/Lib/plat-os2emx/_emx_link.py

 1# _emx_link.py
 2
 3# Written by Andrew I MacIntyre, December 2002.
 4
 5"""_emx_link.py is a simplistic emulation of the Unix link(2) library routine
 6for creating so-called hard links.  It is intended to be imported into
 7the os module in place of the unimplemented (on OS/2) Posix link()
 8function (os.link()).
 9
10We do this on OS/2 by implementing a file copy, with link(2) semantics:-
11  - the target cannot already exist;
12  - we hope that the actual file open (if successful) is actually
13    atomic...
14
15Limitations of this approach/implementation include:-
16  - no support for correct link counts (EMX stat(target).st_nlink
17    is always 1);
18  - thread safety undefined;
19  - default file permissions (r+w) used, can't be over-ridden;
20  - implemented in Python so comparatively slow, especially for large
21    source files;
22  - need sufficient free disk space to store the copy.
23
24Behaviour:-
25  - any exception should propagate to the caller;
26  - want target to be an exact copy of the source, so use binary mode;
27  - returns None, same as os.link() which is implemented in posixmodule.c;
28  - target removed in the event of a failure where possible;
29  - given the motivation to write this emulation came from trying to
30    support a Unix resource lock implementation, where minimal overhead
31    during creation of the target is desirable and the files are small,
32    we read a source block before attempting to create the target so that
33    we're ready to immediately write some data into it.
34"""
35
36import os
37import errno
38
39__all__ = ['link']
40
41def link(source, target):
42    """link(source, target) -> None
43
44    Attempt to hard link the source file to the target file name.
45    On OS/2, this creates a complete copy of the source file.
46    """
47
48    s = os.open(source, os.O_RDONLY | os.O_BINARY)
49    if os.isatty(s):
50        raise OSError, (errno.EXDEV, 'Cross-device link')
51    data = os.read(s, 1024)
52
53    try:
54        t = os.open(target, os.O_WRONLY | os.O_BINARY | os.O_CREAT | os.O_EXCL)
55    except OSError:
56        os.close(s)
57        raise
58
59    try:
60        while data:
61            os.write(t, data)
62            data = os.read(s, 1024)
63    except OSError:
64        os.close(s)
65        os.close(t)
66        os.unlink(target)
67        raise
68
69    os.close(s)
70    os.close(t)
71
72if __name__ == '__main__':
73    import sys
74    try:
75        link(sys.argv[1], sys.argv[2])
76    except IndexError:
77        print 'Usage: emx_link <source> <target>'
78    except OSError:
79        print 'emx_link: %s' % str(sys.exc_info()[1])