Xen devel - Mar 2005 - [PATCH] libxen-3.0 (libxc rewrite)

Hi all,

I''ve been doing a lot of work on libxc.  I''ve got it to the
point that
I''m ready to share.  Below are the major changes.  Feedback is greatly 
appreciated, especially with respect to things that would be required 
for it to be integrated into the xen-unstable tree.

 o Rename libxc => libxen
 o Use pkg-config to control versioning and parallel installs
 o Use autoconf to detect dependencies, provide separate build 
directory, cross-compile
 o Use doxygen to autogenerate HTML documentation
 o Organize hypercalls by groups in separate headers (dom.h, evtchn.h, 
etc.).
 o Provide consistent error semantics for all functions (-errno is 
returned on error).
 o Use pyrex to autogenerate python bindings
 o Provide high-level python interface (Xen() and Domain() classes 
instead of direct hypercalls)

Below is a URL for a tarball and another URL for the doxygen generate 
documentation.

http://www.cs.utexas.edu/users/aliguori/libxen-3.0.0.tar.gz

http://www.cs.utexas.edu/users/aliguori/libxen-3.0.0/files.html

Regards,
Anthony Liguori

Signed-off-by: Anthony Liguori


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Anthony Liguori wrote:
> Hi all,
> 
> I''ve been doing a lot of work on libxc.  I''ve got it to
the point that
> I''m ready to share.  Below are the major changes.  Feedback is
greatly
> appreciated, especially with respect to things that would be required 
> for it to be integrated into the xen-unstable tree.
> 
> o Rename libxc => libxen
Is there any good reason for this? Will this include renaming all the 
xc_* symbols as well? This will break a lot of code in a lot of places, 
so I think the motivation needs to be more than just feel-good value.
> o Use pkg-config to control versioning and parallel installs
Don''t know what that is or why I need it, but I suppose that means yet 
another dependency added. I am against adding dependencies. Used to be I 
could just untar xen and type make, now I need to install most of 
f*cking Gnome, that horrible Twisted-library and I don''t know what, I 
think we are headed in the wrong direction with all this. This is a 
kernel-level project, not Freshmeat Greatest.
> o Use autoconf to detect dependencies, provide separate build directory, 
> cross-compile
I like having a separate build-directory, I still think at non-broken 
build tool (i.e. not make) could have done the job and done it better. 
The whole .d or .deps approach (pollution of source or build-tree with a 
static version of information that could and should be determined at 
run-time) is a gross hack, even MS Visual Studio can do better.

Here is my (a little out of date) Jamfile for libxc btw:

-----------------
SubDir TOP tools libxc  ;
SubDirHdrs $(TOP) tools libxutil ;

Library libxc :
xc_atropos.c
xc_bvtsched.c
xc_domain.c
xc_evtchn.c
xc_io.c
xc_linux_build.c
xc_linux_restore.c
xc_linux_save.c
xc_misc.c
xc_physdev.c
xc_plan9_build.c
xc_private.c
xc_rrobin.c
;
------------------
> o Use doxygen to autogenerate HTML documentation
Will this be optional, or part of the standard build process? Will the 
comments still be human-readable? If find the code and comments in libxc 
fairly easy to read and understand inline, isn''t doxygen overkill for 
this small amount of code? Will I be able to build xen and libxc without 
installing doxygen on my system?
> o Organize hypercalls by groups in separate headers (dom.h, evtchn.h, 
> etc.).
> o Provide consistent error semantics for all functions (-errno is 
> returned on error).
Fine with me.
> o Use pyrex to autogenerate python bindings
If it reduces the code size I guess it makes sense, hopefully I will 
still be able to compile the raw library without compiling the bindings, 
and without having pyrex installed? Please also consider that sometimes 
I and others need to build (not run, obviously) this stuff on boxes 
where we do not have root-access, and the more stuff that needs to be 
installed, the less likely this is to work.  This is not a contrived 
example, when I visited Cambridge (yes, the home of Xen) last year, I 
was doing Xen-development from a regular user account, without having 
root-access.

I still like vm-tools though :-)

Best regards,
Jacob


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Jacob Gorm Hansen wrote:
>> o Rename libxc => libxen
>
>
> Is there any good reason for this? Will this include renaming all the 
> xc_* symbols as well? This will break a lot of code in a lot of 
> places, so I think the motivation needs to be more than just feel-good 
> value.
The API is completely different.  This interface is a thin wrapper 
around the hypercalls.  All the code needs to be rewritten anyway.  I''m
a big believer that things should be self-documenting.  xc seems like a 
bad prefix because it''s pretty much meaningless (I guess it''s
supposed
to be xen-control or something).
>> o Use pkg-config to control versioning and parallel installs
>
>
> Don''t know what that is or why I need it, but I suppose that means
yet
> another dependency added. I am against adding dependencies. Used to be 
> I could just untar xen and type make, now I need to install most of 
> f*cking Gnome, that horrible Twisted-library and I don''t know
what, I
> think we are headed in the wrong direction with all this. This is a 
> kernel-level project, not Freshmeat Greatest.
You may be aware that many libraries distribution custom -config 
scripts.  pkg-config is a unified interface for -config scripts.  If 
your system doesn''t have pkg-config, no harm is done.  It''s
not checked
for in configure because it''s not required to build.  If there really
is
a strong desire to have a -config script instead of using pkg-config, 
that''s fine.  That seems a bit odd though.

