about fork command in linux

[satyanagendra]

Hi
My aim is to create 4 individual childs for a parent and the parent will wait until the 3rd child expires. For that I write the code like this but I am not getting the output correctly. If any one know the problem in code please reply.

[code=c]
main()
{
int i = 4;
for( i =0 ;i < 4 ;i++)
{
if(fork())
{
fflush(stdout);
printf("\nparen t : %d",getpid()) ;
}
else
{
fflush(stdout);
printf("\nchild : %d frm parent:%d",getp id(),getppid()) ;
sleep(5);
exit(0);
}
}
printf("parent waiting for third child");
wait(3);
printf("parent terminating");
}

[/code]
regards

satya

[weaknessforcats]

I corrected your code tags. Use square brackets, [ ], rather than angle brackets, < >. See the reply guidelines message box.

[smalpani]

Hi
My aim is to create 4 individual childs for a parent and the parent will wait until the 3rd child expires. For that I write the code like this but I am not getting the output correctly. If any one know the problem in code please reply.

[code=c]
int main()
{
pid_t pids[4] = {0,0,0,0}; //be careful with these
pid_t parentid,childi d,myid;
int i; //why do you want to initialize these with 4
int status;
parentid = getpid(); //this is the original pid of the parent
for( i =0 ;i < 4 ;i++)
{
pids[i] =fork();
if (pid[i]<0)
{
printf("Fork for %d child failed",pids[i]+1);
if (i==2)
exit(0); //this program is just concerned abt third chld
}
if(pid[i]!=0)
{
fflush(stdout);
printf("\nparen t : %d",getpid()) ;
}
else
{
fflush(stdout);
printf("\nchild : %d frm parent:%d",getp id(),getppid()) ;
sleep(5);
exit(0);
}
}
myid = getpid();
while(myid==par entid) //enter loop only if parent
{
printf("parent waiting for third child\n"); // this line will be printed a lot remove it if you dont want this junk
childid = wait(&status);
if (childid =pids[2] ) //if the child to expire is third child
{
printf("I am parent, my favorite child died, I dont want to live anymore");
break;
}
//else some other child died we dont worry abt him
}
}

[/code]
regards

satya

Ok first lets try to identify whats wrong with your code.

1. A call to wait requires a pointer to an integer which will contain the exit status of the child. Your call will try to write the status to a memory location 3, which is most liable to crash the program.

2. You see to assume that a call like wait(3) will exit when the third process exits, I guess you are absolutely wrong, there is nothing like that.

3. I have modified your code according to me it should work but right now I am not a linux machine so I am not sure if there is still an error.

[smalpani]

int  main()
{
     pid_t pids[4] = {0,0,0,0}; //be careful with these
     pid_t parentid,childid,myid;
      int i;  //why do you want to initialize these with 4
     int status;
     parentid = getpid();   //this is the original pid of the parent
      for( i =0 ;i < 4 ;i++)
      {
          myid = getpid();
          if (myid == parentid)   //only the parent forks
          {
              pids[i] =fork();
          }
    
          if (pids[i]<0) 
          {
               printf("Fork for %d child failed",pids[i]+1);
               if (i==2)
                     exit(0); //this program is just concerned abt third chld
          }
          if(pids[i]!=0)
          {
               fflush(stdout);
               printf("\nparent : %d",getpid());
          }
         else  
         {
             fflush(stdout);
             printf("\nchild : %d frm parent:%d",getpid(),getppid());
             sleep(5);
             exit(0);
          }
      }
     myid = getpid();
     while(myid==parentid)   //enter loop only if parent
     {
         printf("parent  waiting for third child\n"); // this line will be printed a lot remove it if you dont want this junk
         childid = wait(&status);
         if (childid =pids[2] )  //if the child to expire is third child
         {
                 printf("I am parent, my favorite child died, I dont want to live anymore");
                 break;
         }
         //else some other child died we dont worry abt him
     }
}

Modified the code so that only the parent forks

[smalpani]

[CODE=c]
int main()
{
pid_t pids[4] = {0,0,0,0}; //be careful with these
pid_t parentid,childi d,myid;
int i;
int status;
parentid = getpid(); //this is the original pid of the parent
for( i =0 ;i < 4 ;i++)
{
myid = getpid();
if (myid == parentid) //only the parent forks
{
pids[i] =fork();
}

if (pids[i]<0)
{
printf("Fork for %d child failed",pids[i]+1);
if (i==2)
exit(0); //this program is just concerned abt third chld
}
if(pids[i]==0)
{
fflush(stdout);
printf("\nchild : %d frm parent:%d",getp id(),getppid()) ;
sleep(5);
exit(0);
}
}
myid = getpid();
while(myid==par entid) //enter loop only if parent
{
childid = wait(&status);
if (childid =pids[2] ) //if the child to expire is third child
{
printf("I am parent, my favorite child died, I dont want to live anymore");
break;
}
//else some other child died we dont worry abt him
}
}
[/CODE]

Corrected the code syntax and few other things, this should work without much clutter

[satyanagendra]

Corrected the code syntax and few other things, this should work without much clutter

Thank Q i will try this.

regards
Satya