the different effect of system call fork()

the different effect of system call fork()

[Sino]

Hi, all

I don't know whether this is the right place for this email, but I
post it here since it concerns the system call of fork.
here is my fwo source code.

//fork1.cc

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

using namespace std;

int main(int argc, char **argv)
{
   cout << "main process id: "<<  getpid() << ";  Parent process pid:"
<<getppid() <<endl;
   if (pid_t pid = fork() < 0)
   {
       cout << "Faild to fork child" <<endl;
   }
   else if (pid == 0)
   {
       cout << "first level child procss pid: "<< getpid() << "paraent
pid is : "<<getppid()<<endl;
       if(pid_t spid = fork()  < 0)
       {
           cout<<"failed to create second leve child process"<<endl;
       }
       else if (spid == 0)
       {
           cout<<"second level child process pid is: "<<getpid() <<"
its parent pid is: "<<getppid()<<endl;
          while(1){
            sleep(5);
          }
       }
       else
       {
           _exit(0);
       }
   }else
   {
       if(waitpid(pid,NULL,0) != pid)
       {
           cout << "failed to wait process: " << pid <<endl;
       }

   }
   return 0;
}

Here its result after I run this program
main process id: 4831;  Parent process pid:2931
first level child procss pid: 4831paraent pid is : 2931
second level child process pid is: 4831 its parent pid is: 2931
first level child procss pid: 4832paraent pid is : 4831
second level child process pid is: 4834 its parent pid is: 4832
second level child process pid is: 4832 its parent pid is: 4831
second level child process pid is: 4833 its parent pid is: 4831


When I run ps -ef |grep fork
here is the output
work      4831  2931  0 20:45 pts/2    00:00:00 ./fork
work      4832  4831  0 20:45 pts/2    00:00:00 ./fork
work      4833  4831  0 20:45 pts/2    00:00:00 ./fork
work      4834  4832  0 20:45 pts/2    00:00:00 ./fork

//fork2.cc
#include <iostream>
#include <sys/types.h>
#include <unistd.h>
#include <wait.h>

using namespace std;

int main(int argc, char **argv)
{
   cout << "main process id: "<<  getpid() << ";  Parent process pid:
" <<getppid() <<endl;
   pid_t pid = fork();
   if (pid  < 0)
   {
       cout << "Faild to fork child" <<endl;
   }
   else if (pid == 0)
   {
       cout << "first level child procss pid: "<< getpid() << ";
paraent pid is : "<<getppid()<<endl;
       pid_t spid = fork();
       if(spid  < 0)
       {
           cout<<"failed to create second leve child process"<<endl;
       }
       else if (spid == 0)
       {
           cout<<"second level child process pid is: "<<getpid() <<";
 its parent pid is: "<<getppid()<<endl;

          while(1){
            sleep(5);
          }
       }
       else
       {
           _exit(0);
       }
   }else
   {
       if(waitpid(pid,NULL,0) != pid)
       {
           cout << "failed to wait process: " << pid <<endl;
       }

   }
   return 0;
}

the result for the same command are here
./fork
main process id: 4867;  Parent process pid:2931
first level child procss pid: 4868paraent pid is : 4867
second level child process pid is: 4869 its parent pid is: 4868

for ps -ef |grep fork

work      4869     1  0 20:50 pts/2    00:00:00 ./fork

The results are very different. For my understanding the results
should be same. but they are not. Why??

Appreciation for your kindly help.

Please let me know where I should post it if this is not the right.

Thanks very much

BRs
Zongjun

Re: the different effect of system call fork()

[Bernd Petrovitsch]

Hi!

On Wed, 2009-04-08 at 20:53 +0800, Sino wrote:
[...]
> I don't know whether this is the right place for this email, but I

It's probably not.

> post it here since it concerns the system call of fork.
> here is my fwo source code.
> 
> //fork1.cc
[...]
>    if (pid_t pid = fork() < 0)

I don't think that this line does what you want it to do.
Hint: Using a "pid_t" for the boolean result for a comparison seems
strange at best.

	Bernd
-- 
Firmix Software GmbH                   http://www.firmix.at/
mobil: +43 664 4416156                 fax: +43 1 7890849-55
          Embedded Linux Development and Services

Re: the different effect of system call fork()

[Nick Andrew]

On Wed, Apr 08, 2009 at 03:08:48PM +0200, Bernd Petrovitsch wrote:
> >    if (pid_t pid = fork() < 0)
> 
> I don't think that this line does what you want it to do.
> Hint: Using a "pid_t" for the boolean result for a comparison seems
> strange at best.

The "problem" is due to a misunderstanding of C/C++ operator precedence.

"(pid = fork() < 0)" binds like "pid = (fork() < 0)" and so pid always
has a value of zero and so both parent and child process run the "first
level child process" code ... soon there are 4 processes all confused
about who they are.

Nick.