009-5-11 LWIP之API_MSG結構及其實現
從上面一篇的socket實現來看,如果要評起到最關鍵作用的一個結構體,那麼struct api_msg當之無愧。先看下它的定義:
/** This struct contains a function to execute in another thread context and
a struct api_msg_msg that serves as an argument for this function.
This is passed to tcpip_apimsg to execute functions in tcpip_thread context. */
struct api_msg
{
/** function to execute in tcpip_thread context */
void (* function)(struct api_msg_msg *msg);
/** arguments for this function */
struct api_msg_msg msg;
};
功能說的很清楚。但是具體怎麼個操作法還是不知道,沒關係,接着看它的調用。
舉一個例子,剛好是上一篇中調用,但是沒有看具體實現的
err_t netconn_getaddr(struct netconn *conn, struct ip_addr *addr, u16_t *port, u8_t local)
{
struct api_msg msg;
msg.function = do_getaddr;
msg.msg.conn = conn;
msg.msg.msg.ad.ipaddr = addr;
msg.msg.msg.ad.port = port;
msg.msg.msg.ad.local = local;
TCPIP_APIMSG(&msg);
return conn->err;
}
說明一下,api_msg結構幾乎都是在netconn_xxx函數中被調用,方式千篇一律,除了msg.funcion的賦值不一樣外。上面的調用很簡單,對該結構體變量賦值,接着就是調用TCPIP_APIMSG,這個函數上面講過,可過去看下。既然如此,就不得不說mbox及其相關函數了。
static sys_mbox_t mbox = SYS_MBOX_NULL;【tcp.c】
再看sys_mbox_t的定義,在【src\include\lwip\sys.h】中
/* For a totally minimal and standalone system, we provide null
definitions of the sys_ functions. */
typedef u8_t sys_sem_t;
typedef u8_t sys_mbox_t;
typedef u8_t sys_prot_t;
可以看到這裏只是簡單的定義成了u8類型,注意上面的紅色字體的說明,很明顯這個是可移植的一部分,需要根據不同的平臺,不同的操作系統具體定義。可以借鑑焦海波大俠的關於ucos上對lwip的移植筆記來看。
我們可以看到在api_msg結構的處理過程中,所有的信息都是包含在api_msg_msg結構體中的,api_msg只是將其和function簡單的組合了。下面看下這個牛結構的定義:
/** This struct includes everything that is necessary to execute a function
for a netconn in another thread context (mainly used to process netconns
in the tcpip_thread context to be thread safe). */
struct api_msg_msg
{
/** The netconn which to process - always needed: it includes the semaphore
which is used to block the application thread until the function finished. */
struct netconn *conn;
/** Depending on the executed function, one of these union members is used */
union
{
/** used for do_send */
struct netbuf *b;
/** used for do_newconn */
struct {
u8_t proto;
} n;
/** used for do_bind and do_connect */
struct {
struct ip_addr *ipaddr;
u16_t port;
} bc;
/** used for do_getaddr */
struct {
struct ip_addr *ipaddr;
u16_t *port;
u8_t local;
} ad;
/** used for do_write */
struct {
const void *dataptr;
int len;
u8_t apiflags;
} w;
/** used ofr do_recv */
struct {
u16_t len;
} r;
#if LWIP_IGMP
/** used for do_join_leave_group */
struct {
struct ip_addr *multiaddr;
struct ip_addr *interface;
enum netconn_igmp join_or_leave;
} jl;
#endif /* LWIP_IGMP */
#if TCP_LISTEN_BACKLOG
struct {
u8_t backlog;
} lb;
#endif /* TCP_LISTEN_BACKLOG */
} msg;
};
一個很合理的設計,至少筆者是這麼認爲的。關鍵在於msg union的設計。
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