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

[Chad]

<OT>
foo
</OT>

Chad

[Keith Thompson]

ri*****@cogsci. ed.ac.uk (Richard Tobin) writes:

In article <sl************ **********@rand om.yi.org>,
Jordan Abel <ra*******@gmai l.com> wrote:

That's not "emulating pass by reference", that is passing by reference.

You will find yourself talking at cross-purposes if you take that
approach, since most people don't take "has call-by-reference" to mean
"can achieve the effect of call-by-reference by use of a pointer".

You're well on your way to the Turing tar-pit.

The phrase wasn't "has call-by-reference" (referring to the C
language), it was "is passing by referenece" (referring to the
technique of passing a pointer to an object to allow a function to
modify the object).

C doesn't "have" linked lists, but in a program using the usual
technique involving structures and pointers, the linked lists are real
linked lists.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[M.B]

Keith Thompson wrote:

"M.B" <mb*******@gmai l.com> writes:

Chuck F. wrote:

M.B wrote:
> sorry if thjis is a duplicate reply...
>
> I guess this is a scope issue for variable "namelist" I suggest
> to change the function int getFileName(cha r *,struct dirent
> ***); and make necessary changes to code. it may work fine
>
> bcoz scandir i guess sets "struct dirent **" itself. if its
> global all is fine, but local variables 0 its problem-same as
> pass by value

Please use English, not dudespeak, in this newsgroup, and include
adequate context. Between the two your article is totally
meaningless. See below for further advice.
For letting out context, my mistake sorry.
English - hope this is not an english literature group. People want
answers and its enough if they get those in anyway. go hell with
correct English.

You don't need to write flawless English, especially if it doesn't
happen to be your native language, but avoiding abbreviations like "u"
for "you" and "bcoz" for "because" make it *much* easier to understand
you.

Too many SMS s . I will try

Also please check if the advice given in your signature works
correctly.
As far as I know it does. Are you saying it doesn't?

Not for me (and i dont know why).

And finally, noone has time and patience to reply to trolls.
Chuck F. is not a troll. Trust me on this.

I know. Chuck F and others, I apologise

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Jordan Abel]

On 2006-01-09, Richard Tobin <ri*****@cogsci .ed.ac.uk> wrote:

In article <sl************ **********@rand om.yi.org>,
Jordan Abel <ra*******@gmai l.com> wrote:

That's not "emulating pass by reference", that is passing by reference.

You will find yourself talking at cross-purposes if you take that
approach, since most people don't take "has call-by-reference" to mean
"can achieve the effect of call-by-reference by use of a pointer".

Nobody said "has call-by-reference". In the initial context, the mention
of "pass by reference / pass by value" was perfectly clear. It's only
this subsequent arguing in circles that has muddied the waters.

[Default User]

Mark McIntyre wrote:

I have no clue what that babble means, but then, Trolls rarely make
sense.

You're starting to get it. Now, the final enlightenment will be to stop
replying to this cretin.

Brian

[nelu]

Keith Thompson wrote:

Then we need to make it very clear to J Random Newbie that C doesn't
support pass by reference as a languag construct. Don't say that the
"*" means pass by reference; say that the "*" means you're passing a
pointer value, which can be used to do the equivalent of pass by
reference. Until J Random Newbie is able to hold that entire complex
thought in his head, he's not going to be able to be an effective C
programmer.

http://publib.boulder.ibm.com/infoce...dfhp3b0037.htm
Now, the title of the article refers to both C and C++ so I'm not sure
about it but they say: "Passing a pointer is also known as passing a
value by reference.". But then what would be the difference between *
and & in C++?

[aegis]

Jordan Abel wrote:

On 2006-01-07, Keith Thompson <ks***@mib.or g> wrote:

Richard Heathfield <in*****@invali d.invalid> writes:

Keith Thompson said:
(I'm 100% certain that I'm not telling you anything you don't already
know; the point is to figure out just where we disagree.)

I'm for whatever makes C easier to understand. Heaven knows it's hard enough
already, without making it harder.

When you say "pointers" and "pass by reference", yes, OF COURSE I know what
you mean. But J Random Newbie over there, who already has a hundred new
concepts buzzing around his head right now and who does not fully
understand what pointers *are* yet, let alone what they are *for*, is going
to think "pointers, right, the little * thing, okay, pass-by-reference,
right, * means pass by reference, which means I can update it, great..."
and he is going to think he understands things a little better - and then
he is (eventually) going to wonder why this:

void allocstr(char *s, size_t n)
{
s = malloc(n);

if(s == NULL) abort();
}

doesn't seem to do what he wants.

Then we need to make it very clear to J Random Newbie that C doesn't
support pass by reference as a languag construct. Don't say that the
"*" means pass by reference; say that the "*" means you're passing a
pointer value, which can be used to do the equivalent of pass by
reference. Until J Random Newbie is able to hold that entire complex
thought in his head, he's not going to be able to be an effective C
programmer.

Pass by reference is a very common and useful programming technique,
one that C supports quite well (though arguably a little more clumsily
than some other languages do).

And uses it extensively in the standard library:

math.h: frexp, modf,
stdio.h: fscanf (and friends), fprintf (and friends, for %n), fgetpos,
fsetpos,
stdlib.h: strtol (and friends),
time.h: mktime, time, asctime, asctime, ctime, gmtime, localtime

Possibly others i've missed from later standards (this was written by
grepping c89 for (.*\*.*) and manually selecting the functions that use
this mechanism.

