Re: Curl handle as a FILE *

From: Timothe Litt <litt_at_acm.org>
Date: Tue, 6 Dec 2022 09:50:35 -0500

On 06-Dec-22 09:24, Patrick Monnerat via curl-library wrote:
>
> On 12/6/22 13:08, Daniel Stenberg wrote:
>> On Tue, 6 Dec 2022, Patrick Monnerat via curl-library wrote:
>>
>>> Please find the source code and use examples of functions
>>> curl_fopen() and curl_fdopen() in attachement.
>>>
>>> These allow associating an URL or a curl handle with a standard C
>>> FILE pointer and use the standard C IO API on it.
>>
>> Fun!
>>
>> Also a quick reminder to readers that if you want an
>> fopen/fread/frite style API for libcurl, there is also this start =>
>> https://github.com/curl/fcurl
>
>
> Thanks for your feedback. And the reminder was a good idea.
>
> Although the goal is the same, I targetted something really compatible
> with C FILE. This allows to easily insert libcurl use in existing
> programs as old as C iitself and use on a handle a much wider range of
> API without having to write wrappers. The real challenge was "FILE *" !
>
> I know fcurl is not complete yet, but noted the method is a bit
> different: curl_fopen plays with the pause flags and unpause only when
> enough buffer is available (that is: when in read or write). I did it
> this way because I support writes too and to prevent buffer overflows.
>
>
> If you try my code on an Apache+PHP-FPM web service, you might
> experience problems while posting: there currently is an Apache bug on
> handling chunked post input. It works perfectly with nginx.
>
> Patrick
>
It's a good idea.

I don't think your seek() semantics are correct.  seek(SEEK_END)
terminates connection, which isn't what seek does, or what a C program
would expect.  seek just positions.  I think that what you're doing maps
to fclose().

seek should return errno = EBADF if not implemented.  EINVAL is for a
bad 'whence' .

The idea of doing range requests based on seek from fcurll's issues has
merit.  May need some support in the library.

You might also provide these as curl_xxx(), using curl handles. While
transparency is nice, you can't always hook the C RTL. curl_xxx could be
the portable version - it's a pretty simple search-and-replace for
users.  And your hooked version, if available at configure time, can be
used where it's available.

I think that would give you the best of both approaches. curl_portable
API that looks like FILE *, or transparent FILE * handles where they are
available.  The internals will be largely the same.

You may also want to consider other key functions, including delete and
rename.  With ftp, at least: chmod, chown...

There's quite a bit to making network operations look exactly like local
files.  But you're definitely on to a useful subset.


Timothe Litt
ACM Distinguished Engineer
--------------------------
This communication may not represent the ACM or my employer's views,
if any, on the matters discussed.

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Received on 2022-12-06