Asp Forum - Non-constant initializers

Fred the Freshwater Catfish

5/12/2011 10:59:00 PM

Hey guys,

I've been writing some code that I would prefer to be maximally
portable. As I have mostly used Linux, gcc is pretty much the only C
compiler I've ever used under UN*X. I'd like to use non-constant
initializers for some global variables. I'm pretty sure I can get it
to work the way I want under gcc, but what of other compilers? The
gcc documentation in info format says that ISO C99 allows this, and so
I expect some compilers built in the 90's will support the feature,
but to what extent is this true?

What I mean to learn is the general prevalence of this C feature. If
there's no support for certain well-known platforms (maybe SunOS or
OSF/1, whatever) I may consider a different solution to the original
problem, but I'd like to know what I'm getting in to before I start
trying to read the docs for every bloody C compiler that ever hit the
'net in the last twenty years.

Thanks if you can shed any light on this subject.

Fred the Freshwater Catfish

94 Answers

Ian Collins

5/12/2011 11:07:00 PM

On 05/13/11 10:59 AM, Fred the Freshwater Catfish wrote:
> Hey guys,
>
> I've been writing some code that I would prefer to be maximally
> portable. As I have mostly used Linux, gcc is pretty much the only C
> compiler I've ever used under UN*X. I'd like to use non-constant
> initializers for some global variables. I'm pretty sure I can get it
> to work the way I want under gcc, but what of other compilers? The
> gcc documentation in info format says that ISO C99 allows this, and so
> I expect some compilers built in the 90's will support the feature,
> but to what extent is this true?

How do you derive the initialiser values?

--
Ian Collins

Shao Miller

5/12/2011 11:09:00 PM

On 5/12/2011 5:59 PM, Fred the Freshwater Catfish wrote:
> ...I'd like to use non-constant
> initializers for some global variables. I'm pretty sure I can get it
> to work the way I want under gcc, but what of other compilers? The
> gcc documentation in info format says that ISO C99 allows this, and so
> I expect some compilers built in the 90's will support the feature,
> but to what extent is this true?

Could you please share an example of what kind of non-constant you'd be
inclined to initialize a static-duration, file-scoped object with? Are
you talking about using a function's return value? Are you talking
about using the address of something whose address you don't know at
some point in time?

China Blue Veins

5/12/2011 11:16:00 PM

In article <OgcdY5.Lcq.Cst0A@btdt.invalid>,
Fred the Freshwater Catfish <usenet.fred@bdtd.invalid> wrote:

> Hey guys,
>
> I've been writing some code that I would prefer to be maximally
> portable. As I have mostly used Linux, gcc is pretty much the only C
> compiler I've ever used under UN*X. I'd like to use non-constant
> initializers for some global variables. I'm pretty sure I can get it
> to work the way I want under gcc, but what of other compilers? The
> gcc documentation in info format says that ISO C99 allows this, and so
> I expect some compilers built in the 90's will support the feature,
> but to what extent is this true?
>
> What I mean to learn is the general prevalence of this C feature. If
> there's no support for certain well-known platforms (maybe SunOS or
> OSF/1, whatever) I may consider a different solution to the original
> problem, but I'd like to know what I'm getting in to before I start
> trying to read the docs for every bloody C compiler that ever hit the
> 'net in the last twenty years.

I use an alternative strategy: if I want to implement
static double XYZ = nonconstant-expression;

I instead do
#define XYZ (*XYZ_())
static double *XYZ_(void) {
static double xyz;
static bool initialised = false;
LOCK
if (!initialised) {
xyz = nonconstant-expression;
initialised = true;
}
UNLOCK
return &xyz;
}

XYZ will be an lvalue just like any other variable; it gets initialised on the
first use; the initialisation can be any complicated as desired, using any
dependencies desired, without depending on the compiler and loader to sneak in
the initialisation code in the right order.

--
Damn the living - It's a lovely life. I'm whoever you want me to be.
Silver silverware - Where is the love? At least I can stay in character.
Oval swimming pool - Where is the love? Annoying Usenet one post at a time.
Damn the living - It's a lovely life. In 1492....

