what will happen after i use free()???

[neilcancer]

i come from china,and i'm sorry that my english is very poor.

now i'm studing data structure and i met some problem about c language.

could you tell me what will happen after i use free()? i mean once i
use free() on a pointer,what will the pointer points to ?

for example:

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

void main()

{

int *p;
if(!p) printf("good");
else printf("fail");
p=(int *)malloc(100);
if(p) printf("\n\ngoo d");
else printf("\n\nfai l");

free(p);
if(!p) printf("\n\ngoo d");
else printf("\n\nfai l");

*p=100;
printf("\n\n%d" ,*p);
}

the result is:

fail

good

fail

100

why?

[Artie Gold]

ne********@gmai l.com wrote:

i come from china,and i'm sorry that my english is very poor.

now i'm studing data structure and i met some problem about c language.

could you tell me what will happen after i use free()? i mean once i
use free() on a pointer,what will the pointer points to ?

for example:

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

void main() main() returns int
{

int *p;
if(!p) printf("good"); Undefined behavior, the pointer has not been initialized. else printf("fail");
p=(int *)malloc(100); Do not cast the return value of malloc(). It is unnecessary and can
(though, in this case you have included the proper header) hide errors. if(p) printf("\n\ngoo d");
else printf("\n\nfai l");

free(p);
if(!p) printf("\n\ngoo d"); Undefined behavior. You can't even *look* at the value of a pointer (not
to mention dereference it) after it has been free()-ed. else printf("\n\nfai l");

*p=100; ...and now you've gone and dereferenced it... printf("\n\n%d" ,*p);
}

the result is:

fail
Undefined behavior means *anything* can happen. In this case the
`garbage' value of the pointer happened to be non-null.
good
You successfully allocated memory
fail
See above (undefined behavior).
100
Similarly.

HTH,
--ag
--
Artie Gold -- Austin, Texas
http://goldsays.blogspot.com
"You can't KISS* unless you MISS**"
[*-Keep it simple, stupid. **-Make it simple, stupid.]

[Robert Gamble]

neilcan...@gmai l.com wrote:

i come from china,and i'm sorry that my english is very poor.

now i'm studing data structure and i met some problem about c language.

could you tell me what will happen after i use free()? i mean once i
use free() on a pointer,what will the pointer points to ?

for example:

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

void main()
main returns int:

int main (void)

See:
http://www.c-faq.com/ansi/voidmain.html
http://www.c-faq.com/ansi/maindecl.html
http://www.c-faq.com/ansi/voidmainbooks.html
{

int *p;
if(!p) printf("good");
else printf("fail");
At this point p has not been initialized so it's value is indeterminate
and dereferencing the pointer will result in undefined behavior. In
fact, attempting to access it's value is also undefined behavior which
you have invoked in your if statement.
p=(int *)malloc(100);
Don't cast the return value of malloc.
See:
http://www.c-faq.com/malloc/mallocnocast.html

You should check the return value of malloc as it will return a null
pointer on failure.
if(p) printf("\n\ngoo d");
else printf("\n\nfai l");

free(p);
if(!p) printf("\n\ngoo d");
else printf("\n\nfai l");
Dereferencing a free'd pointer is undefined behavior, technically any
use of such a pointer, aside from assigning a new value to p, is
undefined behavior including the test in your if statement. The value
of p is not set to NULL after free is called.
See:
http://www.c-faq.com/malloc/ptrafterfree.html
*p=100;
Undefined behavior.
See:
http://www.c-faq.com/malloc/useafterfree.html
http://www.c-faq.com/malloc/ptrafterfree.html
printf("\n\n%d" ,*p);
}

the result is:

fail
Because automatic variables, including pointers, do not initialize
themselves and can contain any value, apparently that value wasn't 0 in
this case.
good
Because malloc returned a non-null value indicating success.
fail
Because free'd pointers aren't set to NULL.
100

Undefined behavior so anything can happen. In some implementations
attempting to access memory that has been free'd will result in program
termination or worse. Other implementations may not actually release
the memory but mark it as available for future calls to malloc in which
case it may be possible to continue to use the memory successfully but
it would be quite unwise to do so and is still undefined behavior.

