errno reset when using thread enabled gcc

errno reset when using thread enabled gcc

[Anurag Sharma]

Hi All,

I have come across a strange behaviour using native gcc on cygwin. I
understand that this issue is not 100% relevant to this list, but I am
hoping some one might have come across this issue before. Apologies for
any incovinience. I have posted the same on newlib mailing list too.

Consider this short code:

   int fd = ::open("/tmp/errnotest", O_RDONLY | O_BINARY);
    
    if (fd == -1) {
	std::cerr << "open failed, errno=" << errno << ", strerror=" <<
strerror(errno) << std::endl;
	std::cerr << "open failed, errno=" << errno << ", strerror=" <<
strerror(errno) << std::endl;
    }

If I build the gcc compiler by providing --enable-threads=single option
in the configure stage, following is the output :

open failed, errno=2, strerror=No such file or directory
open failed, errno=2, strerror=No such file or directory
 
This is of course expected.

Hoewever if I build the compiler by providing --enable-threads=posix
option in the configure stage, the output is as follows :

open failed, errno=2, strerror=No such file or directory
open failed, errno=0, strerror=No error

That is , after the first call to errno, it is reset!!

Has anyone else experiened this before? Can anyone please help find out
what exactly is going wrong?

Regards,

Anurag

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Re: errno reset when using thread enabled gcc

[Igor Pechtchanski]

On Thu, 17 Oct 2002, Anurag Sharma wrote:

> Hi All,
>
> I have come across a strange behaviour using native gcc on cygwin. I
> understand that this issue is not 100% relevant to this list, but I am
> hoping some one might have come across this issue before. Apologies for
> any incovinience. I have posted the same on newlib mailing list too.
>
> Consider this short code:
>
>    int fd = ::open("/tmp/errnotest", O_RDONLY | O_BINARY);
>
>     if (fd == -1) {
>         std::cerr << "open failed, errno=" << errno << ", strerror=" << strerror(errno) << std::endl;
>         std::cerr << "open failed, errno=" << errno << ", strerror=" << strerror(errno) << std::endl;
>     }
>
> If I build the gcc compiler by providing --enable-threads=single option
> in the configure stage, following is the output :
>
> open failed, errno=2, strerror=No such file or directory
> open failed, errno=2, strerror=No such file or directory
>
> This is of course expected.
>
> Hoewever if I build the compiler by providing --enable-threads=posix
> option in the configure stage, the output is as follows :
>
> open failed, errno=2, strerror=No such file or directory
> open failed, errno=0, strerror=No error
>
> That is , after the first call to errno, it is reset!!
>
> Has anyone else experiened this before? Can anyone please help find out
> what exactly is going wrong?
>
> Regards,
>
> Anurag

Anurag,

While I don't know for sure that this is what's happening, have you
considered that operator<< or endl may (re)set errno?
	Igor
-- 
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      |\      _,,,---,,_		pechtcha@cs.nyu.edu
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     |,4-  ) )-,_. ,\ (  `'-'		Igor Pechtchanski
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Re: errno reset when using thread enabled gcc

[Anurag Sharma]

On Thu, 17 Oct 2002 10:09:26 -0400 (EDT)
Igor Pechtchanski <pechtcha@cs.nyu.edu> wrote:

> On Thu, 17 Oct 2002, Anurag Sharma wrote:
> 
> > Hi All,
> >
> > I have come across a strange behaviour using native gcc on cygwin. I
> > understand that this issue is not 100% relevant to this list, but I am
> > hoping some one might have come across this issue before. Apologies
> > for any incovinience. I have posted the same on newlib mailing list
> > too.
> >
> > Consider this short code:
> >
> >    int fd = ::open("/tmp/errnotest", O_RDONLY | O_BINARY);
> >
> >     if (fd == -1) {
> >         std::cerr << "open failed, errno=" << errno << ", strerror="
> >         << strerror(errno) << std::endl; std::cerr << "open failed,
> >         errno=" << errno << ", strerror=" << strerror(errno) <<
> >         std::endl;
> >     }
> >
> > If I build the gcc compiler by providing --enable-threads=single
> > option in the configure stage, following is the output :
> >
> > open failed, errno=2, strerror=No such file or directory
> > open failed, errno=2, strerror=No such file or directory
> >
> > This is of course expected.
> >
> > Hoewever if I build the compiler by providing --enable-threads=posix
> > option in the configure stage, the output is as follows :
> >
> > open failed, errno=2, strerror=No such file or directory
> > open failed, errno=0, strerror=No error
> >
> > That is , after the first call to errno, it is reset!!
> >
> > Has anyone else experiened this before? Can anyone please help find
> > out what exactly is going wrong?
> >
> > Regards,
> >
> > Anurag
> 
> Anurag,
> 
> While I don't know for sure that this is what's happening, have you
> considered that operator<< or endl may (re)set errno?
> 	Igor

Yes Igor,

It is the STL iostream operators which are resetting it. Using the
threading model of posix does make the STL thread safe, but why should it
affect errno? 

Thanks for your response.

Reagrds,
Anurag

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