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This is an excerpt from the latest version perlfaq5.pod, which
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5.23: All I want to do is append a small amount of text to the end of a file. Do I still have to use locking?

If you are on a system that correctly implements "flock" and you use the
example appending code from "perldoc -f flock" everything will be OK
even if the OS you are on doesn't implement append mode correctly (if
such a system exists.) So if you are happy to restrict yourself to OSs
that implement "flock" (and that's not really much of a restriction)
then that is what you should do.

If you know you are only going to use a system that does correctly
implement appending (i.e. not Win32) then you can omit the "seek" from
the code in the previous answer.

If you know you are only writing code to run on an OS and filesystem
that does implement append mode correctly (a local filesystem on a
modern Unix for example), and you keep the file in block-buffered mode
and you write less than one buffer-full of output between each manual
flushing of the buffer then each bufferload is almost guaranteed to be
written to the end of the file in one chunk without getting intermingled
with anyone else's output. You can also use the "syswrite" function
which is simply a wrapper around your system's write(2) system call.

There is still a small theoretical chance that a signal will interrupt
the system level "write()" operation before completion. There is also a
possibility that some STDIO implementations may call multiple system
level "write()"s even if the buffer was empty to start. There may be
some systems where this probability is reduced to zero, and this is not
a concern when using ":perlio" instead of your system's STDIO.


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