lwip ppp lower physical layer to app layer communication

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lwip ppp lower physical layer to app layer communication

rohanm
Hi,
 
i am working on lwip ppp over serial using gsm.I am successfully able to post to my web server.but

1)On calling lwip_read(s, Get_Post_Buffer_GSM, PPP_BUF_SIZE),it calls the netconn_recv_data function,there is a call to sys_arch_mbox_fetch(&conn->recvmbox, &buf, 0);on line 427
in api_lib.c.when the  function sys_arch.... returns the value inside the buf is zero

2)on the lower side the function call sio_read succesfully reads the expected string and post it using the ip_input
function to the upper layer.

Question:

what would be wrong in my code that the upper TCP layer is not returning the received buffer.that means lwip_read function not returning recevied buffer value. pls suggest.

NOTE:I am succesfully able to post data using this GITHUB code mentioned above.