C Programming vfork return value

Question

I have to create a program that:

ask for a number
create a child process (using vfork)
calculate the square root (in the child process)
show the square root from the parent process

Here is my code

#include <stdio.h>
#include <sys/types.h>
#include <math.h>
#include <unistd.h>
#include <stdlib.h>

int main(int argc, char **argv)
{
    double n=0;
    printf("Number: "); //ask number
    scanf("%d", &n);

    pid_t pid = vfork(); //create child process

    if (pid==0)//if child process
    {
        printf("Child process started\n");
        n = sqrt(n);//calculate square root
    }
    else//parent process
    {
        printf("Returnning to parent process\n");
    printf("Square Root: %d",n);
    }       

    return 0;
}

But my code doesnt work, can anyone help me?

Any reason you're using `vfork()` instead of `fork()`? `vfork()` is generally considered obsolete. — FatalError, Mar 25 '13 at 03:23
Also, your format strings in youf `scanf()` and `printf()` calls do not match the type of `n`. — FatalError, Mar 25 '13 at 03:32

score 2 · Accepted Answer · edited May 23 '17 at 11:45

2

Why would you expect it to work? Doing anything but exec or _exit after vfork results in explicitly undefined behavior. See:

vfork() system call

And some further discussion of the horrors of vfork:

http://ewontfix.com/7/

http://www.openwall.com/lists/musl/2012/12/31/16

In case it's interesting, here is a list of possible problems with your program (manifestations of the UB):

printf in the child could horribly corrupt the parent's stdio state.
n could be stored permanently in a register, in which case there's no way the parent could see the changes made by the child
The compiler can see that n is uninitialized in the else branch, so it need not generate any code to read it at all (this branch unconditionally invoked UB by accessing an object whose value is indeterminate).

edited May 23 '17 at 11:45

Community

1
1

answered Mar 25 '13 at 03:23

R.. GitHub STOP HELPING ICE

208,859
35
376
711

how can I allow the parent to see the "n" variable on the children? – Art Grc Mar 25 '13 at 04:35
1

You can't. The fact that `vfork` *might* allow this is a **very bad side effect** of the way `vfork` works, not an effect you should attempt to use. It's inherently unreliable as I've described above. If you want to dabble in things like this, you should be explicitly allocating (via `mmap`) a shared memory mapping then using `fork`, or using threads. – R.. GitHub STOP HELPING ICE Mar 25 '13 at 04:38

score 0 · Answer 2 · answered Mar 25 '13 at 03:38

0

You're calculating the square root in the child, but trying to print it in the parent. The parent has no idea what the square root is. If you want the parent to print the square root, then you'll need to communicate that value from the child back to the parent process somehow. It seems more likely that you're supposed to print the square root in the child process.

answered Mar 25 '13 at 03:38

Caleb

124,013
19
183
272

OP (or, more likely, OP's ignorant and misguided instructor) is trying to use the fact that the parent and child share address space after `vfork` as a means to pass the result back. This is, of course, not reliable. – R.. GitHub STOP HELPING ICE Mar 25 '13 at 04:38

score 0 · Answer 3 · edited Feb 03 '14 at 14:05

   int main(int argc, char **argv) {    int  n=0;


    pid_t pid = vfork(); //create child process

    if (pid==0)//if child process
    {
        printf("Child process started\n");
       scanf("%d",&n);

        n = sqrt(n);//calculate square root

       _exit(23);
    }
    else//parent process
    {
     printf("Returnning to parent process\n");
    printf("Square Root: %ld",n);
    }

    return 0; }

C Programming vfork return value

3 Answers3