Re: [Mingw-users] Missing pread, pwrite

[Keith M. <kei...@us...>]

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On 22/03/17 20:00, Antonio Diaz Diaz wrote:
> This has been asked before, but never answered AFAICT.

Really?  I could find only one reference:
https://sourceforge.net/p/mingw/mailman/mingw-users/thread/200...@me.../

That, (asked in one of those hideous fora which core MinGW developers 
detested, and mostly ignored), seeks advice on where to find pread() 
and pwrite() for MinGW, (which at the time was nowhere, because they 
don't exist within MSVCRT.DLL), or what alternatives might exist; it 
doesn't appear to explicitly ask:

> Please, could pread/pwrite be implemented in MinGW?

There is one reply to that forum posting: it offers a naive, untested, 
and fundamentally broken, (because of the race conditions it exposes), 
implementation for pread(); there is no evidence of any associated 
feature request, or of any actual code submission to MinGW.org, via 
any other channel.

pread() and pwrite() are POSIX.1 functions, implemented only as XSI 
options, prior to their incorporation into the base specification as 
of POSIX.1-2008.  Windows is not a POSIX conforming system, and MinGW 
does not aim to make it so.  At the time of that original posting, in 
2005, when these functions were specified only as XSI options, it is 
most unlikely that any request to support their incorporation into 
MinGW would have been favourably considered by the lead developers 
of that era; even today, when I have a more favourable attitude to 
adoption of POSIX.1 features, without a formal feature request, 
accompanied by a robust implementation, there may be no compelling 
incentive for MinGW developers to pursue this.

> The work is already done.  Hannes Domani implemented his own
> versions of the pread and pwrite functions in order to compile
> plzip on Windows using MinGW.  AFAIK, they have been working
> in plzip without problems for more than 3 years.

An implementation deployed only within the context of one application 
doesn't attest to its suitability for general adoption; in addition to 
a formal feature request[1], with accompanying implementation, we would 
require unit tests, (ideally implemented via autotest), to confirm its 
conformance to POSIX.1-2008, its immunity to race conditions when used 
in conjunction with open(), read(), and write() in distinct threads, 
and its proper handling of associated error conditions.

[1] https://sourceforge.net/p/mingw/bugs/new/  (set Type = "Feature").

- -- 
Regards,
Keith.

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