Using pkg-config just means that you put a file in a known location 
(${prefix}/lib/pkgconfig) that describes your libraries CFLAGS and 
LDFLAGS.  Otherwise, you have to install your own -config script in 
${prefix}/bin.

Some sort of -config mechanism is necessary for libraries.  Otherwise, a 
developer has no chance of being able to link against your software 
unless you always install in /usr/include and /usr/lib (which is not a 
very good thing to require).

I feel your pain, I agree with you completely :-)  If you don''t believe
me, take a busy box chroot and ./configure libxen.  Everything will work 
just fine :-)
>> o Use doxygen to autogenerate HTML documentation
>
>
> Will this be optional, or part of the standard build process? Will the 
> comments still be human-readable? If find the code and comments in 
> libxc fairly easy to read and understand inline, isn''t doxygen 
> overkill for this small amount of code? Will I be able to build xen 
> and libxc without installing doxygen on my system?
Yes, yes, no, yes.  If you don''t have doxygen, it''s not used. 
The
comments are as human-readable as they could possibly be.  libxc 
currently has almost no interface level comments.  Documentation of all 
public interfaces is a requirement for EAL certification and doing it 
now with doxygen is going to save a lot of headache in the future when 
someone has to do this.

Not to mention the fact that trying to write a robust management tool 
without interface-level documentation is painful.  It basically requires 
sifting through the Xen code to find which errors can be returned in 
what circumstances.

We basically have three options:

1) Use a standard format for documenting APIs and use a tool to extract 
that documentation
2) Maintain a separate document documenting APIs
3) Sparsely document header files

1 & 2 seem to be the only reasonable things to do, and 1 is going to be 
a lot easier to deal with in the long run.
> o Use pyrex to autogenerate python bindings
>
> If it reduces the code size I guess it makes sense, hopefully I will 
> still be able to compile the raw library without compiling the 
> bindings, and without having pyrex installed? Please also consider 
> that sometimes I and others
Yes.  If you don''t have pyrex or python the bindings won''t be
built.
Moreover, pyrex generates C code and the build process is completely 
python free so the generated C files could be included as part of the 
source tarballs (much like configure is autogenerated and yet still 
usually included in source tarballs).
> need to build (not run, obviously) this stuff on boxes where we do not 
> have root-access, and the more stuff that needs to be installed, the 
> less likely this is to work.  This is not a contrived example, when I 
> visited Cambridge (yes, the home of Xen) last year, I was doing 
> Xen-development from a regular user account, without having root-access.
Yes, that''s one of the reasons to use autoconf.  It lets you specify 
non-standard locations for header files so you can do just what you 
describe.

Take a look at this library, I think you''ll find that it provides more 
support for the things you''re concerned about than libxc currently
does.

Regards,
Anthony Liguori


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> I''ve been doing a lot of work on libxc.  I''ve got it to
the
> point that 
> I''m ready to share.  Below are the major changes.  Feedback 
> is greatly 
> appreciated, especially with respect to things that would be required 
> for it to be integrated into the xen-unstable tree.
> 
>  o Rename libxc => libxen
>  o Use pkg-config to control versioning and parallel installs
>  o Use autoconf to detect dependencies, provide separate build 
> directory, cross-compile
>  o Use doxygen to autogenerate HTML documentation
>  o Organize hypercalls by groups in separate headers (dom.h, 
> evtchn.h, 
> etc.).
>  o Provide consistent error semantics for all functions (-errno is 
> returned on error).
>  o Use pyrex to autogenerate python bindings
>  o Provide high-level python interface (Xen() and Domain() classes 
> instead of direct hypercalls)
Is there a good place to get pyrexc from? I couldn''t obviously spot an
RPM in FC2 or FC3.

I''ve built the C stuff and will look through it. There are a bunch of
semantic changes we want to make to the dom0 op interface (SMP and misc
cleanups, finer grained access control etc), but it might make sense to
integrate your stuff first.

Do you reckon you could get xend and friends building against the pyrex
wrapped libraries? 

