memory block containing pointers, aka pointer to pointer i believe
| Author |
Message |
|
Brian Hansel
#1 / 14
|
 memory block containing pointers, aka pointer to pointer i believe Hello,
My question is regards allocating a block of memory to store a list of
pointers to characters. For instance, instead of allocating memory to
store,
say, 5 integers (int *memory = (int *)malloc(sizeof(int)*5);) I would like
the
block of memory to store, say, 5 pointers to characters.
This is what I've tried:
char *memory = (char *)malloc(sizeof(char *)*5);
And then to extract the 2nd pointer I would do something like this:
char *ptr;
memcpy(ptr, memory+sizeof(char *), sizeof(char *));
This doesn't work, obviously. I've tried a number of different things, but
nothing works. Anyone know what I should be doing?? Thanks a lot,
Brian
|
| Wed, 05 May 2004 05:00:43 GMT |
|
 |
|
Emmanuel Delahay
#2 / 14
|
memory block containing pointers, aka pointer to pointer i believe
Quote: > My question is regards allocating a block of memory to store a list of
> pointers to characters. For instance, instead of allocating memory to
> store,
> say, 5 integers (int *memory = (int *)malloc(sizeof(int)*5);) I would
or simply:
int *memory = malloc(sizeof *memory * 5);
be sure you are using a C compiler and including <stdlib.h>
Quote: > like the > block of memory to store, say, 5 pointers to characters. > This is what I've tried: > char *memory = (char *)malloc(sizeof(char *)*5);
char *memory = malloc(sizeof *memory * 5);
as you can see, we have a minimum change here.
Quote: > And then to extract the 2nd pointer I would do something like this:
Note that the pointers are not initialized.
Quote: > char *ptr;
> memcpy(ptr, memory+sizeof(char *), sizeof(char *));
You are not aware of pointer arithmetics. The second location is at memory
+ 1 whatever the type.
char *ptr;
memcpy(ptr, memory + 1, sizeof(char *));
or
memcpy(ptr, memory + 1, sizeof *ptr);
More simply, you can write
char *ptr = *(memory + 1);
which is commonly noted
char *ptr = memory[1];
The latter would be my choice. Don't forget that the pointers are still
unitialized. Initiazing a pointer with the value of an uninitialized
pointer is just a non-sense. Your compiler should complain about it.
--
-ed- emdel at noos.fr
c.l.c.-FAQ http://www.eskimo.com/~scs/C-faq/top.html
C-library: http://www.dinkumware.com/htm_cl/index.html
FAQ de f.c.l.c : http://www.isty-info.uvsq.fr/~rumeau/fclc/
|
| Wed, 05 May 2004 05:25:28 GMT |
|
|
|
Kelsey Bjarnaso
#3 / 14
|
memory block containing pointers, aka pointer to pointer i believe Quote:
> Hello, > My question is regards allocating a block of memory to store a list of
> pointers to characters. For instance, instead of allocating memory to
> store,
> say, 5 integers (int *memory = (int *)malloc(sizeof(int)*5);) I would
> like the
> block of memory to store, say, 5 pointers to characters.
> This is what I've tried:
> char *memory = (char *)malloc(sizeof(char *)*5);
> And then to extract the 2nd pointer I would do something like this:
> char *ptr;
> memcpy(ptr, memory+sizeof(char *), sizeof(char *));
> This doesn't work, obviously. I've tried a number of different things,
> but nothing works. Anyone know what I should be doing?? Thanks a lot,
> Brian
If I'm reading this right, what you want isn't a pointer-to-char, but a
set of pointers-to-char, correct? Okay, so try something like this:
/* NULL tests, etc, elided for brevity */
char **ptrlist = malloc( N * sizeof( char *) );
for ( i = 0; i < N; i++ )
ptrlist[i] = malloc( size of desired char buffer )
now you can, say, strcpy( ptrlist[5], "Hello" ); assuming ptrlist[5] was
allocated to be large enough, etc.
|
| Wed, 05 May 2004 05:27:24 GMT |
|
|
|
Eric Go
#4 / 14
|
memory block containing pointers, aka pointer to pointer i believe typedef char *CharPtr;
typedef unsigned long ULong;
const ULong kNElements = 5;
CharPtr *memory = NULL;
memory = malloc( sizeof( CharPtr ) * kNElements );
memory can be used as an array of 5 character pointers.
is this what you are looking for?
