Accessing multiple values from C

Accessing multiple values from C

[Julian Graham]

Hey all,

I was playing around with some C code that uses the new R6RS
bytevector ports, and I noticed that there doesn't seem to be an easy
way (a la `let-values' or `receive') to access multiple return values
from C.  I've resorted to doing:

  scm_struct_ref (foo, SCM_INUM0);

...which is almost certainly not future-proof.  All I want to do is
access these values as a pair or list.  Is there a Right Way to do
that?


Thanks,
Julian

Re: Accessing multiple values from C

[Ludovic Courtès]

Hi,

Julian Graham <joolean@gmail.com> skribis:

> I was playing around with some C code that uses the new R6RS
> bytevector ports, and I noticed that there doesn't seem to be an easy
> way (a la `let-values' or `receive') to access multiple return values
> from C.  I've resorted to doing:
>
>   scm_struct_ref (foo, SCM_INUM0);
>
> ...which is almost certainly not future-proof.  All I want to do is
> access these values as a pair or list.

Values were already structs in 1.8, so that’s OK.

Perhaps this should be documented though, and C accessors could be
provided.

> Is there a Right Way to do that?

Yes: write Scheme code!  :-)

In 1.8 multiple-values were always a struct, which had to be allocated
each time multiple values were returned.  In 2.0 it’s a struct only if
it has to cross the Scheme/C boundary–otherwise the values are stored on
the VM stack.

Thanks,
Ludo’.

Re: Accessing multiple values from C

[Andy Wingo]

Hi,

On Tue 01 Nov 2011 16:34, Julian Graham <joolean@gmail.com> writes:

> I was playing around with some C code that uses the new R6RS
> bytevector ports, and I noticed that there doesn't seem to be an easy
> way (a la `let-values' or `receive') to access multiple return values
> from C.  I've resorted to doing:
>
>   scm_struct_ref (foo, SCM_INUM0);
>
> ...which is almost certainly not future-proof.  All I want to do is
> access these values as a pair or list.  Is there a Right Way to do
> that?

Not yet.  You interested in adding scm_c_value_ref (SCM, size_t) to the
API, and documentation to api-control.texi?  If you do it soon, it
will make it into 2.0.4.

Andy
-- 
http://wingolog.org/

Re: Accessing multiple values from C

[Julian Graham]

Hey Andy,


> Not yet.  You interested in adding scm_c_value_ref (SCM, size_t) to the
> API, and documentation to api-control.texi?  If you do it soon, it
> will make it into 2.0.4.

Sure.  Gimme a day or so and I'll have something for review.


Regards,
Julian

Re: Accessing multiple values from C

[Julian Graham]

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Howdy,


>> Not yet.  You interested in adding scm_c_value_ref (SCM, size_t) to the
>> API, and documentation to api-control.texi?  If you do it soon, it
>> will make it into 2.0.4.
>
> Sure.  Gimme a day or so and I'll have something for review.

Find attached a patch.  Let me know if I missed anything.


Regards,
Julian

[-- Attachment #2: 0001-Add-scm_c_value_ref-to-allow-access-to-multiple-retu.patch --]
[-- Type: text/x-diff, Size: 5095 bytes --]

From 8d1bf16a607c6e8a0913ef982df88286dd887d51 Mon Sep 17 00:00:00 2001
From: Julian Graham <julian@navigator.(none)>
Date: Wed, 11 Jan 2012 22:08:51 -0500
Subject: [PATCH] Add `scm_c_value_ref' to allow access to multiple returned
 values from C.

* libguile/values.c, libguile/values.h (scm_c_value_ref): New function.
* doc/ref/api-control.texi (Multiple Values): Add documentation for
  `scm_c_value_ref'.
* test-suite/standalone/test-scm-values.c: New file.
* test-suite/standalone/Makefile.am: Ensure that test-scm-values.c gets
  built and executed as part of `make check'.
---
 doc/ref/api-control.texi                |    5 +++
 libguile/values.c                       |    7 ++++
 libguile/values.h                       |    1 +
 test-suite/standalone/Makefile.am       |    7 ++++
 test-suite/standalone/test-scm-values.c |   60 +++++++++++++++++++++++++++++++
 5 files changed, 80 insertions(+), 0 deletions(-)
 create mode 100644 test-suite/standalone/test-scm-values.c

diff --git a/doc/ref/api-control.texi b/doc/ref/api-control.texi
index c1502b0..4077bd9 100644
--- a/doc/ref/api-control.texi
+++ b/doc/ref/api-control.texi
@@ -785,6 +785,11 @@ the current implementation that object shares structure with
 @var{args}, so @var{args} should not be modified subsequently.
 @end deffn
 