Ian






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On Mon, 21 Mar 2005 15:25:09 -0600, Anthony Liguori <aliguori@us.ibm.com>
wrote:
> o Provide consistent error semantics for all functions (-errno is
> returned on error).
Why -errno?  What''s wrong with regular errno, like used everywhere
else in userspace?

    christian


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On 21 Mar 2005, at 21:25, Anthony Liguori wrote:
> I''ve been doing a lot of work on libxc.  I''ve got it to
the point that
> I''m ready to share.  Below are the major changes.  Feedback is
greatly
> appreciated, especially with respect to things that would be required 
> for it to be integrated into the xen-unstable tree.
Is there a particular reason for opening /proc/xen/privcmd on every 
invocation, rather than keeping a handle continuously open? Also I 
rather liked the xc_ name tags: the tag was a nice indication that I 
had reached the bottom of a tools call path and was about to hit Xen. 
The -errno return values are bizarre: user space has the errno variable 
for conveying this information.

Overall the library seems to be a greater modification of libxc than I 
would have thought necessary. It basically looks like a rewrite to 
pretty much the same interface but with names and copyrights changed.

  -- Keir



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Ian Pratt wrote:
>Is there a good place to get pyrexc from? I couldn''t obviously spot
an
>RPM in FC2 or FC3.
>  
>
It seems that the RPMS are part of the standard FC3 distro. 

http://download.fedora.redhat.com/pub/fedora/linux/core/3/i386/os/Fedora/RPMS/Pyrex-0.9.2.1-2.noarch.rpm
>I''ve built the C stuff and will look through it. There are a bunch
of
>semantic changes we want to make to the dom0 op interface (SMP and misc
>cleanups, finer grained access control etc), but it might make sense to
>integrate your stuff first.
>
>Do you reckon you could get xend and friends building against the pyrex
>wrapped libraries? 
>  
>
Sure, it would take a bit more work though.  I''ve not spent much time
on
the python interfaces.

Regards,
Anthony Liguori
>Ian
>
>
>
>
>
>  
>


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Christian Limpach wrote:
>On Mon, 21 Mar 2005 15:25:09 -0600, Anthony Liguori
<aliguori@us.ibm.com> wrote:
>  
>
>>o Provide consistent error semantics for all functions (-errno is
>>returned on error).
>>    
>>
>
>Why -errno?  What''s wrong with regular errno, like used everywhere
>else in userspace?
>  
>
errno''s a global variable and it''s way to easy to lose
it''s value.  For
instance:

if (read(fd, buffer, sizeof(buffer)) == -1) {
    close(fd);
    perror("read");
}

Doesn''t do what you''d expect.  It gets particularly hairy when
you''re
not sure what functions modify errno and which functions don''t.

Regards,
Anthony Liguori
>    christian
>
>  
>


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> It seems that the RPMS are part of the standard FC3 distro. 
> 
> http://download.fedora.redhat.com/pub/fedora/linux/core/3/i386
> /os/Fedora/RPMS/Pyrex-0.9.2.1-2.noarch.rpm
Capital ''P'' in Pyrex. How annoying...

Looks like its not part of FC2, but I guess we could live with this.
 
> >Do you reckon you could get xend and friends building 
> against the pyrex
> >wrapped libraries? 
> >  
> >
> Sure, it would take a bit more work though.  I''ve not spent 
> much time on the python interfaces.
Given that at least 2.1 is going to be based on xend, I think this would
be a worthwhile thing to do.

Ian


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Keir Fraser wrote:
>
> On 21 Mar 2005, at 21:25, Anthony Liguori wrote:
>
>> I''ve been doing a lot of work on libxc.  I''ve got it
to the point
>> that I''m ready to share.  Below are the major changes. 
Feedback is
>> greatly appreciated, especially with respect to things that would be 
>> required for it to be integrated into the xen-unstable tree.
>
>
> Is there a particular reason for opening /proc/xen/privcmd on every 
> invocation, rather than keeping a handle 
It''s not opened on every invocation.  It uses lazy evaluation to only 
open it once for the library.  I should have perhaps documented that a 
little better.
> continuously open? Also I rather liked the xc_ name tags: the tag was 
> a nice indication that I had reached the 
I don''t mind adding a common prefix back.  Do other agree with this?  
What about using xen_ instead of xc_?
> bottom of a tools call path and was about to hit Xen. The -errno 
> return values are bizarre: user space has the errno variable for 
> conveying this information.
I can''t speak for every library, but I think it''s pretty rare
to
actually return errno in userspace.  It just makes life so much easier.
> Overall the library seems to be a greater modification of libxc than I 
> would have thought necessary. It basically looks like a rewrite to 
> pretty much the same interface but with names and copyrights changed.
The goals were to provide a interface tightly tied to the actual 
hypercalls.  I know it''s probably a lot more than you expected but
there
were a lot of problems with libxc.  I can attest to this from trying to 
develop tools off of them.

