Re: [lwip-users] Why does sys_sem_wait_timeout exist?

[Atte Kojo]

Jonathan Larmour wrote:
> Packet-oriented netbufs can be used more efficiently than streams, and 
> you can't do things like netbuf_ref() etc. with streams.
I'm afraid that I don't quite understand this. Could you elaborate a bit 
more?

As far as I understand, when running in flat memory "zero-copy" is more 
of an illusion than anything else. Right now writes/sends on UDP sockets 
with socket API are already blocking and don't copy any data between 
pbufs (uses netbuf_ref i.e. PBUF_REF). TCP writes could very easily be 
made "zero-copy" also with a non-standard option for send (like 
MSG_DONTCOPY, which would then use netbuf_ref/PBUF_REF under the covers).

In any case not copying data for performance reasons is kind of a moot 
point as the data must be gone through at least once when calculating 
checksums. Doing a copy-on-checksum takes almost negligible performance 
hit (depending on memory allocator implementation) so only real case for 
not copying data at all is for very memory-constrained systems where the 
user wants to avoid copying data from external ROM to RAM. And, as I 
said earlier, this can be handled very easily.

PS. The main reason I'm so strongly advocating this is that I don't like 
having a nonstandard API where a standard one would do. Using a socket 
API that behaves just like its real counterpart is a lot easier for new 
users (and us old ones also). Plus the netconn-API lacks select() which 
makes single-thread servers a gazillion times easier to implement.