+@deffn {C Function} scm_c_value_ref (values, idx)
+Returns the value at the position specified by @var{idx} in the
+multiple-values object @var{values}.
+@end deffn
+
 @rnindex call-with-values
 @deffn {Scheme Procedure} call-with-values producer consumer
 Calls its @var{producer} argument with no values and a
diff --git a/libguile/values.c b/libguile/values.c
index ab77731..3a213fa 100644
--- a/libguile/values.c
+++ b/libguile/values.c
@@ -67,6 +67,13 @@ print_values (SCM obj, SCM pwps)
   return SCM_UNSPECIFIED;
 }
 
+SCM
+scm_c_value_ref (SCM obj, size_t idx)
+{
+  SCM values = scm_struct_ref (obj, SCM_INUM0);
+  return scm_list_ref (values, SCM_I_MAKINUM (idx));
+}
+
 SCM_DEFINE (scm_values, "values", 0, 0, 1,
 	    (SCM args),
 	    "Delivers all of its arguments to its continuation.  Except for\n"
diff --git a/libguile/values.h b/libguile/values.h
index 65ad8a1..5f79855 100644
--- a/libguile/values.h
+++ b/libguile/values.h
@@ -33,6 +33,7 @@ SCM_API SCM scm_values_vtable;
 SCM_INTERNAL void scm_i_extract_values_2 (SCM obj, SCM *p1, SCM *p2);
 
 SCM_API SCM scm_values (SCM args);
+SCM_API SCM scm_c_value_ref (SCM values, size_t idx);
 SCM_INTERNAL void scm_init_values (void);
 
 #endif  /* SCM_VALUES_H */
diff --git a/test-suite/standalone/Makefile.am b/test-suite/standalone/Makefile.am
index 08d249c..7b96785 100644
--- a/test-suite/standalone/Makefile.am
+++ b/test-suite/standalone/Makefile.am
@@ -189,6 +189,13 @@ test_scm_to_latin1_string_LDADD = $(LIBGUILE_LDADD)
 check_PROGRAMS += test-scm-to-latin1-string
 TESTS += test-scm-to-latin1-string
 
+# test-values
+test_scm_values_SOURCES = test-scm-values.c
+test_scm_values_CFLAGS = ${test_cflags}
+test_scm_values_LDADD = $(LIBGUILE_LDADD)
+check_PROGRAMS += test-scm-values
+TESTS += test-scm-values
+
 if HAVE_SHARED_LIBRARIES
 
 # test-extensions
diff --git a/test-suite/standalone/test-scm-values.c b/test-suite/standalone/test-scm-values.c
new file mode 100644
index 0000000..25acd2e
--- /dev/null
+++ b/test-suite/standalone/test-scm-values.c
@@ -0,0 +1,60 @@
+/* Copyright (C) 2012 Free Software Foundation, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public License
+ * as published by the Free Software Foundation; either version 3 of
+ * the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301 USA
+ */
+
+#ifdef HAVE_CONFIG_H
+# include <config.h>
+#endif
+
+#include <assert.h>
+#include <libguile.h>
+#include <stdlib.h>
+#include <string.h>
+
+static void
+test_scm_c_value_ref ()
+{
+  SCM values = scm_values (scm_list_3 (scm_from_locale_string ("foo"),
+				       scm_from_locale_string ("bar"),
+				       scm_from_locale_string ("baz")));
+
+  
+  char *foo = scm_to_locale_string (scm_c_value_ref (values, 0));
+  char *bar = scm_to_locale_string (scm_c_value_ref (values, 1));
+  char *baz = scm_to_locale_string (scm_c_value_ref (values, 2));
+
+  assert (strcmp (foo, "foo") == 0);
+  assert (strcmp (bar, "bar") == 0);
+  assert (strcmp (baz, "baz") == 0);
+
+  free (foo);
+  free (bar);
+  free (baz);
+}
+
+static void
+tests (void *data, int argc, char **argv)
+{
+  test_scm_c_value_ref ();
+}
+
+int
+main (int argc, char *argv[])
+{
+  scm_boot_guile (argc, argv, tests, NULL);
+  return 0;
+}
-- 
1.7.5.4

