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      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])
     80