accessing freed memory without error

[sachin_mzn]

Hi,

Why I am not getting any run time error while accessing a freed memory
in following code. This is printing h in std output.

#include<stdio. h>
main()
{
char* buffer = (char*)malloc(6 );
strcpy(buffer," hello");
free(buffer);
printf("buffer= %c\n", *buffer);
}

-Sachin

[buda]

<sa********@yah oo.com> wrote in message
news:11******** *************@z 14g2000cwz.goog legroups.com...

Hi,

Why I am not getting any run time error while accessing a freed memory
in following code. This is printing h in std output.

#include<stdio. h>
main()
{
char* buffer = (char*)malloc(6 );
strcpy(buffer," hello");
free(buffer);
printf("buffer= %c\n", *buffer);
}

You've entered the realm of undefined behaviour with the printf, and
anything at all (including printing h on stdout) can happen. Simply don't do
it.

[dandelion]

<sa********@yah oo.com> wrote in message
news:11******** *************@z 14g2000cwz.goog legroups.com...

Hi,

Why I am not getting any run time error while accessing a freed memory
in following code.
C does not have any "run-time-errors". Accessing freed (i.e. non-allocated)
memory results in undefined behavior. If pink rabbits would jump out of your
computer, there would still be nothing wrong as far as the standard is
concerned.
This is printing h in std output.

#include<stdio. h>
main()
{
char* buffer = (char*)malloc(6 );
strcpy(buffer," hello");
free(buffer); printf("buffer= %c\n", *buffer);
If that would print the content of the last issue of PlayGirl, it would be
right, too. }

Checkout the FAQ.

http://www.eskimo.com/~scs/C-faq/q7.20.html

[Taran]

dandelion wrote:

C does not have any "run-time-errors". Accessing freed (i.e. non-allocated) memory results in undefined behavior. If pink rabbits would jump out of your computer, there would still be nothing wrong as far as the standard is concerned.
If that would print the content of the last issue of PlayGirl, it would be right, too.

}

ROTFL. Pink Rabbits (?), PlayGirl(!) issue...

[Mike Wahler]

<sa********@yah oo.com> wrote in message
news:11******** *************@z 14g2000cwz.goog legroups.com...

Hi,

Why I am not getting any run time error while accessing a freed memory
in following code.
Because your program's behavior is undefined. This means
that from the language perspective, it could do absolutely
anything at all, from 'seems to work', to a crash, anything
in between, or something else.
This is printing h in std output.
It also might have caused the monitor to fall of the desk.
Moral: Don't Do That.

#include<stdio. h>
#include <stdlib.h> /* declares 'malloc()' and 'free()' */
#include <string.h> /* declares 'strcpy()' */
main()
int main(void)
{
char* buffer = (char*)malloc(6 );
You forgot to check if 'malloc()' succeeded. If it
failed, it returns NULL, in which case the call
to 'strcpy()' would give undefined behavior, because
it would try to dereference a null pointer.

Also, your casting of 'malloc()'s return value hides a
serious error: There's no prototype for 'malloc()' in
scope, which will cause a C89 compiler to assume it
returns type 'int'. So your cast tries to convert
a pointer type to an integer type. The language
does not define such a conversion. This conversion
is implementation-defined, but in any case it will
very likely corrupt the returned pointer value, in
which case you have more undefined behavior.
strcpy(buffer," hello");
free(buffer);
printf("buffer= %c\n", *buffer);
return 0;
}

It's been a while since I've seen so many errors in such
a small piece of code. :-)

-Mike

[Martin Ambuhl]

sa********@yaho o.com wrote:

Hi,

Why I am not getting any run time error while accessing a freed memory
in following code.
Dumb luck. Just as using malloc() and free() without the prototypes
found in <stdlib.h> and using strcpy without the prototype <string.h>
gave you no error.
This is printing h in std output.
Not for me. A version of your program with other errors corrected
follows yours; note that the output is not what you claim.
#include<stdio. h>
main()
{
char* buffer = (char*)malloc(6 );
strcpy(buffer," hello");
free(buffer);
printf("buffer= %c\n", *buffer);
}

#include <stdio.h>
#include <stdlib.h> /* note */
#include <string.h> /* note */

int main(void)
{
char *buffer = malloc(6); /* note */
if (!buffer) {
fprintf(stderr, "malloc failed.\n");
exit(EXIT_FAILU RE);
}
strcpy(buffer, "hello");
free(buffer);
printf("buffer= %c\n", *buffer); /* impromper use of pointer */
return 0;
}

[output]
buffer=

BTW: please note the other threads on preserving indenting when using
google.com. You can follow those suggestions or -- even better -- stop
using google for posting.

