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fork() function in unix

hi there
I have a homework that has a class like this
#include csapp.h
void end(void){
printf("2");
}
int main(){
if(Fork()==0)
atexit(end);
}
if(Fork==0)
printf("0");
else
printf("1");
exit(0);
}

the question is to determine which of the input below are possible. The atexit funtiontak apointer to a function and add it to a list of functions(initial empty) that will be called when the exit() is called

A. 112002
B. 211020
C. 102120
D. 122001
E. 100212

In my opinion
D is the correct answer since 1 can be called then 2 2 since fork can be at 0 0 then 00 1 but I am not very sure and wanted to ask you guys

Thanks

ozo

I disagree

dminh01

ASKER

can you guide me a bit? as I understand whenever we call Fork() we will get 2 return: 1 in the parent and one in child. Since atexit(end) will add the print "2" everytime exit() is called so the possible is if FOrk==0 then it will add 22 firstly or 00 else 11. Since the only choice I see is D which has 2200 together it will make sense to me

thanks for your explanation

ozo

Assuming you meant
void end(void){
printf("2");
}
int main(){
if(fork()==0)
atexit(end);
if(fork()==0)
printf("0");
else
printf("1");
exit(0);
}
there are more than one correct answers

dminh01

ASKER

well but can u explain how to find that value. I am new to this and still not quite get it how it is working. does fork() return twice as I mentioned?

thanks

ozo

In each process, some things are always called before exit,
but something in one process can run before or after something in a different processes

dminh01

ASKER

thanks but so does the fork has to return twice when called?

ASKER CERTIFIED SOLUTION

ozo

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dminh01

ASKER

also in process can something be called twice?
thanks

ozo

fork returns twice, once in the parent and once in the child, unless it fails

ozo

What would the possible outputs be if fork did not return twice?
for example if you had had
#define fork() (0)
or
#define fork() (1)
or
#define fork() (rand())

dminh01

ASKER

look like C will be a good choice
102120

dminh01

ASKER

I think I get it C and E
Thanks for your help. So fork alwasy return value of parent first then child

ozo

It is true that 102120 is a possible output

ozo

either the child or the parent can run first. they can run at the same time or go back and forth