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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>34.14. Event System</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="libpq-notice-processing.html" title="34.13. Notice Processing" /><link rel="next" href="libpq-envars.html" title="34.15. Environment Variables" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">34.14. Event System</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="libpq-notice-processing.html" title="34.13. Notice Processing">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="libpq.html" title="Chapter 34. libpq — C Library">Up</a></td><th width="60%" align="center">Chapter 34. <span class="application">libpq</span> — C Library</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="libpq-envars.html" title="34.15. Environment Variables">Next</a></td></tr></table><hr /></div><div class="sect1" id="LIBPQ-EVENTS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">34.14. Event System</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="libpq-events.html#LIBPQ-EVENTS-TYPES">34.14.1. Event Types</a></span></dt><dt><span class="sect2"><a href="libpq-events.html#LIBPQ-EVENTS-PROC">34.14.2. Event Callback Procedure</a></span></dt><dt><span class="sect2"><a href="libpq-events.html#LIBPQ-EVENTS-FUNCS">34.14.3. Event Support Functions</a></span></dt><dt><span class="sect2"><a href="libpq-events.html#LIBPQ-EVENTS-EXAMPLE">34.14.4. Event Example</a></span></dt></dl></div><p>
<span class="application">libpq</span>'s event system is designed to notify
registered event handlers about interesting
<span class="application">libpq</span> events, such as the creation or
destruction of <code class="structname">PGconn</code> and
<code class="structname">PGresult</code> objects. A principal use case is that
this allows applications to associate their own data with a
<code class="structname">PGconn</code> or <code class="structname">PGresult</code>
and ensure that that data is freed at an appropriate time.
</p><p>
Each registered event handler is associated with two pieces of data,
known to <span class="application">libpq</span> only as opaque <code class="literal">void *</code>
pointers. There is a <em class="firstterm">pass-through</em> pointer that is provided
by the application when the event handler is registered with a
<code class="structname">PGconn</code>. The pass-through pointer never changes for the
life of the <code class="structname">PGconn</code> and all <code class="structname">PGresult</code>s
generated from it; so if used, it must point to long-lived data.
In addition there is an <em class="firstterm">instance data</em> pointer, which starts
out <code class="symbol">NULL</code> in every <code class="structname">PGconn</code> and <code class="structname">PGresult</code>.
This pointer can be manipulated using the
<a class="xref" href="libpq-events.html#LIBPQ-PQINSTANCEDATA"><code class="function">PQinstanceData</code></a>,
<a class="xref" href="libpq-events.html#LIBPQ-PQSETINSTANCEDATA"><code class="function">PQsetInstanceData</code></a>,
<a class="xref" href="libpq-events.html#LIBPQ-PQRESULTINSTANCEDATA"><code class="function">PQresultInstanceData</code></a> and
<a class="xref" href="libpq-events.html#LIBPQ-PQRESULTSETINSTANCEDATA"><code class="function">PQresultSetInstanceData</code></a> functions. Note that
unlike the pass-through pointer, instance data of a <code class="structname">PGconn</code>
is not automatically inherited by <code class="structname">PGresult</code>s created from
it. <span class="application">libpq</span> does not know what pass-through
and instance data pointers point to (if anything) and will never attempt
to free them — that is the responsibility of the event handler.
</p><div class="sect2" id="LIBPQ-EVENTS-TYPES"><div class="titlepage"><div><div><h3 class="title">34.14.1. Event Types</h3></div></div></div><p>
The enum <code class="literal">PGEventId</code> names the types of events handled by
the event system. All its values have names beginning with
<code class="literal">PGEVT</code>. For each event type, there is a corresponding
event info structure that carries the parameters passed to the event
handlers. The event types are:
</p><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PGEVT-REGISTER"><span class="term"><code class="literal">PGEVT_REGISTER</code></span></dt><dd><p>
The register event occurs when <a class="xref" href="libpq-events.html#LIBPQ-PQREGISTEREVENTPROC"><code class="function">PQregisterEventProc</code></a>
is called. It is the ideal time to initialize any
<code class="literal">instanceData</code> an event procedure may need. Only one
register event will be fired per event handler per connection. If the
event procedure fails (returns zero), the registration is cancelled.
