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Today's Topics:
1. [mingw] #42561: MSVCRT.DLL implementation of POSIX dup2()
function does not conform to POSIX.1 (MinGW Notification List)
----------------------------------------------------------------------
Message: 1
Date: Fri, 25 Jun 2021 18:47:12 +0100
From: MinGW Notification List <[email protected]>
To: OSDN Ticket System <[email protected]>
Subject: [MinGW-Notify] [mingw] #42561: MSVCRT.DLL implementation of
POSIX dup2() function does not conform to POSIX.1
Message-ID: <[email protected]>
Content-Type: text/plain; charset=UTF-8
#42561: MSVCRT.DLL implementation of POSIX dup2() function does not conform to
POSIX.1
Open Date: 2021-06-22 23:34
Last Update: 2021-06-25 18:47
URL for this Ticket:
https://osdn.net//projects/mingw/ticket/42561
RSS feed for this Ticket:
https://osdn.net/ticket/ticket_rss.php?group_id=3917&tid=42561
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Last Changes/Comment on this Ticket:
2021-06-25 18:47 Updated by: keith
Comment:
A reasonably simple work-around would be to provide an in-line implementation
of dup2():
#define dup2 __mingw_posix_dup2 __CRT_ALIAS __cdecl __MINGW_NOTHROW int dup2
(int __fd1, int __fd2) { return ((__fd1 = _dup2( __fd1, __fd2 )) == 0) ? __fd2
: __fd1; }which would override the declaration in <io.h>, (provided this is
provided after <io.h> has been included).
There are several options, (not exhaustive), for provision of such a
work-around:
Do nothing; leave it to the user's discretion, if and when to implement such a
work-around, in their own code. That may be advantageous to me, as mingwrt
maintainer, but is hardly a convenient solution for end users.
Add the work-around within <io.h>, as an unconditional replacement for the
existing dup2() function declaration. This would ensure POSIX.1 conformity, in
any use of the function, when referred to by its POSIX.1 name, but may come as
a surprise to any user who expects the original Microsoft behaviour, in spite
of using the POSIX.1 function name; (this may not be a significant concern,
since such usage cases really should be using Microsoft's preferred, ugly,
non-standard, non-portable alternative name, _dup2(), rather than the POSIX.1
name in a non-conforming context).
Add the work-around within <io.h>, as for option 2 above, but make replacement
of the existing dup2() declaration conditional on some feature test — either a
new one, created specifically for this purpose, or on #if __UNISTD_H_SOURCED__,
(which is already in scope, when <io.h> is included by <unistd.h>). I don't
like this, as a potential solution, because, on the one hand, it would require
definition of yet another non-standard feature test macro — and we already have
more than enough of those — while on the other hand, it would make the
work-around sensitive to header inclusion order; any inclusion of <io.h>,
(either directly, or possibly indirectly through another header, of which there
are several), before including <unistd.h>, would (possibly unexpectedly)
disable the work-around.
Add the work-around within <unistd.h>, after this has included <io.h> itself.
This has the advantage that any code which includes <unistd.h>, (as might be
expected of any POSIX.1 conformant user of the dup2() function), will
automatically activate the work-around, (regardless of any prior inclusion of
<io.h>). If we also make it conditional, on absence of a prior definition of
dup2, (as a macro):
#ifndef dup2 #define dup2 __mingw_posix_dup2 __CRT_ALIAS __cdecl
__MINGW_NOTHROW int dup2 (int __fd1, int __fd2) { return ((__fd1 = _dup2(
__fd1, __fd2 )) == 0) ? __fd2 : __fd1; } #endifthe the user has the option to
disable the work-around, by defining:
#define dup2 dup2before including <unistd.h>, (or any other header which
might do so).
Of these options, the last is my preferred choice, with the second as a
possible alternative; (note that, in the case of the second option, the
existing declaration of dup2() could simply be deleted from <io.h>, and the
definition of dup2(), as a macro, would become unnecessary).
---------------------------------------------------------------------
Ticket Status:
Reporter: keith
Owner: (None)
Type: Issues
Status: Open
Priority: 5 - Medium
MileStone: (None)
Component: WSL
Severity: 5 - Medium
Resolution: None
---------------------------------------------------------------------
Ticket details:
I've been aware of this non-conformity for several years, yet it continues to
trip me up, on occasion. Consider the code fragment:
int fd = open( "foo.txt", O_CREAT | O_EXCL, S_IREAD | S_IWRITE ); if( dup2(
fd, STDOUT_FILENO ) != STDOUT_FILENO ) perror( "open" );On a POSIX.1
conforming platform, (with added support for the BSD standard S_IREAD and
S_IWRITE macros), assuming that "foo.txt" is successfully opened, and bound to
STDOUT_FILENO by the dup2() call, the condition dup2( fd, STDOUT_FILENO ) ==
STDOUT_FILENO will evaluate as TRUE, and the perror() call will not be invoked.
However, on Windows, the same code, (assuming that errno is zero, immediately
prior to its execution), will likely result in output (from perror(), which is
invoked) similar to:
open: Success.
The reason for this disparity, given a function prototype of int dup2( int fd1,
int fd2 ), is that, whereas POSIX.1 stipulates that the return value, following
a successful invocation, shall be equal to fd2, the Microsoft implementation
always returns zero, on success, regardless of the value of fd2.
To be fair to Microsoft, although their documentation does describe dup2() as a
POSIX function (in name only), it indicates that it is a deprecated ("because
[it doesn't] follow the Standard C rules for implementation-specific names" ...
a rationale which would seem to not be endorsed by ISO/IEC, since dup2() is
ratified as an international standard name, within ISO/IEC 9945, as it is in
IEEE 1003.1-2017) alias for Microsoft's non-standard _dup2(), for which the
documentation does indicate the non-conformity w.r.t. the return value
stipulated by POSIX.1; however, given that this catches me out, every time I
have occasion to use dup2(), and I don't think it is unreasonable to expect
POSIX.1 conformity, when I call any function by its POSIX.1 name, I really
would like to see this fixed in the MinGW implementation.
--
Ticket information of MinGW - Minimalist GNU for Windows project
MinGW - Minimalist GNU for Windows Project is hosted on OSDN
Project URL: https://osdn.net/projects/mingw/
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URL for this Ticket:
https://osdn.net/projects/mingw/ticket/42561
RSS feed for this Ticket:
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