Hi AM,

Different operating systems hande this in different ways. I'm am almost 100% positive that on any UNIX like operating system the append writes are atomic. Meaning as long as you are writting in append mode you don't have to explicitly lock the file. The operating system will make sure that the write from one process will complete before the write from another process begins. From the open() man page (on Mandrake 8.1):

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
[download]

I take this to mean that with the exception of writes to an NFS mounted file system writes in append mode happen atomically and each write should finish before the system allows another to begin. This means your output may be intermingled with other output, but not corrupted.

The only link I could find that explicitely states this is here - a manpage for fopen():

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.
[download]

Sorry this is vague, but I hope it helps. I know I read this somewhere - I just wish I could recall where for you. Good luck,
{NULE}
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