Here were some of the problems with libxc:

1) Inconsistent errors
   - some calls returned errno from the hypercall, some returned errno 
from the call that failed, some set errno themselves, some never touched 
errno and made calls that would squash the hypercall''s errno.
   - some calls just printf()''d an error message and squashed errno

2) Inconsistent mlock()''ing
   - some functions required arguments to be mlock()''d and some did it 
for you.

3) Poor typing
   - some functions used the wrong types in the interface (like passing 
an int instead of a domid_t for xc_domain_create()).

4) Lack of documentation

5) Poor developer support
   - Assumes headers and libs are in /usr/{include, lib}

So, making all of these changes touched every function.  Especially 
documenting every error condition.  So it was significantly easier to 
just start with fresh code.
>  -- Keir
>
>


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Jeremy Katz wrote:
>Note that Pyrex generates some pretty crappy code (to the point that it
>won''t build at all with gcc4).  The dbus guys are seriously
considering
>switching away from pyrex-generated bindings to something hand done.
>  
>
Do you have a suggestion for an alternative?  I know there are a number 
of binding-generators but I thought Pyrex was the most widely used.

Regards,
Anthony Liguori
>Jeremy
>
>
>  
>


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On Tue, 2005-03-22 at 09:18 -0600, Anthony Liguori
wrote:
> Jeremy Katz wrote:
> >Note that Pyrex generates some pretty crappy code (to the point that it
> >won''t build at all with gcc4).  The dbus guys are seriously
considering
> >switching away from pyrex-generated bindings to something hand done.
> >  
> Do you have a suggestion for an alternative?  I know there are a number 
> of binding-generators but I thought Pyrex was the most widely used.
Pyrex is probably the most generally used -- the stuff pygtk uses is
nice, but not very general.

In most cases, though, bindings done by hand end up being higher quality
than something that''s just being generated.

Jeremy



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Jeremy Katz wrote:
>On Tue, 2005-03-22 at 09:18 -0600, Anthony Liguori wrote:
>  
>
>>Do you have a suggestion for an alternative?  I know there are a number 
>>of binding-generators but I thought Pyrex was the most widely used.
>>    
>>
>
>Pyrex is probably the most generally used -- the stuff pygtk uses is
>nice, but not very general.
>
>In most cases, though, bindings done by hand end up being higher quality
>than something that''s just being generated.
>  
>
I was afraid that that would be your answer.

I would really like to automatically generate the bindings.  There is 
more C code in the current libxc/libxu python bindings than all of vm-tools.

gcc4 is still a bit off and I imagine that Pyrex will be updated as 
needed.  If it ever proves to be painful to deal with, I''ll break down 
and add bindings by hand.  Does that seem like a reasonable plan moving 
forward?

Regards,
Anthony Liguori
>Jeremy
>
>
>  
>


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> >Why -errno?  What''s wrong with regular errno, like used
everywhere
> >else in userspace?
> >  
> errno''s a global variable and it''s way to easy to lose
it''s
> value.  For 
> instance:
> 
> if (read(fd, buffer, sizeof(buffer)) == -1) {
>     close(fd);
>     perror("read");
> }
> 
> Doesn''t do what you''d expect.  It gets particularly hairy
when you''re
> not sure what functions modify errno and which functions don''t.
Personally, I think the best approach is to stick with the existing C
convention that everyone is already well familiar with, and have a
separate errno variable. This means we can then have functions return
pointers etc rather than having to pass them by reference, which is
undeniably ugly. 

I notice that you store the fd of the priv_cmd in a static variable. I
guess this is OK, but I think I still prefer a way of explicit way of
closing the fd. You''d also have to be a little bit careful about
someone
forking then two guys trying to open the fd at the same time.

Ian



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On Tue, Mar 22, 2005 at 09:04:47AM -0600, Anthony Liguori
wrote:
> Christian Limpach wrote:
> >On Mon, 21 Mar 2005 15:25:09 -0600, Anthony Liguori
<aliguori@us.ibm.com>
> >wrote:
> >>o Provide consistent error semantics for all functions (-errno is
> >>returned on error).
> >Why -errno?  What''s wrong with regular errno, like used
everywhere
> >else in userspace?
> >
> errno''s a global variable and it''s way to easy to lose
it''s value.  For
> instance:
> 
> if (read(fd, buffer, sizeof(buffer)) == -1) {
>    close(fd);
>    perror("read");
> }
> 
> Doesn''t do what you''d expect.  It gets particularly hairy
when you''re
> not sure what functions modify errno and which functions don''t.
I think it does what I expect.  And it seems to work for a lot of
libraries just fine.  By not using the global errno, you''re preventing
people from using perror, warn, err and the likes.  Also some of the
interfaces in your library are slightly awkward because you''re wasting
the return parameter to return the failure reason.

