alloca bug

alloca bug

[Jon Wilson]

Hi,
So it looks like the blob in libguile/eval.c (lines 40-56) from the 
autoconf manual to define alloca doesn't work on BSD systems.  This is 
in guile 1.8.1.

In the subdirectory libguile:
eval.c includes __scm.h
__scm.h includes scmconfig.h

scmconfig.h seems to only be created while make all is running (try make 
scmconfig after ../configure).  However, on my system, and Gopi's 
FreeBSD system as well, scmconfig.h includes <stdlib.h>.  So, 
indirectly, eval.c includes <stdlib.h>, and does so prior to the 
autoconf alloca blob.  Since <stdlib.h> defines alloca (in some cases), 
the blob redefines alloca.  Is this something we need to fix (by not 
including <stdlib.h> I would suppose) or is it something that we toss 
into the autoconf people's court?

If this message would more properly go on the guile developers mailing 
list, could someone who is a member of that list please forward it for me?
Regards,
Jon


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Re: alloca bug

[Kevin Ryde]

Jon Wilson <j85wilson@fastmail.fm> writes:
>
> scmconfig.h seems to only be created while make all is running (try
> make scmconfig after ../configure).

Yes, it's generated.

> However, on my system, and Gopi's
> FreeBSD system as well, scmconfig.h includes <stdlib.h>.  So,
> indirectly, eval.c includes <stdlib.h>, and does so prior to the
> autoconf alloca blob.  Since <stdlib.h> defines alloca (in some
> cases), the blob redefines alloca.  Is this something we need to fix
> (by not including <stdlib.h> I would suppose) or is it something that
> we toss into the autoconf people's court?

Assuming we're using the blob correctly then it's an autoconf problem.

Is the blob supposed to come before including <stdlib.h>?  Though that
sounds too easy to mess up, you'd hope the blob could be made to cope
with being after.


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Re: alloca bug

[Jon Wilson]

Hi,
I'm still trying to get a shell account on my friend's FreeBSD machine.  
Once I've got that, it shouldn't take very long to file a bug report.
Regards,
Jon


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Re: alloca bug

[Jon Wilson]

Whoops I sent this email just to Kevin instead of the list...  sorry folks.

Hi Kevin,
> Assuming we're using the blob correctly then it's an autoconf problem.
>
> Is the blob supposed to come before including <stdlib.h>?  Though that
> sounds too easy to mess up, you'd hope the blob could be made to cope
> with being after.
>   

What I'm thinking is that perhaps there is a needed blob for using
stdlib?  I haven't been able to find any such thing, however, with a
cursory bit of googling.  I'm thinking that it is most likely as you say
an autoconf problem.  Who wants to file a bug report?  I don't mind
doing so myself, but I suspect that it is best done by one of our
developers.
Regards,
Jon



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Re: alloca bug

[Jon Wilson]

And here was Kevin's reply:

> What I'm thinking is that perhaps there is a needed blob for using
> stdlib?

Perhaps, though the previous version from autoconf didn't.  What it
did used to have though was a wrapping #ifndef,

#ifndef alloca
# ifdef __GNUC__
#  define alloca __builtin_alloca
# else
#  ifdef __DECC
#   define alloca(x) __ALLOCA(x)
#  else
#   ifdef _MSC_VER
#    include <malloc.h>
#    define alloca _alloca
#   else
#    if HAVE_ALLOCA_H
#     include <alloca.h>
#    else
#     if defined (_AIX) || defined (_IBMR2)
....

> Who wants to file a bug report?  I don't
> mind doing so myself, but I suspect that it is best done by one of our
> developers.

Can you put together a minimal failing .c file?  Perhaps what's below
is enough.  I don't have a fleebsd system handy to try.  You can send
it to bug-autoconf saying this has arisen in guile.  Cc guile-devel if
you like.  (You can subscribe to guile-devel but turn off delivery if
you want to post without wanting to read the list.  Nothing exciting
happens on that list anyway. :-)


#include <stdlib.h>

/* This blob per the Autoconf manual (under "Particular Functions"). */
#if HAVE_ALLOCA_H
# include <alloca.h>
#elif defined __GNUC__
# define alloca __builtin_alloca
#elif defined _AIX
# define alloca __alloca
#elif defined _MSC_VER
# include <malloc.h>
# define alloca _alloca
#else
# include <stddef.h>
# ifdef  __cplusplus
extern "C"
# endif
void *alloca (size_t);
#endif


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Re: alloca bug

[Neil Jerram]

Jon Wilson <j85wilson@fastmail.fm> writes:

> Hi,
> So it looks like the blob in libguile/eval.c (lines 40-56) from the
> autoconf manual to define alloca doesn't work on BSD systems.  This is
> in guile 1.8.1.

One thing to be aware of is that the so-called "portable"
implementation of alloca() in alloca.c just doesn't work with Guile,
because it doesn't really allocate memory on the stack.

