memset

[Bill Cunningham]

I think alot of my problem is not understanding what a function is
asking for when it's asking for a pointer. If it's asking for a pointer is
it wanting what the pointer is pointing too. memset for example.

void *memset (void *s, int c, size_t n);

that first parameter. Is it want a pointer to something or to be passed what
the pointer is pointing to. For example.

int *p;

memset(p,0,sizeof(int));
or memset(&p,0,sizeof(int));

Is such a thing as a function that wants this?

function (int &a, short &b);

If something is defined like that would you just pass the pointer?

Bill

 

[Bill Cunningham]

Hum. Is this pass by reference? I want C not C++.

Bill

 

[glen herrmannsfeldt]

Hum. Is this pass by reference? I want C not C++.

C is pass by value (or call by value). If you want a function
call to be able to change something, you pass a pointer to
the something.

But also you often (usually) pass a pointer when you are
passing an array or structure. Before ANSI C, you couldn't pass
structures directly, only through pointers. ANSI allows it, but
it isn't used all that often.

-- glen

 

[Bill Cunningham]

C is pass by value (or call by value). If you want a function
call to be able to change something, you pass a pointer to
the something.

But also you often (usually) pass a pointer when you are
passing an array or structure. Before ANSI C, you couldn't pass
structures directly, only through pointers. ANSI allows it, but
it isn't used all that often.

-- glen

In your ¶2 I believe I understand that. But my parameters if I want an array
is usually define as...

int func(int a[]){}

Then that way the paramter tells you you are wanting to pass an array. It's
more explicit than...

int func (int *a){}

Your first ¶ C is pass by value... I may just be beginning to realize what
that might entail. Like the freeaddrinfo function it wants a pointer. But
you don't pass &value to it. you actually pass the pointer

freeaddrinfo(value);
not freeaddrinfo(&value);

Because the second example is saying you are passing what a pointer is
pointing to. Does this make any sense?

Bill

 

[Barry Schwarz]

I think alot of my problem is not understanding what a function is
asking for when it's asking for a pointer. If it's asking for a pointer is
it wanting what the pointer is pointing too. memset for example.

If the function asks for a pointer value, the corresponding argument
must be a pointer value. If the function wants to access the value
pointed to, it is the function's responsibility to dereference the
pointer value. This is what happens in the vast majority of cases,
including your current favorite memset.

On the other hand, there are functions like free and realloc which ask
for a pointer value and may never dereference it.

For standard library functions, you shouldn't care. The function
description tells you what the parameters are used for and your only
concern should be to insure the argument you pass is suitable for that
use.

void *memset (void *s, int c, size_t n);

that first parameter. Is it want a pointer to something or to be passed what
the pointer is pointing to. For example.

Reason it out. Does the compiler know what a pointer to void points
to? Does memset? Do either care? Have you seen any documentation
anywhere that implies the compiler can or does change a pointer type
argument from an address to the data at that address?

int *p;

int i;
p = &i;

memset(p,0,sizeof(int));

You should be able to explain why this invokes undefined behavior
without the two statements I added.

It should not be a surprise that you can achieve the same effect with
memset(&i, 0, sizeof i);
eliminating both the need for p and dependency in the code on the type
of p or i.

or memset(&p,0,sizeof(int));

memset(&p, 0, sizeof(int*));

You should be able to explain why I made the change and why your code
could potentially invoke undefined behavior depending on certain
system characteristics.

Is such a thing as a function that wants this?

What you coded - no. What I coded is legal but
p = NULL;
accomplishes the same thing much more simply. On the other hand, if p
were a pointer to structure, then
memset(p, 0, sizeof *p);
could be potentially useful but note the lack of an ampersand.

function (int &a, short &b);

If something is defined like that would you just pass the pointer?

You would pass two pointer values, either by specify the names of
pointer objects or by using the & operand on suitable objects. What
other option do you think would work?

 

[Bill Cunningham]

[snip]

I don't know. The only thing I know is to jump into code dealing with
pointers and go from there. If I have problems come here and try to
figure it out. As is said, "C wears well as one's experience with
it..." and so on. But this seems to be a common problem that pointers
cause problems with a lot of people. I guess I'll have to go one step
at a time and when I get burned out take a break.