Even if we don''t use the global errno, I''m still wondering why
you''re
returning -errno and not errno?

    christian



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Christian Limpach wrote:
>I think it does what I expect.  And it seems to work for a lot of
>libraries just fine.  By not using the global errno, you''re
preventing
>people from using perror, warn, err and the likes.  Also some of the
>interfaces in your library are slightly awkward because you''re
wasting
>the return parameter to return the failure reason.
>  
>
You''re right.  Some of the interfaces are a little awkward (especially 
the memory mapping ones).  It seemed like a reasonable trade-off to make 
though.
>Even if we don''t use the global errno, I''m still wondering
why you''re
>returning -errno and not errno?
>  
>
Good question.  I guess since we never returned > 0 it would be 
reasonable to return errno instead of -errno.  -errno is the convention 
used in the Linux kernel.  That''s what I was modelling.

Regards,
Anthony Liguori
>    christian
>
>
>  
>


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On Tue, Mar 22, 2005 at 10:06:44AM -0600, Anthony Liguori
wrote:
> Christian Limpach wrote:
> 
> >I think it does what I expect.  And it seems to work for a lot of
> >libraries just fine.  By not using the global errno, you''re
preventing
> >people from using perror, warn, err and the likes.  Also some of the
> >interfaces in your library are slightly awkward because you''re
wasting
> >the return parameter to return the failure reason.
> > 
> >
> You''re right.  Some of the interfaces are a little awkward
(especially
> the memory mapping ones).  It seemed like a reasonable trade-off to make 
> though.
What about perror, warn, err and the likes, I really like to use those.
It seems very illogical having to stick the returned value into errno to be
able to use those...
> >Even if we don''t use the global errno, I''m still
wondering why you''re
> >returning -errno and not errno?
> > 
> >
> Good question.  I guess since we never returned > 0 it would be 
> reasonable to return errno instead of -errno.  -errno is the convention 
> used in the Linux kernel.  That''s what I was modelling.
Again, it seems illogical having to negate the returned value to be
able to use it, when there''s no reason to return a negative value in
the first place.

    christian



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Ian Pratt wrote:
>Personally, I think the best approach is to stick with the existing C
>convention that everyone is already well familiar with, and have a
>separate errno variable. This means we can then have functions return
>pointers etc rather than having to pass them by reference, which is
>undeniably ugly. 
>  
>
There''s only one problem with this: thread-safety.  I believe errno is 
commonly implemented in thread-local storage.  We''d have to jump
through
major hoops to get our own proper errno.

Returning the error as part of the interface makes life a lot easier for 
threading.  However, it sounds like a lot of people don''t like this.

The best alternative would be to have a context for the library (not 
just a simple file descriptor, but an actual struct that contained the 
file descriptors and the current error condition).  I really was hoping 
to avoid requiring a context (it simplifies application programming 
tremendously).

Does anyone have an idea for making a libxen errno thread safe that 
wouldn''t require a context?
>I notice that you store the fd of the priv_cmd in a static variable. I
>guess this is OK, but I think I still prefer a way of explicit way of
>closing the fd. You''d also have to be a little bit careful about
someone
>  
>
Ok.  Not a problem.
>forking then two guys trying to open the fd at the same time.
>  
>
Yes, I was actually considering the case of two threads when I wrote the 
library.  That''s why I exposed hcall_init() as part of the public 
interface.  It allows a threaded app (or a forking() app) to ensure that 
the library is only initialized once.

Regards,
Anthony Liguori
>Ian
>
>
>  
>


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Christian Limpach wrote:
>>You''re right.  Some of the interfaces are a little awkward
(especially
>>the memory mapping ones).  It seemed like a reasonable trade-off to make
>>though.
>>    
>>
>
>What about perror, warn, err and the likes, I really like to use those.
>It seems very illogical having to stick the returned value into errno to be
>able to use those...
>  
>
You''re not going to like this answer but I don''t think I would
use any
of those functions in a real management application.  Management tools 
should be using standard logging suites (like syslog).

strerror() still works with these return codes btw.  My test code 
usually likes like this:

domid_t domid;
int ret = dom_create_domain(3000, 0, &domid);

if (ret < 0) error("dom_create_domain: failed %s", strerror(-ret));

This would be even easier if the library returned errno instead of 
-errno ;-)

Regards,
Anthony Liguori


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> >Personally, I think the best approach is to stick with the existing C
> >convention that everyone is already well familiar with, and have a
> >separate errno variable. This means we can then have functions return
> >pointers etc rather than having to pass them by reference, which is
> >undeniably ugly. 
> >  
> >
> There''s only one problem with this: thread-safety.  I believe 
> errno is 
> commonly implemented in thread-local storage.  We''d have to 
> jump through major hoops to get our own proper errno.
No -- We want to use the normal libc errno, like other libraries!

