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man: document effects of concurrent truncation

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Co-authored-by: Lennart Poettering <lennart@poettering.net>
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Lucas Werkmeister 2021-01-18 21:58:28 +01:00
parent d15b1a6c22
commit e372584067
1 changed files with 9 additions and 2 deletions
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11
man/systemd.exec.xml
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@ -2429,8 +2429,15 @@ SystemCallErrorNumber=EPERM</programlisting>
it. For units with multiple command lines, e.g. <varname>Type=oneshot</varname> services with
multiple <varname>ExecStart=</varname>, or services with <varname>ExecCondition=</varname>,
<varname>ExecStartPre=</varname> or <varname>ExecStartPost=</varname>, the output file is reopened
and therefore re-truncated for each command line. Any <varname>ExecReload=</varname> will likewise
truncate the file when run.</para>
and therefore re-truncated for each command line. If the output file is truncated while another
process still has the file open, e.g. by an <varname>ExecReload=</varname> running concurrently with
an <varname>ExecStart=</varname>, and the other process continues writing to the file without
adjusting its offset, then the space between the file pointers of the two processes may be filled
with <constant>NUL</constant> bytes, producing a sparse file. Thus,
<option>truncate:<replaceable>path</replaceable></option> is typically only useful for units where
only one process runs at a time, such as services with a single <varname>ExecStart=</varname> and no
<varname>ExecStartPost=</varname>, <varname>ExecReload=</varname>, <varname>ExecStop=</varname> or
similar.</para>
<para><option>socket</option> connects standard output to a socket acquired via socket activation. The
semantics are similar to the same option of <varname>StandardInput=</varname>, see above.</para>