I seem to remember the string had to be less than a certain length for this to be true. (1024, but that might be system dependant)

There are so many "if"s that this doesn't seem to reliable to me. I would at least give the user the option of using flock if I were to try to take advantage of this atomic property of syswrite.

Besides, flock will allow you to do longer operations on the log, such as shrinking in a nightly job.