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.