| Postgres-XC 1.0.4 Documentation | ||||
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Note: The following description applies both to Postgres-XC and PostgreSQL if not described explicitly.
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 passthrough pointer that is provided
by the application when the event handler is registered with a
PGconn. The passthrough 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
PQsetResultInstanceData functions. Note that
unlike the passthrough pointer, instance data of a PGconn
is not automatically inherited by PGresults created from
it. libpq does not know what passthrough
and instance data pointers point to (if anything) and will never attempt
to free them — that is the responsibility of the event handler.
Note: The following description applies both to Postgres-XC and PostgreSQL if not described explicitly.
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:
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, the registration is aborted.
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.
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. If
the event procedure fails, the entire connection reset will fail; the
PGconn is put into
CONNECTION_BAD status and
PQresetPoll will return
PGRES_POLLING_FAILED.
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.
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.
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 the event procedure fails, the result will be cleared and
the failure will be propagated. The event procedure must not try to
PQclear the result object for itself. When returning a
failure code, all cleanup must be performed as no
PGEVT_RESULTDESTROY event will be sent.
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 the event procedure fails, the entire copy operation will fail and the dest result will be cleared. When returning a failure code, all cleanup must be performed as no PGEVT_RESULTDESTROY event will be sent for the destination result.
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.
Note: The following description applies both to Postgres-XC and PostgreSQL if not described explicitly.
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.
| Caution |
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. |
Note: The following description applies both to Postgres-XC and PostgreSQL if not described explicitly.
PQregisterEventProc
Registers 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);
PQresultInstanceData
Returns the result's instanceData associated with proc, or NULL if there is none.
void *PQresultInstanceData(const PGresult *res, PGEventProc proc);
Note: The following description applies both to Postgres-XC and PostgreSQL if not described explicitly.
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");
if (PQstatus(conn) != CONNECTION_OK)
{
fprintf(stderr, "Connection to database failed: %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) */
PQsetResultInstanceData(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) */
PQsetResultInstanceData(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 */
}