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Groups > comp.lang.c++ > #9414

Re: Create an istream from an fd?

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On 08/29/11 07:03 AM, Nobody wrote:
> On Sun, 28 Aug 2011 15:56:58 +1200, Ian Collins wrote:
>
>>>> (The standard could have required an implementation defined
>>>> typedef for the file descriptor in filebuf, and a constructor
>>>> for that, but as far as I know, no one proposed such a thing.)
>>>
>>> It could have required a version of filebuf::open() (or similar) taking
>>> a FILE*, given that<cstdio>   is part of the standard.
>>
>> Then you would end up with two layers of buffering
>
> I see no reason why you would need two layers of buffering. The streambuf
> can rely upon the stdio layer's buffering; it doesn't have to provide its
> own.

Yet more kludgery.

>> and considering the similarity between C's fopen and constructing an
>> fstream,  gain very little.
>
> fopen() and freopen() are typically not the only way to get a FILE*, even
> if they're the only mechanisms specified by the standard.

They also can't be used for things like sockets, pipes and doors.

> It's one thing to only specify the most portable mechanisms for creating a
> stdio stream, but another thing to assume that no other mechanisms exist,
> effectively treating stdio as a hermetically-sealed unit which isn't to be
> extended.
>
> As it stands, any implementation wishing to extend the mechanisms for
> creating streams has to do so twice: once for cstdio, once for ios.

Why?  In C they'd extend stdio, in C++ they can use streambufs.

>> It also wouldn't solve one of the most common (at least for me)
>> requirements: a stream over an existing file descriptor that isn't a
>> regular file.
>
> fdopen() doesn't care whether the descriptor refers to a regular file.
> fdopen() isn't portable, but then neither are descriptors.

So? fopen() can't be used for to open them either.

> And whatever's there for stdin (etc) and cin (etc) already has to deal
> with whatever the standard descriptors are associated with, whether a
> file, tty, pipe, socket, etc.

Which isn't exposed to the user.  Unless the user can create a FILE* 
from one of those, the internal mechanism isn't much use.

C++ already provides the mechanism to extend std::streambuf (the 
appropriate virtual members) and using it it trivial.

-- 
Ian Collins

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Thread

Create an istream from an fd? Noah Roberts <roberts.noah@gmail.com> - 2011-08-26 08:54 -0700
  Re: Create an istream from an fd? Leigh Johnston <leigh@i42.co.uk> - 2011-08-26 17:05 +0100
  Re: Create an istream from an fd? Victor Bazarov <v.bazarov@comcast.invalid> - 2011-08-26 12:20 -0400
  Re: Create an istream from an fd? Vaclav Zeman <vzeman79@sh.cvut.cz> - 2011-08-26 18:34 +0200
  Re: Create an istream from an fd? Ian Collins <ian-news@hotmail.com> - 2011-08-27 08:48 +1200
  Re: Create an istream from an fd? "Alf P. Steinbach" <alf.p.steinbach:usenet@gmail.com> - 2011-08-27 01:26 +0200
    Re: Create an istream from an fd? Ian Collins <ian-news@hotmail.com> - 2011-08-27 11:30 +1200
      Re: Create an istream from an fd? "Alf P. Steinbach" <alf.p.steinbach:usenet@gmail.com> - 2011-08-27 02:46 +0200
        Re: Create an istream from an fd? Ian Collins <ian-news@hotmail.com> - 2011-08-28 10:52 +1200
          Re: Create an istream from an fd? James Kanze <james.kanze@gmail.com> - 2011-08-27 16:35 -0700
            Re: Create an istream from an fd? Nobody <nobody@nowhere.com> - 2011-08-28 04:31 +0100
              Re: Create an istream from an fd? Ian Collins <ian-news@hotmail.com> - 2011-08-28 15:56 +1200
                Re: Create an istream from an fd? Nobody <nobody@nowhere.com> - 2011-08-28 20:03 +0100
                Re: Create an istream from an fd? Ian Collins <ian-news@hotmail.com> - 2011-08-29 07:52 +1200
              Re: Create an istream from an fd? James Kanze <james.kanze@gmail.com> - 2011-08-29 03:54 -0700
                Re: Create an istream from an fd? Leigh Johnston <leigh@i42.co.uk> - 2011-08-29 13:55 +0100
                Re: Create an istream from an fd? Nobody <nobody@nowhere.com> - 2011-08-29 20:12 +0100
                Re: Create an istream from an fd? Ian Collins <ian-news@hotmail.com> - 2011-08-30 07:49 +1200
                Re: Create an istream from an fd? Nobody <nobody@nowhere.com> - 2011-08-30 04:40 +0100
                Re: Create an istream from an fd? Ian Collins <ian-news@hotmail.com> - 2011-08-30 15:51 +1200
                Re: Create an istream from an fd? Nobody <nobody@nowhere.com> - 2011-08-31 00:03 +0100
                Re: Create an istream from an fd? Ian Collins <ian-news@hotmail.com> - 2011-08-31 11:55 +1200
                Re: Create an istream from an fd? Nobody <nobody@nowhere.com> - 2011-09-02 00:50 +0100
            Re: Create an istream from an fd? Ian Collins <ian-news@hotmail.com> - 2011-08-28 16:04 +1200
  Re: Create an istream from an fd? Nobody <nobody@nowhere.com> - 2011-08-27 02:40 +0100

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