How to solve this fork() example in c,

Question

I am supposed to work out this code to tell the output for my homework. Could somebody help me out? I'm not looking for the answer but, step by step instructions of how to understand this.

 int main() 
 { 
     int pid; 
     int val = 5; 

     pid = fork(); 

     if (pid == 0) val += 3; 

     if (val == 5) val++; 

     printf(“val=%d\n”, val); 
     exit(0); 
 }

What are your thoughts? Have you tried to run it yet? – Oliver Charlesworth Mar 10 '13 at 22:12 — Oliver Charlesworth, Mar 10 '13 at 22:12

Dietrich Epp · Answer 1 · 2013-03-10T22:36:48.100

1

The code will print one of the three following options:

val=6

val=8
val=6

val=6
val=8

It depends on which write() syscall completes first: child or parent, and whether the child process is successfully created (it might fail).

edited Mar 10 '13 at 22:36

answered Mar 10 '13 at 22:17

Dietrich Epp

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Can you explain why you think `val=5` will get printed ? I understand the other 2 possibilites though which I right now think are the only valid outputs. – Tuxdude Mar 10 '13 at 22:21
@OliCharlesworth - yes there cannot be a `val = 5`, but why would there be a case where the parent process prints nothing ? – Tuxdude Mar 10 '13 at 22:26
@Tuxdude: Because the fork may fail. – Oliver Charlesworth Mar 10 '13 at 22:27
Oh, never really thought about that case, good catch - and yeah `val=6` for that case makes sens :) – Tuxdude Mar 10 '13 at 22:29

score -1 · Accepted Answer · edited May 23 '17 at 11:49

-1

pid = fork(); 
if (pid == 0) val += 3; 
if (val == 5) val++; 
printf(“val=%d\n”, val);

Case 1: After fork() , parent gets scheduled before the child , and complete's printf() call successfully

    val = 6;         //printed by parent
    val = 8;         //printed by child

Case 2: After fork() , parent gets scheduled before the child , and complete's printf() call successfully

    val = 8;         //printed by child
    val = 6;         //printed by parent

-- Problem with printf() and fork() --

#include<stdio.h>
#include <sys/types.h>

int main()
{
    printf("Before forking");
    pid_t pid = fork();

    if (pid == 0)
        printf("child printing");
    else
        printf("parent printing");
}

The output is

term# ./a.out 
Before forkingparent printingBefore forkingchild printing

The problem above is clear that , the statement "Before forking" , is printed twice , where it should have printed only once.

The solution is

Use '\n' , to buffer each line after every printf() call.
As rightly advised here call fflush(0) , to empty all I/O buffers before forking.

edited May 23 '17 at 11:49

Community

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1

answered Mar 10 '13 at 22:27

Barath Ravikumar

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printf() is not atomic , in between the function call , the process can be preempted , happened to me once – Barath Ravikumar Mar 10 '13 at 22:29
@OliCharlesworth , do have a look here http://stackoverflow.com/questions/2530663/printf-anomaly-after-fork – Barath Ravikumar Mar 10 '13 at 22:31
That wouldn't have any relevance here. `val` is always known in this program. – teppic Mar 10 '13 at 22:34
If stdout is line buffered (which it usually is), then I don't think that can happen here, as all the newline characters are in the "correct" place. – Oliver Charlesworth Mar 10 '13 at 22:34
The `printf()` call is line-buffered, so it will call `write()` once, and the underlying `write()` will be atomic. – Dietrich Epp Mar 10 '13 at 22:38
@OliCharlesworth , yes as long as '\n' is present , it is line buffered and there will be no problems , but just so without '\n' , i thought it was a case to consider – Barath Ravikumar Mar 10 '13 at 22:42
It'd still be fine without the `\n` (both would just appear on the same line). – teppic Mar 10 '13 at 22:49
@DietrichEpp thanks for the info , will look a little more closely at printf() – Barath Ravikumar Mar 10 '13 at 22:52
@OliCharlesworth , i have edited my answer , to clarify my last comment – Barath Ravikumar Mar 10 '13 at 23:17
I'm not sure any of what you've just added is relevant, given that that isn't what the OP's code does. – Oliver Charlesworth Mar 10 '13 at 23:22
@BarathBushan: again, this cannot fail. `printf` makes one system call from each process. Since each process only writes one line, there is no issue at all. – teppic Mar 10 '13 at 23:24
@OliCharlesworth just making the OP aware of the problem between printf() and fork() , and suggesting proper solutions to handle them... – Barath Ravikumar Mar 10 '13 at 23:26

How to solve this fork() example in c,

2 Answers2