最新在使用LWIP + MQTT的應用過程中，出現一個很奇怪的問題，當上位機反送的JSON長度過大時，我的stm32收到的數據就會有問題：

上圖中成功接收到失敗接收口相關1個字節，用wireshark看數據包長度分別是591和590：

發現可能是我的接收BUFF不夠大引起的,程序中有MqttRxBuff大小定義為1024，改成2048還是一樣接收失敗:

看來問題不是在這個BUFF,

接著看MQtt對應的TCP接收BUFF,在LWIP的opt.h中看到有如下一些定義，從圖中看到PBUF_POOL_BUFSIZE大小取決到TCP_MESS大小。當前TCP_MESS大小于536 對應計算出的PBUF_POOL_BUFSIZE為592，剛好與我們之前的591相近，看到就是這個問題我們把

按理把這個參數改大后應該可以解決，但是發現實際發現還是不行！

仿真發現數據從路由器發送到MCU時，就已經分成了兩包，最終將TCP的接收函數改寫成如下問題解決。

?

/**

? * @brief tcp_receiv callback

? * @param arg: argument to be passed to receive callback

? * @param tpcb: tcp connection control block

? * @param err: receive error code

? * @retval err_t: retuned error

? */

err_t MqttTcp_recv(void *arg, struct tcp_pcb *tpcb, struct pbuf *p, err_t err)

{

??? struct mqttclinet *es;

??? err_t ret_err;

??? int i = 0;

?

?

??? LWIP_ASSERT("arg != NULL",arg != NULL);

?

??? es = (struct mqttclinet *)arg;

?

??? /* if we receive an empty tcp frame from server => close connection */

??? if (p == NULL)

??? {

??????? /* remote host closed connection */

??????? es->state = ES_CLOSING;

??????? if(es->p_tx == NULL)

??????? {

??????????? /* we're done sending, close connection */

??????????? MqttTcp_connection_close(tpcb, es);

??????? }

??????? else

??????? {

??????????? /* send remaining data*/

??????????? MqttTcp_send(tpcb, es);

??????? }

??????? ret_err = ERR_OK;

??? }

??? /* else : a non empty frame was received from echo server but for some reason err != ERR_OK */

??? else if(err != ERR_OK)

??? {

??????? /* free received pbuf*/

??????? pbuf_free(p);

?

??????? ret_err = err;

??? }

??? else if(es->state == ES_CONNECTED)

??? {

??????? /* Acknowledge data reception */

??????? tcp_recved(tpcb, p->tot_len);

?

#if 0

??????? // 回顯功能

??????? es->p_tx = p;

??????? MqttTcp_send(tpcb, es);

?

#else

??????? // 將要接收的數據放到BUFF里

??????? if( p->flags == 0 )? //? 如果出現了分包的，在這里時行判斷

??????? {

? ??????????MqttRxOffset = p->len;

????????????? ?? memcpy((void *)MqttRxTemp,p->payload,p->len);

??????? }

??????? else

??????? {

??????????? if( MqttRxOffset )

??????????? {

??????????????? if( (MqttRxBwp + MqttRxOffset) -? MqttRxBrp < MQTTBUFFSIZE?? )

??????????????? {

??????????????????? for( i=0; i<MqttRxOffset; i++ )

??????????????????? {

??????????????????????? MqttRxBuff[MqttRxBwp] = MqttRxTemp[i];

????????????????????? ??MqttRxBwp = (MqttRxBwp+1)%MQTTBUFFSIZE;

??????????????????? }

??????????????? }

???????????????????? ? MqttRxOffset = 0;

??????????? }

??????????? if( (MqttRxBwp + p->tot_len) -? MqttRxBrp < MQTTBUFFSIZE?? )

??????????? {

??????????????? for( i=0; i<p->tot_len; i++)

? ??????????????{

??????????????????? MqttRxBuff[MqttRxBwp] = *((uint8_t *)(p->payload)+i);

??????????????????? MqttRxBwp = (MqttRxBwp+1)%MQTTBUFFSIZE;

??????????????? }

??????????? }??

??????? }

??????? pbuf_free(p);

#endif

??????? ret_err = ERR_OK;

??? }

?

??? /* data received when connection already closed */

??? else

??? {

??????? /* Acknowledge data reception */

??????? tcp_recved(tpcb, p->tot_len);

?

??????? /* free pbuf and do nothing */

??????? pbuf_free(p);

??????? ret_err = ERR_OK;

??? }

??? return ret_err;

}

總結

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