_PROGRAMMER'S BOOKSHELF COLUMN_
by Andrew Schulman

[FIGURE 1: (a) non-networking ``hello world'' (b) networking
``hello world'']


(a)

/* HELLO.C */
main()
{
    puts("hello world");
}


(b) 

typedef struct {
    int request;
    int len;
    char buf[MAX];
    } REQUEST_PACKET;           /* client requests to server */
typedef unsigned REPLY_PACKET;  /* server replies to client */

/* H_CLIENT.C */
main()
{
    REQUEST_PACKET requ;
    REPLY_PACKET reply;
    char *msg = "hello world";
    conn = call(machine_running_H_SERVER);
    requ.request = DISPLAY;
    requ.len = strlen(msg);
    strcpy(requ.buf, msg);
    send(conn, &requ, sizeof(REQUEST_PACKET));
    receive(conn, &reply, sizeof(REPLY_PACKET));
    puts(reply == OK ? "ok" : "fail");
    hangup(conn);
}

/* H_SERVER.C */
main()
{
    REQUEST_PACKET requ;
    REPLY_PACKET ok = OK;
    for (;;) /* endless loop */
    {
        conn = listen(anyone);
        receive(conn, &requ, sizeof(REQUEST_PACKET));
        switch (requ.request)
        {
            case DISPLAY:
                printf("%*s\n", requ.len, requ.buf);
                send(conn, &ok, sizeof(REPLY_PACKET)); 
                break;
            // handle other requests
        }
        hangup(conn);
    }
}


[FIGURE 2: No-wait networking ``hello'' server]


/* H_SERVER.C */
hangup_handler()        /* called when hangup completes */
{
    // free any buffers allocated in listen_handler, etc.
}
send_handler()          /* called when send completes */
{
    // do nothing
}
receive_handler(requ)   /* called when receive completes */
{
    REPLY_PACKET ok = OK;
    switch (requ.request)
    {
        case DISPLAY:
            printf("%*s\n", requ.len, requ.buf);
            send_nowait(conn, &ok, sizeof(REPLY_PACKET),
                send_handler);
            break;
        // handle other requests
    }
    hangup_nowait(conn, hangup_handler);
}
listen_handler(conn)    /* called when listen completes */
{
    REQUEST_PACKET requ;
    receive_nowait(conn, &requ, sizeof(REQUEST_PACKET), 
        receive_handler);
}
main()
{
    for (;;) /* endless loop */
        listen_nowait(anyone, listen_handler);
}