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

     6 for creating so-called hard links.  It is intended to be imported into

     7 the os module in place of the unimplemented (on OS/2) Posix link()

     8 function (os.link()).

     9 

    10 We 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 

    15 Limitations 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 

    24 Behaviour:-

    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 

    36 import os

    37 import errno

    38 

    39 __all__ = ['link']

    40 

    41 def 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 

    72 if __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])