Global pointer pointing to locally defined type

[Andrej Prsa]

Hi!

What happens to a globally defined pointer, e.g.

void *value;

that points to a particular type in a function:

int dummy_func (int a)
{
value = &a;
return 0;
}

after the function returns to main? If this doesn't work, how could I make
it work? Do I have to allocate memory for this int and copy the original
int to that and then point value to this newly allocated int?

Thanks for your time,

Andrej

[Joona I Palaste]

Andrej Prsa <an*********@gu est.arnes.si> scribbled the following:

Hi! What happens to a globally defined pointer, e.g. void *value; that points to a particular type in a function: int dummy_func (int a)
{
value = &a;
return 0;
} after the function returns to main?
It points to a variable that is no longer in scope, thus no longer
accessible. Any attempt to indirect through that pointer will cause
undefined behaviour. In short, the pointer points to garbage and
cannot be used until reassigned.
If this doesn't work, how could I make
it work? Do I have to allocate memory for this int and copy the original
int to that and then point value to this newly allocated int?

That could be one way, yes.

--
/-- Joona Palaste (pa*****@cc.hel sinki.fi) ------------- Finland --------\
\-- http://www.helsinki.fi/~palaste --------------------- rules! --------/
"'It can be easily shown that' means 'I saw a proof of this once (which I didn't
understand) which I can no longer remember'."
- A maths teacher

[Joona I Palaste]

Andrej Prsa <an*********@gu est.arnes.si> scribbled the following:

Hi! What happens to a globally defined pointer, e.g. void *value; that points to a particular type in a function: int dummy_func (int a)
{
value = &a;
return 0;
} after the function returns to main?
It points to a variable that is no longer in scope, thus no longer
accessible. Any attempt to indirect through that pointer will cause
undefined behaviour. In short, the pointer points to garbage and
cannot be used until reassigned.
If this doesn't work, how could I make
it work? Do I have to allocate memory for this int and copy the original
int to that and then point value to this newly allocated int?

That could be one way, yes.

--
/-- Joona Palaste (pa*****@cc.hel sinki.fi) ------------- Finland --------\
\-- http://www.helsinki.fi/~palaste --------------------- rules! --------/
"'It can be easily shown that' means 'I saw a proof of this once (which I didn't
understand) which I can no longer remember'."
- A maths teacher

[Andrej Prsa]

Hi!

If this doesn't work, how could I make
it work? Do I have to allocate memory for this int and copy the original
int to that and then point value to this newly allocated int?

That could be one way, yes.

Is there any better way you'd suggest?

Andrej

[Andrej Prsa]

Hi!

If this doesn't work, how could I make
it work? Do I have to allocate memory for this int and copy the original
int to that and then point value to this newly allocated int?

That could be one way, yes.

Is there any better way you'd suggest?

Andrej

[Joona I Palaste]

Andrej Prsa <an*********@gu est.arnes.si> scribbled the following:

Hi!

> If this doesn't work, how could I make
> it work? Do I have to allocate memory for this int and copy the original
> int to that and then point value to this newly allocated int?
That could be one way, yes.

Is there any better way you'd suggest?

Pass the address of a variable as a parameter to your function.

--
/-- Joona Palaste (pa*****@cc.hel sinki.fi) ------------- Finland --------\
\-- http://www.helsinki.fi/~palaste --------------------- rules! --------/
"All that flower power is no match for my glower power!"
- Montgomery Burns

[Joona I Palaste]

Andrej Prsa <an*********@gu est.arnes.si> scribbled the following:

Hi!

> If this doesn't work, how could I make
> it work? Do I have to allocate memory for this int and copy the original
> int to that and then point value to this newly allocated int?
That could be one way, yes.

Is there any better way you'd suggest?

Pass the address of a variable as a parameter to your function.

--
/-- Joona Palaste (pa*****@cc.hel sinki.fi) ------------- Finland --------\
\-- http://www.helsinki.fi/~palaste --------------------- rules! --------/
"All that flower power is no match for my glower power!"
- Montgomery Burns

[Martin Dickopp]

Andrej Prsa <an*********@gu est.arnes.si> writes:

> If this doesn't work, how could I make it work? Do I have to
> allocate memory for this int and copy the original int to that and
> then point value to this newly allocated int?

That could be one way, yes.

Is there any better way you'd suggest?

I recommend to avoid "global" (file scope) objects whenever possible.

What's the best solution to your problem depends on what your problem
is. :) If you describe a bit more elaborately what you're actually
trying to achieve, we might be able to make better suggestions.

Martin
--
,--. Martin Dickopp, Dresden, Germany ,= ,-_-. =.
/ ,- ) http://www.zero-based.org/ ((_/)o o(\_))
\ `-' `-'(. .)`-'
`-. Debian, a variant of the GNU operating system. \_/

[Martin Dickopp]

Andrej Prsa <an*********@gu est.arnes.si> writes:

> If this doesn't work, how could I make it work? Do I have to
> allocate memory for this int and copy the original int to that and
> then point value to this newly allocated int?

That could be one way, yes.

Is there any better way you'd suggest?

I recommend to avoid "global" (file scope) objects whenever possible.

What's the best solution to your problem depends on what your problem
is. :) If you describe a bit more elaborately what you're actually
trying to achieve, we might be able to make better suggestions.

Martin
--
,--. Martin Dickopp, Dresden, Germany ,= ,-_-. =.
/ ,- ) http://www.zero-based.org/ ((_/)o o(\_))
\ `-' `-'(. .)`-'
`-. Debian, a variant of the GNU operating system. \_/

[Andrej Prsa]

Hi!

> If this doesn't work, how could I make it work? Do I have to
> allocate memory for this int and copy the original int to that and
> then point value to this newly allocated int?

That could be one way, yes.

Is there any better way you'd suggest?

I recommend to avoid "global" (file scope) objects whenever possible.

What's the best solution to your problem depends on what your problem
is. :) If you describe a bit more elaborately what you're actually
trying to achieve, we might be able to make better suggestions.

Sure! I'm making a script language for modeling of eclipsing binaries. I
want to be able to reach a parameter table from all kinds of plug-ins to
this language, thus I have a global table like this:

typedef struct PHOEBE_paramete r_tag
{
GtkWidget *widget;
char *qualifier;
char *keyword;
char *description;
PHOEBE_type type;
void *valueptr;
} PHOEBE_paramete r_tag;

PHOEBE_paramete r_tag *PHOEBE_paramet ers;
int PHOEBE_paramete rs_no;

Here PHOEBE_type is a simple enumeration that defines all types that a
particular argument can be and GtkWidget is a GTK+ widget (GUI related).
PHOEBE_paramete rs is a global array, to which parameters are added by
calling a function:

void declare_paramet er (char *qualifier, GtkWidget *parent, char *widget,
char *keyword, char *description, ...)

The variable argument list holds the type and the initial value, e.g.:

declare_paramet er ("phoebe_name_v alue", PHOEBE, "data_star_name _entry",
"NAME", "Star name", TYPE_STRING, "New star");

As you may imagine, I want the valueptr of the global PHOEBE_paramete rs[i]
variable to point to whatever value corresponds to the given parameter.
Since it can be int, double, char *, but also an array of those, I hoped a
generic pointer void * would do the trick for me. And since this is a
back-end for a very large project (PHOEBE - PHysics Of Eclipsing
BinariEs), I want it to be, well, perfect! That's why I'm consulting you
guys!

Thanks for your time reading and answering!

Best wishes,

Andrej