GATE CSE 2005 | Question: 72

Question

Consider the following code fragment:
if (fork() == 0)
{
   a = a + 5;
   printf("%d, %p n", a, &a);
}
else
{
   a = a - 5;
   printf ("%d, %p n", a,& a);
}

Let $u,v$ be the values printed by the parent process and $x,y$ be the values printed by the child process. Which one of the following is TRUE?

• A. $u = x + 10  \text{ and } v = y$
• B. $u = x + 10 \text{ and } v != y$
• C. $u + 10 = x \text{ and } v = y$
• D. $u + 10 = x \text{ and } v != y$

Comments

Video solution:

https://youtu.be/aq2ZiD6ztZI?list=PLIPZ2_p3RNHixlIaarIXGPy-eggJQMxd_&t=6047

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Key idea:     printf() prints logical addresses.

When we fork a new process, then the new child process so created (which is an exact copy of parent, except PID etc.) has same logical addresses of global and global static variables. Also local variables and static local variables of functions share same logical addresses.

In below image, we can see child and parent has same logical addresses for different kinds of variables.

Link to code

But we need to be careful in terms of block scope due to if-else constructs or others.

In below image, we see same variables within if-else has different addresses in parent and child (Same variables [eg. local, slocal ] appear to be one after other)

Link to code

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https://web.stanford.edu/class/archive/cs/cs110/cs110.1204/static/lectures/cs110-lecture-05-understanding-execvp.pdf

GOOD READ ON FORK 

Answer 1

$u, v$ values printed by the parent process.
$u=a-5; v$ be address of $a$
$a=u+5;$
$x, y$ values printed by the child process.
$x=a+5; y$ be the address of $a$
$x=u+5+5; v=y$
$x=u+10$

Answer 2

 

option c

Answer 3

Child will execute else part as it will be if(0) so it will be a-5 , parent will execute if part as it will be if(>0) so it will be a+5 so u = x+10 ...by fork call parent and child will have identical address space so v =y so answer is C

Answer 4

•  

THIS MAY HELP YOU !!............