PostgreSQL
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# libpq Control Functions

- `PQsetNoticeProcessor` Control reporting of notice and warning messages generated by libpq.

      typedef void (*PQnoticeProcessor) (void *arg, const char *message);

      PQnoticeProcessor
      PQsetNoticeProcessor(PGconn *conn,
                           PQnoticeProcessor proc,
                           void *arg);

By default, libpq prints "notice" messages from the backend on `stderr`, as well as a few error messages that it generates by itself. This behavior can be overridden by supplying a callback function that does something else with the messages. The callback function is passed the text of the error message (which includes a trailing newline), plus a void pointer that is the same one passed to `PQsetNoticeProcessor`. (This pointer can be used to access application-specific state if needed.) The default notice processor is simply

    static void
    defaultNoticeProcessor(void * arg, const char * message)
    {
        fprintf(stderr, "%s", message);
    }

To use a special notice processor, call `PQsetNoticeProcessor` just after creation of a new PGconn object.

The return value is the pointer to the previous notice processor. If you supply a callback function pointer of NULL, no action is taken, but the current pointer is returned.

Once you have set a notice processor, you should expect that that function could be called as long as either the PGconn object or PGresult objects made from it exist. At creation of a PGresult, the PGconn's current notice processor pointer is copied into the PGresult for possible use by routines like `PQgetvalue`.

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