struggling to use calloc and realloc

[dagger]

Hi there. I'm using C under FreeBSD with the gcc compiler and am having a
bit of trouble using the calloc and realloc calls.

As an example the code snippet:

#include <stdio.h>

int main() {

char *ptr;

ptr = (char *) calloc(1, sizeof(char));

printf( "initial size (1 char) = %d\n", sizeof(ptr) );

ptr = (char *) realloc(ptr, sizeof(char)*10 );

printf( "new size (10 chars) = %d\n", sizeof(ptr) );

return 0;
}
and yet when I run it I get a size of 4 for each printf even though
initially I allocated only the size of 1 char, and after reallocation there
should be room for 10 bytes...? Or where am I making a mistake in my
reasoning?

Thanks for any help :)

[Alex]

Alex <al*******@hotm ail.com> wrote:

Christopher Benson-Manica <at***@nospam.c yberspace.org> wrote:

Kevin Goodsell <us************ *********@never box.com> spoke thus:

all-bits-zero is not
guaranteed to be the representation of 0 for any type other than the
character types.

It may be a trap representation for integer types, yes?

Signed integer types. Yes.

No!

I should have read the question first.

All bits ONE can be a trap representation in one's complement.

Alex

[Richard Heathfield]

dagger wrote:

Hi there. I'm using C under FreeBSD with the gcc compiler and am having a
bit of trouble using the calloc and realloc calls.

As an example the code snippet:

#include <stdio.h>
OH NO! You forgot to get yourself a prototype for calloc and realloc. You
can get these most simply by doing this:

#include <stdlib.h>

int main() {

char *ptr;

ptr = (char *) calloc(1, sizeof(char));
If you hadn't added this cast, you would have got a very useful diagnostic.
Note that (provided you remember to #include <stdlib.h>) the above is
functionally equivalent to:

ptr = calloc(1, sizeof *ptr);

which is rather neater, easier to type, and easier to maintain.
printf( "initial size (1 char) = %d\n", sizeof(ptr) );

You are misinterpreting the result of the sizeof operator. It yields the
size of the /pointer/, not the size of the thing to which it points.

--
Richard Heathfield : bi****@eton.pow ernet.co.uk
"Usenet is a strange place." - Dennis M Ritchie, 29 July 1999.
C FAQ: http://www.eskimo.com/~scs/C-faq/top.html
K&R answers, C books, etc: http://users.powernet.co.uk/eton

[Kevin Goodsell]

Alex wrote:

Alex <al*******@hotm ail.com> wrote:

Christopher Benson-Manica <at***@nospam.c yberspace.org> wrote:

It may be a trap representation for integer types, yes?

Signed integer types. Yes.

No!

I should have read the question first.

All bits ONE can be a trap representation in one's complement.

You seem to be implying that all-bits-0 may not be a trap representation
for any integer type. If I am misunderstandin g, please clarify.
Otherwise, I'd be interested in how you justify this claim. My reading
of the standard so far strongly suggests that any non-character integer
type can have padding bits, and that those bits can be used to form a
trap representation. Furthermore, it does not specify what values these
padding bits take in any circumstance, and as far as I've seen does not
specify that all-padding-bits-0 (and all value bits "don't cares")
cannot be a trap representation.

-Kevin
--
My email address is valid, but changes periodically.
To contact me please use the address from a recent posting.

[Martin Ambuhl]

dagger wrote:

Hi there. I'm using C under FreeBSD with the gcc compiler and am having a
bit of trouble using the calloc and realloc calls.

As an example the code snippet:

#include <stdio.h>
You forgot
#include <stdlib.h>

int main() {

char *ptr;

ptr = (char *) calloc(1, sizeof(char));
I think you mean
ptr = calloc(1,1);
or
ptr = malloc(1);
or
ptr = calloc(1, sizeof *ptr);
or
ptr = malloc(*ptr);
Your cast serves no useful function; all it does is mask your error in
failing to #include <stdlib.h>.
sizeof(char) is 1 by definition. The 3rd and 4th options above are in case
the type of ptr should be changed later to some other kind of pointer.
Calling calloc() instead of malloc() when you don't need to initialize the
allocated array to "all bit zero" is wasteful.
Failing to check whether malloc(), calloc(), or realloc() succeeded is a
severe design error.

printf( "initial size (1 char) = %d\n", sizeof(ptr) );
This tells you what the size of a pointer to char is, so should be rewritten
printf("Size of pointer to char is %u\n", (unsigned) sizeof ptr);
Since sizeof yields a size_t, an unsigned integer (not necessarily an
unsigned int), a cast should always be used unless you have the new C99
specifier modifiers "%zu" and family. It would be safer to use "%lu" with
(unsigned long) or "%llu" with (unsigned long long) rather than plain "%d"
or "%u".
ptr = (char *) realloc(ptr, sizeof(char)*10 );
See comments on the calloc() call.
printf( "new size (10 chars) = %d\n", sizeof(ptr) );
See comments on the previous printf(). You are _still_ printing the size
of a pointer to char.
return 0;
}

All of this is covered in the FAQ. You should learn to check the FAQ
always before posting.
--
Martin Ambuhl

[Martin Ambuhl]

Servé Lau wrote:

"dagger" <fe****@mweb.co .za> wrote in message
news:3f******** @news1.mweb.co. za...

Hi there. I'm using C under FreeBSD with the gcc compiler and am having a
bit of trouble using the calloc and realloc calls.

As an example the code snippet:

#include <stdio.h>

Didn't you get a warning? For realloc and calloc you need stdlib.h too.

His casting the return values from realloc and calloc would, for many
compilers, supress any warning. And make sure that his code was wrong, too.

