<chapter id="signals">
<DocInfo>
<AuthorGroup>
<Author>
<FirstName>Massimo</FirstName>
<Surname>Dal Zotto</Surname>
</Author>
</AuthorGroup>
<Date>Transcribed 1998-10-16</Date>
</DocInfo>

<title><productname>Postgres</productname> Signals</title>

<Para>
<Note>
<Para>
Contributed by <ULink url="mailto:dz@cs.unitn.it">Massimo Dal Zotto</ULink>
</Para>
</Note>
</para>

<para>
<productname>Postgres</productname> uses the following signals for
 communication between the postmaster and backends:
</para>

<para>
<table tocentry="1">
<title><productname>Postgres</productname> Signals</title>
<titleabbrev>Signals</titleabbrev>

<tgroup cols="2">
<thead>
<row>
<entry>
Signal
</entry>
<entry>
<application>postmaster</application> Action
</entry>
<entry>
Server Action
</entry>
</row>
</thead>

<tbody>
<row>
<entry>
SIGHUP
</entry>
<entry>
kill(*,sighup)
</entry>
<entry>
read_pg_options
</entry>
</row>

<row>
<entry>
SIGINT
</entry>
<entry>
die
</entry>
<entry>
cancel query
</entry>
</row>

<row>
<entry>
SIGQUIT
</entry>
<entry>
kill(*,sigterm)
</entry>
<entry>
handle_warn
</entry>
</row>

<row>
<entry>
SIGTERM
</entry>
<entry>
kill(*,sigterm), kill(*,9), die
</entry>
<entry>
die
</entry>
</row>

<row>
<entry>
SIGPIPE
</entry>
<entry>
ignored
</entry>
<entry>
die
</entry>
</row>

<row>
<entry>
SIGUSR1
</entry>
<entry>
kill(*,sigusr1), die
</entry>
<entry>
quickdie
</entry>
</row>

<row>
<entry>
SIGUSR2
</entry>
<entry>
kill(*,sigusr2)
</entry>
<entry>
async notify (SI flush)
</entry>
</row>

<row>
<entry>
SIGCHLD
</entry>
<entry>
reaper
</entry>
<entry>
ignored (alive test)
</entry>
</row>

<row>
<entry>
SIGTTIN
</entry>
<entry>
ignored
</entry>
<entry>
</entry>
</row>

<row>
<entry>
SIGTTOU
</entry>
<entry>
ignored
</entry>
<entry>
</entry>
</row>

<row>
<entry>
SIGCONT
</entry>
<entry>
dumpstatus
</entry>
<entry>
</entry>
</row>

<row>
<entry>
SIGFPE
</entry>
<entry>
</entry>
<entry>
FloatExceptionHandler
</entry>
</row>

</tbody>
</tgroup>
</table>

<note>
<para>
"<literal>kill(*,signal)</literal>" means sending a signal to all backends.
</para>
</note>
</para>

<para>
The main changes to the old signal handling are the use of SIGQUIT instead
of SIGHUP to handle warns, SIGHUP to re-read the pg_options file and the
redirection to all active backends of SIGHUP, SIGTERM, SIGUSR1 and SIGUSR2
sent to the postmaster.
In this way these signals sent to the postmaster can be sent
automatically to all the backends without need to know their pids.
To shut down postgres one needs only to send a SIGTERM to postmaster
and it will stop automatically all the backends.
</para>

<para>
The SIGUSR2 signal is also used to prevent SI cache table overflow
which happens when some backend doesn't process SI cache for a long period.
When a backend detects the SI table full at 70% it simply sends a signal
to the postmaster which will wake up all idle backends and make them flush
the cache.
</para>

<para>
The typical use of signals by programmers could be the following:

<programlisting>
# stop postgres
kill -TERM $postmaster_pid
</programlisting>

<programlisting>
# kill all the backends
kill -QUIT $postmaster_pid
</programlisting>

<programlisting>
# kill only the postmaster
kill -INT $postmaster_pid
</programlisting>

<programlisting>
# change pg_options
cat new_pg_options > $DATA_DIR/pg_options
kill -HUP $postmaster_pid
</programlisting>

<programlisting>
# change pg_options only for a backend
cat new_pg_options > $DATA_DIR/pg_options
kill -HUP $backend_pid
cat old_pg_options > $DATA_DIR/pg_options
</programlisting>
</para>
</chapter>

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