Thread: fork() and zombies

run the following code, at first attempt, enter the number when asked...everything works fine
in the second attempt dont enter any number, let the timer process expire. then...all hell breaks loose...

Code:

/* This program checks if the user has "5" as input before the countdown is over or not*/
#include<stdio.h>
#include<stdlib.h>
#include<signal.h>

int main()
{
    int x,i=0,pid;
    char ch;
    system("clear");
    for(x=0;x<3;x++)
    {
            system("clear");
            if((pid=fork()) == 0)
            {
               
                for(i=5;i>0;i--)
                {                   
                    printf("X=%d\tenter number(ppid=%d,pid=%d)",x,getppid(),getpid());
                    printf("\t\t\t\t\t\t\t%d\a\n",i);
                    sleep(1);
                    system("clear");                                                           
                }                
            }

            else
            {       
                scanf("%d",&i);      //<--------this is my problem...
                system("clear");
                kill(pid,SIGKILL);                                  
                printf("i=%d",i);                                 
            }
  
             
            if(i==5)
            {
                printf("YES i=5\n");                
            }
            else
            {
                printf("NO i!=5\n");                
            }
            sleep(3);                
    }
    printf("THE END\n");
    return 0;    
}

This program checks if the user has "5" as input before the countdown is over or not.

when i enter a number before the timer exits the for loop...everything is ok.
but suppose i dont enter anything and the child process finishes its execution, but the parent process continues to wait for user input...
but if i dont enter anything for all the three iterations,
this is what i have

Code:

[c_d@localhost C scratchpad]$ ps -A | grep "a.out"
30245 pts/4    00:00:00 a.out
30248 pts/4    00:00:00 a.out
30269 pts/4    00:00:00 a.out
30291 pts/4    00:00:00 a.out <defunct>        #Scooby Doo?! Where are you?

each of these process would further spawn more process...it goes out of control and beyond my comprehension ,if user does not consistently give the input...

what i would want is to somehow be able to take the user input,for him,like 0 when the countdown expires and child process is ready to terminate...is that possible?

i have modified the code...

Code:

/* This program checks if the user has "5" as input before the countdown is over or not*/

#include<stdio.h>
#include<stdlib.h>
#include<signal.h>

int main()
{
    int x,i=0,pid,pid2;
    system("clear");
    for(x=0;x<3;x++)
    {
        pid=-1;pid2=-1;
        if((pid=fork()) == 0)   //child process- the countdown timer
        {
            pid=getpid();
            for(i=5;i>0;i--)    //start a 5 sec countdown
            {                   
                printf("X=%d\tenter number(ppid=%d,pid=%d)",x,getppid(),getpid());
                printf("\t\t\t\t\t\t\t%d\a\n",i);
                sleep(1);                                                                     
            }
            exit(666+x);      //exit when 5 secs are over
        }

        else      //parent process
        {       
           if((pid2=fork()) == 0)   //create a seperate child process for input
           { 
                pid2=getpid();
                printf("\n\nscanf pid2=%d\n\n",pid2);
                scanf("%d",&i);
                kill(pid,SIGKILL);     //when input is recieved kill the timer process
                 if(i==5)             //check the user input 
                {
                   printf("\nYES i=5(pid=%d) for x=%d\n\n",getpid(),x);                
                }
                else
                {
                    printf("\nNO i!=5(pid=%d) for x=%d\n\n",getpid(),x);                
                }                 
                exit(777+i);
           }     
           wait();      //wait for child process...timer process.         
           sleep(3);     //take a deep breath
           printf("\n\npid2=%d(i must execute only after wait() is over)\n\n",pid2);                                       
           if((kill(pid2,SIGKILL))==0)  //after the timer child process finishes ,if input process is still running...kill it
           {
               printf("\nNO i!=5(pid=%d) for x=%d\n\n",getpid(),x);//default msg
           }   
        }
    }
    printf("THE END(pid=%d\n",getpid());       //this should be executed when all other child processes are over.
    return 0;    
}

but still, i get a lot of unexpected behaviour this one for example

Code:

X=0	enter number(ppid=24800,pid=24802)							5


scanf pid2=24803

X=0	enter number(ppid=24800,pid=24802)							4
X=0	enter number(ppid=24800,pid=24802)							3
X=0	enter number(ppid=24800,pid=24802)							2
X=0	enter number(ppid=24800,pid=24802)							1


pid2=24803(i must execute only after wait() is over)


NO i!=5(pid=24800) for x=0 #its ok till here...it waited till timer exited then executed the rest,killing scanf process.

