Re: [lwip-users] function recv

[David Empson]

Whether or not you do a blocking lwip_recv(), the amount of data returned can be any number of bytes up to the number you specified.

There is no option to make lwip_recv() block until a certain minimum number of bytes have been received.

 

You cannot control the pattern of receive data, as it is affected by many factors beyond your control. You should never assume that data will be received in the same sized chunks as were written by the sender.

 

Your application must be able to deal with receiving only a few of the bytes you need, storing them in your own buffer, then calling lwip_recv() again to get the next chunk, and repeating until you have the number of bytes you want.

 

For the protocol as you describe it, I suggest you set up an 8 byte buffer to receive the length and command code, and do repeated lwip_recv() calls as required to fill that buffer, initially asking for 8 bytes, then if the initial call returns fewer than 8 bytes (and no error), repeat as required, asking for the remaining bytes and adjusting the memory pointer to allow for the bytes already received. Once you have the full 8 bytes, you can interpret the length and command fields, and decide what to do about the data field which follows.

 

If you can't cope with 4 gigabytes of data at once, you will have to do sanity checks on the received length field, and for acceptable but large lengths you may need to use a relatively small buffer and process the data in chunks, rather than trying to receive all the data into a single contiguous buffer. You will also have to make repeated lwip_recv() calls to receive all of the data (up to the total number of bytes specified in your protocol's length field).

 

Error checking will also be required, e.g. to detect premature closure of the connection.

 

----- Original Message -----

From: Oscar F

To: Mailing list for lwIP users

Sent: Wednesday, September 16, 2009 5:30 PM

Subject: [lwip-users] function recv

Hello everybody, i have a question about function recv in a TCP socket connection. In my application i receive several command with diferent lenght. the command is composed for 4 byte of lenght + 4 byte comamnd code + N byte of data.

Does the recv function return me the lenght total, or can be posible to return me less data of the total lenght?

What is the best form to implement this buffer, create a buffer of maximun size (i don´t known exactly the total lenght for command, and i don´t want to create a buffer of 4 byte because is very big?

any suggestion?

thanks
Oscar