why i value doesn't change ?

[sophia.agnes]

Dear all,

consider the following program

#include<stdio.h>
#include<unistd.h>

main()
{
int pid,*i,j;

i = &j;
*i= 10;

pid = fork();

if(pid == 0)
{
printf("\n address of i = %p",i);
printf("\n initially i value in child = %d",*i);
*i = *i +10;
printf("\n after incrementation i value in child = %d",*i);
puts("Child terminated....");
}
else
{
wait( (int*) 0);

printf("\n address of i = %p",i);
printf("\n value of i in parent = %d",*i);
}

i am getting o/p as:-

address of i = 0xbfe6c01c
initially i value in child = 10
after incrementation i value in child = 20
Child terminated....

address of i = 0xbfe6c01c
value of i in parent = 10

the address of i is same in both parent and child process but
the change made in child process is not reflected in the
parent process why ?

 

[Marco Manfredini]

the address of i is same in both parent and child process but
the change made in child process is not reflected in the
parent process why ?

Looks like you have yet to understand what "process" means or fork()
does. On many system, in particular those which have "fork()", every
process has it's own memory, the OS sets things up that the memory
address 0xbfe6c01c in process A points to a different RAM address than
the same address in process B. That way every process can assume that
nobody else will write into the memory of the process.

You need threads or shared memory on these systems, if you want
concurrent modification.

 

[Willem]

(e-mail address removed) wrote:
) the address of i is same in both parent and child process but
) the change made in child process is not reflected in the
) parent process why ?

Suppose you're living in a house. You turn the bathroom light on. Then,
your kid goes to live on her own, a few houses down the block. She decides
to turn the bathroom light off, and she then writes in her blog 'I just
turned off the bathroom light'. You check if this is true, but for some
odd reason the bathroom light is still on.

How is this possible ? The bathroom light switch is at exactly the same
location in the house, after all.


SaSW, Willem
--
Disclaimer: I am in no way responsible for any of the statements
made in the above text. For all I know I might be
drugged or something..
No I'm not paranoid. You all think I'm paranoid, don't you !
#EOT

 

[CBFalconer]

consider the following program

#include<stdio.h>
#include<unistd.h>

Reading stops here. There is no <unistd.h> in ISO standard C, so
you are off topic. Try a Unix or Linux group.

 

[Kenny McCormack]

Looks like you have yet to understand what "process" means or fork()
does. On many system, in particular those which have "fork()", every
process has it's own memory, the OS sets things up that the memory
address 0xbfe6c01c in process A points to a different RAM address than
the same address in process B. That way every process can assume that
nobody else will write into the memory of the process.

What? No standard CLC answers? Nobody pointing out that some of the
putative header files mentioned in this post do not exist - are not C,
etc, etc? No pointing out that we have no idea what "fork" (and, for
that matter, what about "spoon" and "knife") and "wait" (and "hesitate",
"stop", and others...) are? Pointing out that we would need the OP to
supply complete source code (in completely compliant ANSI C, of course)
for these totally unknown functions, before we can even consider
responding?

Come on, guys. You're slipping...

You need threads or shared memory on these systems, if you want
concurrent modification.

All completely OT, of course.

 

[Marco Manfredini]

>
> What? No standard CLC answers? Nobody pointing out that some of the
> putative header files mentioned in this post do not exist - are not C,
> etc, etc? No pointing out that we have no idea what "fork" (and, for
> that matter, what about "spoon" and "knife") and "wait" (and "hesitate",
> "stop", and others...) are? Pointing out that we would need the OP to
> supply complete source code (in completely compliant ANSI C, of course)
> for these totally unknown functions, before we can even consider
> responding?

I'm just a curios visitor, bewildered by the customs of the local
natives. Anyway, I could point out that this question *is* on-topic,
because the OP found an external library function which seems to
undermine C's memory model *completely*, an inexplicable behavior. A
beginner may even wonder, if his/her inchoate proficiency of the
language is the cause of a fallacy only resolvable by the courtesy of
the knowing.

cheers

 

[jacob navia]

What? No standard CLC answers? Nobody pointing out that some of the
putative header files mentioned in this post do not exist - are not C,
etc, etc? No pointing out that we have no idea what "fork" (and, for
that matter, what about "spoon" and "knife") and "wait" (and "hesitate",
"stop", and others...) are? Pointing out that we would need the OP to
supply complete source code (in completely compliant ANSI C, of course)
for these totally unknown functions, before we can even consider
responding?

Come on, guys. You're slipping...

Its sunday...

Happily they are sleeping late

 

[CBFalconer]

.... snip ...

I'm just a curios visitor, bewildered by the customs of the local
natives. Anyway, I could point out that this question *is*
on-topic, because the OP found an external library function which
seems to undermine C's memory model *completely*, an inexplicable
behavior. A beginner may even wonder, if his/her inchoate
proficiency of the language is the cause of a fallacy only
resolvable by the courtesy of the knowing.

This has nothing to do with the topicality, which is restricted to
ISO standard C. The 'finding' of another external library doesn't
affect that. As long as questions etc. deal with standard C we all
know what the basis is, and no guesswork is needed.

The 'knowing' can be restricted to the reading of the standard,
which is generally available. However, this may be fairly tough on
the newbie.

 

[santosh]

What? No standard CLC answers? Nobody pointing out that some of the
putative header files mentioned in this post do not exist - are not C,
etc, etc? No pointing out that we have no idea what "fork" (and, for
that matter, what about "spoon" and "knife") and "wait" (and
"hesitate", "stop", and others...) are? Pointing out that we would
need the OP to supply complete source code (in completely compliant
ANSI C, of course) for these totally unknown functions, before we can
even consider responding?

