如何创建文件洞的文件？ [英] How to create a file with file holes?

问题描述

文件孔在文件中的空白，然而，不占用任何磁盘空间和包含null字节。因此，该文件的大小比在磁盘上的实际大小更大的

File holes are the empty spaces in file, which, however, doesn't take up any disk space and contains null bytes. Therefore, the file size is larger than its actual size on disk.

不过，我不知道如何创建文件洞的文件用于试验。

However, I don't know how to create a file with file holes for experimenting with.

推荐答案

使用与 DD 命令征求参数。

dd if=/dev/urandom bs=4096 count=2 of=file_with_holes
dd if=/dev/urandom bs=4096 seek=7 count=2 of=file_with_holes

这是你创建与8192字节一个不错的孔文件字节28671。

That creates you a file with a nice hole from byte 8192 to byte 28671.

下面是一个例子，证明确实是文件有它（ LS -S 命令告诉你磁盘块有多少正在使用的文件）孔：

Here's an example, demonstrating that indeed the file has holes in it (the ls -s command tells you how many disk blocks are being used by a file):

$ dd if=/dev/urandom bs=4096 count=2 of=fwh # fwh = file with holes
2+0 records in
2+0 records out
8192 bytes (8.2 kB) copied, 0.00195565 s, 4.2 MB/s

$ dd if=/dev/urandom seek=7 bs=4096 count=2 of=fwh
2+0 records in
2+0 records out
8192 bytes (8.2 kB) copied, 0.00152742 s, 5.4 MB/s

$ dd if=/dev/zero bs=4096 count=9 of=fwnh # fwnh = file with no holes
9+0 records in
9+0 records out
36864 bytes (37 kB) copied, 0.000510568 s, 72.2 MB/s

$ ls -ls fw*
16 -rw-rw-r-- 1 hopper hopper 36864 Mar 15 10:25 fwh
36 -rw-rw-r-- 1 hopper hopper 36864 Mar 15 10:29 fwnh

正如你所看到的，有孔的文件占用较少的磁盘块，尽管是相同的大小。

As you can see, the file with holes takes up fewer disk blocks, despite being the same size.

如果你想有一个计划，这样做，那就是：

If you want a program that does it, here it is:

#include <unistd.h>
#include <sys/types.h>
#include <stdio.h>
#include <fcntl.h>

int main(int argc, const char *argv[])
{
    char random_garbage[8192]; /* Don't even bother to initialize */
    int fd = -1;
    if (argc < 2) {
        fprintf(stderr, "Usage: %s <filename>\n", argv[0]);
        return 1;
    }
    fd = open(argv[1], O_WRONLY | O_CREAT | O_TRUNC, 0666);
    if (fd < 0) {
        perror("Can't open file: ");
        return 2;
    }
    write(fd, random_garbage, 8192);
    lseek(fd, 5 * 4096, SEEK_CUR);
    write(fd, random_garbage, 8192);
    close(fd);
    return 0;
}

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