</p><pre class="synopsis">
typedef struct
{
PGconn *conn;
} PGEventRegister;
</pre><p>
When a <code class="literal">PGEVT_REGISTER</code> event is received, the
<em class="parameter"><code>evtInfo</code></em> pointer should be cast to a
<code class="structname">PGEventRegister *</code>. This structure contains a
<code class="structname">PGconn</code> that should be in the
<code class="literal">CONNECTION_OK</code> status; guaranteed if one calls
<a class="xref" href="libpq-events.html#LIBPQ-PQREGISTEREVENTPROC"><code class="function">PQregisterEventProc</code></a> right after obtaining a good
<code class="structname">PGconn</code>. When returning a failure code, all
cleanup must be performed as no <code class="literal">PGEVT_CONNDESTROY</code>
event will be sent.
</p></dd><dt id="LIBPQ-PGEVT-CONNRESET"><span class="term"><code class="literal">PGEVT_CONNRESET</code></span></dt><dd><p>
The connection reset event is fired on completion of
<a class="xref" href="libpq-connect.html#LIBPQ-PQRESET"><code class="function">PQreset</code></a> or <code class="function">PQresetPoll</code>. In
both cases, the event is only fired if the reset was successful.
The return value of the event procedure is ignored
in <span class="productname">PostgreSQL</span> v15 and later.
With earlier versions, however, it's important to return success
(nonzero) or the connection will be aborted.
</p><pre class="synopsis">
typedef struct
{
PGconn *conn;
} PGEventConnReset;
</pre><p>
When a <code class="literal">PGEVT_CONNRESET</code> event is received, the
<em class="parameter"><code>evtInfo</code></em> pointer should be cast to a
<code class="structname">PGEventConnReset *</code>. Although the contained
<code class="structname">PGconn</code> was just reset, all event data remains
unchanged. This event should be used to reset/reload/requery any
associated <code class="literal">instanceData</code>. Note that even if the
event procedure fails to process <code class="literal">PGEVT_CONNRESET</code>, it will
still receive a <code class="literal">PGEVT_CONNDESTROY</code> event when the connection
is closed.
</p></dd><dt id="LIBPQ-PGEVT-CONNDESTROY"><span class="term"><code class="literal">PGEVT_CONNDESTROY</code></span></dt><dd><p>
The connection destroy event is fired in response to
<a class="xref" href="libpq-connect.html#LIBPQ-PQFINISH"><code class="function">PQfinish</code></a>. It is the event procedure's
responsibility to properly clean up its event data as libpq has no
ability to manage this memory. Failure to clean up will lead
to memory leaks.
</p><pre class="synopsis">
typedef struct
{
PGconn *conn;
} PGEventConnDestroy;
</pre><p>
When a <code class="literal">PGEVT_CONNDESTROY</code> event is received, the
<em class="parameter"><code>evtInfo</code></em> pointer should be cast to a
<code class="structname">PGEventConnDestroy *</code>. This event is fired
prior to <a class="xref" href="libpq-connect.html#LIBPQ-PQFINISH"><code class="function">PQfinish</code></a> performing any other cleanup.
The return value of the event procedure is ignored since there is no
way of indicating a failure from <a class="xref" href="libpq-connect.html#LIBPQ-PQFINISH"><code class="function">PQfinish</code></a>. Also,
an event procedure failure should not abort the process of cleaning up
unwanted memory.
</p></dd><dt id="LIBPQ-PGEVT-RESULTCREATE"><span class="term"><code class="literal">PGEVT_RESULTCREATE</code></span></dt><dd><p>
The result creation event is fired in response to any query execution
function that generates a result, including
<a class="xref" href="libpq-async.html#LIBPQ-PQGETRESULT"><code class="function">PQgetResult</code></a>. This event will only be fired after
the result has been created successfully.
</p><pre class="synopsis">
typedef struct
{
PGconn *conn;
PGresult *result;
} PGEventResultCreate;
</pre><p>
When a <code class="literal">PGEVT_RESULTCREATE</code> event is received, the
<em class="parameter"><code>evtInfo</code></em> pointer should be cast to a
<code class="structname">PGEventResultCreate *</code>. The
<em class="parameter"><code>conn</code></em> is the connection used to generate the
result. This is the ideal place to initialize any
<code class="literal">instanceData</code> that needs to be associated with the
result. If an event procedure fails (returns zero), that event
procedure will be ignored for the remaining lifetime of the result;
that is, it will not receive <code class="literal">PGEVT_RESULTCOPY</code>
or <code class="literal">PGEVT_RESULTDESTROY</code> events for this result or
results copied from it.
</p></dd><dt id="LIBPQ-PGEVT-RESULTCOPY"><span class="term"><code class="literal">PGEVT_RESULTCOPY</code></span></dt><dd><p>
The result copy event is fired in response to
<a class="xref" href="libpq-misc.html#LIBPQ-PQCOPYRESULT"><code class="function">PQcopyResult</code></a>. This event will only be fired after
the copy is complete. Only event procedures that have
successfully handled the <code class="literal">PGEVT_RESULTCREATE</code>
or <code class="literal">PGEVT_RESULTCOPY</code> event for the source result
will receive <code class="literal">PGEVT_RESULTCOPY</code> events.