--
== Eric Gorr == http://www.*-*-*.com/ :9293199 ===
"Therefore the considerations of the intelligent always include both
benefit and harm." - Sun Tzu
== Insults, like {*filter*}, are the last refuge of the incompetent... ===
|
| Wed, 05 May 2004 05:33:18 GMT |
|
|
|
Dann Corbi
#5 / 14
|
memory block containing pointers, aka pointer to pointer i believe Have a look at the C FAQ.
6.16 and related questions may prove helpful
--
C-FAQ: http://www.eskimo.com/~scs/C-faq/top.html
"The C-FAQ Book" ISBN 0-201-84519-9
C.A.P. FAQ: ftp://cap.connx.com/pub/Chess%20Analysis%20Project%20FAQ.htm
|
| Wed, 05 May 2004 05:34:49 GMT |
|
|
|
Default Use
#6 / 14
|
memory block containing pointers, aka pointer to pointer i believe Quote:
> > My question is regards allocating a block of memory to store a list of
> > pointers to characters. For instance, instead of allocating memory to
> > store,
> > say, 5 integers (int *memory = (int *)malloc(sizeof(int)*5);) I would
> or simply:
> int *memory = malloc(sizeof *memory * 5);
> be sure you are using a C compiler and including <stdlib.h>
> > like the
> > block of memory to store, say, 5 pointers to characters.
> > This is what I've tried:
> > char *memory = (char *)malloc(sizeof(char *)*5);
> or
> char *memory = malloc(sizeof *memory * 5);
This gives a block of five characters, pointed to by memory. Not the
list of character POINTERS that was desired.
Quote: > > And then to extract the 2nd pointer I would do something like this: > Note that the pointers are not initialized.
Quote: > > char *ptr; > > memcpy(ptr, memory+sizeof(char *), sizeof(char *)); > You are not aware of pointer arithmetics. The second location is at memory
> + 1 whatever the type.
> char *ptr;
> memcpy(ptr, memory + 1, sizeof(char *));
> or
> memcpy(ptr, memory + 1, sizeof *ptr);
> More simply, you can write
> char *ptr = *(memory + 1);
> which is commonly noted
> char *ptr = memory[1];
What the OP needs is this (assuming his problem description was
correct):
char **memory;
int i;
char *ptr;
memory = malloc (sizeof *memory * 5); /* now there are 5 pointers to
char */
/* error checking ignored, don't you forget it */
for (i = 0; i < 5; i++)
{
memory[i] = malloc (80); /* pull a string size out of my hat */
Quote: }
ptr = malloc[1];
For a "ragged array" of differing string lengths, you don't use a fixed
memory allocation. Say if you were reading strings from a file, and
wanted exact lengths.
Brian Rodenborn
|
| Wed, 05 May 2004 06:26:36 GMT |
|
|
|
Default Use
#7 / 14
|
memory block containing pointers, aka pointer to pointer i believe Quote:
> Hello,
> My question is regards allocating a block of memory to store a list of
> pointers to characters.
This same message was also posted to comp.lang.c++. First of all, figure
out where the message belongs. You're going to get different answers
here than there (I'll bet they tell you to drop malloc and go with
new[]);
If you decide it needs to be in multiple groups, crosspost, don't
multipost. That way, the two groups know what's going on.
Brian Rodenborn
|
| Wed, 05 May 2004 06:18:09 GMT |
|
|
|
Emmanuel Delahay
#8 / 14
|
memory block containing pointers, aka pointer to pointer i believe
Quote: >> char *memory = malloc(sizeof *memory * 5); > This gives a block of five characters, pointed to by memory. Not the
> list of character POINTERS that was desired.
--
-ed- emdel at noos.fr
c.l.c.-FAQ http://www.eskimo.com/~scs/C-faq/top.html
C-library: http://www.dinkumware.com/htm_cl/index.html
FAQ de f.c.l.c : http://www.isty-info.uvsq.fr/~rumeau/fclc/
|
| Wed, 05 May 2004 12:40:04 GMT |
|
|
|
Emmanuel Delahay
#9 / 14
|
memory block containing pointers, aka pointer to pointer i believe
Forget that post, I misread the question. Sorry about that.
