segfault w/ block, but not file scope

[Dieter]

Hi.

In the snippet of code below, I'm trying to understand why when the

struct dirent ** namelist

is declared with "file" scope, I don't have a problem freeing the
allocated memory. But when the struct is declared in main (block scope)
it will segfault when passing namelist to freeFileNames() .

Since this seems to be just a matter of my understanding scope and
pointer parameter passing better, I only included what thought to be
relevant code. I'll happily provide compilable code if deemed necessary.

Please see commented lines:
struct dirent **namelist; /* file scope works */

int main(void)
{
/* struct dirent **namelist */ /* block scope doesn't work */
int n;

n = getFileNames(H5 DIR, namelist); /* included from mylib.h */
freeFileNames(n amelist, n); /* included from mylib.h */

return 0;
}
Thank you very much for your comments,
Dieter

[Jordan Abel]

On 2006-01-09, Keith Thompson <ks***@mib.or g> wrote:

[context is the term "pass by reference]

What do you call the corresponding programming technique? (Ideally,
I'd like to see a pity and language-independent term, just as we use
the term "linked list" regardless of the implementation language.)

If some other language, say C+++ (sic),
Or, say, lisp.
had built-in support for linked lists, would you insist on not using
the term "linked list" in the context of a C program?

Apparently not.

[Jordan Abel]

On 2006-01-09, aegis <ae***@mad.scie ntist.com> wrote:

Jordan Abel wrote:

On 2006-01-08, aegis <ae***@mad.scie ntist.com> wrote:

> Passing by reference has to do with passing around actual containers
> or in C's case, objects.

I disagree.

> We don't do that in C. We instead pass the
> values stored in objects around to functions. The standard is clear
> here and supports absolutely diddly squat in regards to a concept
> called "pass by reference".

The language itself supports absolutely squat in regards to another
concept called "linked list", or "binary tree", or any number of other
things, which means absolutely squat to people who use those concepts
anyway in C regardless.

What a terrible comparison. The actual passing mechanism supported
by C is defined by the standard. Because it is a mechanism of the
language. Concepts such as 'linked list' and 'binary tree' are
implemented with the language. Not /into/ the language. So you
compare to a concept not associated with making up the actual
language. Call by value or pass by value is clearly outlined in
section 3.3.2.2 of c89. Which section describes this so called
concept 'pass by reference' or 'call by reference'? I suspect
there is no section as there is no mechanism of such implemented
into the language.

yet there _is_ a technique, which has been described in detail,
which can be implemented with C, and which is used extensively in
the standard library, and you have repeatedly failed to put forth a
better name for it than "pass by reference".

[Dik T. Winter]

In article <dp**********@m alatesta.hpl.hp .com> Chris Dollin <ke**@hpl.hp.co m> writes:

Dik T. Winter wrote:

The distinction between passing by reference and passing by value is
that at the callers end you can not distinguish the two, it is at
the callees end that the two are distinguished. So in Algol 60
(which has proper pass by reference (actually pass by name, but the
distinction is minor)

Surely not, as Jensen's Device illustrates. (And trying to compile
it.)

In the context the difference is minor. And I know exactly how call
by name was compiled on the system I did use.
--
dik t. winter, cwi, kruislaan 413, 1098 sj amsterdam, nederland, [phone]
home: bovenover 215, 1025 jn amsterdam, nederland; http://www.cwi.nl/~dik/

[Dik T. Winter]

In article <dp***********@ pc-news.cogsci.ed. ac.uk> ri*****@cogsci. ed.ac.uk (Richard Tobin) writes:

In article <Is********@cwi .nl>, Dik T. Winter <Di********@cwi .nl> wrote:

(which has proper pass by reference (actually pass by name, but the
distinction is minor) and pass by value)
The distinction is minor when the actual parameter is a variable, but
not when it is an expression.

I know all that. I have quite a bit of experience with Algol 60 (it was
the first computer language I ever used). My "minor" was intended in
context.