Ian


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On Tue, 2005-03-22 at 09:55 -0600, Anthony Liguori
wrote:
> Jeremy Katz wrote:
> >On Tue, 2005-03-22 at 09:18 -0600, Anthony Liguori wrote:
> >>Do you have a suggestion for an alternative?  I know there are a
number
> >>of binding-generators but I thought Pyrex was the most widely used.
> >
> >Pyrex is probably the most generally used -- the stuff pygtk uses is
> >nice, but not very general.
> >
> >In most cases, though, bindings done by hand end up being higher
quality
> >than something that''s just being generated.
> >
> I was afraid that that would be your answer.
> 
> I would really like to automatically generate the bindings.  There is 
> more C code in the current libxc/libxu python bindings than all of
vm-tools.
That doesn''t make it bad :)  You''re just moving where the code
is to
another place by generating the bindings.
> gcc4 is still a bit off and I imagine that Pyrex will be updated as 
> needed.  If it ever proves to be painful to deal with, I''ll break
down
> and add bindings by hand.  Does that seem like a reasonable plan moving 
> forward?
gcc4 isn''t that far off.  It''s in the Fedora development tree
and we''re
using it to compile everything now.  See Rik''s multiple patches to fix
warnings that pop up with gcc4.  And counting on Pyrex being fixed based
on the complete lack of response to questions on this topic may be
wishful thinking.  Maybe I''ll look at Pyrex a little this afternoon and
see if I can figure out how to fix what it''s doing.

Jeremy



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Anthony Liguori wrote:
> There''s only one problem with this: thread-safety.  I believe
errno is
> commonly implemented in thread-local storage.  We''d have to jump
through
> major hoops to get our own proper errno.
Anthony - not sure what you are referring to here, because
the common use is to use the global errno made available
in libc. Is that not a proper errno?

thanks,
Nivedita




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Nivedita Singhvi wrote:
> Anthony Liguori wrote:
>
>> There''s only one problem with this: thread-safety.  I believe
errno
>> is commonly implemented in thread-local storage.  We''d have to
jump
>> through major hoops to get our own proper errno.
>
>
> Anthony - not sure what you are referring to here, because
> the common use is to use the global errno made available
> in libc. Is that not a proper errno?
Some libraries implement their own psuedo-errno to store library 
specific errors.  To the best of my knowledge, with the exception of 
system libraries (libc) errno is not used and not really supposed to be 
used.

I could be wrong, but I can''t think of any libraries that use it.
> thanks,
> Nivedita
>
>
>


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> Nivedita Singhvi wrote:
> 
> > Anthony Liguori wrote:
> >
> >> There''s only one problem with this: thread-safety.  I
believe errno
> >> is commonly implemented in thread-local storage.  We''d
have to jump
> >> through major hoops to get our own proper errno.
> >
> >
> > Anthony - not sure what you are referring to here, because
> > the common use is to use the global errno made available
> > in libc. Is that not a proper errno?
> 
> Some libraries implement their own psuedo-errno to store library 
> specific errors.  To the best of my knowledge, with the exception of 
> system libraries (libc) errno is not used and not really supposed to be 
> used.
> 
> I could be wrong, but I can''t think of any libraries that use it.
I think we can just use the regular errno as defined in errno.h (and
in the Posix standard). 

The convention of using negative numbers in the kernel is
counterbalanced by another convention of using positive numbers 
in user space. Shouldn''t be too hard to fix I guess. 

cheers,

S.


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On Tue, Mar 22, 2005 at 10:21:19AM -0600, Anthony Liguori
wrote:
> Christian Limpach wrote:
> 
> >>You''re right.  Some of the interfaces are a little awkward
(especially
> >>the memory mapping ones).  It seemed like a reasonable trade-off to
make
> >>though.
> >>   
> >>
> >
> >What about perror, warn, err and the likes, I really like to use those.
> >It seems very illogical having to stick the returned value into errno
to be
> >able to use those...
> > 
> >
> You''re not going to like this answer but I don''t think I
would use any
> of those functions in a real management application.  Management tools 
> should be using standard logging suites (like syslog).
You mean like using `%m'' in your format string to syslog(3)?
> strerror() still works with these return codes btw.  My test code 
> usually likes like this:
> 
> domid_t domid;
> int ret = dom_create_domain(3000, 0, &domid);
> 
> if (ret < 0) error("dom_create_domain: failed %s",
strerror(-ret));
While most other people''s test code would look like this:

domid = dom_create_domain(3000, 0);
if (domid < 0)
  err(1, "dom_create_domain");

I claim that this is what more people are used to since most libraries
support this interface, i.e. return _value_ with documented specific
error values, failure condition in errno and error reporting using one
of the various standard functions.

    christian



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Christian Limpach wrote:
>You mean like using `%m'' in your format string to syslog(3)?
>  
>
''%m'' is a GNU extension.
>While most other people''s test code would look like this:
>
>domid = dom_create_domain(3000, 0);
>if (domid < 0)
>  err(1, "dom_create_domain");
>
>I claim that this is what more people are used to since most libraries
>support this interface, i.e. return _value_ with documented specific
>error values, failure condition in errno and error reporting using one
>of the various standard functions.
>  
>
There seems to be wide consensus on this one though and I don''t seem to
have the popular opinion.  So perhaps we can comprimise.  What about:

1) Have most functions return an integer.  An error is indicated if the 
return is < 0.  In the event of an error, the function will return 
-errno but make sure to set errno properly.