--
-ed- emdel at noos.fr
c.l.c.-FAQ http://www.eskimo.com/~scs/C-faq/top.html
C-library: http://www.dinkumware.com/htm_cl/index.html
FAQ de f.c.l.c : http://www.isty-info.uvsq.fr/~rumeau/fclc/
|
| Wed, 05 May 2004 12:38:59 GMT |
|
|
|
Dave Vandervi
#10 / 14
|
memory block containing pointers, aka pointer to pointer i believe
Quote: >What the OP needs is this (assuming his problem description was >correct): >char **memory;
>int i;
>char *ptr;
>memory = malloc (sizeof *memory * 5); /* now there are 5 pointers to
>char */
>/* error checking ignored, don't you forget it */
>for (i = 0; i < 5; i++)
>{
> memory[i] = malloc (80); /* pull a string size out of my hat */
>}
>ptr = malloc[1];
ITYM `ptr = memory[1]'.
dave
--
[T]he author of this code makes no warrenty as to its effectiveness.
Is his paranoia justified?
--Chris Dollin in comp.lang.c
|
| Wed, 05 May 2004 17:24:56 GMT |
|
|
|
Emmanuel Delahay
#11 / 14
|
memory block containing pointers, aka pointer to pointer i believe
Quote: >>for (i = 0; i < 5; i++) >>{ >> memory[i] = malloc (80); /* pull a string size out of my hat */ } >>ptr = malloc[1];
> ^^^^^^
> ITYM `ptr = memory[1]'.
--
-ed- emdel at noos.fr
c.l.c.-FAQ http://www.eskimo.com/~scs/C-faq/top.html
C-library: http://www.dinkumware.com/htm_cl/index.html
FAQ de f.c.l.c : http://www.isty-info.uvsq.fr/~rumeau/fclc/
|
| Wed, 05 May 2004 18:23:44 GMT |
|
|
|
Simon Bibe
#12 / 14
|
memory block containing pointers, aka pointer to pointer i believe Quote:
> This same message was also posted to comp.lang.c++. First of all, figure
> out where the message belongs. You're going to get different answers
> here than there (I'll bet they tell you to drop malloc and go with
> new[]);
Perhaps, but any answer which pertains to C++ is off-topic here, and I
wouldn't want to have people crosspost C++ answers back here.
Quote: > If you decide it needs to be in multiple groups, crosspost, don't
> multipost. That way, the two groups know what's going on.
Assuming the OP did want to know how to do this in both C and C++ (two very
different languages), I'd say that the OP was right in multiposting in this
case. An answer which describes how to do it in C++ is quite off-topic here,
and while the answers given here probably also work in C++, they're also
probably not optimal or stylistic.
--
Simon.
|
| Wed, 05 May 2004 19:52:37 GMT |
|
|
|
Default Use
#13 / 14
|
memory block containing pointers, aka pointer to pointer i believe Quote:
> >ptr = malloc[1];
> ^^^^^^
> ITYM `ptr = memory[1]'.
Hehe. Yes of course. Too much typing, too little proofreading.
Brian Rodenborn
|
| Thu, 06 May 2004 03:22:20 GMT |
|
|
|
Default Use
#14 / 14
|
memory block containing pointers, aka pointer to pointer i believe Quote:
> > This same message was also posted to comp.lang.c++. First of all, figure
> > out where the message belongs. You're going to get different answers
> > here than there (I'll bet they tell you to drop malloc and go with
> > new[]);
> Perhaps, but any answer which pertains to C++ is off-topic here, and I
> wouldn't want to have people crosspost C++ answers back here.
No, but the C++ people shouldn't be wasting time showing him how to do
it using new[] and delete[] when he really wants a C answer.
Quote: > > If you decide it needs to be in multiple groups, crosspost, don't > > multipost. That way, the two groups know what's going on. > Assuming the OP did want to know how to do this in both C and C++ (two very
> different languages), I'd say that the OP was right in multiposting in this
> case.
More likely, he didn't have any idea how to post it correctly. I think
my assumption is far more probable than yours, so my admonishment
stands.
Brian Rodenborn
|
| Thu, 06 May 2004 03:26:18 GMT |
|
| |
|
