间 */
{
	struct m_inode * inode;
	long actime,modtime;

	if (!(inode=namei(filename)))
		return -ENOENT;
	if (times) {
		actime = get_fs_long((unsigned long *) &times->actime);
		modtime = get_fs_long((unsigned long *) &times->modtime);
	} else
		actime = modtime = CURRENT_TIME;
	inode->i_atime = actime;
	inode->i_mtime = modtime;
	inode->i_dirt = 1;
	iput(inode);
	return 0;
}

/*
 * XXX should we use the real or effective uid?  BSD uses the real uid,
 * so as to make this call useful to setuid programs.
 */
int sys_access(const char * filename,int mode)/*检查系统访问权限 ？？？（p517）*/
{
	struct m_inode * inode;
	int res, i_mode;

	mode &= 0007;/*检测的访问属性在底3位 */
	if (!(inode=namei(filename)))
		return -EACCES;
	i_mode = res = inode->i_mode & 0777;
	iput(inode);
	if (current->uid == inode->i_uid)
		res >>= 6;
	else if (current->gid == inode->i_gid)
		res >>= 6;
	if ((res & 0007 & mode) == mode)
		return 0;
	/*
	 * XXX we are doing this test last because we really should be
	 * swapping the effective with the real user id (temporarily),
	 * and then calling suser() routine.  If we do call the
	 * suser() routine, it needs to be called last. 
	 */
	if ((!current->uid) &&
	    (!(mode & 1) || (i_mode & 0111)))
		return 0;
	return -EACCES;
}

int sys_chdir(const char * filename)/*改变当前工作目录 */
{
	struct m_inode * inode;

	if (!(inode = namei(filename)))
		return -ENOENT;
	if (!S_ISDIR(inode->i_mode)) {
		iput(inode);
		return -ENOTDIR;
	}
	iput(current->pwd);
	current->pwd = inode;
	return (0);
}

int sys_chroot(const char * filename)/*改变当前进程根目录 */
{
	struct m_inode * inode;

	if (!(inode=namei(filename)))
		return -ENOENT;
	if (!S_ISDIR(inode->i_mode)) {
		iput(inode);
		return -ENOTDIR;
	}
	iput(current->root);
	current->root = inode;
	return (0);
}

int sys_chmod(const char * filename,int mode)/*修改文件属性 */
{
	struct m_inode * inode;

	if (!(inode=namei(filename)))
		return -ENOENT;
	if ((current->euid != inode->i_uid) && !suser()) {
		iput(inode);
		return -EACCES;
	}
	inode->i_mode = (mode & 07777) | (inode->i_mode & ~07777);
	inode->i_dirt = 1;
	iput(inode);
	return 0;
}

int sys_chown(const char * filename,int uid,int gid)/*修改文件宿主用户 */
{
	struct m_inode * inode;

	if (!(inode=namei(filename)))
		return -ENOENT;
	if (!suser()) {
		iput(inode);
		return -EACCES;
	}
	inode->i_uid=uid;
	inode->i_gid=gid;
	inode->i_dirt=1;
	iput(inode);
	return 0;
}

int sys_open(const char * filename,int flag,int mode)/*打开文件：获得指定路径名的i节点，在内存文件表中申请一项使该项指向该i节点*/
{                                                    /*使当前进程的文件表中的一项指向刚才申请到的文件表项，并返回该项的索引（句柄）*/
	struct m_inode * inode;
	struct file * f;
	int i,fd;

	mode &= 0777 & ~current->umask;/*？？将用户设置的文件模式和进程模式屏蔽码相与产生许可的文件模式 */
	for(fd=0 ; fd<NR_OPEN ; fd++)
		if (!current->filp[fd])
			break;
	if (fd>=NR_OPEN)
		return -EINVAL;
	current->close_on_exec &= ~(1<<fd);/*？？设置当前进程的执行时关闭文件句柄位图，复位对应比特位 */
	f=0+file_table;                     /*即设置当子进程调用execve时关闭的文件（默认情况下新建的子进程将打开所有父进程打开的文件） */
	for (i=0 ; i<NR_FILE ; i++,f++)
		if (!f->f_count) break;
	if (i>=NR_FILE)
		return -EINVAL;
	(current->filp[fd]=f)->f_count++;
	if ((i=open_namei(filename,flag,mode,&inode))<0) {
		current->filp[fd]=NULL;
		f->f_count=0;
		return i;
	}
/* ttys are somewhat special (ttyxx major==4, tty major==5) */
	if (S_ISCHR(inode->i_mode))      /*为进程分配终端？？？*/
		if (MAJOR(inode->i_zone[0])==4) {
			if (current->leader && current->tty<0) {
				current->tty = MINOR(inode->i_zone[0]);
				tty_table[current->tty].pgrp = current->pgrp;
			}
		} else if (MAJOR(inode->i_zone[0])==5)
			if (current->tty<0) {
				iput(inode);
				current->filp[fd]=NULL;
				f->f_count=0;
				return -EPERM;
			}
/* Likewise with block-devices: check for floppy_change */
	if (S_ISBLK(inode->i_mode))
		check_disk_change(inode->i_zone[0]);
	f->f_mode = inode->i_mode;  /*内存文件表项初始化*/
	f->f_flags = flag;
	f->f_count = 1;
	f->f_inode = inode;
	f->f_pos = 0;
	return (fd);
}

int sys_creat(const char * pathname, int mode)  /*创建文件*/
{
	return sys_open(pathname, O_CREAT | O_TRUNC, mode);
}

int sys_close(unsigned int fd)  /*关闭文件（参数为文件句柄）。当前进程的文件数组中相应项置null；*/
{	                            /*内存文件表结构中相应项引用计数减1；iput该i节点*/
	struct file * filp;

	if (fd >= NR_OPEN)
		return -EINVAL;
	current->close_on_exec &= ~(1<<fd);
	if (!(filp = current->filp[fd]))
		return -EINVAL;
	current->filp[fd] = NULL;
	if (filp->f_count == 0)
		panic("Close: file count is 0");
	if (--filp->f_count)  /*申请内存文件表项时的依据是引用计数为0，这里出现引用计数>1是由于创建子进程时增加了文件引用计数*/
		return (0);
	iput(filp->f_inode);
	return (0);
}