Fred the Freshwater Catfish

5/12/2011 11:17:00 PM

On Fri, May 13, 2011 at 11:07:09AM +1200, Ian Collins wrote:
> On 05/13/11 10:59 AM, Fred the Freshwater Catfish wrote:
> >Hey guys,
> >
> >I've been writing some code that I would prefer to be maximally
> >portable. As I have mostly used Linux, gcc is pretty much the only C
> >compiler I've ever used under UN*X. I'd like to use non-constant
> >initializers for some global variables. I'm pretty sure I can get it
> >to work the way I want under gcc, but what of other compilers? The
> >gcc documentation in info format says that ISO C99 allows this, and so
> >I expect some compilers built in the 90's will support the feature,
> >but to what extent is this true?
>
> How do you derive the initialiser values?

It should end up being a function call most of the time.

Fred the Freshwater Catfish

Ian Collins

5/12/2011 11:21:00 PM

On 05/13/11 11:17 AM, Fred the Freshwater Catfish wrote:
> On Fri, May 13, 2011 at 11:07:09AM +1200, Ian Collins wrote:
>> On 05/13/11 10:59 AM, Fred the Freshwater Catfish wrote:
>>> Hey guys,
>>>
>>> I've been writing some code that I would prefer to be maximally
>>> portable. As I have mostly used Linux, gcc is pretty much the only C
>>> compiler I've ever used under UN*X. I'd like to use non-constant
>>> initializers for some global variables. I'm pretty sure I can get it
>>> to work the way I want under gcc, but what of other compilers? The
>>> gcc documentation in info format says that ISO C99 allows this, and so
>>> I expect some compilers built in the 90's will support the feature,
>>> but to what extent is this true?
>>
>> How do you derive the initialiser values?
>
> It should end up being a function call most of the time.

Then you either want C++, or a singleton fiddle as posted else-thread.

--
Ian Collins

Uncle Steve

5/12/2011 11:23:00 PM

On Thu, May 12, 2011 at 06:08:46PM -0500, Shao Miller wrote:
> On 5/12/2011 5:59 PM, Fred the Freshwater Catfish wrote:
> >...I'd like to use non-constant
> >initializers for some global variables. I'm pretty sure I can get it
> >to work the way I want under gcc, but what of other compilers? The
> >gcc documentation in info format says that ISO C99 allows this, and so
> >I expect some compilers built in the 90's will support the feature,
> >but to what extent is this true?
>
> Could you please share an example of what kind of non-constant you'd be
> inclined to initialize a static-duration, file-scoped object with? Are
> you talking about using a function's return value? Are you talking
> about using the address of something whose address you don't know at
> some point in time?

Essentially, I'm looking to seed a few critical global variables with
run-time computed values. Obviously, this will be the product of a
function call. I would like to declare:

int foops[] = howmuchfoo(void);

and have everything work as I expect. Pie in the sky for non-recent
UN*Xes?

Regards,

Uncle Steve

--
Should a professional politician be charged with molestation if he
kisses babies while attending political rallies?

Fred the Freshwater Catfish

5/12/2011 11:29:00 PM

On Thu, May 12, 2011 at 04:15:36PM -0700, Columbus sailed the ocean China Blue wrote:
> In article <OgcdY5.Lcq.Cst0A@btdt.invalid>,
> Fred the Freshwater Catfish <usenet.fred@bdtd.invalid> wrote:
>
> > Hey guys,
> >
> > I've been writing some code that I would prefer to be maximally
> > portable. As I have mostly used Linux, gcc is pretty much the only C
> > compiler I've ever used under UN*X. I'd like to use non-constant
> > initializers for some global variables. I'm pretty sure I can get it
> > to work the way I want under gcc, but what of other compilers? The
> > gcc documentation in info format says that ISO C99 allows this, and so
> > I expect some compilers built in the 90's will support the feature,
> > but to what extent is this true?
> >
> > What I mean to learn is the general prevalence of this C feature. If
> > there's no support for certain well-known platforms (maybe SunOS or
> > OSF/1, whatever) I may consider a different solution to the original
> > problem, but I'd like to know what I'm getting in to before I start
> > trying to read the docs for every bloody C compiler that ever hit the
> > 'net in the last twenty years.
>
> I use an alternative strategy: if I want to implement
> static double XYZ = nonconstant-expression;
>
> I instead do
> #define XYZ (*XYZ_())
> static double *XYZ_(void) {
> static double xyz;
> static bool initialised = false;
> LOCK
> if (!initialised) {
> xyz = nonconstant-expression;
> initialised = true;
> }
> UNLOCK
> return &xyz;
> }
>
> XYZ will be an lvalue just like any other variable; it gets initialised on the
> first use; the initialisation can be any complicated as desired, using any
> dependencies desired, without depending on the compiler and loader to sneak in
> the initialisation code in the right order.

