returning address of stack variable

[Manish Tomar]

Hi All,

The following code as per my knowledge should not work:

int* some()
{
int b = 10;
return &b;
}
int main(int argc, char** argv)
{
int* test = NULL;

test = some();
printf("test ; %d\n", *test);
*test = 20;

return 0;
}

as it is returning the address of a variable on stack which should not
be valid once the function returns. However, this code seems to be
working absolutely fine in VC++ 6.0 on x86 processor. Could anyone
please explain what is correct or wrong and why is it working.

Also, in the following statement
char *p = "Hello world";
where is the memory for p allocated? is it on stack, heap or global
area and when is it freed, if so?

Thanks in advance,
Manish

[Ian Collins]

Manish Tomar wrote:

Hi All,

The following code as per my knowledge should not work:

int* some()
{
int b = 10;
return &b;
}
int main(int argc, char** argv)
{
int* test = NULL;

test = some();
printf("test ; %d\n", *test);
*test = 20;

return 0;
}

as it is returning the address of a variable on stack which should not
be valid once the function returns. However, this code seems to be
working absolutely fine in VC++ 6.0 on x86 processor. Could anyone
please explain what is correct or wrong and why is it working.
You have undefined behaviour. The value is still valid because nothing
has had a chance to overwrite it. Add a couple more function calls
before the printf and see what happens.
Also, in the following statement
char *p = "Hello world";
where is the memory for p allocated? is it on stack, heap or global
area and when is it freed, if so?

It isn't, "Hello world" is a string literal, look it up in the FAQ.

--
Ian Collins.

[Richard Heathfield]

Manish Tomar said:

Hi All,

The following code as per my knowledge should not work:

And it doesn't. Unfortunately, "not work" can have a variety of outcomes.
One of those outcomes is "what someone who thinks the code is correct would
expect to happen, up until the cost of it not working exceeds the value of
continuing to hire the person writing the code".

In other words, the code will continue to work until your boss is
demonstrating the program to an important customer, at which point it will
fail in a highly visible and embarrassing way.

--
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)

[Manish Tomar]

In statement printf("Hello world"), the printf function receives the
string literal as char* which means it receives some memory address.
Hence some memory should be used to store this string. Where is it?

[pete]

Manish Tomar wrote:

In statement printf("Hello world"), the printf function receives the
string literal as char* which means it receives some memory address.
Hence some memory should be used to store this string. Where is it?

printf("It's at %p\n", (void *)"Hello world");

--
pete

[Suman]

pete wrote:

Manish Tomar wrote:

In statement printf("Hello world"), the printf function receives the
string literal as char* which means it receives some memory address.
Hence some memory should be used to store this string. Where is it?

printf("It's at %p\n", (void *)"Hello world");

IMHO, the OP was looking for stack/heap as an answer. Which of course
if one of the most commonly occuring questions around here.I remember
having read a rather excellent post by Mr. Paul Hsieh regarding this,
sometime
back.STFW.

[Richard Bos]

"Suman" <sk*****@gmail. com> wrote:

pete wrote:

Manish Tomar wrote:

In statement printf("Hello world"), the printf function receives the
string literal as char* which means it receives some memory address.
Hence some memory should be used to store this string. Where is it?
printf("It's at %p\n", (void *)"Hello world");

IMHO, the OP was looking for stack/heap as an answer. Which of course
if one of the most commonly occuring questions around here.

The question misses the point; and any definite answer is likely to be
unreliable.
(FWIW, any string literal has static duration; the most logical place to
put them where they occur normally is neither on the stack nor on what
is inaccurately termed the heap; but this is a function parameter, so
putting _this_ string literal where normal function parameters are
passed is also a viable option; other options are not inconceivable; and
most importantly, any program which cares the slightest _where_ string
literals reside is designed broken.)
I remember having read a rather excellent post by Mr. Paul Hsieh regarding
this, sometime back.

I would be most surprised.

Richard

[Flash Gordon]

Manish Tomar wrote:

In statement printf("Hello world"), the printf function receives the
string literal as char* which means it receives some memory address.
Hence some memory should be used to store this string. Where is it?

Where ever the implementation chooses to store it. On a number of the
systems I've dealt with it would be burnt in to an EEPROM, but this is
slightly less likely on a desktop PC.

All you need to know is that the string exists for the lifetime of the
program.
--
Flash Gordon, living in interesting times.
Web site - http://home.flash-gordon.me.uk/
comp.lang.c posting guidelines and intro:
http://clc-wiki.net/wiki/Intro_to_clc

Inviato da X-Privat.Org - Registrazione gratuita http://www.x-privat.org/join.php

[pete]

Suman wrote:

pete wrote:

Manish Tomar wrote:

In statement printf("Hello world"),
the printf function receives the
string literal as char* which means it
receives some memory address.
Hence some memory should be
used to store this string. Where is it?

printf("It's at %p\n", (void *)"Hello world");

IMHO, the OP was looking for stack/heap as an answer.

"The stack" and "the heap"
aren't part of the C programming language.

--
pete

[Suman]

pete wrote:

Suman wrote:

[...]

IMHO, the OP was looking for stack/heap as an answer.

"The stack" and "the heap"
aren't part of the C programming language.

Of course not, which is why I avoided any allusion to them in
my reply. I somehow missed the whole thread, and now that
I have it, I do see that the OP _did_ mention stack/heap etc
in early on.

The problem is your reply is technically sound and correct.
But I have a gut feeling it fails to communicate with the OP.
Whence my lament.