The file is opened  in  append  mode.  Before  each
write, the file pointer is positioned at the end of
the file, as if with lseek.  O_APPEND may  lead  to
corrupted  files  on  NFS file systems if more than
one process appends data to a file at  once.   This
is  because  NFS  does  not  support appending to a
file, so the client  kernel  has  to  simulate  it,
which can't be done without a race condition

##</code><code>##

Opening a file with append mode (a as the first character in
the  mode argument) causes all subsequent writes to the file
to be forced to the then current end-of-file, regardless  of
intervening  calls  to fseek(3S).  If two separate processes
open the same file for append, each process may write freely
to  the file without fear of destroying output being written
by the other.  The output from the  two  processes  will  be
intermixed in the file in the order in which it is written.