34.14. Event System

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.

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.

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 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.

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.

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 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.

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 .

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);

Returns the connection conn 's instanceData associated with procedure proc , or NULL if there is none.

void *PQinstanceData(const PGconn *conn, PGEventProc proc);

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.)

Returns the result's instanceData associated with proc , or NULL if there is none.

void *PQresultInstanceData(const PGresult *res, PGEventProc proc);