Robert Gamble

[Thunderbird]

my textbook is <the c programming language> by Kernighan and Ritchie.so
when i read your answer i felt astonished!!

first , i think in that book ,main can return void, but you told me it
was wrong!!

second ,i that book , malloc() is casted,cuz malloc() returns void.

the third is that thank you for teaching me the knowledge about free().
from now on i will never handle a pointer after it's free()ed. thank
you!

[Vladimir S. Oka]

On Thursday 16 March 2006 08:13, Thunderbird opined (in
<11************ **********@i40g 2000cwc.googleg roups.com>):

my textbook is <the c programming language> by Kernighan and
Ritchie.so when i read your answer i felt astonished!!
Provide context! Read: <http://cfaj.freeshell. org/google/>.
first , i think in that book ,main can return void, but you told me it
was wrong!!

second ,i that book , malloc() is casted,cuz malloc() returns void.

I think you should get the Second Edition of the above title...

(I can't check all your claims now, as my K&R2 copy is in the office --
I should be there as well, but hey, it's a gray day out there...).

--
BR, Vladimir

Good night, Mrs. Calabash, wherever you are.

[Richard Heathfield]

Thunderbird said:

my textbook is <the c programming language> by Kernighan and Ritchie.so
when i read your answer i felt astonished!!

first , i think in that book ,main can return void, but you told me it
was wrong!!
Page reference, please. (Don't spend forever trying to find it, though. K&R
does not in fact have any programs in it that abuse main's return type as
you suggest.)
second ,i that book , malloc() is casted,cuz malloc() returns void.
No, malloc returns void *, not void; and it requires no cast in C. K&R, for
some strange reason, decided to test their code using a C++ compiler. In
C++, the cast is required. In C, it is not. (But in C++, you almost
certainly won't be using malloc anyway.)
the third is that thank you for teaching me the knowledge about free().
from now on i will never handle a pointer after it's free()ed. thank
you!

free(p);
p = NULL;

is okay, though. (And wise, in most cases.)
--
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)

[CBFalconer]

Thunderbird wrote:

my textbook is <the c programming language> by Kernighan and
Ritchie.so when i read your answer i felt astonished!!

first , i think in that book ,main can return void, but you told
me it was wrong!!

second ,i that book , malloc() is casted,cuz malloc() returns
void.

Look up the published errata from dmr for the book. All those
things are covered.

--
"If you want to post a followup via groups.google.c om, don't use
the broken "Reply" link at the bottom of the article. Click on
"show options" at the top of the article, then click on the
"Reply" at the bottom of the article headers." - Keith Thompson
More details at: <http://cfaj.freeshell. org/google/>
Also see <http://www.safalra.com/special/googlegroupsrep ly/>

[Jordan Abel]

On 2006-03-16, Vladimir S. Oka <no****@btopenw orld.com> wrote:

On Thursday 16 March 2006 08:13, Thunderbird opined (in
<11************ **********@i40g 2000cwc.googleg roups.com>):

my textbook is <the c programming language> by Kernighan and
Ritchie.so when i read your answer i felt astonished!!

Provide context! Read: <http://cfaj.freeshell. org/google/>.

first , i think in that book ,main can return void, but you told me it
was wrong!!

second ,i that book , malloc() is casted,cuz malloc() returns void.

I think you should get the Second Edition of the above title...

(I can't check all your claims now, as my K&R2 copy is in the office --
I should be there as well, but hey, it's a gray day out there...).

K&R2 does cast malloc in some places. Main never returns void, but
sometimes it doesn't return a value.

[Thunderbird]

Richard Heathfield 写�：

Page reference, please. (Don't spend forever trying to find it, though. K&R
does not in fact have any programs in it that abuse main's return type as
free(p);
p = NULL;

is okay, though. (And wise, in most cases.)

thank u very much.
--
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)

[Ravi Nakidi]

Hi,

This is a concept called Dangling pointer. Once u free the pointer
pointer does't point any memory location. So, the program gives a
problem. In real life after int *p; now person born then can u offer
some food correct, then he will work fine. Once that person died means
free(p) free of pointer. I can't offer any food to him correct. But, u
are offering *p=100. some food. Think, how it is possible in general
life. The same thing here. If u want once again allocate memory and use
it.
I think by this u will got. If, u want more and detailed explanation
conatct. Always welcome.

Regards,
Ravi Nakidi,
S/w Enginner.