2) For functions that want to return addresses, they can do so and 
return NULL on error.  Since all functions set errno properly, errno 
will have to be used for extended error conditions on these functions.

This should make everyone happy.   Your code will work just fine and an 
application developer can avoid using errno.  Thoughts?

Regards,
Anthony Liguori
>    christian
>
>
>  
>


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> > Do you have a suggestion for an alternative?  I know there 
> are a number 
> > of binding-generators but I thought Pyrex was the most widely used.
> 
> Pyrex is probably the most generally used -- the stuff pygtk uses is
> nice, but not very general.
> 
> In most cases, though, bindings done by hand end up being 
> higher quality
> than something that''s just being generated.
I''ve just looked at Pyrex and it doesn''t look like a good
soloution to
me. It doesn''t seem to offer much over the current manual bindings in
Xc.

Has anyone any recent experience with SWIG?

Ian


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On 22 Mar 2005, at 17:05, Anthony Liguori wrote:
> There seems to be wide consensus on this one though and I don''t
seem
> to have the popular opinion.  So perhaps we can comprimise.  What 
> about:
>
> 1) Have most functions return an integer.  An error is indicated if 
> the return is < 0.  In the event of an error, the function will return 
> -errno but make sure to set errno properly.
>
> 2) For functions that want to return addresses, they can do so and 
> return NULL on error.  Since all functions set errno properly, errno 
> will have to be used for extended error conditions on these functions.
>
> This should make everyone happy.   Your code will work just fine and 
> an application developer can avoid using errno.  Thoughts?
I''ll complicate the argument some more by suggesting that errno, and 
the standard errno values, aren''t the best things to be returning from 
Xen across the hypercall interface anyway. Many errno values are not 
applicable to a hypervisor; also, if we defined our own then we could 
return a richer set of error notifications to guests and to control 
tools.
Currently EINVAL, EPERM, EFAULT and ESRCH are approx the limit of our 
''richness''.

The reason I used errno''s originally in Xen was laziness and time 
constraints. :-)

I guess we''d need strong motivation (some cases where better error 
returns would be really useful) to be bothered to change the status quo 
though.

  -- Keir



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Ian Pratt wrote:
>Has anyone any recent experience with SWIG?
>  
>
We''re using it in Samba4.  It seems to work quite nicely.  I was 
considering using it instead of pyrex but I didn''t want to rock the
boat
too much.

Here''s a link to the current swig code in Samba:

http://websvn.samba.org/cgi-bin/viewcvs.cgi/branches/SAMBA_4_0/source/scripting/swig/

Regards,
Anthony Liguori
>Ian
>
>  
>


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On Tue, 22 Mar 2005, Anthony Liguori wrote:
> 1) Have most functions return an integer.  An error is indicated if the
return
> is < 0.  In the event of an error, the function will return -errno but
make
> sure to set errno properly.
works for me.
> 2) For functions that want to return addresses, they can do so and return
NULL
> on error.  Since all functions set errno properly, errno will have to be
used
> for extended error conditions on these functions.
sure. 

ron


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On Tue, Mar 22, 2005 at 11:05:03AM -0600, Anthony Liguori
wrote:
> Christian Limpach wrote:
> 
> >You mean like using `%m'' in your format string to syslog(3)?
> > 
> >
> ''%m'' is a GNU extension.
It exists in the BSD''s implementations and it is documented in SUS V3.
In any case, the point was that while you can argue that management
applications might not use perror, warn or err, you can''t really
argue that there won''t be at least one library usable for developing
management applications which supports printing error messages depending
on the value of errno.
> >While most other people''s test code would look like this:
> >
> >domid = dom_create_domain(3000, 0);
> >if (domid < 0)
> > err(1, "dom_create_domain");
> >
> >I claim that this is what more people are used to since most libraries
> >support this interface, i.e. return _value_ with documented specific
> >error values, failure condition in errno and error reporting using one
> >of the various standard functions.
> > 
> >
> There seems to be wide consensus on this one though and I don''t
seem to
> have the popular opinion.  So perhaps we can comprimise.  What about:
> 
> 1) Have most functions return an integer.  An error is indicated if the 
> return is < 0.  In the event of an error, the function will return 
> -errno but make sure to set errno properly.
> 
> 2) For functions that want to return addresses, they can do so and 
> return NULL on error.  Since all functions set errno properly, errno 
> will have to be used for extended error conditions on these functions.
> 
> This should make everyone happy.   Your code will work just fine and an 
> application developer can avoid using errno.  Thoughts?
It''s inconsistent.  Why would anybody examine errno for the 2nd type
of functions and then not do so for the 1st type?

