35.14. Event System #
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 PGresult
s 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 PGresult
s 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.
35.14.1. Event Types #
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 anyinstanceData
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, theevtInfo
pointer should be cast to aPGEventRegister *
. This structure contains aPGconn
that should be in theCONNECTION_OK
status; guaranteed if one callsPQregisterEventProc
right after obtaining a goodPGconn
. When returning a failure code, all cleanup must be performed as noPGEVT_CONNDESTROY
event will be sent.PGEVT_CONNRESET
#The connection reset event is fired on completion of
PQreset
orPQresetPoll
. In both cases, the event is only fired if the reset was successful. The return value of the event procedure is ignored in Postgres Pro 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, theevtInfo
pointer should be cast to aPGEventConnReset *
. Although the containedPGconn
was just reset, all event data remains unchanged. This event should be used to reset/reload/requery any associatedinstanceData
. Note that even if the event procedure fails to processPGEVT_CONNRESET
, it will still receive aPGEVT_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, theevtInfo
pointer should be cast to aPGEventConnDestroy *
. This event is fired prior toPQfinish
performing any other cleanup. The return value of the event procedure is ignored since there is no way of indicating a failure fromPQfinish
. 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, theevtInfo
pointer should be cast to aPGEventResultCreate *
. Theconn
is the connection used to generate the result. This is the ideal place to initialize anyinstanceData
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 receivePGEVT_RESULTCOPY
orPGEVT_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 thePGEVT_RESULTCREATE
orPGEVT_RESULTCOPY
event for the source result will receivePGEVT_RESULTCOPY
events.typedef struct { const PGresult *src; PGresult *dest; } PGEventResultCopy;
When a
PGEVT_RESULTCOPY
event is received, theevtInfo
pointer should be cast to aPGEventResultCopy *
. Thesrc
result is what was copied while thedest
result is the copy destination. This event can be used to provide a deep copy ofinstanceData
, sincePQcopyResult
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 receivePGEVT_RESULTCOPY
orPGEVT_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, theevtInfo
pointer should be cast to aPGEventResultDestroy *
. This event is fired prior toPQclear
performing any other cleanup. The return value of the event procedure is ignored since there is no way of indicating a failure fromPQclear
. Also, an event procedure failure should not abort the process of cleaning up unwanted memory.
35.14.2. Event Callback Procedure #
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 beint eventproc(PGEventId evtId, void *evtInfo, void *passThrough)
The
evtId
parameter indicates whichPGEVT
event occurred. TheevtInfo
pointer must be cast to the appropriate structure type to obtain further information about the event. ThepassThrough
parameter is the pointer provided toPQregisterEventProc
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.
35.14.3. Event Support Functions #
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 lookupinstanceData
. Thename
argument is used to refer to the event procedure in error messages. This value cannot beNULL
or a zero-length string. The name string is copied into thePGconn
, so what is passed need not be long-lived. ThepassThrough
pointer is passed to theproc
whenever an event occurs. This argument can beNULL
.PQsetInstanceData
#Sets the connection
conn
'sinstanceData
for procedureproc
todata
. This returns non-zero for success and zero for failure. (Failure is only possible ifproc
has not been properly registered inconn
.)int PQsetInstanceData(PGconn *conn, PGEventProc proc, void *data);
PQinstanceData
#Returns the connection
conn
'sinstanceData
associated with procedureproc
, orNULL
if there is none.void *PQinstanceData(const PGconn *conn, PGEventProc proc);
PQresultSetInstanceData
#Sets the result's
instanceData
forproc
todata
. This returns non-zero for success and zero for failure. (Failure is only possible ifproc
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 byPQresultMemorySize
, unless it is allocated usingPQresultAlloc
. (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 withproc
, orNULL
if there is none.void *PQresultInstanceData(const PGresult *res, PGEventProc proc);
35.14.4. Event Example #
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 */ }