file truncation on open with O_WRONLY

[Anticipating a Reply <ruxyz@yahoo.com>]

Hello All !

   I was reading Linux Device Drivers , by 
A.Rubini and found the below lines on page 71
near the end . 

     " The only real operation performed on the 
device is truncating it to a length of zero when 
the device is opened for writing. This is 
performed because, by design, overwriting a pscull 
device with a shorter file results in a shorter 
device data area. This is similar to the way 
opening a regular file for writing truncates it to 
zero length. The operation does nothing if the 
device is opened for reading. "

  As said above in the last lines , I tried 
the open system call on a regular file ,with 
O_WRONLY option , without doing any further 
operations . I was expecting the regular file to 
truncate to zero after the open call as said above,
but I  found that the regular file is
intact .
 
  Can anybody please explain ? 
Or maybe I have not understood the above lines
properly .

   Thanks in advance .

Regards !
     


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