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
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
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.