_PG_init首先使用DefineCustomTypeVariable定义GUC变量，以pgaudit.log为例，auditLog是pgaudit.c中定义的char *指针变量，check_pgaudit_log和assign_pgaudit_log是pgaudit.c中定义的函数。

/* Define GUC variables and install hooks upon module load. */
void _PG_init(void) {
    ...
    /* Define pgaudit.log */
    DefineCustomStringVariable(
        "pgaudit.log",
        "Specifies which classes of statements will be logged by session audit "
        "logging. Multiple classes can be provided using a comma-separated "
        "list and classes can be subtracted by prefacing the class with a "
        "- sign.",
        NULL,
        &auditLog,
        "none",
        PGC_SUSET,
        GUC_LIST_INPUT | GUC_NOT_IN_SAMPLE,
        check_pgaudit_log,
        assign_pgaudit_log,
        NULL);
    ....

将我们定义的钩子加入PG的钩子链。如下图就是pgaudit.c中定义的临时钩子指针变量，用于保存PG已经引入的钩子变量，然后将我们定义的钩子挂在标准的钩子上，也就是我们熟悉的单链表头插法。

    /* Install our hook functions after saving the existing pointers to preserve the chains. */
    next_ExecutorStart_hook = ExecutorStart_hook;
    ExecutorStart_hook = pgaudit_ExecutorStart_hook;

    next_ExecutorCheckPerms_hook = ExecutorCheckPerms_hook;
    ExecutorCheckPerms_hook = pgaudit_ExecutorCheckPerms_hook;

    next_ProcessUtility_hook = ProcessUtility_hook;
    ProcessUtility_hook = pgaudit_ProcessUtility_hook;

    next_object_access_hook = object_access_hook;
    object_access_hook = pgaudit_object_access_hook;

    /* The following hook functions are required to get rows */
    next_ExecutorRun_hook = ExecutorRun_hook;
    ExecutorRun_hook = pgaudit_ExecutorRun_hook;

    next_ExecutorEnd_hook = ExecutorEnd_hook;
    ExecutorEnd_hook = pgaudit_ExecutorEnd_hook;

以pgaudit_ExecutorStart_hook为例，先执行pgaudit_ExecutorStart_hook钩子函数，最后判断后续有没有钩子需要继续执行，没有就执行执行PG标准应该执行的函数。

下一篇博客集中讲解pgaudit_ExecutorCheckPerms_hook、pgaudit_ExecutorStart_hook、pgaudit_ProcessUtility_hook、pgaudit_object_access_hook、pgaudit_ExecutorRun_hook和pgaudit_ExecutorEnd_hook的实现。
https://github.com/pgaudit/pgaudit/blob/master/pgaudit.c