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    <p><strong>UPDATE:</strong></p> <p>While logging is safe to use in multithreading environment:</p> <blockquote> <p>The logging module is intended to be thread-safe without any special work needing to be done by its clients. It achieves this though using threading locks; there is one lock to serialize access to the module’s shared data, and each handler also creates a lock to serialize access to its underlying I/O.</p> </blockquote> <p><a href="http://docs.python.org/2/library/logging.html#thread-safety" rel="nofollow">http://docs.python.org/2/library/logging.html#thread-safety</a></p> <p>it appears to be risky in multiprocessing environment since it doesn't protect the logging file with file locks (all it uses for protection is a bunch of mutexes). See the <a href="http://hg.python.org/cpython/file/d489394a73de/Lib/logging/__init__.py#l1000" rel="nofollow">source code</a> of FileHandler class (used by WatchedFileHandler). To guaranty writes atomicity you'll have to protect your log file with file locks yourself or you can use something like <a href="https://pypi.python.org/pypi/ConcurrentLogHandler/0.9.1" rel="nofollow">ConcurrentLogHandler</a></p> <p>So you was just lucky enough to avoid data overlaps in your log file. Usually operating systems don't provide any guaranties about writes atomicity, moreover different versions of libc may have different default values of buffer size.</p> <p>You can get default stream buffer size on your system using the following C code</p> <pre class="lang-c prettyprint-override"><code>#include &lt;stdio.h&gt; int main(void) { printf("%d\n", BUFSIZ); return 0; } </code></pre>
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    1. POWhy don’t I see interleaved lines when writing to a file in parallel?
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    1. USDan Kruchinin
    1. USDan Kruchinin
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    1. COPlease re-read my question. I have several processes, not several threads in one process.
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      1. USVasiliy Faronov
    2. CO@VasiliyFaronov From a brief look at the source code I can conclude that I was wrong, logging doesn't use file locks => it's not safe in multiprocess environment. So you was just lucky enough to avoid data overlaps. In general OSes don't guaranty you "writes atomicity", each libc may have different default values for internal buffer sizes (afaik, there's no any standard on this). I guess you'll have to protect your files with file locks yourself.
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      1. USDan Kruchinin
    3. COYou can post that as an answer and I’ll accept it if nobody else weighs in.
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      1. USVasiliy Faronov
 

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