Bill

 

[David Brown]

[snip]

I don't know. The only thing I know is to jump into code dealing with
pointers and go from there. If I have problems come here and try to
figure it out. As is said, "C wears well as one's experience with
it..." and so on. But this seems to be a common problem that pointers
cause problems with a lot of people. I guess I'll have to go one step
at a time and when I get burned out take a break.

Bill

You are trying to learn about C pointers by guessing, randomly snipping
sections of sample code, and trial-and-error, and then expecting other
people to spoon-feed you fixes for the resulting mess. People in this
group have been extraordinarily patient with you, but there is no
possibility that you will ever learn C properly in this way.

Buy a book on C for beginners, and study it /properly/. I don't know
what your native language is (by the grammar in your posts, I'm guessing
it is not English), but you might be lucky and get one that is easy
enough for you to follow.

When you have some of the basics, and have more questions than are
answered in the book, many people here will be willing to help answer you.

 

[Ben Bacarisse]

But also you often (usually) pass a pointer when you are
passing an array or structure. Before ANSI C, you couldn't pass
structures directly, only through pointers.

It was allowed before ANSI C. However, there is (as far as I know) no
official definition of C with structure passing/returning/assigning
(they all happened at the same time) that predates ANSI C. In those
days, C was, by and large, what the Unix compiler accepted.

The C described in K&R does not have these things, but the book
explicitly flags them up as coming in a future version. Sadly, I
remember the release and the joy it brought to all who compiled with it.

<snip>

 

[Bill Cunningham]

Barry Schwarz wrote:
On Mon, 14 Apr 2014 20:32:11 -0400, "Bill Cunningham"
[snip]

I don't know. The only thing I know is to jump into code dealing
with pointers and go from there. If I have problems come here and
try to figure it out. As is said, "C wears well as one's experience
with it..." and so on. But this seems to be a common problem that
pointers cause problems with a lot of people. I guess I'll have to
go one step at a time and when I get burned out take a break.

Bill

You are trying to learn about C pointers by guessing, randomly
snipping sections of sample code, and trial-and-error, and then
expecting other people to spoon-feed you fixes for the resulting
mess. People in this group have been extraordinarily patient with
you, but there is no possibility that you will ever learn C properly
in this way.

Buy a book on C for beginners, and study it /properly/. I don't know
what your native language is (by the grammar in your posts, I'm
guessing it is not English), but you might be lucky and get one that
is easy enough for you to follow.

When you have some of the basics, and have more questions than are
answered in the book, many people here will be willing to help answer
you.

I do not know why someone would want to help someone who knew what they were
doing. That makes no sense at all to me. Or why they would want or need help
if they knew C?

 

[Bill Cunningham]

I do not know why someone would want to help someone who knew what
they were doing. That makes no sense at all to me. Or why they would
want or need help if they knew C?

So in order to get help one must not need help then. I see.

Rationality is not a popular trait in clc.

 

[glen herrmannsfeldt]

David Brown wrote:
(snip)

I do not know why someone would want to help someone who knew what
they were doing. That makes no sense at all to me. Or why they
would want or need help if they knew C?

Well, some of the discussion here is on the very fine points in the
language, some might never come up in real programs.

You will still get help here, but you are expected to do some of
the help yourself. OK, since it is baseball season and I have an
mlb.com gameday window open, you wouldn't send your little-league
kid out to batting practice with an MLB pitcher. For one, he wouldn't
learn anything, and for another it could be very dangerous if
he got hit by a pitch.

On the other hand, even the best programmers make dumb mistakes
once in a while.

In my early assembler programs, I wasn't so sure what I was doing,
but tried some things and learned how they worked. I mostly did it
from IBM manuals, and not much asking, but there was some guessing.

You should find some good books, including explanations on how
and when to use pointers in C. If you can't understand the
explanation, ask here.

-- glen

 

[Bill Cunningham]

and when to use pointers in C. If you can't understand the
explanation, ask here.

I have read C book after C book. And learned alittle. I don't know if
I'm just one of those people that learns hands on or what. It seems like
that's the way I learn.

