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

[Richard Heathfield]

Jordan Abel said:

On 2006-01-08, Richard Heathfield <in*****@invali d.invalid> wrote:

[...] Jordan is wrong about time() which does not in fact
take its argument by reference. It takes its parameter by value.

It takes a time_t by reference.

No, it doesn't. It takes a time_t * by value.

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at above domain (but drop the www, obviously)

[Jordan Abel]

On 2006-01-08, Richard Heathfield <in*****@invali d.invalid> wrote:

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).

I know what you're saying, but I think that's an own-foot-shooting way to
say it.

"own-foot-shooting" wording is something that C has its fair share of,
so best not to cast the first stone. See "equivalenc e of pointers and
arrays" (in, of course, all but three contexts), %f in printf vs scanf
(traceable to the problem of float-to-double argument promotion in
general), the example rand(), etc, etc.

To name an issue that recently came up in here, I think it would have
been better if %s for scanf took "pointer to array of char" rather than
"pointer to char as first member of array".

In particular, the use of the term "object" in C to refer to something
other than what it means to those familiar with other languages is
comparable to this use of "pass by reference" to refer to a programming
technique rather than the language feature it sometimes means instead.

[Jordan Abel]

On 2006-01-08, Chuck F. <cb********@yah oo.com> wrote:

Fortunately for JRN the use of pass-by-name has virtually
disappeared. In fact fortunately for most of us, IMO.

However, see also C Preprocessor macro expansion.

[Jordan Abel]

On 2006-01-08, Richard Heathfield <in*****@invali d.invalid> wrote:

Jordan Abel said:

On 2006-01-08, Richard Heathfield <in*****@invali d.invalid> wrote:

[...] Jordan is wrong about time() which does not in fact
take its argument by reference. It takes its parameter by value.

It takes a time_t by reference.

No, it doesn't. It takes a time_t * by value.

Which is the means by which it implements a reference to a time_t.
Again, the programming technique (implemented quite easily in C) is
different from the language feature (which C, of course, does not have).

[Keith Thompson]

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

Jordan Abel said:

On 2006-01-08, Richard Heathfield <in*****@invali d.invalid> wrote:

[...] Jordan is wrong about time() which does not in fact
take its argument by reference. It takes its parameter by value.

It takes a time_t by reference.

No, it doesn't. It takes a time_t * by value.

It does that too.

--
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.

[Mark McIntyre]

On Sun, 08 Jan 2006 04:51:47 GMT, in comp.lang.c , Joseph Dionne
<jd*****@hotmai l.com> wrote:

It is just as amusing as you assertion, backed by no well known authority of
the c language specification, that c does not support pass by reference.
Well, jackass, it doesn't. It supports pass by value, and you may pass
the value of a pointer to emulate pass by reference.
Your continued reference to 'object' during replies proves my assertions

no, it proves he has more patience than most folk.

You're a troll.

*plonk*
Mark McIntyre
--

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[Mark McIntyre]

On Sun, 08 Jan 2006 00:01:42 GMT, in comp.lang.c , Joseph Dionne
<jd*****@hotmai l.com> wrote:

Mark McIntyre wrote:

On Sat, 07 Jan 2006 23:23:22 GMT, in comp.lang.c , Joseph Dionne
<jd*****@hotmai l.com> wrote:

So when enough people refer to the period from sunrise to sunshine (the day)
as night, we all need to use their incorrect definition?

If you want to avoid confusion, and being thought of as a pompous ass,
yes IMHO. YMMV. HAND.

Sorry, Mr. McIntyre, software is a precision art, not hand grenades or
horseshoes. It has a history of development that ought not be forgotten or
convoluted by future misrepresentati ons simply because the originating concept
is difficult to grasp.

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

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[Jordan Abel]

On 2006-01-08, Mark McIntyre <ma**********@s pamcop.net> wrote:

On Sun, 08 Jan 2006 04:51:47 GMT, in comp.lang.c , Joseph Dionne
<jd*****@hotmai l.com> wrote:

It is just as amusing as you assertion, backed by no well known authority of
the c language specification, that c does not support pass by reference.

Well, jackass, it doesn't. It supports pass by value, and you may pass
the value of a pointer to emulate pass by reference.

That's not "emulating pass by reference", that is passing by reference.
The fact that the language doesn't have special syntax for it doesn't
mean it's suddenly a different programming technique.

[Chris Torek]

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.
The fact that the language doesn't have special syntax for it doesn't
mean it's suddenly a different programming technique.

The phrase "programmin g technique" is a little bit slippery (in
much the same sense that an elephant is a little bit large :-) ).
C does not have garbage collection either, but you can "emulate"
it by replacing all operations of the form:

p = malloc(nbytes);
...
q = p;
...
q = r;
...
free(q);
...

with:

record_release( p); p = malloc(nbytes); record_ref(p);
...
record_release( q); q = p; record_ref(q);
...
record_release( q); q = r; record_ref(q);
...
record_release( q); free(q); q = NULL; /* for future record_release */

(make sure that p==NULL and q==NULL initially, of course).

If you do this, would you claim that C now "has" garbage collection?

You can emulate by-reference more easily than you can emulate
automatic garbage collection. Does the difficulty of emulation
play a role in deciding whether the language "has" the thing being
emulated?
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) [phone]
email: forget about it http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers.

[Richard Tobin]

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.

-- Richard