</p><pre class="synopsis">
typedef struct
{
const PGresult *src;
PGresult *dest;
} PGEventResultCopy;
</pre><p>
When a <code class="literal">PGEVT_RESULTCOPY</code> event is received, the
<em class="parameter"><code>evtInfo</code></em> pointer should be cast to a
<code class="structname">PGEventResultCopy *</code>. The
<em class="parameter"><code>src</code></em> result is what was copied while the
<em class="parameter"><code>dest</code></em> result is the copy destination. This event
can be used to provide a deep copy of <code class="literal">instanceData</code>,
since <code class="literal">PQcopyResult</code> cannot do that. If an event
procedure fails (returns zero), that event procedure will be
ignored for the remaining lifetime of the new result; that is, it
will not receive <code class="literal">PGEVT_RESULTCOPY</code>
or <code class="literal">PGEVT_RESULTDESTROY</code> events for that result or
results copied from it.
</p></dd><dt id="LIBPQ-PGEVT-RESULTDESTROY"><span class="term"><code class="literal">PGEVT_RESULTDESTROY</code></span></dt><dd><p>
The result destroy event is fired in response to a
<a class="xref" href="libpq-exec.html#LIBPQ-PQCLEAR"><code class="function">PQclear</code></a>. It is the event procedure's
responsibility to properly clean up its event data as libpq has no
ability to manage this memory. Failure to clean up will lead
to memory leaks.
</p><pre class="synopsis">
typedef struct
{
PGresult *result;
} PGEventResultDestroy;
</pre><p>
When a <code class="literal">PGEVT_RESULTDESTROY</code> event is received, the
<em class="parameter"><code>evtInfo</code></em> pointer should be cast to a
<code class="structname">PGEventResultDestroy *</code>. This event is fired
prior to <a class="xref" href="libpq-exec.html#LIBPQ-PQCLEAR"><code class="function">PQclear</code></a> performing any other cleanup.
The return value of the event procedure is ignored since there is no
way of indicating a failure from <a class="xref" href="libpq-exec.html#LIBPQ-PQCLEAR"><code class="function">PQclear</code></a>. Also,
an event procedure failure should not abort the process of cleaning up
unwanted memory.
</p></dd></dl></div></div><div class="sect2" id="LIBPQ-EVENTS-PROC"><div class="titlepage"><div><div><h3 class="title">34.14.2. Event Callback Procedure</h3></div></div></div><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PGEVENTPROC"><span class="term"><code class="literal">PGEventProc</code><a id="id-1.7.3.21.5.2.1.1.2" class="indexterm"></a></span></dt><dd><p>
<code class="literal">PGEventProc</code> is a typedef for a pointer to an
event procedure, that is, the user callback function that receives
events from libpq. The signature of an event procedure must be
</p><pre class="synopsis">
int eventproc(PGEventId evtId, void *evtInfo, void *passThrough)
</pre><p>
The <em class="parameter"><code>evtId</code></em> parameter indicates which
<code class="literal">PGEVT</code> event occurred. The
<em class="parameter"><code>evtInfo</code></em> pointer must be cast to the appropriate
structure type to obtain further information about the event.
The <em class="parameter"><code>passThrough</code></em> parameter is the pointer
provided to <a class="xref" href="libpq-events.html#LIBPQ-PQREGISTEREVENTPROC"><code class="function">PQregisterEventProc</code></a> when the event
procedure was registered. The function should return a non-zero value
if it succeeds and zero if it fails.
</p><p>
A particular event procedure can be registered only once in any
<code class="structname">PGconn</code>. This is because the address of the procedure
is used as a lookup key to identify the associated instance data.
</p><div class="caution"><h3 class="title">Caution</h3><p>
On Windows, functions can have two different addresses: one visible
from outside a DLL and another visible from inside the DLL. One
should be careful that only one of these addresses is used with
<span class="application">libpq</span>'s event-procedure functions, else confusion will
result. The simplest rule for writing code that will work is to
ensure that event procedures are declared <code class="literal">static</code>. If the
procedure's address must be available outside its own source file,
expose a separate function to return the address.
