lwip: Add a configuration option to enable the lwip socket API.

[infinnovation-dev]

Description

The lwip library includes an API to provide functions similar to the
BSD / POSIX socket functions. Porting existing socket code to use
lwip can be considerably simplified by using this API.

mbed-os uses the alternative netconn API, and so by default disables
the socket API.

This change adds a configuration option allowing the user to enable
the lwip socket API.

It also allows the user to define LWIP_COMPAT_SOCKETS, which enables
macro definitions of "connect" as "lwip_connect", etc. To add another
mbed-os option defining LWIP_COMPAT_SOCKETS would be unworkable, since
it would break code such as "eth.connect()".

Status

READY

Migrations

NO

Related PRs

None known.

Todos

• Tests
• Documentation

[0xc0170]

cc @kjbracey-arm @geky

[kjbracey]

Fine by me. @mikaleppanen should double-check too.

(Arguably a lot of stuff in there should be "ifndef", if you want to support people trying to turn stuff on to use it directly.)

[geky]

This seems fine to me as well if users find it useful.

Though we can't make a garuntee that this api will be stable or always exposed. Perhaps a better solution for users who need this api would be to disable the lwip in mbed-os and add lwip as an external library with the user's configuration?

[infinnovation-dev]

@kjbracey-arm Adding some ifndef's for various lwip configuration values was the subject of PR #2550 but I need to do some work on that.

@geky Lack of guarantee noted. We'll cross that bridge if/when we come to it. In the meantime I would find it advantageous to make use of the existing device level glue and allow co-existence of the mbed-os C++ API and the lwip C socket API.

[mikaleppanen]

Fine to me.

[sg-]

I'd prefer we hold off on this for now. We don't expose, document or support C interfaces. We will consider this for the future when if/when docs/tests/a conscious decision to expose C interfaces is had.