libpq's event system is designed to notify
registered event handlers about interesting
libpq events, such as the creation or
destruction of PGconn and
PGresult objects. A principal use case is that
this allows applications to associate their own data with a
PGconn or PGresult
and ensure that that data is freed at an appropriate time.
Each registered event handler is associated with two pieces of data,
known to libpq only as opaque void *
pointers. There is a pass-through pointer that is provided
by the application when the event handler is registered with a
PGconn. The pass-through pointer never changes for the
life of the PGconn and all PGresults
generated from it; so if used, it must point to long-lived data.
In addition there is an instance data pointer, which starts
out NULL in every PGconn and PGresult.
This pointer can be manipulated using the
PQinstanceData,
PQsetInstanceData,
PQresultInstanceData and
PQresultSetInstanceData functions. Note that
unlike the pass-through pointer, instance data of a PGconn
is not automatically inherited by PGresults created from
it. libpq 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.
The enum PGEventId names the types of events handled by
the event system. All its values have names beginning with
PGEVT. For each event type, there is a corresponding
event info structure that carries the parameters passed to the event
handlers. The event types are:
PGEVT_REGISTER
The register event occurs when PQregisterEventProc
is called. It is the ideal time to initialize any
instanceData 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.
typedef struct
{
PGconn *conn;
} PGEventRegister;
When a PGEVT_REGISTER event is received, the
evtInfo pointer should be cast to a
PGEventRegister *. This structure contains a
PGconn that should be in the
CONNECTION_OK status; guaranteed if one calls
PQregisterEventProc right after obtaining a good
PGconn. When returning a failure code, all
cleanup must be performed as no PGEVT_CONNDESTROY
event will be sent.
PGEVT_CONNRESET
The connection reset event is fired on completion of
PQreset or PQresetPoll. In
both cases, the event is only fired if the reset was successful.
The return value of the event procedure is ignored
in PostgreSQL v15 and later.
With earlier versions, however, it's important to return success
(nonzero) or the connection will be aborted.
typedef struct
{
PGconn *conn;
} PGEventConnReset;
When a PGEVT_CONNRESET event is received, the
evtInfo pointer should be cast to a
PGEventConnReset *. Although the contained
PGconn was just reset, all event data remains
unchanged. This event should be used to reset/reload/requery any
associated instanceData. Note that even if the
event procedure fails to process PGEVT_CONNRESET, it will
still receive a PGEVT_CONNDESTROY event when the connection
is closed.
PGEVT_CONNDESTROY
The connection destroy event is fired in response to
PQfinish. 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.
typedef struct
{
PGconn *conn;
} PGEventConnDestroy;
When a PGEVT_CONNDESTROY event is received, the
evtInfo pointer should be cast to a
PGEventConnDestroy *. This event is fired
prior to PQfinish performing any other cleanup.
The return value of the event procedure is ignored since there is no
way of indicating a failure from PQfinish. Also,
an event procedure failure should not abort the process of cleaning up
unwanted memory.
PGEVT_RESULTCREATE
The result creation event is fired in response to any query execution
function that generates a result, including
PQgetResult. This event will only be fired after
the result has been created successfully.
typedef struct
{
PGconn *conn;
PGresult *result;
} PGEventResultCreate;
When a PGEVT_RESULTCREATE event is received, the
evtInfo pointer should be cast to a
PGEventResultCreate *. The
conn is the connection used to generate the
result. This is the ideal place to initialize any
instanceData 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 PGEVT_RESULTCOPY
or PGEVT_RESULTDESTROY events for this result or
results copied from it.
PGEVT_RESULTCOPY
The result copy event is fired in response to
PQcopyResult. This event will only be fired after
the copy is complete. Only event procedures that have
successfully handled the PGEVT_RESULTCREATE
or PGEVT_RESULTCOPY event for the source result
will receive PGEVT_RESULTCOPY events.
typedef struct
{
const PGresult *src;
PGresult *dest;
} PGEventResultCopy;
When a PGEVT_RESULTCOPY event is received, the
evtInfo pointer should be cast to a
PGEventResultCopy *. The
src result is what was copied while the
dest result is the copy destination. This event
can be used to provide a deep copy of instanceData,
since PQcopyResult 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 PGEVT_RESULTCOPY
or PGEVT_RESULTDESTROY events for that result or
results copied from it.
PGEVT_RESULTDESTROY
The result destroy event is fired in response to a
PQclear. 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.
typedef struct
{
PGresult *result;
} PGEventResultDestroy;
When a PGEVT_RESULTDESTROY event is received, the
evtInfo pointer should be cast to a
PGEventResultDestroy *. This event is fired
prior to PQclear performing any other cleanup.
The return value of the event procedure is ignored since there is no
way of indicating a failure from PQclear. Also,
an event procedure failure should not abort the process of cleaning up
unwanted memory.
PGEventProc
PGEventProc 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
int eventproc(PGEventId evtId, void *evtInfo, void *passThrough)
The evtId parameter indicates which
PGEVT event occurred. The
evtInfo pointer must be cast to the appropriate
structure type to obtain further information about the event.
The passThrough parameter is the pointer
provided to PQregisterEventProc when the event
procedure was registered. The function should return a non-zero value
if it succeeds and zero if it fails.
A particular event procedure can be registered only once in any
PGconn. This is because the address of the procedure
is used as a lookup key to identify the associated instance data.
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
libpq'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 static. If the
procedure's address must be available outside its own source file,
expose a separate function to return the address.
PQregisterEventProcRegisters an event callback procedure with libpq.
int PQregisterEventProc(PGconn *conn, PGEventProc proc,
const char *name, void *passThrough);
An event procedure must be registered once on each
PGconn 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.
The proc argument will be called when a libpq
event is fired. Its memory address is also used to lookup
instanceData. The name
argument is used to refer to the event procedure in error messages.
This value cannot be NULL or a zero-length string. The name string is
copied into the PGconn, so what is passed need not be
long-lived. The passThrough pointer is passed
to the proc whenever an event occurs. This
argument can be NULL.
PQsetInstanceData
Sets the connection conn's instanceData
for procedure proc to data. This
returns non-zero for success and zero for failure. (Failure is
only possible if proc has not been properly
registered in conn.)
int PQsetInstanceData(PGconn *conn, PGEventProc proc, void *data);
PQinstanceData
Returns the
connection conn's instanceData
associated with procedure proc,
or NULL if there is none.
void *PQinstanceData(const PGconn *conn, PGEventProc proc);
PQresultSetInstanceData
Sets the result's instanceData
for proc to data. This returns
non-zero for success and zero for failure. (Failure is only
possible if proc has not been properly registered
in the result.)
int PQresultSetInstanceData(PGresult *res, PGEventProc proc, void *data);
Beware that any storage represented by data
will not be accounted for by PQresultMemorySize,
unless it is allocated using PQresultAlloc.
(Doing so is recommendable because it eliminates the need to free
such storage explicitly when the result is destroyed.)
PQresultInstanceData
Returns the result's instanceData associated with proc, or NULL
if there is none.
void *PQresultInstanceData(const PGresult *res, PGEventProc proc);
Here is a skeleton example of managing private data associated with libpq connections and results.
/* 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 */
}