Anyhow the major distinction between call by value, reference, name,
value-return, or whatever is not at the callers side. It is at the
callees side. Even in Algol 68 you could write:
function(argume nt)
without knowing whether "argument" could be changed or not, unless you
looked at the definition of "function". In this sense C is an improvement
on Algol 68; if I write:
function(argume nt)
I am sure that argument will not change, at most what argument points to.
In Algol 68 this is quite a bit more subtle.
not when it is an expression. In the latter case you have something
like a closure (traditionally called a "thunk" in this context).

Or was your use of "proper" meant to imply that pass by reference
ought to work like this?

-- Richard

--
dik t. winter, cwi, kruislaan 413, 1098 sj amsterdam, nederland, [phone]
home: bovenover 215, 1025 jn amsterdam, nederland; http://www.cwi.nl/~dik/

[aegis]

Jordan Abel wrote:

On 2006-01-09, aegis <ae***@mad.scie ntist.com> wrote:

Jordan Abel wrote:

On 2006-01-08, aegis <ae***@mad.scie ntist.com> wrote:
> Passing by reference has to do with passing around actual containers
> or in C's case, objects.

I disagree.

> We don't do that in C. We instead pass the
> values stored in objects around to functions. The standard is clear
> here and supports absolutely diddly squat in regards to a concept
> called "pass by reference".

The language itself supports absolutely squat in regards to another
concept called "linked list", or "binary tree", or any number of other
things, which means absolutely squat to people who use those concepts
anyway in C regardless.

What a terrible comparison. The actual passing mechanism supported
by C is defined by the standard. Because it is a mechanism of the
language. Concepts such as 'linked list' and 'binary tree' are
implemented with the language. Not /into/ the language. So you
compare to a concept not associated with making up the actual
language. Call by value or pass by value is clearly outlined in
section 3.3.2.2 of c89. Which section describes this so called
concept 'pass by reference' or 'call by reference'? I suspect
there is no section as there is no mechanism of such implemented
into the language.

yet there _is_ a technique, which has been described in detail,
which can be implemented with C, and which is used extensively in
the standard library, and you have repeatedly failed to put forth a
better name for it than "pass by reference".

My original post was in reply to your opposition of the claim
that C does not support 'pass by reference'. I thought it was clear
that this was about a mechanism of the language and not
something you implement with the language. If instead,
you mean to say the value of a pointer object can
also be called a reference and that when passing it to
a function therefore qualifies the language as supporting
'pass by reference', then I think that you are confused. What
you really want to say is 'pass a reference'. I know this is
an international forum so English may not be your native
language but do you understand the difference?
--
aegis

[Chris Dollin]

Jordan Abel wrote:

On 2006-01-09, Chris Dollin <ke**@hpl.hp.co m> wrote:

the use "pass by" instead of "call by" distinguishes the programming
technique from the language feature.

That's interesting distinction, but the only place I ever seen it made
is in your post - have you a (preferably online) reference to update me?

I coined it just now because I think the programming technique deserves
a name. What would you call the technique used in the list of standard
library functions i named upthread?

If you could be more explicit about the list (I looked back a way but
found it not), then I could offer a name.

I note that the pass by / call by distinction you offer is idiosyncratic
to you.

--
Chris "believes seventeen improbable things before coffee" Dollin
Seventeen, forty-two - what else is there?

[Chris Dollin]

Keith Thompson wrote:

Here are a couple of questions for those of you who insist that the
term "pass by reference" should apply only to a built-in language
feature:

What do you call the corresponding programming technique? (Ideally,
I'd like to see a pity and language-independent term, just as we use
the term "linked list" regardless of the implementation language.)
"passing a pointer". Or "indirectio n". Or "passing a reference". Or
even (as I'm an old fogey who defaults, as we have seen, to the term
"call by reference" for the language feature) "pass by reference",
but since that appears to be becomming the term for the feature, I'd
best change my habits.
If some other language, say C+++ (sic), had built-in support for
linked lists, would you insist on not using the term "linked list" in
the context of a C program?

No. I'd just say that C+++ "had" linked lists, and C didn't. (Actually
I'd be likely to say that they had/had not "lists", because my own
idiosyncracy is to think of "list" as /meaning/ linked list; these
funny psuedo-arrays that eg Java call "lists" always seem to me to
be misnamed. But I know that's Just Me.)

