lseek 和空洞文件

先来看看如何在shell下创建空洞文件

dd if=/dev/zero of=/1k.file bs=1k count=1 seek=9

使用lseek则如下，apue上的例子跟这个倒也差不多。

#include <apue.h>

int main(int argc, char *argv[])
{
    int fd;
    char buffer[]="this begins";
    char enddata[]="this ends!";

    // create a new file named file_with_hold in the pwd
    if((fd=creat("file_with_hole",0644))<0)
        exit(1);

    //put a small string at the beginning
    if(write(fd,buffer,10)!=10)
        exit(1);

    //seek 131072=2**17 bytes past the beginning
    if(lseek(fd,131072,SEEK_SET)==-1)
        exit(1);

    // write a small string there as well
    if(write(fd,enddata,10)!=10)
        exit(1);

    // we now have a large file with a big hole
    exit(0);
    }

然后查看下文件内容：

od -c file_with_hole
0000000   t   h   i   s       b   e   g   i   n  \0  \0  \0  \0  \0  \0
0000020  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0  \0
*
0400000   t   h   i   s       e   n   d   s !
0400012

然后用另外一个程序删除这个文件

./unlinkdemo ./ file_with_hole

#include <apue.h>
#include <fcntl.h>
/* Usage: unlinkat demo directory file
where path is the path to a file relative to the
given directory
*/

int main(int argc, char *argv[])
{
    int dirfd;

    /* check args */
    if(argc < 3){
        fprintf(stderr,"usage: %s directory file\n",*argv);
        exit(1);
        }

    /* open directory */
    if((dirfd=open(argv[1],O_RDONLY))==-1){
        fprintf(stderr,"could not open %s\n",argv[1]);
        exit(1);
        }

    /* we could check here if argv[2] refers to a directory or a file
    to make this more robust , but we do not */
    /*unlink the file relative to dirfd */
    if(-1==unlinkat(dirfd,argv[2],0)){
        fprintf(stderr,"could not unlink %s/%s\n",argv[1],argv[2]);
        exit(1);
        }
    return 0;
    }