Determine if there is Data left on the socket and discard it

I'm writing an Interface under Linux which gets Data from a TCP socket. The user provides a Buffer in which the received Data is stored. If the provided Buffer is to small I just want to return an Error. First problem is to determine if the Buffer was to small. The recv() function just returns me the amount of bytes actually written into the Buffer. If I use the MSG_TRUNC flag stated on the recv() manpage it still returns me the same. Second problem is to discard the data still queued in the socket. So if I would determine that my provided Buffer was to small I just want to erase everyth开发者_StackOverflow社区ing which is left on the socket. Is there any other ways to do so except Closing and opening the socket again or just receive until nothing is left? Best Regards

Toby

As documented in the man page, MSG_TRUNC is only valid for packet sockets (e.g. UDP) so this will not work as you want for your TCP socket which is stream based. There are literally hundreds of posts on stackoverflow and elsewhere that talk about preserving application message boundaries on TCP (hint: you need to do this yourself, TCP is a byte stream interface and doesn't) so I won't go into the details here, suffice it to say, you need a mechanism to know how big an application "message" or "packet" is on the recv() side to enable you to do what you want over TCP (or you need to switch over to UDP).

For TCP, if you need to "drain" the socket, reading until there is no data left would work, however, again, you need to consider message boundaries as mentioned above so that you do not read through one "message" and start eating into the next (again, most important point to remember is that TCP provides a byte stream interface and will not necessarily preserve your concept of application level packets or messages).