## 38.3. Writing Trigger Functions in C

This section describes the low-level details of the interface to a trigger function. This information is only needed when writing 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 triggers in C. The documentation of each procedural language explains how to write a trigger in that language.

Trigger functions must use the “version 1” function manager interface.

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

    CALLED_AS_TRIGGER(fcinfo)

which expands to:

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

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

`struct TriggerData` is defined in `commands/trigger.h`:

    typedef struct TriggerData
    {
        NodeTag          type;
        TriggerEvent     tg_event;
        Relation         tg_relation;
        HeapTuple        tg_trigtuple;
        HeapTuple        tg_newtuple;
        Trigger         *tg_trigger;
        TupleTableSlot  *tg_trigslot;
        TupleTableSlot  *tg_newslot;
        Tuplestorestate *tg_oldtable;
        Tuplestorestate *tg_newtable;
    } TriggerData;

where the members are defined as follows:

`type`

  Always `T_TriggerData`.

`tg_event`

  Describes the event for which the function is called. You can use the following macros to examine `tg_event`:

  `TRIGGER_FIRED_BEFORE(tg_event)`

    Returns true if the trigger fired before the operation.

  `TRIGGER_FIRED_AFTER(tg_event)`

    Returns true if the trigger fired after the operation.

  `TRIGGER_FIRED_INSTEAD(tg_event)`

    Returns true if the trigger fired instead of the operation.

  `TRIGGER_FIRED_FOR_ROW(tg_event)`

    Returns true if the trigger fired for a row-level event.

  `TRIGGER_FIRED_FOR_STATEMENT(tg_event)`

    Returns true if the trigger fired for a statement-level event.

  `TRIGGER_FIRED_BY_INSERT(tg_event)`

    Returns true if the trigger was fired by an `INSERT` command.

  `TRIGGER_FIRED_BY_UPDATE(tg_event)`

    Returns true if the trigger was fired by an `UPDATE` command.

  `TRIGGER_FIRED_BY_DELETE(tg_event)`

    Returns true if the trigger was fired by a `DELETE` command.

  `TRIGGER_FIRED_BY_TRUNCATE(tg_event)`

    Returns true if the trigger was fired by a `TRUNCATE` command.

`tg_relation`

  A pointer to a structure describing the relation that the trigger fired for. Look at `utils/rel.h` for details about this structure. The most interesting things are `tg_relation->rd_att` (descriptor of the relation tuples) and `tg_relation->rd_rel->relname` (relation name; the type is not `char*` but `NameData`; use `SPI_getrelname(tg_relation)` to get a `char*` if you need a copy of the name).

`tg_trigtuple`

  A pointer to the row for which the trigger was fired. This is the row being inserted, updated, or deleted. If this trigger was fired for an `INSERT` or `DELETE` then this is what you should return from the function if you don't want to replace the row with a different one (in the case of `INSERT`) or skip the operation. For triggers on foreign tables, values of system columns herein are unspecified.

`tg_newtuple`

  A pointer to the new version of the row, if the trigger was fired for an `UPDATE`, and `NULL` if it is for an `INSERT` or a `DELETE`. This is what you have to return from the function if the event is an `UPDATE` and you don't want to replace this row by a different one or skip the operation. For triggers on foreign tables, values of system columns herein are unspecified.

`tg_trigger`

  A pointer to a structure of type `Trigger`, defined in `utils/reltrigger.h`:

      typedef struct Trigger
      {
          Oid         tgoid;
          char       *tgname;
          Oid         tgfoid;
          int16       tgtype;
          char        tgenabled;
          bool        tgisinternal;
          Oid         tgconstrrelid;
          Oid         tgconstrindid;
          Oid         tgconstraint;
          bool        tgdeferrable;
          bool        tginitdeferred;
          int16       tgnargs;
          int16       tgnattr;
          int16      *tgattr;
          char      **tgargs;
          char       *tgqual;
          char       *tgoldtable;
          char       *tgnewtable;
      } Trigger;

  where `tgname` is the trigger's name, `tgnargs` is the number of arguments in `tgargs`, and `tgargs` is an array of pointers to the arguments specified in the `CREATE TRIGGER` statement. The other members are for internal use only.

`tg_trigslot`

  The slot containing `tg_trigtuple`, or a `NULL` pointer if there is no such tuple.

`tg_newslot`

  The slot containing `tg_newtuple`, or a `NULL` pointer if there is no such tuple.

`tg_oldtable`

  A pointer to a structure of type `Tuplestorestate` containing zero or more rows in the format specified by `tg_relation`, or a `NULL` pointer if there is no `OLD TABLE` transition relation.

`tg_newtable`

  A pointer to a structure of type `Tuplestorestate` containing zero or more rows in the format specified by `tg_relation`, or a `NULL` pointer if there is no `NEW TABLE` transition relation.

To allow queries issued through SPI to reference transition tables, see SPI_register_trigger_data.

A trigger function must return either a `HeapTuple` pointer or a `NULL` pointer (not an SQL null value, that is, do not set `isNull` true). Be careful to return either `tg_trigtuple` or `tg_newtuple`, as appropriate, if you don't want to modify the row being operated on.