[dandelion]

"Taran" <ta************ @honeywell.com> wrote in message
news:11******** **************@ f14g2000cwb.goo glegroups.com.. .

dandelion wrote:

C does not have any "run-time-errors". Accessing freed (i.e.

non-allocated)

memory results in undefined behavior. If pink rabbits would jump out

of your

computer, there would still be nothing wrong as far as the standard

is

concerned.
If that would print the content of the last issue of PlayGirl, it

would be

right, too.

}

ROTFL. Pink Rabbits (?), PlayGirl(!) issue...

Hey! Can't a girl have some fun, too? ;-).

[Lawrence Kirby]

On Wed, 12 Jan 2005 18:13:15 +0000, Mike Wahler wrote:

<sa********@yah oo.com> wrote in message
news:11******** *************@z 14g2000cwz.goog legroups.com...
....
int main(void)

{
char* buffer = (char*)malloc(6 );

You forgot to check if 'malloc()' succeeded. If it
failed, it returns NULL, in which case the call
to 'strcpy()' would give undefined behavior, because
it would try to dereference a null pointer.

Also, your casting of 'malloc()'s return value hides a
serious error: There's no prototype for 'malloc()' in
scope, which will cause a C89 compiler to assume it
returns type 'int'. So your cast tries to convert
a pointer type to an integer type.

Well, an int to a pointer. But that's not the problem. The code invokes
undefined behaviour because it tries to call a function with a type that
is not compatible with the function's definition. So the code has left the
rails before the cast is executed.

Lawrence

[Mike Wahler]

"dandelion" <da*******@mead ow.net> wrote in message
news:41******** *************@d reader10.news.x s4all.nl...

"Taran" <ta************ @honeywell.com> wrote in message
news:11******** **************@ f14g2000cwb.goo glegroups.com.. .

dandelion wrote:

C does not have any "run-time-errors". Accessing freed (i.e.

non-allocated)

memory results in undefined behavior. If pink rabbits would jump out

of your

computer, there would still be nothing wrong as far as the standard

is

concerned.
If that would print the content of the last issue of PlayGirl, it

would be

right, too.
> }

ROTFL. Pink Rabbits (?), PlayGirl(!) issue...

Hey! Can't a girl have some fun, too? ;-).

In that case, I'd think you'd want not the 'content',
but the pictures.

"Honey, I buy 'em for the articles, honest!"

:-)

-Mike

[Krishanu Debnath]

Lawrence Kirby wrote:

On Wed, 12 Jan 2005 18:13:15 +0000, Mike Wahler wrote:

<sa********@y ahoo.com> wrote in message
news:11****** *************** @z14g2000cwz.go oglegroups.com. ..
....

int main(void)

{
char* buffer = (char*)malloc(6 );

You forgot to check if 'malloc()' succeeded. If it
failed, it returns NULL, in which case the call
to 'strcpy()' would give undefined behavior, because
it would try to dereference a null pointer.

Also, your casting of 'malloc()'s return value hides a
serious error: There's no prototype for 'malloc()' in
scope, which will cause a C89 compiler to assume it
returns type 'int'. So your cast tries to convert
a pointer type to an integer type.

Well, an int to a pointer. But that's not the problem. The code invokes

Just wondering what will happen in a 64 bit m/c? 32 bit integer may yield a
junk 64 bit pointer value. Because 64 bit return value of malloc has now
truncated to 32 bit integer.
undefined behaviour because it tries to call a function with a type that
is not compatible with the function's definition. So the code has left the
rails before the cast is executed.

Lawrence

Krishanu