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man: document effects of concurrent truncation
Co-authored-by: Lennart Poettering <lennart@poettering.net>
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@ -2429,8 +2429,15 @@ SystemCallErrorNumber=EPERM</programlisting>
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it. For units with multiple command lines, e.g. <varname>Type=oneshot</varname> services with
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multiple <varname>ExecStart=</varname>, or services with <varname>ExecCondition=</varname>,
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<varname>ExecStartPre=</varname> or <varname>ExecStartPost=</varname>, the output file is reopened
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and therefore re-truncated for each command line. Any <varname>ExecReload=</varname> will likewise
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truncate the file when run.</para>
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and therefore re-truncated for each command line. If the output file is truncated while another
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process still has the file open, e.g. by an <varname>ExecReload=</varname> running concurrently with
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an <varname>ExecStart=</varname>, and the other process continues writing to the file without
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adjusting its offset, then the space between the file pointers of the two processes may be filled
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with <constant>NUL</constant> bytes, producing a sparse file. Thus,
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<option>truncate:<replaceable>path</replaceable></option> is typically only useful for units where
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only one process runs at a time, such as services with a single <varname>ExecStart=</varname> and no
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<varname>ExecStartPost=</varname>, <varname>ExecReload=</varname>, <varname>ExecStop=</varname> or
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similar.</para>
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<para><option>socket</option> connects standard output to a socket acquired via socket activation. The
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semantics are similar to the same option of <varname>StandardInput=</varname>, see above.</para>
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