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
Jan 6 '06
165  6933 
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.
You are the one doing the conflating. C supports pass by value
/only/. See "call by value: 3.3.2.2" 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./"
Passing by reference has to do with passing around actual
containers or in C's case, objects. 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".
--
aegis
Jordan Abel wrote: 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*****@hotm ail.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.
Here, here! Just because K&R describes the process of how a function is
called, i.e. copies are placed on the stack before the function call, does not
imply c is only pass by value.
Under the hood, Pascal and c++ do exactly the same technique. However, I will
concede the syntax in Pascal and c++ supports pass by reference, making it
easier for developers to not "think" about what they code.
I guess learning assembler as my first language has endeared me to the c
language and I *might* be hypersensitive.
Jordan Abel wrote: 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.
Ok if your argument is correct,
do you say C has (not emulate) linked lists,trees,... .because we can
create them iusing C
In that case C supports object orientation also.
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.
On 2006-01-09, aegis <ae***@mad.scie ntist.com> wrote:
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!
You just said the same thing, in response to the same message, three
different times, in three different ways. Did you forget about your two
earlier posts?
On 2006-01-09, M.B <mb*******@gmai l.com> wrote:
Jordan Abel wrote: 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.
Ok if your argument is correct,
do you say C has (not emulate) linked lists,trees,... .because we can
create them iusing C
In that case C supports object orientation also.
C neither supports nor does not support it, because it's not a language
feature, it's a programming technique. THE SAME is true for pass by
reference.
"emulate" isn't the proper word. it's something the programmer _can
implement_ in C. Just like a million other things.
Jordan Abel wrote: On 2006-01-09, M.B <mb*******@gmai l.com> wrote:
Jordan Abel wrote: 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.
Ok if your argument is correct,
do you say C has (not emulate) linked lists,trees,... .because we can
create them iusing C
In that case C supports object orientation also.
C neither supports nor does not support it, because it's not a language
feature, it's a programming technique.
No; it's a language feature; that's the use for which the term was
created, and the use it still retains. A language has "call by
reference" when a call f(x) can change the value of the variable
x as a result of assignments in f to the formal parameter corresponding
to x.
It is a point of distinction between Pascal (I speak of the non-Delphic
variety) and C (K&R, 89, and 99) that you can do this in Pascal (by
declaring the formal parameter to be `VAR`) and you can't do it in C
(and you can do it in C++).
You can certainly get a similar /effect/ in C. You can use an argument
type of pointer-to-Spoo instead of Spoo, replace all references to the
formal argument spoo by *spoo, and insert the necessary & in all the calls.
The fact that you have to /do/ this, consistently, everywhere, is part
of the reason for saying that C doesn't "support" (I would say "have",
as "support" is such an unnecessarily vulnerable word) call-by-reference.
The term "call-by-reference" is part of a family of terms [1] describing
the ways in which parameters may pass into and out of procedures. That
any of these ways might /also/ be done explicitly by additional code
in the language - as a technique as opposed to a feature - doesn't make
the terms valueless and the distinctions they draw irrelevant.
[1] Call by value, call by result, call by value-result, call by reference,
call by value, call by name. "Pass by" would have been better ...
--
Chris "believes seventeen improbable things before coffee" Dollin
Seventeen, forty-two - what else is there?
On 2006-01-09, Chris Dollin <ke**@hpl.hp.co m> wrote: Jordan Abel wrote:
On 2006-01-09, M.B <mb*******@gmai l.com> wrote:
Jordan Abel wrote:
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.
Ok if your argument is correct,
do you say C has (not emulate) linked lists,trees,... .because we can
create them iusing C
In that case C supports object orientation also.
C neither supports nor does not support it, because it's not a language
feature, it's a programming technique.
No; it's a language feature; that's the use for which the term was
created, and the use it still retains. A language has "call by
reference" when a call f(x) can change the value of the variable
x as a result of assignments in f to the formal parameter corresponding
to x.
What does the use for which the term "call by reference" was created
have to do with the original poster's use of the term "pass by
reference"? [which, i might add, was completely clear in context.]
The term "call-by-reference" is part of a family of terms [1] describing
the ways in which parameters may pass into and out of procedures. That
any of these ways might /also/ be done explicitly by additional code
in the language - as a technique as opposed to a feature - doesn't make
the terms valueless and the distinctions they draw irrelevant.
[1] Call by value, call by result, call by value-result, call by reference,
call by value, call by name. "Pass by" would have been better ...
the use "pass by" instead of "call by" distinguishes the programming
technique from the language feature.
Jordan Abel wrote: On 2006-01-09, Chris Dollin <ke**@hpl.hp.co m> wrote: Jordan Abel wrote:
On 2006-01-09, M.B <mb*******@gmai l.com> wrote:
Jordan Abel wrote:
> 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.
Ok if your argument is correct,
do you say C has (not emulate) linked lists,trees,... .because we can
create them iusing C
In that case C supports object orientation also.
C neither supports nor does not support it, because it's not a language
feature, it's a programming technique.
No; it's a language feature; that's the use for which the term was
created, and the use it still retains. A language has "call by
reference" when a call f(x) can change the value of the variable
x as a result of assignments in f to the formal parameter corresponding
to x.
What does the use for which the term "call by reference" was created
have to do with the original poster's use of the term "pass by
reference"? [which, i might add, was completely clear in context.]
Because "pass by reference" means - as far as I am aware - "call by
reference" (and vice versa). "call by reference" is the earlier term,
the one which comes to my fingers; sorry if I've injected confusion
by using one rather than t'other. The term "call-by-reference" is part of a family of terms [1] describing
the ways in which parameters may pass into and out of procedures. That
any of these ways might /also/ be done explicitly by additional code
in the language - as a technique as opposed to a feature - doesn't make
the terms valueless and the distinctions they draw irrelevant.
[1] Call by value, call by result, call by value-result, call by
[reference,
call by value, call by name. "Pass by" would have been better ...
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?
--
Chris "believes seventeen improbable things before coffee" Dollin
Seventeen, forty-two - what else is there?
In article <sl************ **********@rand om.yi.org> Jordan Abel <ra*******@gmai l.com> writes: 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.
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) and pass by value), you can not see from the
line:
routine(argumen t)
whether the argument is passed by reference or by value. Note also
that in some languages (Fortran) pass by reference is indistinguishab le
from pass by value-return (i.e. the value of the argument is passed to
the routine and when the routine terminates a new value is assigned to
the argument).
In C pass by references is only performed with array's (but it is actually
simulated by The Rule).
--
dik t. winter, cwi, kruislaan 413, 1098 sj amsterdam, nederland, +31205924131
home: bovenover 215, 1025 jn amsterdam, nederland; http://www.cwi.nl/~dik/ This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
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