Hey, is that how you ancients did things in the 80's era? I hope
there's a better way.

Fred the Freshwater Catfish

China Blue Veins

5/12/2011 11:30:00 PM

In article <iqhpa0$n31$1@dont-email.me>, Shao Miller <sha0.miller@gmail.com>
wrote:

> On 5/12/2011 5:59 PM, Fred the Freshwater Catfish wrote:
> > ...I'd like to use non-constant
> > initializers for some global variables. I'm pretty sure I can get it
> > to work the way I want under gcc, but what of other compilers? The
> > gcc documentation in info format says that ISO C99 allows this, and so
> > I expect some compilers built in the 90's will support the feature,
> > but to what extent is this true?
>
> Could you please share an example of what kind of non-constant you'd be
> inclined to initialize a static-duration, file-scoped object with? Are
> you talking about using a function's return value? Are you talking
> about using the address of something whose address you don't know at
> some point in time?

This is allowed in C++. The compiler writes out an initialisation section to the
object file. The loader then puts all these initialisation sections together and
runs them before running main(). It would be a simple extension to gcc since g++
and gcc use the same code generator.

--
Damn the living - It's a lovely life. I'm whoever you want me to be.
Silver silverware - Where is the love? At least I can stay in character.
Oval swimming pool - Where is the love? Annoying Usenet one post at a time.
Damn the living - It's a lovely life. In 1492....

Fred the Freshwater Catfish

5/12/2011 11:31:00 PM

On Fri, May 13, 2011 at 11:20:30AM +1200, Ian Collins wrote:
> On 05/13/11 11:17 AM, Fred the Freshwater Catfish wrote:
> >On Fri, May 13, 2011 at 11:07:09AM +1200, Ian Collins wrote:
> >>On 05/13/11 10:59 AM, Fred the Freshwater Catfish wrote:
> >>>Hey guys,
> >>>
> >>>I've been writing some code that I would prefer to be maximally
> >>>portable. As I have mostly used Linux, gcc is pretty much the only C
> >>>compiler I've ever used under UN*X. I'd like to use non-constant
> >>>initializers for some global variables. I'm pretty sure I can get it
> >>>to work the way I want under gcc, but what of other compilers? The
> >>>gcc documentation in info format says that ISO C99 allows this, and so
> >>>I expect some compilers built in the 90's will support the feature,
> >>>but to what extent is this true?
> >>
> >>How do you derive the initialiser values?
> >
> >It should end up being a function call most of the time.
>
> Then you either want C++, or a singleton fiddle as posted else-thread.

Sorry, I'm not using C++.

Fred the Freshwater Catfish

Ian Collins

5/12/2011 11:34:00 PM

On 05/13/11 11:30 AM, Fred the Freshwater Catfish wrote:
> On Fri, May 13, 2011 at 11:20:30AM +1200, Ian Collins wrote:
>> On 05/13/11 11:17 AM, Fred the Freshwater Catfish wrote:
>>> On Fri, May 13, 2011 at 11:07:09AM +1200, Ian Collins wrote:
>>>> On 05/13/11 10:59 AM, Fred the Freshwater Catfish wrote:
>>>>> Hey guys,
>>>>>
>>>>> I've been writing some code that I would prefer to be maximally
>>>>> portable. As I have mostly used Linux, gcc is pretty much the only C
>>>>> compiler I've ever used under UN*X. I'd like to use non-constant
>>>>> initializers for some global variables. I'm pretty sure I can get it
>>>>> to work the way I want under gcc, but what of other compilers? The
>>>>> gcc documentation in info format says that ISO C99 allows this, and so
>>>>> I expect some compilers built in the 90's will support the feature,
>>>>> but to what extent is this true?
>>>>
>>>> How do you derive the initialiser values?
>>>
>>> It should end up being a function call most of the time.
>>
>> Then you either want C++, or a singleton fiddle as posted else-thread.
>
> Sorry, I'm not using C++.

But you require a C++ feature...

You could compile all but main() as C and compile the source file
containing main() and your initialisers as C++.

--
Ian Collins

comp.lang.c

Non-constant initializers

Fred the Freshwater Catfish

Ian Collins

Shao Miller

China Blue Veins

Fred the Freshwater Catfish

Ian Collins

Uncle Steve

Fred the Freshwater Catfish

China Blue Veins

Fred the Freshwater Catfish

Ian Collins

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