 

[Michael Angelo Ravera]

I think alot of my problem is not understanding what a function is
asking for when it's asking for a pointer. If it's asking for a pointer is
it wanting what the pointer is pointing too. memset for example.

void *memset (void *s, int c, size_t n);

that first parameter. Is it want a pointer to something or to be passed what
the pointer is pointing to. For example.

int *p;
memset(p,0,sizeof(int));

or memset(&p,0,sizeof(int));

Is such a thing as a function that wants this?
function (int &a, short &b);

If something is defined like that would you just pass the pointer?

OK. Your fundamental lack of understanding is that of data representation. Programming in C is designed for people who understand data representation and not for people who don't.

What's a pointer? A pointer is a value that represents where something is stored.

For most (it is intended and they are working on "all") data items in C, there is a way to obtain a pointer to it. This is often referred to as "dereferencing". For most pointers, there is a way to obtain the data (or first item of data) to which it points, if such a reference would refer to something in the representation (otherwise you will have some kind of a problem --ranging from "that's not what I meant" on up to "halt and catch fire".)

So, let's look at memset: The first argument is a pointer to the beginning of the of the area of memory that you want to set to the particular value.
This area may represent an array, a structure, a single data item ("scalar"), or a memory map of some device. The function doesn't know or really careall that much. If you tell it to blast '*'s to the memory map of a printerand your program has proper access to it, you will probably get a bunch of'*'s on the paper (or whatever the '*'s represent to the printer). The function memset just puts however many copies you say of whatever character you say in an area of memory that you say. It doesn't know what it is doing or whether it is doing what you want. It is very much like a screwdriver -- really good for tightening or removing screws, usable for drilling holes, can be made to pry things off of each other, really bad for jamming into your eye.

So, the trick in getting memset to do what you want (like putting spaces into a character array where you are going to place a name of something, or putting '*'s into any eye catcher string), is to obtain a pointer to the beginning of the area, determining to what value you want to set everything, and obtaining a byte count for the thing that you want to set.

Usually, you can obtain a pointer to the beginning of the representation ofsomething simply dereferencing it with the "&" operator and you can obtainthe size of the representation of the that thing by using either "sizeof" or, with some more complicated things, by using "offsetof". But, you need to supply sizeof with the representation of the entire thing that you want to set. In some cases, you will use offsetof (thing, last_variable_thing[number_of_last_variable_things]).

 

[Kaz Kylheku]

On Monday, April 14, 2014 5:32:11 PM UTC-7, Bill Cunningham wrote:

OK. Your fundamental lack of understanding ...

.... is a trolling project that is over a decade long, and counting.

Here is post from January 2003:

https://groups.google.com/forum/#!original/comp.lang.c/XCNaLai1vmk/kccdq6h7B0kJ

The news article posting personality calling itself "Bill Cunningham" does not
retain a single fucking new shred of knowledge ever thrown at it.

 

[Bill Cunningham]

OK. Your fundamental lack of understanding is that of data
representation. Programming in C is designed for people who
understand data representation and not for people who don't.

What's a pointer? A pointer is a value that represents where
something is stored.

For most (it is intended and they are working on "all") data items in
C, there is a way to obtain a pointer to it. This is often referred
to as "dereferencing". For most pointers, there is a way to obtain
the data (or first item of data) to which it points, if such a
reference would refer to something in the representation (otherwise
you will have some kind of a problem -- ranging from "that's not what
I meant" on up to "halt and catch fire".)

So, let's look at memset: The first argument is a pointer to the
beginning of the of the area of memory that you want to set to the
particular value.
This area may represent an array, a structure, a single data item
("scalar"), or a memory map of some device. The function doesn't know
or really care all that much. If you tell it to blast '*'s to the
memory map of a printer and your program has proper access to it, you
will probably get a bunch of '*'s on the paper (or whatever the '*'s
represent to the printer). The function memset just puts however many
copies you say of whatever character you say in an area of memory
that you say. It doesn't know what it is doing or whether it is doing
what you want. It is very much like a screwdriver -- really good for
tightening or removing screws, usable for drilling holes, can be made
to pry things off of each other, really bad for jamming into your
eye.

So, the trick in getting memset to do what you want (like putting
spaces into a character array where you are going to place a name of
something, or putting '*'s into any eye catcher string), is to obtain
a pointer to the beginning of the area, determining to what value you
want to set everything, and obtaining a byte count for the thing that
you want to set.

