1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
|
# Example: Managing hard-links on an ISO
PyCdlib supports an advanced concept called hard-links, which is multiple names for the same piece of data (this is somewhat similar to Unix hard-links). Most users will not need to use this functionality and should stick with the standard [add_file](pycdlib-api.html#PyCdlib-add_file) and [rm_file](pycdlib-api.html#PyCdlib-rm_file) APIs. However, for those that want to do more advanced things like hiding a file from Joliet while having it remain visible in ISO9660, this functionality can be useful.
On an ISO, a piece of data can be referred to (possibly several times) from four different contexts:
1. From the original ISO9660 context, including the Rock Ridge extensions.
1. From the Joliet context, since this is a separate context.
1. From the El Torito boot record, since this is effectively a separate context.
1. From the UDF context, since this is a separate context.
The data can be referred to zero, one, or many times from each of these contexts. The most classic example of hard-links happens when an ISO has the Joliet extensions. In that case, there is implicitly a hard-link from the ISO9660 (and Rock Ridge) context to the file contents, and a hard-link from the Joliet context to the file contents. When a piece of data has zero entries in a context, it is effectively hidden from that context. For example, a file could be visible from ISO9660/Rock Ridge, but hidden from Joliet, or vice-versa. A file could be used for booting, but be hidden from both ISO9660/Rock Ridge and Joliet, etc. Management of these hard-links is done via the PyCdlib APIs [add_hard_link](pycdlib-api.html#PyCdlib-add_hard_link) and [rm_hard_link](pycdlib-api.html#PyCdlib-rm_hard_link). Adding or removing a file through the [add_file](pycdlib-api.html#PyCdlib-add_file) and [rm_file](pycdlib-api.html#PyCdlib-rm_file) APIs implicitly manipulates hard-links behind the scenes. Note that hard-links only make sense for files, since directories have no direct data (only metadata).
An example should help to illustrate the concept. Here's the complete code for the example:
```
try:
from cStringIO import StringIO as BytesIO
except ImportError:
from io import BytesIO
import pycdlib
iso = pycdlib.PyCdlib()
iso.new(joliet=3)
foostr = b'foo\n'
iso.add_fp(BytesIO(foostr), len(foostr), '/FOO.;1', joliet_path='/foo')
iso.add_hard_link(iso_old_path='/FOO.;1', iso_new_path='/BAR.;1')
iso.rm_hard_link(joliet_path='/foo')
outiso = BytesIO()
iso.write_fp(outiso)
iso.close()
```
Let's take a closer look at the code.
```
try:
from cStringIO import StringIO as BytesIO
except ImportError:
from io import BytesIO
import pycdlib
```
As in earlier examples, import the relevant libraries, including pycdlib itself.
```
iso = pycdlib.PyCdlib()
iso.new(joliet=3)
```
As in earlier examples, create a PyCdlib object, and then create a new, empty ISO with the Joliet extensions.
```
foostr = b'foo\n'
iso.add_fp(BytesIO(foostr), len(foostr), '/FOO.;1', joliet_path='/foo')
```
As in earlier examples, add a new file to the ISO. Here we have provided both the ISO path '/FOO.;1' and the Joliet path '/foo', so the file implicitly has two links; one from the ISO context, and one from the Joliet context.
```
iso.add_hard_link(iso_old_path='/FOO.;1', iso_new_path='/BAR.;1')
```
Add a hard-link from the original '/FOO.;1' location in the ISO context to a second location in the ISO context '/BAR.;1'. This takes up no additional space on the ISO for the data, only for the metadata.
```
iso.rm_hard_link(joliet_path='/foo')
```
Remove the link from the Joliet context for the file. Now this file is effectively hidden from the Joliet context, while still being visible in the ISO context.
```
outiso = BytesIO()
iso.write_fp(outiso)
```
As in earlier examples, write the ISO out to the BytesIO object.
```
iso.close()
```
Since we are done with the ISO object, close it out.
---
<div style="width: 100%; display: table;">
<div style="display: table-row;">
<div style="width: 33%; display: table-cell; text-align: left;">
<a href="example-modifying-file-in-place.html"><-- Example: Modifying a file in place</a>
</div>
<div style="width: 33%; display: table-cell; text-align: center;">
<a href="https://clalancette.github.io/pycdlib/">Top</a>
</div>
<div style="width: 33%; display: table-cell; text-align: right;">
<a href="example-forcing-consistency.html">Example: Forcing consistency --></a>
</div>
</div>
|
