basic fork() program returns either seg fault or floating pt exception

Question

i have tried both fork() and vfork() and get a float pt excep with fork and a seg fault with vfork. for no apparent reason when i use vfork() it exits the child but doesnt enter the parent and then seg faults. when i use fork() it enters the parent but gives me a float pt excep. any ideas?

    #include <iostream>
    #include <unistd.h>
    #include <sys/types.h>
    #include <sys/wait.h>

    int sum = 0;
    int i = 0;
    pid_t pID;

    int main()
    {   
         pID = vfork();
         if (pID > 0)
         {    
              std::cout<< "b4 NULL"<< i<< "__"<< sum << std::endl;
              wait(NULL);
              std::cout<< "after NULL"<< i<< "__"<< sum << std::endl;
              int avg = sum/i;
              std::cout<<avg;
         }    
         else if (pID == 0)
         {    
              int J = 0;
              std::cout<<sum<<"__"<<i<<std::endl;
              i=0;
              sum=0;
              std::cout<<"enter a num:";
              std::cin>>J;      
         while(J != -1)
         {    
              sum += J;
              std::cout<<"enter a num:";
              std::cin>>J;
              i++;
         }      
         //int avg = sum/i;
         //std::cout<<avg;
         std::cout<<"exit child"<< i << "__" << sum << "__" << sum/i << std::endl;
         //return(sum);

         }
         else
         {
             std::cerr << "Failed to fork" << std::endl;
             return 1;
         }

      return 0;
    }

David Schwartz · Answer 1 · 2014-09-26T03:54:20.793

2

You can't use vfork here.

"... the behavior is undefined if the process created by vfork() either modifies any data other than a variable of type pid_t used to store the return value from vfork(), or returns from the function in which vfork() was called, or calls any other function before successfully calling _exit() or one of the exec family of functions." -- vfork

Your child modifies data, calls operator<< and all kinds of things that are not allowed. So that's not going to work.

With fork, you divide by zero:

int i = 0;
pid_t pID;

int main()
{   
     pID = fork(); /* changed to fork */
     if (pID > 0)
     {    
          std::cout<< "b4 NULL"<< i<< "__"<< sum << std::endl;
          wait(NULL);
          std::cout<< "after NULL"<< i<< "__"<< sum << std::endl;
          int avg = sum/i; /* division by zero */

No code changes i, and i's value is zero. So you divide by zero here.

edited Sep 26 '14 at 03:54

answered Sep 26 '14 at 03:49

David Schwartz

179,497
17
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the cut of code you showed is my parent function. its only supposed to modify (sum) and display avg. the child is the (pID == 0). – user2782246 Sep 26 '14 at 05:19
@user2782246 Right. Do you think that changes anything? – David Schwartz Sep 26 '14 at 15:04
i was just unsure because you mentioned my child process but showed the parent. – user2782246 Sep 29 '14 at 22:27
In the `vfork` case, the child breaks the rules. In the `fork` case, the parent does something nonsensical, dividing by zero. So neither case works. If you want to play with shared memory, use shared memory. – David Schwartz Sep 29 '14 at 22:31

score 1 · Answer 2 · answered Sep 26 '14 at 03:56

1

You are dividing by zero within parent code:

int avg = sum/i;

The block if (pID == 0) is for the child, and when you fork, the child process is created as a clone of the original running process but in separate address space, the variable sum is not shared, rather have separate copies in child and parent, no sharing. For sharing, you need to us shared memory or pipe, To use global data in same address space, you need to create / use threads and not processes.

answered Sep 26 '14 at 03:56

Dr. Debasish Jana

6,980
4
30
69

so basically processes dont share changes to global variables? i asked a couple of my old profs and they said the code should work. because when i use fork() the child joins the parent just fine but refuses to take the values from the child. when i used vfork() the child process exited but didnt seem to join the parent. are you sure theres no way to do it with processes? id rather not redo the entire prgram for threads. – user2782246 Sep 26 '14 at 05:15
@user2782246, yes processes run in separate address space, they have cloned copy of variables, but essentially they are different variables which cannot be shared. Read Shared variables among processes, pipe etc for inter-process communication mechanisms – Dr. Debasish Jana Sep 26 '14 at 07:18
@user2782246 You have to use some shared memory mechanism. Your operating system probably has lots of them. You can memory map a file before forking, you can use `shm_open`, or several other mechanisms. – David Schwartz Sep 26 '14 at 15:05
thank you for your help. im finally able to sit down with it again tonight. ill give it a shot. – user2782246 Sep 29 '14 at 22:34

basic fork() program returns either seg fault or floating pt exception

2 Answers2