core.sys.posix.unistd: Add XSI legacy bindings

[ibuclaw]

In the unistd module, there are a number of legacy XSI functions, not all are presently available on all supported platforms.

std.experimental also declares a couple of these functions locally (they shouldn't be doing this).

Some thoughts:

1. Should these be labelled as deprecated?
2. Should they be moved to core.sys.<platform-specific>.unistd? Existing functions in core.sys.posix.unistd will then be labelled as deprecated.

[dlang-bot]

Thanks for your pull request, @ibuclaw!

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[maxhaton]

What does not supported mean in this case - completely missing or a runtime error? If we are silently assuming they are there when they aren't, deprecation seems the way (then they should be moved to a platform specific alternative).

Also, what platforms are effected specifically (I'm not all that familiar with XSI beyond the very basics)?

[ibuclaw]

What does not supported mean in this case - completely missing or a runtime error?

Depends on the platform. For Musl, calling sbrk() only sets errno to ENOMEM and nothing else, whereas on DragonFly, it is completely absent.

If we are silently assuming they are there when they aren't, deprecation seems the way (then they should be moved to a platform specific alternative).

Also, what platforms are effected specifically (I'm not all that familiar with XSI beyond the very basics)?

All posix-compliant platforms should implement XSI, but when the baseline supported revision is adjusted - and most probably support version 7 by now - then these functions are #ifdef'd out of the headers.

[RazvanN7]

I think that the simplest way would be to just deprecate them. @ibuclaw care to do that in this PR?

[RazvanN7]

ping @ibuclaw

[RazvanN7]

@ibuclaw How do we move forward here?

[RazvanN7]

@ibuclaw Should we close this?

[Geod24]

Druntime have been merged into DMD. Please re-submit your PR to dlang/dmd repository.