Re: [boost] [modular boost] non-linked headers

Subject: Re: [boost] [modular boost] non-linked headers
From: Daniel James (daniel_at_[hidden])
Date: 2013-12-03 03:52:11

On 3 December 2013 02:02, Beman Dawes <bdawes_at_[hidden]> wrote:
> On Mon, Dec 2, 2013 at 7:11 PM, Daniel James <daniel_at_[hidden]> wrote:
>
>> On 3 December 2013 00:08, Bjørn Roald <bjorn_at_[hidden]> wrote:
>> >
>> > yes, but b2 headers create hard links
>>
>> It really should use soft links. Most programs don't change files in
>> place, so as soon as such a change is made the two entries will be
>> pointing to different inodes. Which defeats the purpose, since they
>> should always be the same.
>>
>
> I'm missing something. Could you give an example of what you mean by "Most
> programs don't change files in place"? Hard links ensure that a change is
> always seen by both the entries because there is only one underlying file.
> I ran into that with Visual Studio when I tried symlinks and found that as
> a result Visual Studio failed to realize when a dependency had changed.

I corrected myself a bit later, it isn't "most" programs, but is many,
especially things that involve unix shell scripting or want to update
file atomically. I was also talking about using linux and os x, I
don't know anything about hard links on windows. The problem is that
different programs do different things. There's no consensus on what
the semantic of hard links are - sometimes they are used as a cheap
copy, sometimes to link two directory entries. And most programmers
don't even think about this when writing software, so they usually
stumble into one unknowingly (or even both). If you have a system you
completely control you can ensure one or the other, but once your
software is exposed to the world the other behaviour easily sneaks in.
And because it's subtle, people often don't realise what's happening.

Git deliberately breaks hard links, so that you can create a fast
local clone using hardlinks and then checkout another branch without
disrupting the original. Someone proposed making that an option, and
it was rejected:

http://thread.gmane.org/gmane.comp.version-control.git/97974

For a demonstration, take a clean checkout of master and do this:

./b2 headers
cd libs/detail
git checkout develop -- include/boost/detail/iterator.hpp
cd ../..
diff boost/detail/iterator.hpp libs/detail/include/boost/detail/iterator.hpp

This can be fixed by running 'b2 headers' again, but this isn't
something that should be relied on - people use other build tools. I
also wonder if there's a possibility that 'b2 headers' will choose the
wrong file.

Soft links also have other advantages, they make it clear which module
the header file came from, and it's more immediately obvious that file
is linked - especially in graphical tools.