34.14. Event System
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## 34.14. 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 `PQsetResultInstanceData` 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.

### 34.14.1. 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 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.

`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. 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.

`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 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.

`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 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.

`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.

### 34.14.2. 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 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.

### 34.14.3. 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 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);

### 34.14.4. 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) */
                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 */
    }

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