37.3. Writing Event Trigger Functions in C

This section describes the low-level details of the interface to an event trigger function. This information is only needed when writing event trigger functions in C. If you are using a higher-level language then these details are handled for you. In most cases you should consider using a procedural language before writing your event triggers in C. The documentation of each procedural language explains how to write an event trigger in that language.

Event trigger functions must use the "version 1" function manager interface.

When a function is called by the event trigger manager, it is not passed any normal arguments, but it is passed a "context" pointer pointing to a EventTriggerData structure. C functions can check whether they were called from the event trigger manager or not by executing the macro:

CALLED_AS_EVENT_TRIGGER(fcinfo)

which expands to:

((fcinfo)->context != NULL && IsA((fcinfo)->context, EventTriggerData))

If this returns true, then it is safe to cast fcinfo->context to type EventTriggerData * and make use of the pointed-to EventTriggerData structure. The function must not alter the EventTriggerData structure or any of the data it points to.

struct EventTriggerData is defined in commands/event_trigger.h:

typedef struct EventTriggerData
{
    NodeTag     type;
    const char *event;      /* event name */
    Node       *parsetree;  /* parse tree */
    const char *tag;        /* command tag */
} EventTriggerData;

where the members are defined as follows:

An event trigger function must return a NULL pointer (not an SQL null value, that is, do not set isNull true).