Re: Accessing multiple values from C

[Ludovic Courtès]

Hi Julian,

Julian Graham <joolean@gmail.com> skribis:

>>> Not yet.  You interested in adding scm_c_value_ref (SCM, size_t) to the
>>> API, and documentation to api-control.texi?  If you do it soon, it
>>> will make it into 2.0.4.
>>
>> Sure.  Gimme a day or so and I'll have something for review.
>
> Find attached a patch.  Let me know if I missed anything.

Looks good to me, thanks!

> +@deffn {C Function} scm_c_value_ref (values, idx)
> +Returns the value at the position specified by @var{idx} in the

Should be “Return” here.

Andy: OK to apply?

Ludo’.

Re: Accessing multiple values from C

[Andy Wingo]

On Sun 15 Jan 2012 22:47, ludo@gnu.org (Ludovic Courtès) writes:

>>>> Not yet.  You interested in adding scm_c_value_ref (SCM, size_t) to the
>>>> API, and documentation to api-control.texi?  If you do it soon, it
>>>> will make it into 2.0.4.
>>>
>>> Sure.  Gimme a day or so and I'll have something for review.
>>
>> Find attached a patch.  Let me know if I missed anything.
>
> Looks good to me, thanks!
>
> Andy: OK to apply?

If it looks good to you, sure!  I haven't had a chance to review
anything besides local-eval in a few days :/

Happy hacking,

Andy
-- 
http://wingolog.org/

Re: Accessing multiple values from C

[Mark H Weaver]

Hi Julian, thanks for working on this!

Julian Graham <joolean@gmail.com> writes:
> +SCM
> +scm_c_value_ref (SCM obj, size_t idx)
> +{
> +  SCM values = scm_struct_ref (obj, SCM_INUM0);
> +  return scm_list_ref (values, SCM_I_MAKINUM (idx));
> +}
> +

Should this verify that `obj' is a values object?

Should it verify that `idx' can fit in an inum?

Also, if `obj' is _not_ a values object and `idx' is 0, should this
simply return `obj'?  Since a single value is conceptually no different
than multiple values (but is represented very differently within Guile),
I'd think that this function should handle that case gracefully.

What do other people think?

    Thanks,
      Mark

Re: Accessing multiple values from C

[Andy Wingo]

On Wed 18 Jan 2012 04:15, Mark H Weaver <mhw@netris.org> writes:

> Should this verify that `obj' is a values object?
>
> Should it verify that `idx' can fit in an inum?

Agreed!

> Also, if `obj' is _not_ a values object and `idx' is 0, should this
> simply return `obj'?  Since a single value is conceptually no different
> than multiple values (but is represented very differently within Guile),
> I'd think that this function should handle that case gracefully.

Hum, good question.  I'm inclined to agree.

Regards,

Andy
-- 
http://wingolog.org/

Re: Accessing multiple values from C

[Ludovic Courtès]

Hi Mark,

Mark H Weaver <mhw@netris.org> skribis:

> Julian Graham <joolean@gmail.com> writes:
>> +SCM
>> +scm_c_value_ref (SCM obj, size_t idx)
>> +{
>> +  SCM values = scm_struct_ref (obj, SCM_INUM0);
>> +  return scm_list_ref (values, SCM_I_MAKINUM (idx));
>> +}
>> +
>
> Should this verify that `obj' is a values object?
>
> Should it verify that `idx' can fit in an inum?

Yes, good point.  Using an ‘SCM_VALIDATE’, which will throw an exception
upon error, right?

> Also, if `obj' is _not_ a values object and `idx' is 0, should this
> simply return `obj'?

Yes, makes sense to me.

Julian: could you change these bits and send an updated patch?

Thanks,
Ludo’.