X=1	enter number(ppid=24800,pid=24812)	#new timer process						5


scanf pid2=24813    #new child scanf process

X=1	enter number(ppid=24800,pid=24812)							4
X=1	enter number(ppid=24800,pid=24812)							3


pid2=24813(i must execute only after wait() is over) #why didnt it wait till timer process exited?


NO i!=5(pid=24800) for x=1 #this killed my scanf before the timer expired.

X=1	enter number(ppid=24800,pid=24812)							2
X=2	enter number(ppid=24800,pid=24817)							5


scanf pid2=24818   # x=2 has started before x=1 even finished

X=1	enter number(ppid=24800,pid=24812)							1   
#the sleep(3) statement never executed again
X=2	enter number(ppid=24800,pid=24817)							4
X=2	enter number(ppid=24800,pid=24817)							3
X=2	enter number(ppid=24800,pid=24817)							2


pid2=24818(i must execute only after wait() is over)


NO i!=5(pid=24800) for x=2

THE END(pid=24800    #the parent exits , leaving the timer process an orphan
[c_d@localhost C scratchpad]$ X=2	enter number(ppid=1,pid=24817)							1

the unexpected behaviour is highlighed with red...and commentaries are written by side of output in blue...

please comment.

well,

i just changed from wait to waitpid() and there has been a lot of improvement.

Code:

#include<stdio.h>
#include<stdlib.h>
#include<signal.h>

int main()
{
    int x,i=0,pid,pid2,status,r;
    system("clear");
    for(x=0;x<3;x++)
    {
        pid=-1;pid2=-1;
        if((pid=fork()) == 0)   //child process- the countdown timer
        {
            pid=getpid();
            for(i=5;i>0;i--)    //start a 5 sec countdown
            {                   
                printf("X=%d\tenter number(ppid=%d,pid=%d)",x,getppid(),getpid());
                printf("\t\t\t\t\t\t\t%d\a\n",i);
                sleep(1);                                                                     
            }
            exit(111+x);      //exit when 5 secs are over
        }

        else      //parent process
        {       
           if((pid2=fork()) == 0)   //create a seperate child process for input
           { 
                pid2=getpid();
                //printf("\n\nscanf pid2=%d\n\n",pid2);
                scanf("%d",&i);
                kill(pid,SIGKILL);     //when input is recieved kill the timer process
                 if(i==5)             //check the user input 
                {
                   printf("\nYES i=5(pid=%d) for x=%d\n\n",getpid(),x);                
                }
                else
                {
                    printf("\nNO i!=5(pid=%d) for x=%d\n\n",getpid(),x);                
                }                 
                exit(0);
           }     
        
           r=waitpid(pid,&status); //wait for child process...timer process.         
           
           sleep(3);     //take a deep breath
           printf("\n\npid=%d,pid2=%d(wait status=%d,r=%d)\n\n",pid,pid2,status,r);                                       
           if((kill(pid2,SIGKILL))==0)  //after the timer child process finishes ,if input process is still running...kill it
           {
               printf("\nNO i!=5(pid=%d) for x=%d\n\n",getpid(),x);//since the user did not enter anything within time.display this
           }   
        }
    }
    printf("THE END(pid=%d\n",getpid());       //this should be executed when all other child processes are over.
    return 0;    
}

output:

Code:

X=0	enter number(ppid=7095,pid=7097)							5
X=0	enter number(ppid=7095,pid=7097)							4
X=0	enter number(ppid=7095,pid=7097)							3
X=0	enter number(ppid=7095,pid=7097)							2
X=0	enter number(ppid=7095,pid=7097)							1


pid=7097,pid2=7098(wait status=28416,r=7097)


NO i!=5(pid=7095) for x=0

X=1	enter number(ppid=7095,pid=7115)							5
X=1	enter number(ppid=7095,pid=7115)							4
X=1	enter number(ppid=7095,pid=7115)							3
X=1	enter number(ppid=7095,pid=7115)							2
X=1	enter number(ppid=7095,pid=7115)							1


pid=7115,pid2=7116(wait status=28672,r=7115)


NO i!=5(pid=7095) for x=1

X=2	enter number(ppid=7095,pid=7141)							5
X=2	enter number(ppid=7095,pid=7141)							4
X=2	enter number(ppid=7095,pid=7141)							3
X=2	enter number(ppid=7095,pid=7141)							2


pid=7141,pid2=7142(wait status=28672,r=0) #why is r=0 when pid=7141,in this case?