So far your only point for not using errno has been that the programmer
can inadvertently overwrite it by calling a function which resets it before
using its value.  But even that now seems a moot point since it can still
happen with the 2nd type of functions.  What''s your problem with errno?

    christian



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Christian Limpach wrote:
>So far your only point for not using errno has been that the programmer
>can inadvertently overwrite it by calling a function which resets it before
>using its value.  But even that now seems a moot point since it can still
>happen with the 2nd type of functions.  What''s your problem with
errno?
>  
>
It''s too easy to overwrite.  Error paths are rarely invoked so if
you''re
overwriting errno in your error path the chances of you realizing
you''re
doing that is low.

This is a real problem.  If you don''t believe me, look at libxc.  In 
fact, the first file I opened, xc_private.c, has that problem exact 
problem in the first function (xc_map_foreign_batch()).  If the function 
fails for any reason, it does cleanup (while also calling perror).  The 
cleanup function (munmap) will squash errno.

This function would have to save errno after the ioctl, then restore it 
after the cleanup was done.  That''s ugly and most programmers
don''t
think to do it.

Regards,
Anthony Liguori
>    christian
>
>
>  
>


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On Mon, 21 Mar 2005, Jacob Gorm Hansen wrote:
> > o Use autoconf to detect dependencies, provide separate build
directory,
> > cross-compile
>
> I like having a separate build-directory, I still think at non-broken
> build tool (i.e. not make) could have done the job and done it better.
> The whole .d or .deps approach (pollution of source or build-tree with a
> static version of information that could and should be determined at
> run-time) is a gross hack, even MS Visual Studio can do better.
runtime?  how do you mean?  The .d is source file deps.

ps: autoconf itself doesn''t give a separate build dir, so
don''t naively assume
so.



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Adam Heath wrote:
>>I like having a separate build-directory, I still think at non-broken
>>build tool (i.e. not make) could have done the job and done it better.
>>The whole .d or .deps approach (pollution of source or build-tree with a
>>static version of information that could and should be determined at
>>run-time) is a gross hack, even MS Visual Studio can do better.
> 
> 
> runtime?  how do you mean?  The .d is source file deps.
Jam and MS Visual Studio (for example, not that I am advocating the use
of the latter in the scope of this project) both know how to deduce
header-dependencies from the C-source, without littering the tree with
.d files. Since this can be done in virtually zero time, there is no
reason to keep all the .d-files around. Basically that means you no
longer strictly need a separate configure-step, because your source tree
no longer contains any build-specific persistent state, and all your
object files and executables go somewhere if you want them to.

A good build tool will be quick enough that you can just always run it
''just in case'', every time you run your program, e.g.

$ jam && ./run

appears to be just as quick as

$ ./run
> ps: autoconf itself doesn''t give a separate build dir, so
don''t naively assume
> so.
Sorry if I was being naive. There is even less reason to choose autoconf
over Jam then.

Jacob



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On Tue, 2005-03-22 at 11:25 -0500, Jeremy Katz wrote:
> On Tue, 2005-03-22 at 09:55 -0600, Anthony Liguori wrote:
> > gcc4 is still a bit off and I imagine that Pyrex will be updated as 
> > needed.  If it ever proves to be painful to deal with, I''ll
break down
> > and add bindings by hand.  Does that seem like a reasonable plan
moving
> > forward?
> 
> gcc4 isn''t that far off.  It''s in the Fedora development
tree and we''re
> using it to compile everything now.  See Rik''s multiple patches to
fix
> warnings that pop up with gcc4.  And counting on Pyrex being fixed based
> on the complete lack of response to questions on this topic may be
> wishful thinking.  Maybe I''ll look at Pyrex a little this
afternoon and
> see if I can figure out how to fix what it''s doing.
Okay, I got Pyrex happier with gcc4 and sent that patch off this
afternoon, so that''s not an argument against it at this point.

I guess it''s down to the aesthetics of binding generators, which since
I''m not really a fan of any of them, I won''t argue much about
between
the evils.

Jeremy



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Jeremy Katz wrote:
>Okay, I got Pyrex happier with gcc4 and sent that patch off this
>afternoon, so that''s not an argument against it at this point.
>  
>
Awesome! Thanks!

I''m currently playing with SWIG.  I haven''t really decided
myself which
method I like the most.

Regards,
Anthony Liguori
>I guess it''s down to the aesthetics of binding generators, which
since
>I''m not really a fan of any of them, I won''t argue much
about between
>the evils.
>
>Jeremy
>
>
>  
>


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