Come on, guys. You're slipping...

Not to worry. It was a slight oversight, now rectified.

 

[Joachim Schmitz]

Reading stops here. There is no <unistd.h> in ISO standard C, so
you are off topic. Try a Unix or Linux group.

Which she did...

Bye, Jojo

 

[Kelsey Bjarnason]

[snips]

I'm just a curios visitor, bewildered by the customs of the local
natives. Anyway, I could point out that this question *is* on-topic,
because the OP found an external library function which seems to
undermine C's memory model *completely*, an inexplicable behavior.

Not really.

Each process ends up being, in essence, its own separate animal, using its
own variables - so one wouldn't expect data modified in B to be reflected
in A. Taken alone, each of A and B work consistently with what one would
expect from the language - apart from the use of the non-standard elements.

That is, the parent and child each work as if they were completely
unrelated programs, self-contained - exactly what you'd expect. The fact
one is spawned by the other doesn't really enter into it.

What *would* be weird is if modifying variables in B actually _did_ cause
variables in A to be altered; that _would_ mess with the whole
conceptualization of how variables work in C. Usually you have to use
some sort of inter-process communication, shared memory or suchlike to
accomplish that sort of thing.

 

[Marco Manfredini]

[snips]

I'm just a curios visitor, bewildered by the customs of the local
natives. Anyway, I could point out that this question *is* on-topic,
because the OP found an external library function which seems to
undermine C's memory model *completely*, an inexplicable behavior.

Not really.

[...]

Then please give an implementation of fork() in ISO-C.

That's the point about "undermine C's memory model". The OP hit an
library function which exposes a functionality that can not be explained
or done within ISO-C's memory model and now she tries to find out if she
possibly misunderstood something.

Or put in a different form:

--- Exercise 117 (advanced) ---
Write a function v such that the following program:

extern int v();
int main(int argc, const char**argv)
{
const char *s=argv[1];
if (v()) s=argv[2];
printf("%s\n",s);
}

prints any two command line arguments in reverse order.

Can it be done? Please cite from ISO/IEC 9899:1990 to support your
arguments.
---

 

[James Kuyper]

Kelsey Bjarnason wrote:
....

That is, the parent and child each work as if they were completely
unrelated programs, self-contained - exactly what you'd expect. ...

That is what some people would expect; contrary expectations are the
reason for the message that started this thread.

 

[pete]

I'm just a curios visitor, bewildered by the customs of the local
natives. Anyway, I could point out that this question *is* on-topic,
because the OP found an external library function which seems to
undermine C's memory model *completely*, an inexplicable behavior. A
beginner may even wonder, if his/her inchoate proficiency of the
language is the cause of a fallacy only resolvable by the courtesy of
the knowing.

As far as posted C code goes:
If posted C code can be part of the kind of
code that can be translated and executed
on any conforming C implementation, then it's on topic.
If posted C code can't be part of that kind of code,
then only the reason why it can't be, is on topic.

I couldn't compile original posted code.

I don't have unistd.h
It's possible that various individuals may each have
a different version of unistd.h
unistd.h isn't described in the C standard.

On the other hand, I do have stdio.h
I can discuss code which depends on stdio.h with *anyone*
because the C standard specifies the minimum required
characteristics of stdio.h.

 

[Kenny McCormack]

As far as posted C code goes:
If posted C code can be part of the kind of
code that can be translated and executed
on any conforming C implementation, then it's on topic.
If posted C code can't be part of that kind of code,
then only the reason why it can't be, is on topic.

Translation: CLC is, in and of itself, a completely useless group; it's
only purpose is to serve as a gateway to other, useful groups.

 

[Marco Manfredini]

As far as posted C code goes:
If posted C code can be part of the kind of
code that can be translated and executed
on any conforming C implementation, then it's on topic.

A conforming implementation allows the user to use code that is declared
in header files and defined elsewhere. Quiz: Judging only the behavior
described by the OP, can this mysterious "fork()" function be
implemented in ISO-C? If not, what does that mean? Undefined behavior?

 

[Willem]

Marco wrote:
) Then please give an implementation of fork() in ISO-C.

Why does the implementation have to be in ISO-C ?


SaSW, Willem
--
Disclaimer: I am in no way responsible for any of the statements
made in the above text. For all I know I might be
drugged or something..
No I'm not paranoid. You all think I'm paranoid, don't you !
#EOT

 

[Marco Manfredini]

Marco wrote:
) Then please give an implementation of fork() in ISO-C.

Why does the implementation have to be in ISO-C ?

Is this thread crossposted to comp.lang.ada?

 

[Kenny McCormack]

Marco wrote:
) Then please give an implementation of fork() in ISO-C.

Why does the implementation have to be in ISO-C ?

If it isn't, we can't talk about it here.

 

[Willem]

Kenny wrote:
) In article <[email protected]>,
)>Marco wrote:
)>) Then please give an implementation of fork() in ISO-C.
)>
)>Why does the implementation have to be in ISO-C ?
)
) If it isn't, we can't talk about it here.

The original claim was that fork() somehow *broke* standard-C.
I don't see how having to implement it in C has anything to do with it.


SaSW, Willem
--
Disclaimer: I am in no way responsible for any of the statements
made in the above text. For all I know I might be
drugged or something..
No I'm not paranoid. You all think I'm paranoid, don't you !
#EOT