Usually, you can obtain a pointer to the beginning of the
representation of something simply dereferencing it with the "&"
operator and you can obtain the size of the representation of the
that thing by using either "sizeof" or, with some more complicated
things, by using "offsetof". But, you need to supply sizeof with the
representation of the entire thing that you want to set. In some
cases, you will use offsetof (thing,
last_variable_thing[number_of_last_variable_things]).

Thanks Michael but I will have to admit; a lot of this is a little over
my head right now. There's just something about these pointers. But I guess
that's a common problem with persons who use C. I am a hobbyist. I have been
trying to learn C on and off for a long time. I take a break for a while and
I am finding learn as you go seems to slowly work. I appreciate any help
though. It doesn't go unnoticed.

Bill

 

[Kaz Kylheku]

You have to say "represents" dont you? Why not just say it points to
where the data is? Sheesh.

Because then he would be dropping less of a clue that he's not a genuine
retard.

 

[Kenny McCormack]

Thanks Michael but I will have to admit; a lot of this is a little over
my head right now. There's just something about these pointers. But I guess
that's a common problem with persons who use C. I am a hobbyist. I have been
trying to learn C on and off for a long time. I take a break for a while and
I am finding learn as you go seems to slowly work. I appreciate any help
though. It doesn't go unnoticed.

Bill

No. It doesn't ... lol.[/QUOTE]

What I love is the implication in Bill's posts (the one above and others
over the years) that there is some big do-ha to learning C. For crying out
loud, it's a programming language; it's not like it's a spoken foreign
language (which really does take years of study and effort to learn). In
the real world, people are expected to pick up programming languages and
start using them in a matter of days.

BTW, welcome "Michael Angelo Ravera" to the small group of twisted nuts who
post in both CLC and RGB. There's a lot of similarity between the two
groups - both arenas (C and Bridge) offer a lot of opportunities for
cliquishness and snarkiness. As far as I know, there are 3 in the club at
present (me, Martin Ambuhl, and now you).

P.S. I'm pretty sure that "Bill Cunningham" is one of the regs in
disguise. The reasons for this should be obvious.

--
Is God willing to prevent evil, but not able? Then he is not omnipotent.
Is he able, but not willing? Then he is malevolent.
Is he both able and willing? Then whence cometh evil?
Is he neither able nor willing? Then why call him God?
~ Epicurus

 

[Kenny McCormack]

Because then he would be dropping less of a clue that he's not a genuine
retard.

I'm still trying to parse this last sentence. What does it mean?

Help me out here, please...

--
"I heard somebody say, 'Where's Nelson Mandela?' Well,
Mandela's dead. Because Saddam killed all the Mandelas."

George W. Bush, on the former South African president who
is still very much alive, Sept. 20, 2007

 

[Bill Cunningham]

Kenny McCormack wrote:

[snip]

What I love is the implication in Bill's posts (the one above and
others over the years) that there is some big do-ha to learning C.

Try working with not well documented functions. Or ones that are new to
you in some cases. A non standard C function getaddrinfo() gave me fits when
I first started working with it. And a lot of it was not fully understanding
not so much C syntax as what to do with the function.

I do not consider myself a regular poster here on clc but a regular
lurker. And I do post from time to time, though I've been here a while.

Bill

 

[Osmium]

Try working with not well documented functions. Or ones that are new to
you in some cases. A non standard C function getaddrinfo() gave me fits
when I first started working with it. And a lot of it was not fully
understanding not so much C syntax as what to do with the function.

Of course the documentation is very often bad, I think we all agree on that.

But you have two problems that precedes that problem. You refuse to follow
a structured, reasonable approach to learning the fundamentals. You refuse
to buy a book appropriate to your mind set and background. Your are like
someone trying to learn Finnish and your method is to do so by writing the
great Finnish novel. It Doesn't Work That Way!

It is as though you had a combination lock you were trying to open. You
keep trying to guess the combination. Meanwhile, there is this great book
on lock picking setting on the table. Open the damn book and start reading.
The author's name is King.

http://www.amazon.com/Programming-Modern-Approach-2nd-Edition/dp/0393979504

You can even get an old edition for 97 cents plus postage "and stuff", less
than $5 total.
Search "k n king" amazon.

PS. Yes, I know lock picking is kind of incompatible with a combination
lock.