In fact, alloca.c was removed for this reason in 2004:

2004-09-08  Marius Vollmer  <marius.vollmer@uni-dortmund.de>

	* Makefile.am (EXTRA_libguile_la_SOURCES): Removed "alloca.c".

Does anyone know what caused it to reappear?  ChangeLog doesn't reveal
anything.

Guile fundamentally requires a real alloca, which means it has to be a
compiler builtin.

Regards,
     Neil



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Re: alloca bug

[Jon Wilson]

Hi Neil,

Neil Jerram wrote:
> One thing to be aware of is that the so-called "portable"
> implementation of alloca() in alloca.c just doesn't work with Guile,
> because it doesn't really allocate memory on the stack.
>   
alloca.c from where?  From libc?  If so, which libc?  Or do you mean 
some file alloca.c within the guile source?
> In fact, alloca.c was removed for this reason in 2004:
>
> 2004-09-08  Marius Vollmer  <marius.vollmer@uni-dortmund.de>
>
> 	* Makefile.am (EXTRA_libguile_la_SOURCES): Removed "alloca.c".
>
> Does anyone know what caused it to reappear?  ChangeLog doesn't reveal
> anything.
>   
I don't see any file called alloca.c in the 1.8.1 source tree.
> Guile fundamentally requires a real alloca, which means it has to be a
> compiler builtin.

I'm puzzled now.  Could you please clarify exactly what you are talking 
about?

What I was referring to was the fact that when you #include stdlib.h on 
a BSD system, and then use autoconf's blob of code to define alloca, 
using the BSD default libc and cc, you get a preprocessor error that 
says that alloca has already been defined.  Or, at the least, guile's 
libguile/eval.c does #include stdlib.h, and does use autoconf's blob, 
and produces such an error.  The location of the previous definition is 
given as stdlib.h.  Are we talking about the same thing?  If so, I need 
some help understanding what you've said.
Regards,
Jon


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Re: alloca bug

[Kevin Ryde]

Jon Wilson <j85wilson@fastmail.fm> writes:
>
> I need some help understanding what you've said.

AC_FUNC_ALLOCA in configure normally uses an alloca.c
(libguile/alloca.c) to provide alloca() if the compiler or C library
doesn't otherwise have one.  Should be unrelated to whatever happens
on bsd.

Not sure why libguile/alloca.c has to exist.  Does autoconf insist on
it being present, even if we then reject that version?


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Re: alloca bug

[Jon Wilson]

Hi Kevin,

Kevin Ryde wrote:
> Jon Wilson <j85wilson@fastmail.fm> writes:
>   
>> I need some help understanding what you've said.
>>     
>
> AC_FUNC_ALLOCA in configure normally uses an alloca.c
> (libguile/alloca.c) to provide alloca() if the compiler or C library
> doesn't otherwise have one.  Should be unrelated to whatever happens
> on bsd.
>
> Not sure why libguile/alloca.c has to exist.  Does autoconf insist on
> it being present, even if we then reject that version?
>   

Thanks.  Now I get what Neil was talking about.  I've got that shell 
account now, but it is getting late.  I'll take a look at making a bug 
report tomorrow.  Plenty of time now classes are all over, and all I 
have left are grad school apps.  Speaking of which, if anyone is at Ohio 
State and could put in a good word for me with the physics department, 
that would be favorite.</shameless plug>
Regards,
Jon


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Re: alloca bug

[Neil Jerram]

Jon Wilson <j85wilson@fastmail.fm> writes:

> alloca.c from where?  From libc?  If so, which libc?  Or do you mean
> some file alloca.c within the guile source?

Yes, there's a libguile/alloca.c in Guile CVS.  It probably doesn't
survive into any distributions though (and shouldn't).

> I'm puzzled now.  Could you please clarify exactly what you are
> talking about?

After reviewing the thread again, I think my interjection was a red
herring - sorry.

I noticed that libguile/alloca.c was still around, when I thought it
had been removed, and got worried about that.  In fact I don't think
it can ever get built in, so it can't do any harm.

> What I was referring to was the fact that when you #include stdlib.h
> on a BSD system, and then use autoconf's blob of code to define
> alloca, using the BSD default libc and cc, you get a preprocessor
> error that says that alloca has already been defined.  Or, at the
> least, guile's libguile/eval.c does #include stdlib.h, and does use
> autoconf's blob, and produces such an error.  The location of the
> previous definition is given as stdlib.h.  Are we talking about the
> same thing?  If so, I need some help understanding what you've said.

Thanks, I understand now.  Agree that this is a bug with the autoconf
blob for BSD.  Has anyone reported it yet?

In the meantime, can we fix this for guile by just adding #ifndef
alloca around the whole blob?  Would that have bad effects on other
OSs?

Regards,
     Neil



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Re: alloca bug

[Kevin Ryde]

Neil Jerram <neil@ossau.uklinux.net> writes:
>
> In the meantime, can we fix this for guile by just adding #ifndef
> alloca around the whole blob?  Would that have bad effects on other
> OSs?

Sounds good.


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