fcntl.c

一份精简的linux内核源代码

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

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

#include <fcntl.h>
#include <sys/stat.h>

extern int sys_close(int fd);

static int dupfd(unsigned int fd, unsigned int arg)/*复制文件句柄（即，使另一个新句柄也指向该文件结构项），arg为新句柄的最小值，对应文件结构项引用计数加1*/
{
	if (fd >= NR_OPEN || !current->filp[fd])
		return -EBADF;
	if (arg >= NR_OPEN)
		return -EINVAL;
	while (arg < NR_OPEN)
		if (current->filp[arg])
			arg++;
		else
			break;
	if (arg >= NR_OPEN)
		return -EMFILE;
	current->close_on_exec &= ~(1<<arg);/*复位该句柄位图，即以后执行exec时不会关闭该句柄所对应文件*/
	(current->filp[arg] = current->filp[fd])->f_count++;
	return arg;
}

int sys_dup2(unsigned int oldfd, unsigned int newfd)/*复制文件句柄，若newfd非空，则先释放之，然后将原句柄复制到newfd*/
{
	sys_close(newfd);/*关闭文件（参数为文件句柄）*/
	return dupfd(oldfd,newfd);/*复制文件句柄*/
}

int sys_dup(unsigned int fildes)/*复制文件句柄，新句柄为最小的空句柄*/
{
	return dupfd(fildes,0);
}

int sys_fcntl(unsigned int fd, unsigned int cmd, unsigned long arg)/*文件控制系统调用，fd为文件句柄，cmd为控制命令，arg对应不同cmd有不同含义（p465）*/
{	
	struct file * filp;

	if (fd >= NR_OPEN || !(filp = current->filp[fd]))
		return -EBADF;
	switch (cmd) {
		case F_DUPFD:/*复制文件句柄*/
			return dupfd(fd,arg);
		case F_GETFD:/*取文件句柄的close_on_exec标志*/
			return (current->close_on_exec>>fd)&1;
		case F_SETFD:/*设置close_on_exec标志*/
			if (arg&1)
				current->close_on_exec |= (1<<fd);
			else
				current->close_on_exec &= ~(1<<fd);
			return 0;
		case F_GETFL:/*取文件状态*/
			return filp->f_flags;
		case F_SETFL:/*设置文件状态*/
			filp->f_flags &= ~(O_APPEND | O_NONBLOCK);
			filp->f_flags |= arg & (O_APPEND | O_NONBLOCK);
			return 0;
		case F_GETLK:	case F_SETLK:	case F_SETLKW:
			return -1;
		default:
			return -1;
	}
}