Re: Accessing multiple values from C

[Mark H Weaver]

ludo@gnu.org (Ludovic Courtès) writes:

> Mark H Weaver <mhw@netris.org> skribis:
>
>> Julian Graham <joolean@gmail.com> writes:
>>> +SCM
>>> +scm_c_value_ref (SCM obj, size_t idx)
>>> +{
>>> +  SCM values = scm_struct_ref (obj, SCM_INUM0);
>>> +  return scm_list_ref (values, SCM_I_MAKINUM (idx));
>>> +}
>>> +
>>
>> Should this verify that `obj' is a values object?
>>
>> Should it verify that `idx' can fit in an inum?
>
> Yes, good point.  Using an ‘SCM_VALIDATE’, which will throw an exception
> upon error, right?

Well, there's a complication with using an `SCM_VALIDATE' to check for
values objects.  If `idx' is 0 (make sure it's exact!), then we must
_not_ throw an exception if `obj' is not a values object.  However, in
that case we still need to know whether `obj' is a values object or not.

Also, a fine point about desirable error messages: asking for `idx' 5 of
a values object with only 3 elements is, conceptually, the same kind of
error as asking for `idx' 5 of something that is _not_ a values object.

Therefore, I think in both cases the error should be something to the
effect of "too few values".

    Thanks,
      Mark

Re: Accessing multiple values from C

[Ludovic Courtès]

Hi!

Mark H Weaver <mhw@netris.org> skribis:

> ludo@gnu.org (Ludovic Courtès) writes:
>
>> Mark H Weaver <mhw@netris.org> skribis:
>>
>>> Julian Graham <joolean@gmail.com> writes:
>>>> +SCM
>>>> +scm_c_value_ref (SCM obj, size_t idx)
>>>> +{
>>>> +  SCM values = scm_struct_ref (obj, SCM_INUM0);
>>>> +  return scm_list_ref (values, SCM_I_MAKINUM (idx));
>>>> +}
>>>> +
>>>
>>> Should this verify that `obj' is a values object?
>>>
>>> Should it verify that `idx' can fit in an inum?
>>
>> Yes, good point.  Using an ‘SCM_VALIDATE’, which will throw an exception
>> upon error, right?
>
> Well, there's a complication with using an `SCM_VALIDATE' to check for
> values objects.  If `idx' is 0 (make sure it's exact!), then we must
> _not_ throw an exception if `obj' is not a values object.  However, in
> that case we still need to know whether `obj' is a values object or not.

Yes, I already agreed on this one.  ;-)

> Also, a fine point about desirable error messages: asking for `idx' 5 of
> a values object with only 3 elements is, conceptually, the same kind of
> error as asking for `idx' 5 of something that is _not_ a values object.
>
> Therefore, I think in both cases the error should be something to the
> effect of "too few values".

Right.

Ludo’.

Re: Accessing multiple values from C

[Mark H Weaver]

I think something along these lines be ideal,
though this code is UNTESTED.

     Mark


SCM
scm_c_value_ref (SCM obj, size_t idx)
{
  if (SCM_VALUESP (obj))
    {
      SCM values = scm_struct_ref (obj, SCM_INUM0);
      size_t i = idx;
      while (SCM_LIKELY (!scm_is_null (values)))
        {
          if (i == 0)
            return SCM_CAR (values);
          values = SCM_CDR (values);
          i--;
        }
    }
  else if (idx == 0)
    return obj;

  scm_error (scm_out_of_range_key,
	     "scm_c_value_ref",
	     "Too few values in ~S to access index ~S",
             scm_list_2 (obj, scm_from_unsigned_integer (idx)),
             scm_list_1 (scm_from_unsigned_integer (idx)));
}

Re: Accessing multiple values from C

[Mark H Weaver]

At Julian's request, I went ahead and pushed my implementation of
`scm_c_value_ref' to the stable-2.0 branch, so it will be in 2.0.4.

    Thanks,
      Mark

Re: Accessing multiple values from C

[Julian Graham]

Hi all,

Thanks for the review!

On Wed, Jan 18, 2012 at 3:19 PM, Ludovic Courtès <ludo@gnu.org> wrote:
> Julian: could you change these bits and send an updated patch?

Will do.


Regards,
Julian