NO i!=5(pid=7095) for x=2

THE END(pid=7095
[c_d@localhost C scratchpad]$ X=2	enter number(ppid=1,pid=7141)							1 #orphaned yet again

[c_d@localhost C scratchpad]$

any ideas why the third time why return value of waitpid() was 0?

SUCCESS!!

forgot to include<sys/wait.h>

final code

Code:

#include<stdio.h>
#include<stdlib.h>
#include<signal.h>
#include <sys/wait.h>
#define CL system
#define C "clear" 

int main()
{
    int x,i=0,pid,pid2,status;
    FILE *temp,*rf;         //temp will contain the marks,rf will contain if user gave an input or not
    temp=fopen("temp","w");
    fputc(0,temp);          //set marks to 0
    fputc(0,temp);
    fclose(temp);
    CL(C);                  //CL(C)=clear screen(see #define)
    for(x=0;x<3;x++)
    {
        CL(C);
        pid=-1;pid2=-1;
        if((pid=fork()) == 0)   //child process- the countdown timer
        {
            for(i=5;i>0;i--)    //start a 5 sec countdown
            {                   
                CL(C);
                printf("X=%d\tenter a number(ppid=%d,pid=%d)",x,getppid(),getpid());
                printf("\t\t\t\t\t\t\t%d\a\n",i);
                sleep(1);                                                                                     
            }
            exit(0);      //exit when 5 secs are over
        }
        else      //parent process
        {       
           if((pid2=fork()) == 0)   //create a seperate child process for input
           { 
                int noattempts,nocorrect,rval;
                rf=fopen("rval","w");   //set value in rval file to -1
                fclose(rf);               
                rval=scanf("%d",&i);
                rf=fopen("rval","w");   
                fputc(rval,rf);        //if scanf worked correctly then value in rval file will be 1
                fclose(rf);
                kill(pid,SIGKILL);     //when input is recieved kill the timer process
                temp=fopen("temp","r");
                noattempts=fgetc(temp);
                nocorrect=fgetc(temp);
                fclose(temp);
                
                if(i==5)               //check the user input 
                {
                   printf("\nYES i=5(pid=%d) for x=%d\n\n",getpid(),x);
                   noattempts++;
                   nocorrect++;                                   
                }
                else
                {
                    printf("\nNO i!=5(pid=%d) for x=%d\n\n",getpid(),x);                
                    noattempts++;
                }
                temp=fopen("temp","w");
                fputc(noattempts,temp);  //update marks
                fputc(nocorrect,temp);
                fclose(temp);                 
                exit(0);        //exit the scanf process.no need to kill this if user entered input appropriately
           }                
           waitpid(pid,&status,0); //wait for child process...timer process.                                 
           rf=fopen("rval","r");
           if(fgetc(rf)!=1)   //check if user entered input appropriately
           {
                printf("\nAppropriate input not specified\n");
                kill(pid2,SIGKILL);   
           }
           fclose(rf);           
        }        
        printf("\n\nNext question...");
        fflush(stdout);
        sleep(3);           //take a deep breath
    }
    CL(C);
    temp=fopen("temp","r");
    printf("THE END(pid=%d)\nNo of Correct=%d\nNo of Attempts=%d\n",getpid(),fgetc(temp),fgetc(temp)); //this should be executed when all other child processes are over.
    fclose(temp);
    remove("temp");      //cleanup
    remove("rval");      
    return 0;    
}

it works nicely, but here i m using files, to "pass" values from one process to another. what other alternative do i have?

Originally Posted by cyberfish

Memory-mapped file I/O?

That's still using files, but could be a lot more efficient.

any hyperlinks you could direct me to(please dont say google it or wiki it)