</p></div></dd></dl></div></div><div class="sect2" id="LIBPQ-EVENTS-FUNCS"><div class="titlepage"><div><div><h3 class="title">34.14.3. Event Support Functions</h3></div></div></div><div class="variablelist"><dl class="variablelist"><dt id="LIBPQ-PQREGISTEREVENTPROC"><span class="term"><code class="function">PQregisterEventProc</code><a id="id-1.7.3.21.6.2.1.1.2" class="indexterm"></a></span></dt><dd><p>
Registers an event callback procedure with libpq.
</p><pre class="synopsis">
int PQregisterEventProc(PGconn *conn, PGEventProc proc,
const char *name, void *passThrough);
</pre><p>
</p><p>
An event procedure must be registered once on each
<code class="structname">PGconn</code> you want to receive events about. There is no
limit, other than memory, on the number of event procedures that
can be registered with a connection. The function returns a non-zero
value if it succeeds and zero if it fails.
</p><p>
The <em class="parameter"><code>proc</code></em> argument will be called when a libpq
event is fired. Its memory address is also used to lookup
<code class="literal">instanceData</code>. The <em class="parameter"><code>name</code></em>
argument is used to refer to the event procedure in error messages.
This value cannot be <code class="symbol">NULL</code> or a zero-length string. The name string is
copied into the <code class="structname">PGconn</code>, so what is passed need not be
long-lived. The <em class="parameter"><code>passThrough</code></em> pointer is passed
to the <em class="parameter"><code>proc</code></em> whenever an event occurs. This
argument can be <code class="symbol">NULL</code>.
</p></dd><dt id="LIBPQ-PQSETINSTANCEDATA"><span class="term"><code class="function">PQsetInstanceData</code><a id="id-1.7.3.21.6.2.2.1.2" class="indexterm"></a></span></dt><dd><p>
Sets the connection <em class="parameter"><code>conn</code></em>'s <code class="literal">instanceData</code>
for procedure <em class="parameter"><code>proc</code></em> to <em class="parameter"><code>data</code></em>. This
returns non-zero for success and zero for failure. (Failure is
only possible if <em class="parameter"><code>proc</code></em> has not been properly
registered in <em class="parameter"><code>conn</code></em>.)
</p><pre class="synopsis">
int PQsetInstanceData(PGconn *conn, PGEventProc proc, void *data);
</pre><p>
</p></dd><dt id="LIBPQ-PQINSTANCEDATA"><span class="term"><code class="function">PQinstanceData</code><a id="id-1.7.3.21.6.2.3.1.2" class="indexterm"></a></span></dt><dd><p>
Returns the
connection <em class="parameter"><code>conn</code></em>'s <code class="literal">instanceData</code>
associated with procedure <em class="parameter"><code>proc</code></em>,
or <code class="symbol">NULL</code> if there is none.
</p><pre class="synopsis">
void *PQinstanceData(const PGconn *conn, PGEventProc proc);
</pre><p>
</p></dd><dt id="LIBPQ-PQRESULTSETINSTANCEDATA"><span class="term"><code class="function">PQresultSetInstanceData</code><a id="id-1.7.3.21.6.2.4.1.2" class="indexterm"></a></span></dt><dd><p>
Sets the result's <code class="literal">instanceData</code>
for <em class="parameter"><code>proc</code></em> to <em class="parameter"><code>data</code></em>. This returns
non-zero for success and zero for failure. (Failure is only
possible if <em class="parameter"><code>proc</code></em> has not been properly registered
in the result.)
</p><pre class="synopsis">
int PQresultSetInstanceData(PGresult *res, PGEventProc proc, void *data);
</pre><p>
</p><p>
Beware that any storage represented by <em class="parameter"><code>data</code></em>
will not be accounted for by <a class="xref" href="libpq-misc.html#LIBPQ-PQRESULTMEMORYSIZE"><code class="function">PQresultMemorySize</code></a>,
unless it is allocated using <a class="xref" href="libpq-misc.html#LIBPQ-PQRESULTALLOC"><code class="function">PQresultAlloc</code></a>.
(Doing so is recommendable because it eliminates the need to free
such storage explicitly when the result is destroyed.)
</p></dd><dt id="LIBPQ-PQRESULTINSTANCEDATA"><span class="term"><code class="function">PQresultInstanceData</code><a id="id-1.7.3.21.6.2.5.1.2" class="indexterm"></a></span></dt><dd><p>
Returns the result's <code class="literal">instanceData</code> associated with <em class="parameter"><code>proc</code></em>, or <code class="symbol">NULL</code>
if there is none.