This is suppose to be suggestive of C supporting
pass by reference? Are you kidding? Chapter and verse
please!

--
aegis

[aegis]

Jordan Abel wrote:

On 2006-01-07, Keith Thompson <ks***@mib.or g> wrote:

Richard Heathfield <in*****@invali d.invalid> writes:

Keith Thompson said:
(I'm 100% certain that I'm not telling you anything you don't already
know; the point is to figure out just where we disagree.)

I'm for whatever makes C easier to understand. Heaven knows it's hard enough
already, without making it harder.

When you say "pointers" and "pass by reference", yes, OF COURSE I know what
you mean. But J Random Newbie over there, who already has a hundred new
concepts buzzing around his head right now and who does not fully
understand what pointers *are* yet, let alone what they are *for*, is going
to think "pointers, right, the little * thing, okay, pass-by-reference,
right, * means pass by reference, which means I can update it, great..."
and he is going to think he understands things a little better - and then
he is (eventually) going to wonder why this:

void allocstr(char *s, size_t n)
{
s = malloc(n);

if(s == NULL) abort();
}

doesn't seem to do what he wants.

Then we need to make it very clear to J Random Newbie that C doesn't
support pass by reference as a languag construct. Don't say that the
"*" means pass by reference; say that the "*" means you're passing a
pointer value, which can be used to do the equivalent of pass by
reference. Until J Random Newbie is able to hold that entire complex
thought in his head, he's not going to be able to be an effective C
programmer.

Pass by reference is a very common and useful programming technique,
one that C supports quite well (though arguably a little more clumsily
than some other languages do).

And uses it extensively in the standard library:

math.h: frexp, modf,
stdio.h: fscanf (and friends), fprintf (and friends, for %n), fgetpos,
fsetpos,
stdlib.h: strtol (and friends),
time.h: mktime, time, asctime, asctime, ctime, gmtime, localtime

Possibly others i've missed from later standards (this was written by
grepping c89 for (.*\*.*) and manually selecting the functions that use
this mechanism.

How does this make your case? This in no way
suggests that C supports pass by reference.
If you keep on insisting that C supports this concept
then I will have to ask you for a chapter and
verse please.

--
aegis

[aegis]

Jordan Abel wrote:

On 2006-01-07, Joe Wright <jo********@com cast.net> wrote:

M.B wrote:

Richard Heathfield wrote:

M.B said:
>its a pass by value v/s pass by reference issue

Since C doesn't have pass by reference, it is clear to me that you are
talking nonsense.

I presume u area c++ guy to talk like this.
pass by reference is a generic term for ur info.
C iimplements this by pointers.

M.B., you clearly don't know what 'pass by reference' means. C doesn't
do it, even with pointers. Whether C++ does it is of no interest here.

Yes it does. time() takes its argument by reference [as do many other
functions in time.h, for historical reasons] You are conflating a
generic concept with a specific feature C++ has that C does not.

No it does not. See "call by value: 3.3.2.2." straight from c89.
Specifically paragraph two. Which states:

"If the expression that denotes the called function has a type that
includes a prototype, the number of arguments shall agree with the
number of parameters. /Each argument shall have a type such that [its
value] may be assigned to an object with the unqualified version of the
type of its corresponding parameter./"

Pass by reference is about passing around actual containers
and not values. The standard is clear here.
There are no two ways about it.

--
aegis

[aegis]

Jordan Abel wrote:

On 2006-01-07, Keith Thompson <ks***@mib.or g> wrote:

Richard Heathfield <in*****@invali d.invalid> writes:

Keith Thompson said:
(I'm 100% certain that I'm not telling you anything you don't already
know; the point is to figure out just where we disagree.)

I'm for whatever makes C easier to understand. Heaven knows it's hard enough
already, without making it harder.

When you say "pointers" and "pass by reference", yes, OF COURSE I know what
you mean. But J Random Newbie over there, who already has a hundred new
concepts buzzing around his head right now and who does not fully
understand what pointers *are* yet, let alone what they are *for*, is going
to think "pointers, right, the little * thing, okay, pass-by-reference,
right, * means pass by reference, which means I can update it, great..."
and he is going to think he understands things a little better - and then
he is (eventually) going to wonder why this:

void allocstr(char *s, size_t n)
{
s = malloc(n);

if(s == NULL) abort();
}

doesn't seem to do what he wants.

Then we need to make it very clear to J Random Newbie that C doesn't
support pass by reference as a languag construct. Don't say that the
"*" means pass by reference; say that the "*" means you're passing a
pointer value, which can be used to do the equivalent of pass by
reference. Until J Random Newbie is able to hold that entire complex
thought in his head, he's not going to be able to be an effective C
programmer.

Pass by reference is a very common and useful programming technique,
one that C supports quite well (though arguably a little more clumsily
than some other languages do).

And uses it extensively in the standard library:

math.h: frexp, modf,
stdio.h: fscanf (and friends), fprintf (and friends, for %n), fgetpos,
fsetpos,
stdlib.h: strtol (and friends),
time.h: mktime, time, asctime, asctime, ctime, gmtime, localtime

Possibly others i've missed from later standards (this was written by
grepping c89 for (.*\*.*) and manually selecting the functions that use
this mechanism.

This is suppose to be suggestive of C supporting
pass by reference? You will have to try harder than that.
Chapter and verse please!

--
aegis