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Date: Mon, 13 Dec 2004 12:09:43 +0100 (MET) 

From: "Michael Kerrisk" <mtk-manpages@gmx.net>
To: Andries Brouwer <Andries.Brouwer@cwi.nl>
Subject: Re: errno

Hi Andries,

> On Fri, Dec 10, 2004 at 05:07:36PM +0100, Michael Kerrisk wrote:
> 
> > I added this text to fcntl.2:
> > 
> >     BUGS
> >        A  limitation of the Linux system call conventions means that
> >        if a (negative) process group ID to be returned  by  F_GETOWN
> >        falls  in  the  range  -1  to -4095, then the return value is
> >        wrongly interpreted by glibc as an error in the system  call;
> >        that  is,  the  return value of fcntl() will be -1, and errno
> >        will contain the (positive) process group ID.
> 
> Yes.
> 
> (Maybe glibc always did this, early libc considered any negative
> return value an error. On the other hand, not all the world is an i386 -
> IBM has just decided that we don't need any i386's anymore
> and sold their stuff to the Chinese - we must use PPC, as Linus
> does already - and on other architectures we do not have this
> ugliness, I think.)
> 
> You might consider adding "i386" somewhere:
>  A limitation of the Linux i386 system call conventions ...

Some testing on ia64 (RedHat EL 3.0, 2.4.21) and 
alpha (2.4.18, Debian 3.0) showed that any negative PGID value
causes F_GETOWN to fail.

My limited reading of the ia64 source:

sysdeps/unix/sysv/linux/ia64/sysdep.h

shows that there is a comment about the -4095 value there, 
but that doesn't seem to reflect the reality of the code.

Reading the source, the -4095 limit seems to hold on some 
other architectures, e.g.:

sysdeps/unix/sysv/linux/m68k/sysdep.h
sysdeps/unix/sysv/linux/hppa/sysdep.h
sysdeps/unix/sysv/linux/s390/s390-32/sysdep.h
sysdeps/unix/sysv/linux/s390/s390-64/sysdep.h
sysdeps/unix/sysv/linux/x86_64/sysdep.h

Unfortunately, I have no non-x86 systems other than the above 
alpha and ia64 (HP-testdrive) on which I can test.

I modified the text a little:

   BUGS
       A  limitation  of the Linux system call conventions on some
       architectures (notably x86) means that if a (negative) pro&#8208;
       cess group ID to be returned by F_GETOWN falls in the range
       -1 to -4095, then the return value is  wrongly  interpreted
       by  glibc  as  an  error  in  the system call; that is, the
       return value of fcntl() will be -1, and errno will  contain
       the (positive) process group ID.

I've left a FIXME in the man page source noting that details have
yet to be sorted out for ia64, alpha, etc.
This commit is contained in:
Michael Kerrisk 2004-12-13 11:32:37 +00:00
parent d3c8b3e9b7
commit 8505c1f1d6
1 changed files with 8 additions and 2 deletions
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10
man2/fcntl.2
View File

@ -729,16 +729,22 @@ Clearly,
alone is not going to be very useful if the process holding the lock
may live on a different machine.
.SH BUGS
A limitation of the Linux system call conventions means that if
a (negative) process group ID to be returned by
A limitation of the Linux system call conventions on some
architectures (notably x86) means that if a (negative)
process group ID to be returned by
.B F_GETOWN
falls in the range -1 to -4095, then the return value is wrongly
interpreted by glibc as an error in the system call;
.\" glibc source: sysdeps/unix/sysv/linux/i386/sysdep.h
that is, the return value of
.BR fcntl ()
will be \-1, and
.I errno
will contain the (positive) process group ID.
.\" FIXME Dec 04: some limited testing on alpha and ia64 seems to
.\" indicate that ANY negative PGID value will cause F_GETOWN
.\" to misintepret the return as an error.Some other architectures
.\" seem to have the same range check as x86. -- MTK
.SH "CONFORMING TO"
SVr4, SVID, POSIX, X/OPEN, BSD 4.3. Only the operations F_DUPFD,
F_GETFD, F_SETFD, F_GETFL, F_SETFL, F_GETLK, F_SETLK and F_SETLKW are