typedetruct BackgroundWorker
{
    char        bgw_name[BGW_MAXLEN];
    int         bgw_flags;
    BgWorkerStartTime bgw_start_time;
    int         bgw_restart_time;       /* in seconds, or BGW_NEVER_RESTART */
    bgworker_main_type bgw_main;
    char        bgw_library_name[BGW_MAXLEN];   /* only if bgw_main is NULL */
    char        bgw_function_name[BGW_MAXLEN];  /* only if bgw_main is NULL */
    Datum       bgw_main_arg;
    char        bgw_extra[BGW_EXTRALEN];
    pid_t       bgw_notify_pid; /* SIGUSR1 this backend on start/stop */
} BackgroundWorker;

/*
     * If bgw_main is set, we use that value as the initial entrypoint.
     * However, if the library containing the entrypoint wasn't loaded at
     * postmaster startup time, passing it as a direct function pointer is not
     * possible.  To work around that, we allow callers for whom a function
     * pointer is not available to pass a library name (which will be loaded,
     * if necessary) and a function name (which will be looked up in the named
     * library).
     */
    if (worker->bgw_main != NULL)
        entrypt = worker->bgw_main;
    else
        entrypt = (bgworker_main_type)
            load_external_function(worker->bgw_library_name,
                                   worker->bgw_function_name,
                                   true, NULL);

    /*
     * Note that in normal processes, we would call InitPostgres here.  For a
     * worker, however, we don't know what database to connect to, yet; so we
     * need to wait until the user code does it via
     * BackgroundWorkerInitializeConnection().
     */

    /*
     * Now invoke the user-defined worker code
     */
    entrypt(worker->bgw_main_arg);

/*  编写bgw，最少需要添加的头文件，如果需要别的功能再添加别的头文件
    比如，如果想对数据库进程操作，借助spi manager就需要添加spi.h*/
#include "postgres.h"
#include "postmaster/bgworker.h"
#include "storage/ipc.h"
#include "storage/latch.h"
#include "storage/proc.h"
#include "fmgr.h"

/* Essential for shared libs! */
PG_MODULE_MAGIC;

/* Entry point of library loading */
void _PG_init(void);

/* Signal handling */
static bool got_sigterm = false;

void
_PG_init(void)
{
    BackgroundWorker    worker;

    /* Register the worker processes */
    worker.bgw_flags = BGWORKER_SHMEM_ACCESS;
    worker.bgw_start_time = BgWorkerStart_RecoveryFinished;
    worker.bgw_main = hello_main;
    worker.bgw_sighup = NULL;
    worker.bgw_sigterm = hello_sigterm;
    worker.bgw_name = "hello world";
    worker.bgw_restart_time = BGW_NEVER_RESTART;
    worker.bgw_main_arg = NULL;
    RegisterBackgroundWorker(&worker);
}

static void
hello_main(void *main_arg)
{
    /* ready to receive signals */
    BackgroundWorkerUnblockSignals();
    while (!got_sigterm)
    {
        int     rc;
        /* Wait 10s */
        rc = WaitLatch(&MyProc-;>procLatch,
                WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH,
                10000L);
        ResetLatch(&MyProc-;>procLatch);
        elog(LOG, "Hello World!"); 	/* Say Hello to the world */
    }
    proc_exit(0);
}

static void
hello_sigterm(SIGNAL_ARGS)
{
    int         save_errno = errno;
    got_sigterm = true;
    if (MyProc)
        SetLatch(&MyProc-;>procLatch);
    errno = save_errno;
}