I'd try to be /clear/, in any case. "Well, C+++ has linked lists as
a built-in feature; C doesn't have them, but you can implement them,
or something pretty close."

--
Chris "believes seventeen improbable things before coffee" Dollin
(But the coffee machine is broken. Belief overload!)

[M.B]

Chris Dollin wrote:

Keith Thompson wrote:

Here are a couple of questions for those of you who insist that the
term "pass by reference" should apply only to a built-in language
feature:

What do you call the corresponding programming technique? (Ideally,
I'd like to see a pity and language-independent term, just as we use
the term "linked list" regardless of the implementation language.)
"passing a pointer". Or "indirectio n". Or "passing a reference". Or
even (as I'm an old fogey who defaults, as we have seen, to the term
"call by reference" for the language feature) "pass by reference",
but since that appears to be becomming the term for the feature, I'd
best change my habits.

If some other language, say C+++ (sic), had built-in support for
linked lists, would you insist on not using the term "linked list" in
the context of a C program?

No. I'd just say that C+++ "had" linked lists, and C didn't. (Actually
I'd be likely to say that they had/had not "lists", because my own
idiosyncracy is to think of "list" as /meaning/ linked list; these
funny psuedo-arrays that eg Java call "lists" always seem to me to
be misnamed. But I know that's Just Me.)

I'd try to be /clear/, in any case. "Well, C+++ has linked lists as
a built-in feature; C doesn't have them, but you can implement them,
or something pretty close."

Does this answer this long argument
http://c-faq.com/ptrs/passbyref.html
--
Chris "believes seventeen improbable things before coffee" Dollin
(But the coffee machine is broken. Belief overload!)

[Jordan Abel]

On 2006-01-10, aegis <ae***@mad.scie ntist.com> wrote:

My original post was in reply to your opposition of the claim that C
does not support 'pass by reference'. I thought it was clear that this
was about a mechanism of the language and not something you implement
with the language. If instead, you mean to say the value of a pointer
object can also be called a reference and that when passing it to a
function therefore qualifies the language as supporting 'pass by
reference', then I think that you are confused. What you really want
to say is 'pass a reference'.
yes - you "pass a reference" _by which reference_ you are thus passing
the object whose address is taken. thus "pass by [that] reference". the
'reference' or 'pointer' is something you are _using_ as a mechanism to
pass something, which is why the word "by" is used. The only difference
is whether you, the programmer, are doing it, vs the compiler doing it.
I know this is an international forum so English may not be your
native language but do you understand the difference?

I think _you_ are the one who is not looking at the actual meanings of
the words, and unjustifiably seeing the phrase "pass by reference" as
indivisible.

[Jordan Abel]

On 2006-01-10, Chris Dollin <ke**@hpl.hp.co m> wrote:

Keith Thompson wrote:

Here are a couple of questions for those of you who insist that the
term "pass by reference" should apply only to a built-in language
feature:

What do you call the corresponding programming technique? (Ideally,
I'd like to see a pity and language-independent term, just as we use
the term "linked list" regardless of the implementation language.)

"passing a pointer". Or "indirectio n". Or "passing a reference". Or
even (as I'm an old fogey who defaults, as we have seen, to the term
"call by reference" for the language feature) "pass by reference",
but since that appears to be becomming the term for the feature, I'd
best change my habits.

If some other language, say C+++ (sic), had built-in support for
linked lists, would you insist on not using the term "linked list" in
the context of a C program?

No. I'd just say that C+++ "had" linked lists, and C didn't. (Actually
I'd be likely to say that they had/had not "lists", because my own
idiosyncracy is to think of "list" as /meaning/ linked list; these
funny psuedo-arrays that eg Java call "lists" always seem to me to
be misnamed. But I know that's Just Me.)

I'd try to be /clear/, in any case. "Well, C+++ has linked lists as
a built-in feature; C doesn't have them, but you can implement them,
or something pretty close."

But saying "C doesn't have linked lists, this isn't C+++" in response to
EVERY mention of "linked list", even when it was clear that it wasn't
about a language feature, would be widely seen as a troll, wouldn't it?