--
Martin Ambuhl

[Randy Howard]

In article <br**********@s parta.btinterne t.com>,
do******@addres s.co.uk.invalid says...

printf( "initial size (1 char) = %d\n", sizeof(ptr) );

You are misinterpreting the result of the sizeof operator. It yields the
size of the /pointer/, not the size of the thing to which it points.

Note also that it returns a size_t, which is not guaranteed to fit in
the "%d" specified in the printf() call.

--
Randy Howard _o
2reply remove FOOBAR \<,
_______________ _______()/ ()_____________ _______________ _______________ ___
SCO Spam-magnet: po********@sco. com

[Irrwahn Grausewitz]

Kevin Goodsell <us************ *********@never box.com> wrote:

Christopher Benson-Manica wrote:

Kevin Goodsell <us************ *********@never box.com> spoke thus:

all-bits-zero is not
guaranteed to be the representation of 0 for any type other than the
character types.

It may be a trap representation for integer types, yes?

I think I've heard conflicting reports from different experts, or
possibly I've misunderstood some of what I've heard. It's one of those
things I've been meaning to look up for myself, but I'm afraid it'll
take me hours to find and decrypt all the relevant sections. :/

I recall there is this Defect Report #263 filed:
( http://anubis.dkuug.dk/jtc1/sc22/wg1...ocs/dr_263.htm )

[ idiomatic memset(..., 0, ...) and calloc examples ]

Suggested[1] Technical Corrigendum

Append to 6.2.6.2#5:

For any integer type, the object representation where all the
bits are zero shall be a representation of the value zero in
that type.

[1] The last subheading in this DR should read: "Proposed
Technical Corrigendum", checked with comp.std.c recently.
Regards
--
Irrwahn Grausewitz (ir*******@free net.de)
welcome to clc : http://www.angelfire.com/ms3/bchambl...me_to_clc.html
clc faq-list : http://www.eskimo.com/~scs/C-faq/top.html
acllc-c++ faq : http://www.contrib.andrew.cmu.edu/~a...acllc-c++.html

[Kevin Goodsell]

Irrwahn Grausewitz wrote:

I recall there is this Defect Report #263 filed:
( http://anubis.dkuug.dk/jtc1/sc22/wg1...ocs/dr_263.htm )

[ idiomatic memset(..., 0, ...) and calloc examples ]

Suggested[1] Technical Corrigendum

Append to 6.2.6.2#5:

For any integer type, the object representation where all the
bits are zero shall be a representation of the value zero in
that type.

[1] The last subheading in this DR should read: "Proposed
Technical Corrigendum", checked with comp.std.c recently.

So what is the status of this report? Is it accepted as a defect? I'm
not familiar with how the C committee handles defect reports, but C++
defects are usually given a status that indicates if it has been
accepted (or rejected) as a defect and what plans are in the works to
fix it. I don't see anything like that on the page you linked.

-Kevin
--
My email address is valid, but changes periodically.
To contact me please use the address from a recent posting.

[Kevin Goodsell]

Randy Howard wrote:

In article <br**********@s parta.btinterne t.com>,
do******@addres s.co.uk.invalid says...

printf( "initial size (1 char) = %d\n", sizeof(ptr) );

You are misinterpreting the result of the sizeof operator. It yields the
size of the /pointer/, not the size of the thing to which it points.

Note also that it returns a size_t, which is not guaranteed to fit in
the "%d" specified in the printf() call.

size_t is guaranteed NOT to fit %d... unless size_t happens to promote
to int, which is probably unlikely.

-Kevin
--
My email address is valid, but changes periodically.
To contact me please use the address from a recent posting.

[Keith Thompson]

Kevin Goodsell <us************ *********@never box.com> writes:

Randy Howard wrote:

In article <br**********@s parta.btinterne t.com>,
do******@addres s.co.uk.invalid says...

printf( "initial size (1 char) = %d\n", sizeof(ptr) );

You are misinterpreting the result of the sizeof operator. It yields the
size of the /pointer/, not the size of the thing to which it points.

Note also that it returns a size_t, which is not guaranteed to fit in
the "%d" specified in the printf() call.

size_t is guaranteed NOT to fit %d... unless size_t happens to promote
to int, which is probably unlikely.

It depends on what you mean by "fit". On many systems, size_t happens
to be the same size as int (say, 32 bits), and the following:

printf("sizeof( foo) = %d\n", sizeof(foo));

is likely to work as expected as long as sizeof(foo) doesn't exceed
INT_MAX (assuming an appropriate declaration of foo, of course).

Having said that, it's still undefined behavior; working as expected
is just one of the infinitely many possible results of undefined
behavior. To avoid undefined behavior, you can do one of the
following:

/* Approach 1 */
printf("sizeof( foo) = %d\n", (int)sizeof(foo ));

which will work if you happen to know that sizeof(foo) will never
exceed INT_MAX; or

/* Approach 2 */
printf("sizeof( foo) = %lu\n", (unsigned long)sizeof(foo ));

which is guaranteed to work in C90, but could conceivably fail in C99
if size_t is bigger than unsigned long and sizeof(foo) > ULONG_MAX; or

/* Approach 3 */
printf("sizeof( foo) = %zu\n", sizeof(foo));

which will work in C99 (more precisely, if your C library's printf
implementation supports the 'z' length modifier), but not in C90.

In general, approach 2 is better than approach 1, and is unlikely to
fail in practice.

--
Keith Thompson (The_Other_Keit h) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://www.sdsc.edu/~kst>
Schroedinger does Shakespeare: "To be *and* not to be"
(Note new e-mail address)