</p><pre class="synopsis">
void *PQresultInstanceData(const PGresult *res, PGEventProc proc);
</pre><p>
</p></dd></dl></div></div><div class="sect2" id="LIBPQ-EVENTS-EXAMPLE"><div class="titlepage"><div><div><h3 class="title">34.14.4. Event Example</h3></div></div></div><p>
Here is a skeleton example of managing private data associated with
libpq connections and results.
</p><pre class="programlisting">
/* required header for libpq events (note: includes libpq-fe.h) */
#include <libpq-events.h>
/* The instanceData */
typedef struct
{
int n;
char *str;
} mydata;
/* PGEventProc */
static int myEventProc(PGEventId evtId, void *evtInfo, void *passThrough);
int
main(void)
{
mydata *data;
PGresult *res;
PGconn *conn =
PQconnectdb("dbname=postgres options=-csearch_path=");
if (PQstatus(conn) != CONNECTION_OK)
{
/* PQerrorMessage's result includes a trailing newline */
fprintf(stderr, "%s", PQerrorMessage(conn));
PQfinish(conn);
return 1;
}
/* called once on any connection that should receive events.
* Sends a PGEVT_REGISTER to myEventProc.
*/
if (!PQregisterEventProc(conn, myEventProc, "mydata_proc", NULL))
{
fprintf(stderr, "Cannot register PGEventProc\n");
PQfinish(conn);
return 1;
}
/* conn instanceData is available */
data = PQinstanceData(conn, myEventProc);
/* Sends a PGEVT_RESULTCREATE to myEventProc */
res = PQexec(conn, "SELECT 1 + 1");
/* result instanceData is available */
data = PQresultInstanceData(res, myEventProc);
/* If PG_COPYRES_EVENTS is used, sends a PGEVT_RESULTCOPY to myEventProc */
res_copy = PQcopyResult(res, PG_COPYRES_TUPLES | PG_COPYRES_EVENTS);
/* result instanceData is available if PG_COPYRES_EVENTS was
* used during the PQcopyResult call.
*/
data = PQresultInstanceData(res_copy, myEventProc);
/* Both clears send a PGEVT_RESULTDESTROY to myEventProc */
PQclear(res);
PQclear(res_copy);
/* Sends a PGEVT_CONNDESTROY to myEventProc */
PQfinish(conn);
return 0;
}
static int
myEventProc(PGEventId evtId, void *evtInfo, void *passThrough)
{
switch (evtId)
{
case PGEVT_REGISTER:
{
PGEventRegister *e = (PGEventRegister *)evtInfo;
mydata *data = get_mydata(e->conn);
/* associate app specific data with connection */
PQsetInstanceData(e->conn, myEventProc, data);
break;
}
case PGEVT_CONNRESET:
{
PGEventConnReset *e = (PGEventConnReset *)evtInfo;
mydata *data = PQinstanceData(e->conn, myEventProc);
if (data)
memset(data, 0, sizeof(mydata));
break;
}
case PGEVT_CONNDESTROY:
{
PGEventConnDestroy *e = (PGEventConnDestroy *)evtInfo;
mydata *data = PQinstanceData(e->conn, myEventProc);
/* free instance data because the conn is being destroyed */
if (data)
free_mydata(data);
break;
}
case PGEVT_RESULTCREATE:
{
PGEventResultCreate *e = (PGEventResultCreate *)evtInfo;
mydata *conn_data = PQinstanceData(e->conn, myEventProc);
mydata *res_data = dup_mydata(conn_data);
/* associate app specific data with result (copy it from conn) */
PQresultSetInstanceData(e->result, myEventProc, res_data);
break;
}
case PGEVT_RESULTCOPY:
{
PGEventResultCopy *e = (PGEventResultCopy *)evtInfo;
mydata *src_data = PQresultInstanceData(e->src, myEventProc);
mydata *dest_data = dup_mydata(src_data);
/* associate app specific data with result (copy it from a result) */
PQresultSetInstanceData(e->dest, myEventProc, dest_data);
break;
}
case PGEVT_RESULTDESTROY:
{
PGEventResultDestroy *e = (PGEventResultDestroy *)evtInfo;
mydata *data = PQresultInstanceData(e->result, myEventProc);
/* free instance data because the result is being destroyed */
if (data)
free_mydata(data);
break;
}
/* unknown event ID, just return true. */
default:
break;
}
return true; /* event processing succeeded */
}
</pre></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="libpq-notice-processing.html" title="34.13. Notice Processing">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="libpq.html" title="Chapter 34. libpq — C Library">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="libpq-envars.html" title="34.15. Environment Variables">Next</a></td></tr><tr><td width="40%" align="left" valign="top">34.13. Notice Processing </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 34.15. Environment Variables</td></tr></table></div></body></html>
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